DISSERTATION

THE EFFECT OF PELVIC MOBILITY ON MUSCLE
COACTIVATION DURING THE ACTIVE STRAIGHT
LEG RAISE TEST
TYLER J COLLINGS
A dissertation submitted in partial fulfilment of the requirements for the Bachelor of Physical
Education with Honours at the University of Otago, Dunedin, New Zealand
2015

I
ABSTRACT
Chronic lower back pain (LBP) is a very common, debilitating and costly condition.
Many cases of LBP are not specifically diagnosed and are labelled as non specific. The
sacroiliac joint (SIJ) may be a key structure in causing pain in a number of individuals,
leading to the classification, pelvic girdle pain (PGP). PGP is suggested to arise from
ineffective load transfer through the pelvis, which is associated with SIJ instability and
aberrant muscle activity. The aim of this study was to investigate differences in muscle
coactivation in stabilising muscles of the lumbopelvic region, for healthy individuals of
different pelvic mobility, during the active straight leg raise (ASLR) test. Twenty five
participants were assigned to one of two groups: reciprocal or unilateral, based on pelvic
movement patterns during a hip abduction and external rotation (HABER) test. The reciprocal
group included thirteen participants (7 male, 6 female; age, 29 ± 7 years; height, 169.12 ±
8.67 cm; mass, 68.55 ± 10.49 kg). The unilateral group included twelve participants (5 male,
7 female; age, 28 ± 7 years; height, 172.45 ± 12.62 cm; mass, 76.68 ± 12.03 kg). All
participants were free of pain in the spine, pelvis, hip or legs. Participants were initially
assessed using the HABER test. The HAER test was conducted in a prone position, while the
hip was passively moved through increments of 10° to a maximum of 50° abduction and
external rotation. Innominate position was measured at each increment using a digitizing
stylus and Polhemus® electromagnetic tracking system. Participants selected for use in this
study demonstrated either a reciprocal opening innominate movement pattern or a right / left
unilateral innominate movement pattern during the HABER test. Participants were then asked
to perform an ASLR using both the left and right legs, while surface electromyography
(sEMG) was conducted on selected muscles. Muscles recorded from included the right and
left external oblique (OE), internal oblique (OI), multifidus (MF), gluteus maximus (GM) and
bicep femoris (BF) of the non-lifted leg. Muscle activity was used to calculate a coactivation

II
coefficient index (CCI) between selected muscle pairings, during the raised position of the
ASLR. The reciprocal group had significantly greater coactivation of the left and right OI and
the BF and GM than the unilateral group, while the unilateral group had greater coactivation
of the left and right MF. Results suggest that participants who have greater pelvic mobility
(reciprocal group), require greater coactivation of pelvic stabilisers, such as the OI, GM and
BF to increase force closure and compress the SIJ. Greater MF coactivation was associated
with the unilateral group, who may endure more loading of the lumbar spine during the
ASLR, and require greater bracing of the vertebrae. Greater GM and BF coactivation may
reflect ineffective load transfer across the pelvis due to separation of the pubis symphysis
during the ASLR in the reciprocal group.

III
ACKNOWLEDGEMENTS
I would like to thank my supervisor Melanie Bussey for all her help in organising and
directing me in this project, and for providing feedback on my work.
Thank you to Gavin Kennedy for his contribution to data analysis and programming of
Matlab coding.
Thank you to Divya Adhia and Neil Anderson for their work in collecting participant data for
use in this project.

IV
TABLE OF CONTENTS
ABSTRACT .............................................................................................................................. I
ACKNOWLEDGEMENTS.................................................................................................. III
TABLE OF CONTENTS...................................................................................................... IV
LIST OF TABLES................................................................................................................VII
LIST OF FIGURES............................................................................................................ VIII
ABBREVIATIONS..................................................................................................................X
CHAPTER I: Introduction......................................................................................................1
Rationale.........................................................................................................................1
Aims ...............................................................................................................................3
Hypotheses .....................................................................................................................3
CHAPTER II: Literature review............................................................................................4
Structure and function of pelvis .....................................................................................4
Pelvic bony structure..........................................................................................4
Pelvic ligaments..................................................................................................5
Lumbopelvic muscles..........................................................................................6
Biomechanics of the pelvis.............................................................................................8
Load Transfer .....................................................................................................8
Self-bracing mechanism ...................................................................................10
Instability..........................................................................................................13
Motor control................................................................................................................13
Neural contribution to stability ........................................................................13

V
Dysfunctional motor control.............................................................................15
The Active Straight Leg Raise......................................................................................16
Muscle activity during ASLR............................................................................17
Co-activation during ASLR ..............................................................................18
Summary.......................................................................................................................19
CHAPTER III: Method .........................................................................................................21
Study design .................................................................................................................21
Participants ...................................................................................................................21
Equipment.....................................................................................................................22
Participant data................................................................................................22
HABER test.......................................................................................................22
Muscle EMG.....................................................................................................23
Procedure......................................................................................................................24
HABER test.......................................................................................................25
Sub MVC...........................................................................................................26
Active straight leg raise test .............................................................................27
Data Analysis................................................................................................................28
HABER .............................................................................................................28
Muscle EMG.....................................................................................................29
Co-activation coefficient...................................................................................30
Statistical analysis............................................................................................30
CHAPTER IV: Results ..........................................................................................................32

VI
Participant characteristics.............................................................................................32
Muscle coactivation......................................................................................................34
Right and left external oblique .........................................................................35
Right and left internal oblique..........................................................................36
Right and left multifidus ...................................................................................37
Gluteus maximus and bicep femoris.................................................................38
CHAPTER V: Discussion ......................................................................................................39
Muscle coactivation......................................................................................................40
External oblique ...............................................................................................40
Internal oblique ................................................................................................41
Multifidus..........................................................................................................42
Gluteus maximus & bicep femoris....................................................................44
Clinical relevance .........................................................................................................46
Limitations....................................................................................................................47
Practical implications ...................................................................................................48
Future studies................................................................................................................49
Conclusion....................................................................................................................50
REFERENCES .......................................................................................................................51

VII
LIST OF TABLES
Table 1. Participant demographic information for reciprocal and unilateral groups 33
Table 2. Mean and 95% confidence interval CCI value for the ROE_LOE, ROI_LOI,
RMF_LMF and GM_BF muscle pairings during a left and right leg ASLR 34

VIII
LIST OF FIGURES
Figure 1. Bony structure of the pelvic girdle, pelvic joints and significant anatomical
landmarks (Drake et al., 2015, p. 221). ......................................................................................5
Figure 2. Location of the posteriors ligaments of the pelvis, which provide support to the
sacroiliac joint (Drake et al., 2015, p219). .................................................................................6
Figure 3. Schematic representation of form closure of the sacrum between the innominate
bones (Snijders et al., 1993). ....................................................................................................11
Figure 4. Schematic representation of force closure of the sacrum between the innominate
bones (Snidjers et al., 1993). ....................................................................................................12
Figure 5. A model of the spinal stabilising system (adapted from Panjabi, 2006) ..................14
Figure 6. End point of the active straight leg raise test (ASLR) (Image adapted from
www.thera-bandacademy.com)................................................................................................16
Figure 7. Equipment set up of the HABER test including: custom built table, leg supports,
Polehmus system, digitizing stylus and senor (Adhia et al., 2015)..........................................23
Figure 8. Digitizing the ASIS using the stylus, while participant lies prone, with right leg
strapped into the support and the hip is abducted and externally rotated (Adhia et al., 2015).25
Figure 9. Participant performing a left active straight leg raise (ASLR) underneath the sonar,
located in the top right corner. Markers located on lower leg not applicable to current study.27
Figure 10. Innominate movement patterns observed during the passive loading of the hip
from the neutral position to the maximum HABER position...................................................29
Figure 11. Mean muscle co-activation index (CCI) for the right external oblique (ROE) and
left external oblique (LOE), during a left and right leg active straight leg raise (ASLR), for
participants in the reciprocal and unilateral groups..................................................................35
Figure 12. Mean muscle coactivation index (CCI) for the right internal oblique (ROI) and the
left internal oblique (LOI), during a left and right leg active straight leg raise (ASLR), for
participants in the reciprocal and unilateral groups..................................................................36

IX
Figure 13. Mean muscle coactivation index (CCI) for the right multifidus (RMF) and the left
multifidus (LMF), during a left and right leg active straight leg raise (ASLR), for participants
in the reciprocal and unilateral groups. ...................................................................................37
Figure 14. Mean muscle co-activation index for the gluteus maximus (GM) and the biceps
femoris (BF), during a left and right leg active straight leg raise (ASLR), for participants in
the reciprocal and unilateral groups..........................................................................................38
Figure 15. Stepping of the pubic symphysis during the active straight leg raise (ASLR) (Mens
et al., 1999)...............................................................................................................................45

X
ABBREVIATIONS
LBP
PGP
SIJ
ASLR
HABER
sEMG
OE
OI
MF
BF
GM
CNS
ASIS
PSIS
CCI
MVC
Lower back pain
Pelvic girdle pain
Sacroiliac joint
Active straight leg raise
Hip abduction, external rotation
Surface electromyography
External oblique
Internal oblique
Multifidus
Biceps femoris
Gluteus maximus
Central nervous system
Anterior superior iliac spine
Posterior superior iliac spine
Coactivation coefficient
Maximum voluntary contraction

1
CHAPTER I
Introduction
Rationale
Lower back pain (LBP) is a significant health problem with a lifetime prevalence of
79.2% (Walker, 2004). The source of pain is often ambiguous in 85-90% of cases, where the
cause of the pain is not identified and is therefore termed non-specific low back pain (Hoy,
Brooks, Blyth, & Buchbinder, 2010). Pain may originate from the lumbar spine, while many
categorised as having non-specific LBP in fact have symptoms which arise from the pelvic
joints (O’Sullivan & Beales, 2007). It is estimated that of those with LBP, 15–30% are caused
by mechanisms surrounding the sacroiliac joint (SIJ) (Prather & Hunt, 2004). This finding has
lead to the term pelvic girdle pain (PGP), in order to define this more specific sub group of
LBP. PGP symptoms include pain around the sacroiliac joints, posterior iliac crest or pubis
symphysis, with possible loss of stability, mobility and function (Vleeming, Albert, Östgaard,
Sturesson, & Stuge, 2008). This can greatly affect one’s ability to stand, walk and sit for
extended periods of time. The occurrence of PGP associated with pregnancy is very high,
with 1 in 2 women reporting symptoms (Wu et al., 2004). This is often resolved after the
pregnancy period; however pain may persist during post-partum and possibly become
chronic. Development of PGP may also come about as the result of trauma, arthritis or
osteoarthritis (Vleeming et al., 2008). Diagnosis and treatment of PGP is not well established,
as it is unclear what the underlying mechanisms causing PGP are. Without a definitive
aetiology and source of pain, it is hard to develop an effective treatment protocol. The current
focus of research uses a broad scope, considering the complex interaction of biomechanical,
neurological, genetic or psychosocial factors contributing to PGP (Beales, O’Sullivan, &
Briffa, 2009b).

2
Given the role of the pelvis in connecting the upper and lower body, the underlying
issues of PGP are likely to reside with the mechanics of pelvic load transfer (Pool-
Goudzwaard, Vleeming, Stoeckart, Snijders, & Mens, 1998). The SIJ has been observed as a
potential vulnerable structure in the pelvis, which requires stability to safely conduct load
through the pelvis (Snijders, Vleeming & Stoeckart., 1993a). Stability of the SIJ is determined
by the complex interaction of many structures within the pelvis, including muscle activity,
ligament tension, fascia tension and bony congruence (Pool-Goudzwaard et al., 1998). A
common test used by clinicians to evaluate load transfer through the SIJ and diagnosing PGP
is the active straight leg raise test (ASLR) (Mens, Vleeming, Snijders, Koes, & Stam, 2002).
Inability to complete the ASLR without pain, weakness or heaviness of the leg indicates a
lack of pelvic stability (Mens et al., 2002). The ASLR is therefore a functional test which
allows muscle activity to be assessed in relation to stability of the SIJ. Muscle coactivation
across a joint, or between bilateral muscles of the trunk are observed as strategies to increase
stability during the ASLR (Beales, O’Sullivan, & Briffa, 2009a). Coactivation of bilateral
trunk muscles is an indication of SIJ instability, as the individual attempts to increase pelvic
stability during load transfer ASLR (Palsson, Hirata, & Graven-Nielsen, 2014).
Stability of the pelvic joints is more inherent to some individuals than it is to others,
depending on flexibility of the ligaments, muscles, connective tissue and the articulate
surfaces (Bussey, Bell & Milosavljevic, 2009). Based on the mobility-stability trade off, joints
with greater flexibility are able to move through a larger range of motion, but require greater
stabilising to prevent injury (Veeger, & Van Der Helm, 2007). To further increase our
understanding of the role of muscle activity on SIJ stability, this study evaluated the
differences in muscle coactivation during the ASLR, between healthy individuals who have a
flexible pelvis compared to those with a rigid pelvis.

3
Aims
The aims of the study included: To investigate the effect of pelvic mobility on
coactivation of the external oblique (OE), internal oblique (OI), multifidus (MF), gluteus
maximus (GM) and biceps femoris (BF) muscles during the ASLR test. To determine any
differences in coactivation between muscles when performing a left ASLR compared to a
right ASLR.
Hypotheses
It was hypothesised that: The reciprocal group who have a more flexible pelvis will
require greater muscular coactivation to stabilise the pelvis during the ASLR, than the
unilateral group, who has a more rigid pelvis. The OE and OI would be activated ipslateral to
the lifted leg, therefore producing a low amount of coactivation for both groups. The MF
would perform coactivation, yet produce a low amount of muscle activity for both groups.
The BF and GM would produce the highest amount of coactivation compared to the other
muscle pairs.

4
CHAPTER II
Literature Review
Structure and function of pelvis
Pelvic bony structure
The pelvic girdle is formed by three bones (2 innominate bones and the sacrum)
creating an enclosed ring structure. The innominate bones (hip bones) are formed of three
fused bone; the ilium, ishium and pubis bone (Drake, Vogl, & Mitchell, 2015b, p. 216).The
sacrum connects the hip bones posteriorly via the SIJ, acting as the anchoring point for the
base of the vertebral column. The SIJ is synovial plane joint, which is believed to be capable
of a small range of motion in the sagittal plane only (Bussey et al., 2009). Unlike many other
synovial joints, the articulate surfaces of the joints have many depressions and elevations,
which can lock together when positioned in the closed pack position (Pool-Goudzwaard et al.,
1998). The hip bones then fuse anteriority at the pubic symphysis completing a closed ring
structure. The pubic symphysis is a cartilaginous joint which is largely fixed, yet allows up to
2mm of separation and 1° of rotation in most adults (Becker, Woodley, & Stringer, 2010).
The other major joint of the pelvis is the articulation of the head of the femur with the
acetabulum on both sides of the innominate bones (Drake et al., 2015b, p. 216). This joint is a
synovial ball and socket joint, which allows the leg a large range of motion.

5
Figure 1. Structure of the pelvic girdle, pelvic joints and significant anatomical landmarks
(Drake et al., 2015b, p. 221).
Pelvic ligaments
The joints of the pelvis are supported by an extensive network of ligaments, which
help to maintain joint stability by restricting movement. The major ligaments of the pelvis
which support the SIJ include the anterior sacroiliac ligament, posterior sacroiliac ligament,
interosseous sacroiliac ligament, sacrotuberous ligament and the sacrospinous ligament
(figure 2). The many ligaments form a wide spread web of connections, that are orientated in
different directions. The anterior sacroiliac ligament connects the anterior surface of the
sacrum to the ilium, while the posterior sacroiliac ligament connects the posterior surface of
the sacrum to the posterior superior iliac spine (PSIS) of the ilium (Drake et al., 2015a, p.
219). The posterior sacroiliac ligament increases in tension during counter-nutation (posterior
rotation of the sacrum relative to the innominate bones) helping to restrict movement
(Vleeming et al., 2002). The posterior sacroiliac ligament is significant in the study of LBP,
as the area caudal to the PSIS is often reported as being painful during palpation in those with
peripartum pelvic girdle pain (Vleeming et al., 1996). The posterior sacroiliac ligament shares
connections with the gluteus maximus muscle and is influenced by the sacrotuberous ligament

6
tension (Vleeming et al., 1996). The sacrotuberous ligament originates from the inferior
sacrum and joins to the tuberosity of the ischium (Drake et al., 2015a). The orientation of the
sacrotuberous ligament restricts nutation of the pelvis (anterior rotation of the sacrum to the
ilium) (Vleeming et al, 2002). The sacrotuberous ligament shares connections with the gluteus
maximus, bicep femoris and sacrospinous ligament, which effect the tension of the ligament
(Vleeming et al. 2002). The interosseous sacroiliac ligament is made up of many short fibres
that cross horizontally from the sacrum to the iliac tuberosity (Drake et al., 2015a). The
orientation of the interosseous sacroiliac ligament restricts lateral movement of the
innominate from the sacrum. The sacrospinous ligament originates from the sacrum and
attaches to the spine of the ischium (Drake et al., 2015a).
Figure 2. Location of the posterior ligaments of the pelvis which provide support to the
sacroiliac joint (Drake et al., 2015b, p219).
Lumbopelvic muscles
The muscles associated with the lumbopelvic region have a role in generating
movement of the legs and torso, as well maintaining stability of the pelvic and lumbar joints
(Pool-Goudzwaard et al., 1998). Muscles of interest in the study of PGP are those which

7
effect the position of the pelvic bones, the tension of the ligaments and ultimately affect
pelvic stability (Richardson & Jull, 1995). The multifidus muscle is a posterior, deep back
muscle, which is orientated longitudinally along the spinal process of the vertebral column
(Drake et al., 2015a). The multifidus attaches to each segment of the spinal column down to
the sacrum, where it shares connections with the fascia and ligaments of the pelvis (Drake et
al., 2015a). The multifidus provides stability to the lumbar spine by increasing stiffness
between vertebrae to increase bracing, as well as altering the mechanics of the sacrum and
pelvic ligaments (Hides, Stanton, Mendis, & Sexton, 2011). The multifidus however is often
observed in individuals with pain as being atrophied and unable to be voluntarily contracted
(Hides et al., 2011). Reduced multifidus activation may cause spinal instability and less
control of the sacrum, potentially promoting ongoing pain (Wallwork, Stanton, Freke, &
Hides, 2009). The multifidus has an important feedback role in the proprioception of
lumbosacral positioning that may be affected in LBP (Drysdale, Earl, & Hertel, 2004). The
long head of the bicep femoris directly connects to the ischium of the pelvic girdle (Drake et
al., 2015a). Therefore shortening of the bicep femoris is capable of producing a posterior
pelvic tilt (Hu et al., 2012). The gluteus maximus attaches along the illium and sacrum to the
femur (Drake et al., 2015a). The gluteus maximus impacts stability of the pelvis via
connections to the sacrotuberous ligament (Van Wingerden, Vleeming, Snijders, & Stoeckart,
1993). In some individuals the long head of the bicep femoris may also attach to the
sacrotuberous ligament, while in others it may not (Van Wingerden et al., 1993). The
anatomical connections of the gluteus maximus and bicep femoris to the sacrotuberous
ligament, may affect the tension and loading of the ligament during different postures (Van
Wingerden et al., 1993).
The abdominal muscles are also of interest in the study of LBP as they influence
bracing of lumbar spine and the SIJ. The transverse abdominis muscle is reported as having
the greatest influence on SIJ stability (Richardson et al., 2002). The transverse abdominis

8
attaches to the lower ribs, thoracolumbar fascia, iliac crest and the pubis crest (Drake et al.,
2015a). The large sheet like structure and the transverse orientation of the fibres create a
‘corset’ around the spine and abdominal region (Richardson et al., 2002). The attachment of
the transverses abdominis to the pelvic bones is particularly important for increasing SIJ
stability (Richardson et al., 2002). Contraction of the transverses abdominis compresses the
SIJ, as well as increases abdominal pressure, helping to brace the spinal column and the pelvis
(Richardson et al., 2002). The internal and external oblique attach from the lower ribs to the
iliac crest. The internal oblique fibres are orientated superiorly and medially, while the
external oblique fibres are orientated superior and lateral (Drake et al., 2015a). The internal
oblique lies deep near the transverse abdominis and is considered to influence lumbopelvic
stability (Drysdale et al., 2004). The external oblique is responsible for more global
movements of the vertebral column and the pelvis, such as trunk rotation and flexion
(Drysdale et al., 2004).
Biomechanics of the pelvis
Load Transfer
The downward acceleration of gravity produces a force through the body proportional
to the mass of the individual, known as weight force (Watkins, 2014). Weight force is
counteracted by the reaction forces of the ground, equating in both downwards and upwards
force through the body (Watkins, 2014). The sum of forces applied to the structures of the
body is known as load (Bartlett & Bussey, 2013). Load is also generated by contracting
muscles, which apply force to bones during movement (Snijders et al., 1993a). Mechanical
load causes tissue to undergo stress and deformation, which has the potential to become
injurious if the load exceeds tissue tolerance (Bartlett & Bussey, 2013). Tissues tolerance to
load varies greatly based on the type of tissue and its structure (Bartlett & Bussey, 2013). To

9
avoid injury, load must be attenuate through the body in a way which protects low tolerance
structures. Failure to do so may lead to injury and pain, originating soft tissue structures
(Bartlett & Bussey, 2013).
The positioning of the pelvis between the upper and lower body, and the large muscles
that attach to the pelvic bones, mean that it has a pivotal role in transferring load (Snijders,
Vleeming, & Stoeckart, 1993b). During standing load travels down the spinal column,
through the sacrum and is spread across the pelvic girdles to the head of the femurs. The
bones of the pelvis consist of a ‘sandwich construction’ of inner trabecular bone and outer
cortical bone (Dalstra & Huiskes, 1995). This structure provides high amounts of strength,
while remaining light. The cortical bone undergoes 50x the stress of the trabecular bone,
while the trabecular bone maintains structural integrity (Dalstra & Huiskes, 1995). Depending
on the orientation of loading, the SIJ of the pelvis may undergo compression force, shear
stress or bending moments (Snijders et al., 1993a). The flat surfaces that articulate at the SIJ
are well suited to the transfer of compressive forces, while the length and thickness of the
sacrum increase resistance to bending moments (Snijders et al., 1993a). The SIJ however is
prone to shear stress as load is applied downwards on the sacrum from the spinal column
(Snijders et al., 1993a). The pelvis undergoes constant changes in load orientation and
magnitude during dynamic movement such as walking. Movements such as walking cause
repeated increased asymmetric loading, which alters the requirements in load transfer.
Together the muscles, ligaments and fascia of the pelvis contribute to the successful transfer
of load by providing joint stability (van Dijke, Snijders, Stoeckart, & Stam, 1999).
The ligament structures of the pelvis contribution to load transfer by resisting
displacement of bony surfaces in order to attenuate forces (Snijders et al., 1993a). The
influence of ligaments on load transfer is dependent on the position of the sacrum relative to
the innominate bones, and is influenced by connecting muscles (Pool-Goudzwaard et al.,

10
1998; Vleeming et al., 1996). As the pelvis reaches an end point of range of motion during
pelvic nutation and counter-nutation, ligaments become taut and resist joint seperation
(Vleeming et al., 1996). For example the sacrotuberous, sacrospinous and interossous
ligament increases in tension during pelvic nutation, while the posterior sacroiliac ligament
becomes tauter during sacrum counter-nutation (Van Wingerden et al., 1993; Vleeming et al.,
1996). The large number of ligaments that become taunt during nutation, makes this position
the most stable position and suitable for effective load transfer (Van Wingerden et al., 1993).
For example sacrum nutation occurs in an upright position, which is ideal for increasing SIJ
stability to conduct weight force (Hungerford, Gilleard, & Lee, 2004). However the ligaments
alone are insufficient to maintain stability during load transfer, especially under high load
circumstances (Vleeming et al., 1990).
Self-bracing mechanism
Stability of the SIJ is important to ensure load is transferred through the pelvis with
little disruption to any of the surrounding soft tissue structures (Pool-Goudzwaard et al.,
1998). However stability is not inherent to the SIJ, and requires contribution from the
surrounding musculature (Pool-Goudzwaard et al., 1998). Stability of the pelvic joints is
achieved through what has been described as a self-bracing mechanism, constituted by force
and form closure (Snijders et al., 1993a). Form closure is provided by the shape and structure
of the bones, while force closure is the contribution of the muscles and ligaments to stability
(Pool-Goudzwaard et al., 1998). Form closure is produced by the shape of the joint and the
interaction between the sacrum and the innominate bones (Pool-Goudzwaard et al., 1998).
The wedge shape of the sacrum increases stability during longitudinal loading, as it is
supported by the innominate bones (Pool-Goudzwaard et al., 1998). Maximum bony
congruence and form closure of the SIJ is achieved during nutation of the sacrum (Hungerford
et al., 2004). The articulate surfaces of the SIJ are rough, increasing adherence between

11
surfaces and increasing resistance to movement under loading (Pool-Goudzwaard et al.,
1998). Form closure alone is not sufficient to provide optimal stability of the SIJ (Pool-
Goudzwaard et al., 1998). If this was so the SIJ would be entirely rigid, making any mobility
impossible. However this is not the case, as mobility at the SIJ has value in assisting in shock
absorption during loading (Snijders et al., 1993a).
Figure 3. Schematic representation of form closure of the sacrum between the innominate
bones (Snijders et al., 1993a).
During loading of the SIJ, force closure is required to provide additional joint stability
(Snijders et al., 1993a). Force closure is generated by the muscles and ligaments of the pelvis
(Pool-Goudzwaard et al., 1998). Both the muscles and ligaments increase stability through
compressing the SIJ and increasing friction between articulate surfaces (Snijders, 1993a).
Greater friction increases resistance to vertical displacement and shear stress during loading.
The transverse abdominis, internal oblique and periforms have been identified as having the
greatest contribution to force closure (Richardson et al., 2002). These muscles are transversely
orientated and have direct connections across the pelvis, which compress the SIJ during
contraction (Richardson et al., 2002). While the external oblique, rectus abdominis, gluteus
maximus, bicep femoris and latissimus dorsi are also capable of contributing to force closure
by altering pelvic tilt or surrounding ligament tension (Pool-Goudzwaard et al., 1998). The
abdominal muscles are also capable of producing force closure indirectly, by increasing intra
abdominal pressure to compress the pelvis (O’Sullivan et al., 2002). The amount of force

12
closure produced by the muscle is constantly altered to meet the load requirements of the task
(Pool-Goudzwaard et al., 1998).
Figure 4. Schematic representation of force closure of the sacrum between the innominate
bones (Snidjers et al., 1993).
The ligaments of the pelvis influence force closure additional via muscle activation.
The interconnected nature of the muscles and ligaments of the pelvis mean that muscle
activity can alter ligament tension and provide force closure (van Wingerden et al., 1993).
Activation of the bicep femoris and gluteus maximus increases sacrotuberous ligament
tension and counter-nutation of the pelvis (van Wingerden et al., 1993). The erector spinae
muscles increase posterior sacroiliac ligament tension and nutation of the pelvis (Vleeming et
al., 1993). The thoracolumbar fascia also has an interactive role with the musculature of the
pelvis and influences load transfer (Vleeming, Pool-Goudzwaard, Stoeckart, van Wingerden,
& Snijders, 1995). The tension of the thoracolumbar fascia is influenced by activity of the
latissimus dorsi, gluteus maximus, spinae erector muscles and biceps femoris (Vleeming et
al., 1995). Therefore muscle activation of these muscles will manipulate fascia tension.
Vleeming et al (1995) found the coupling of the latissimus dorsi and gluteus maximus to
increase force closure, by increasing perpendicular compression of the SIJ.

13
Instability
The self locking mechanism is required to maintain stability of the SIJ during load
transfer. Instability of the SIJ increases the amount of shear stress within the soft tissues of the
joint and surrounding structures (Snijders et al., 1993a). Instability of the joint may also cause
greater loading of ligaments, which results in pain after prolonged periods of standing or
sitting (Vleeming et al., 2008). Therefore instability of the SIJ becomes a major focus in the
study of PGP. Instability of the SIJ may occur for a number of reasons relating to muscle
activity and lack of force closure (Pool-Goudzwaard et al., 1998). Lack of force closure may
be the result of a dysfunctional self-bracing mechanism, in which stabilising muscles fail to
respond with sufficient compression of the SIJ during loading. (Pool-Goudzwaard et al.,
1998). As a result friction is decreased and greater shear force occurs at the SIJ. This may
result in greater loading being placed on the ligaments of the pelvis, causing micro-failure and
the sensation of pain (Panjabi, 2006). Instability may also arise from the viscoelastic
properties of supporting ligaments, which undergo ‘creep’ during prolonged loading (such as
standing or sitting) (Pool-Goudzwaard et al., 1998). Creep reduces stiffness of ligaments
causing joint laxity and reduces force closure (Pool-Goudzwaard et al., 1998). Reductions in
pelvic ligament stiffness may be a significant contributor to pelvic girdle pain in pregnancy
(de Groot, Pool-Goudzwaard, Spoor, & Snijders, 2008).
Motor control
Neural contribution to stability
The central nervous system (CNS) plays an integrated role in the self locking
mechanism and resulting stability of the pelvis. Panjabi (1992) suggests a model of stability
for the spinal column, involving the interaction of three sub systems: passive structures, active
structures, and neural control. This model is applicable to the stability of the SIJ also, as it

14
resembles a similar to that of Pool-Goudzwaard et al (1998) model of force and form closure.
In Panjabi (1992) model form closure is represented in the passive sub system while force
closure is included in the active sub system (figure 5). Panjabi’s (1992) model however has
added value when considering stability of the SIJ, by including the effect neural control has
on both passive structures (form closure) and active structures (force closure).
Figure 5. A model of the spinal stabilising system (adapted from Panjabi, 2006)
The three subsystems not only work independently, but also work together to achieve
stability. Information regarding joint positioning and loading is transmitted via sensory nerves
located in the ligament, muscles, tendons and joint capsules to the neural control centres of
the CNS (Panjabi, 1992). The central nervous system responds by adjusting muscle activity,
thereby increasing force closure, but also adjusts form closure by altering joint positioning.
This feedback cycle is continuously active through dynamic movement and requires
immediate adjustments in response to loading, to maintain stability (Hungerford, Gilleard, &
Hodges, 2003).
Force
Closure
Form
Closure

15
Dysfunctional motor control
Dysfunction of anyone of the three sub systems may contribute to the development
and ongoing mechanism of chronic LBP (Panjabi, 1992). In the absence of feedback from the
passive and active components of the pelvis, the CNS may respond with inappropriate muscle
activity or adopt alternative coping strategies (Panjabi, 1992). Studies comparing people with
PGP against pain free subjects demonstrate differences in the motor strategies during load
transfer tasks (Beales et al., 2009a; Beales et al., 2009b). Those with pain adopt strategies
which aim to increase stability of the pelvis. Acutely alterations in neural control may achieve
stability, but development of chronic aberrant motor patterns may place excessive loading on
vulnerable tissues, cause pain and dysfunction (Panjabi, 1992). Observations of coping
strategies to increase stability include splinting the diaphragm and bracing the chest wall
(Beales et al., 2009a). Beales et al (2009a) showed patients with PGP increase bracing
through the chest wall, during the ASLR, to increases intra abdominal pressure. Greater
pressure in the abdominal region may assist in compressing the pelvis and increasing force
closure (Beales et al., 2009a). During the ASLR, subjects with pain co-activate the OI and OE
on both sides (bilaterally) opposed to pain free subjects who showed activation on one side
(ipslateral) (Beales et al., 2009a). The use of co-activation in the oblique muscles may help
increase force closure of the SIJ.
Motor control and pain share a complex interaction which is not largely agreed upon
(Hodges & Moseley, 2003). Pain may be the result of altered motor control strategies as
mentioned previously, but alternatively altered motor control strategies may be the result of
pain (Hodges & Moseley, 2003). Chronic pain has been theorised to cause adaptations to the
CNS in an attempt to reduce pain experienced (Lund, Donga, Widmer, & Stohler, 1991). It
has been suggested that changes in motor patterns can become permanent as pain alters the
organisation of motor control centres in the brain (Lund et al., 1991). This may be a key
reason for the persistence of PGP following pregnancy or other traumatic events, as aberrant

16
motor control becomes fixed at a cortical level (Lund et al., 1991). Pain can provoke emotions
of fear and anxiety, which also alter the motor centres of the brain and the subsequent
activation of muscles (Hodges & Moseley, 2003). Altered mechanics of the lumbopelvic
region may lead to dysfunction in stability and the way the structures of the pelvis are loaded.
The Active Straight Leg Raise
The active straight leg raise test (ASLR) is an asymmetric, functional load transfer
task used in a clinical setting to diagnosis and examines disability of patients with PGP (Mens
et al., 2002). Individuals who suffer from PGP find performing the ASLR difficult, commonly
reporting heaviness of the leg, a paralysed like feeling and pain in the lower back (Mens,
Vleeming, Snijders, Stam, & Ginai, 1999). The ASLR is performed in a supine position, by
lifting a single leg 20cm above the ground, while maintaining a straight knee and a still pelvis.
The results of the test are based on a point scale of level of impairment and difficulty
completing the task, as well as observations made of excessive bracing strategies, such as
abdominal splinting, holding breath and twisting of the pelvis (Mens et al., 1999). The ASLR
has been shown to be an accurate and reliable measure level of impairment in posterior pelvic
girdle pain (Mens et al., 2002).
Figure 6. End point of the active straight leg raise test (ASLR) (Image adapted from
www.thera-bandacademy.com).

17
During the lying positing of the ASLR, gravity no longer acts to pull the sacrum
downwards into the innominate bones, reducing form closure (Bussey et al., 2009).
Furthermore the spine is flattened and the sacrum is counter-nutated, ‘unlocking’ the SIJ and
reducing stability (Bussey et al., 2009). In this position, stability of the SIJ is reliant on the
generation of force closure of the muscles to compress the joint during load transfer.
Therefore the ASLR is an appropriate test to evaluate the contribution of muscle activity to
the stability of the SIJ. The theory of Insufficient force closure as a source of PGP, is
supported by the use of compression belt for an individual with pain during the ASLR. During
the ASLR individuals with pain will experience pain and heaviness of the leg (Beales et al.,
2009b). The amount of impairment during the ASLR is reduced through the use of a pelvic
belt (Mens et al., 1999; O’ Sulivan et al., 2002). A pelvic belt increases the compression of
the SIJ and increases force closure, as well as reduces load placed on pelvic ligaments. This
indicates that pain and disability during load transfer of the ASLR coincides with instability
of the pelvis; specifically at the SIJ (Mens et al., 1999). This is also important in diagnosing
the origin of LBP, as an improvement in pain from applying a compression belt during the
ASLR, separates pain as a SIJ issue from that of the lumbar spine or hips (Hu et al., 2012).
Muscle activity during ASLR
The hip flexion of the ASLR is generated by the ipslateral hip flexors: rectus femoris
and iliopsoas (Hu et al., 2012). Individuals with pain struggle to lift their leg while having
greater activation of the hip flexors than pain free individuals (de Groot et al., 2008). This
indicates inefficient load transfer in individuals with pain compared to pain free individuals,
which requires greater muscle activity to achieve the task (de Groot et al., 2008). Activity of
the ipslateral hip flexors produces an anterior rotation moment of the innominate bones,
which is countered by a posterior rotation moment of the contralateral bicep femoris and

18
gluteus maximus (Pool-Goudzwaard et al., 1998; Hu et al., 2012). The relative activity
between the hip flexors and the hip extensors aims to minimize pelvic tilt during the ASLR
(Hu et al., 2012). In order for this to be effective the sides of the pelvis must move as a single
unit and avoid asymmetric twisting of the pelvis in the sagittal plane (Hu et al., 2012).
Furthermore the bicep femoris and gluteus maximus assist in optimising sacrotuberous
ligament tension to provide stability during the ASLR (Shadmehr, Jafarian, & Talebian,
2012). Healthy individuals performing an ASLR test will utilise primarily asymmetrical
muscle activation strategies with higher activity in the ipslateral trunk muscles (Beales et al.,
2009a). Thus, the internal and external oblique and rectus abdominis all show more ipslateral
activation compared to the contralateral side (Beales et al,. 2009a). The activity of the
abdominals during the ASLR is likely to provide effective load transfer by increasing stability
of the SIJ. However persons with pelvic girdle pain and instability tend to utilise aberrant
activation patterns which are reliant on bilateral activation or co-activation of bilateral
muscles effectively bracing the pelvis (de Groot et al., 2008; Beales et al,. 2009a).
Co-activation during ASLR
Co-activation is the simultaneous activation of two muscles surrounding a joint
(Cholewicki, Juluru, & McGill, 1999). The timing and amount of coactivation between
muscles is controlled by the CNS through feed forward and feedback mechanisms in response
to movement and anticipation of loading (Richardson & Jull, 1995; Hungerford et al., 2003).
The control of coactivation is not well understood, however it would make sense that the
amount of muscle co-activation is in response to the stability required (Granata & Orishimo,
2001). Tasks involving heavy loads or unstable postures will require high coactivation
between stabilising muscles to facilitate load transfer through the pelvis (Granata, &
Orishimo, 2001). While a low load asymmetrical task such as the ASLR should not be
perceived as a threat to stability (SIJ or lumbopelvic), as such should require little

19
coactivation between bilateral muscles associated with force closure (Granata & Orishimo,
2001). Coactivation of the bilateral muscles is considered aberrant because the amount of
force closure or mechanical stability provided to the system is larger than what is required to
perform such a low load task (Beales et al., 2009a). Such excessive lumbopelvic bracing is
common in low back pain and has been shown to increases the stiffness of the intervertebral
joints, thereby reducing movement (Brown & McGill, 2008; Granata, & Marras, 2000;
Stokes, Gardner-Morse, & Henry, 2011). Coactivation between the transverse abdominis and
the multifidus assist in bracing of the spine, by reducing flexion and rotation moments
(Richardson et al., 2002). Bilateral coactivation is observed in the internal and external
oblique and transverse abdominis muscles to increase intra abdominal pressure and
compression of the SIJ (Richardson et al., 2002). Greater bilateral coactivation of the
abdominals in order to brace the pelvis may be a compensatory response to SIJ instability, as
a result of lack of form closure and force closure (Beales., 2009b). Compensatory strategies
may also be a response to pain or fear of pain, which has been shown to effect trunk muscle
control during the ASLR (Palsson et al., 2014).
Summary
The high prevalence and debilitating nature of chronic LBP in both daily tasks and
sport make it a topic of great interest to practitioners (Vleeming et al., 2008). The lack of an
identifiable source of pain causes most cases of LBP to be labelled as non-specific
(O’Sullivan & Beales, 2007). Without an identified aetiology, treatment options are limited in
their effectiveness (Vleeming et al., 2008). The literature suggests that a significant number of
non-specific LBP cases may be of sacroiliac origin, coining the term PGP (Vleeming et al.,
2008). The SIJ is vulnerable to shear stress during loading and requires stabilising through the
self bracing mechanism, constituted of form and force closure (Pool-Goudzwaard et al.,
1998). It is hypothesised that pain may arise if stability is not achieved by the total

20
contribution of these mechanisms (Pool-Goudzwaard et al.,1998). In recent research the focus
of chronic PGP has focused on the role of force closure, which is determined by motor control
signals from the CNS (Panjabi, 1992). Dysfunctional control of muscles may produce
insufficient pelvic stability and load transfer, placing additional stress on soft tissue (Panjabi,
1992). Alterations in motor control may be due to a lack of stability but have also been shown
to occur in response to pain and fear of pain (Hodges & Moseley, 2003). The ASLR is a
functional movement test which provides an avenue for the analysis of muscle contribution to
pelvic stability, during load transfer (Mens et al., 1999). Based on previous studies by Bussey
et al (2009), it was recognised that individuals experience different pelvic movement patterns
during loading of the hip. How the pelvis moves under loading is attributed to pelvic
flexibility and is specific to the individual (Bussey et al., 2009). It is therefore of interest to
analyse how stability control is altered in those with greater flexibility compared to those with
low flexibility. An appropriate way to evaluate muscle contribution to stability is to measure
coactivation.

21
CHAPTER III
Method
Study design
This study followed a cross sectional design, providing insight in to the given
population, at a given time. The aim was to investigate coactivation of muscles associated
with pelvic stability during the ASLR test, in individuals with different levels of pelvic
flexibility. The study included two groups, one made up of participants considered to have a
high amount of pelvic mobility and another who had a more rigid pelvis. A cross sectional
study including two groups allows comparisons of measured variables to be made between
groups. Measured variables will be made up of qualitative measures, allowing statistical
analysis for significance in difference between groups.
Participants
Participants were recruited from adults located in Dunedin (Otago, New Zealand),
with the majority of participants associated with the School of Physical Education, Otago
University. Participants were selected from a larger pool of subjects who had undergone
pelvic movement and muscle activity tests. Participants were selected based on two specific
innominate kinematic patterns observed during a specialised pelvic movement tracking test,
referred to as a hip abduction, external rotation (HABER) test. Innominate movement was
measured during the HABER position leading to four possible classifications (figure 10).
Participants of interest to this study demonstrated either a reciprocal pelvic movement or
unilateral pelvic movement (either left or right). 13 participants were included in the
reciprocal group and 12 participants were included in the unilateral group. No reward as such

22
will be offered, however, each participant will be offered a $20 petrol voucher to help with
the transportation costs of attending the testing session.
All participants of the study were of healthy status, including no history of LBP,
pelvis, hip, knee, or ankle disorders in the past 12 months. Participants were excluded from
the study if they had non-specific LBP for more than 3 months, and have sought treatment for
their pain within the last 12 months. Participants would also be excluded if they had any of
the following conditions; known localised spinal pathology (viz., tumour, infection, fracture),
known congenital anomalies of the hip, pelvis or spine that limits mobility, known systemic
arthropathy, neuropathy or metabolic disorder, Diagnosed acute disc herniation / prolapse
with or without radiculopathy, pregnancy, less than 6 months post partum, post-menopausal
women.
Equipment
Participant data
Participant standing height was measured using a stadiometer (School of Physical
Endurance, University of Otago, New Zealand), and mass using scales. Hip range of motion
was measured using a chair, physio table and gonometer. Consent forms were used to ensure
participants could safely take part in the study and were aware of what was involved. A
questionnaire sheet was also filled out by participants, to collect personal information
including age, physical activity per week, intense physical activity per week and ethnicity.
HABER test
The HABER test and hip range of motion were performed on a custom made table
which supported the individual in a prone position. The table included a T shaped opening
under the pelvis allowing access to the anterior pelvic landmarks. A leg rotation frame was

23
attached to the table, allowing the leg to be fixed in place while manipulating the amount of
hip abduction and external rotation (Adhia, Milosavljevic, Tumilty, & Bussey, 2015). The leg
rotation frame included an angle measurement scale allowing different hip angles to be
measured and locked into place. Therefore standardizing hip position and the amount of stress
applied to be the joints (Adhia et al., 2015). Pelvic kinematics were collected and recorded
throughout the HABER test using a Polhemus Liberty TM
electromagnetic tracking system
(Polhemus Incorporated, Colchester, USA) and palpation technique (Bussey, Yanai, &
Milburn, 2004). This system consists of a transmitter, systems electronics unit and 3D digital
stylus pen sensor and additional local sensor (Bussey et al., 2009). The transmitter was fixed
under the table, while a sensor was on the spinous process of the 3rd lumbar (L3) vertebra
using adhesive tape (Adhia et al., 2015)
Figure 7. Equipment set up of the HABER test including: custom built table, leg supports,
Polehmus system, digitizing stylus and senor (Adhia et al., 2015).
Muscle EMG
Surface electromyography was performed on various postural muscles of the lower
trunk and proximal leg that contribute to pelvic stability. The muscles recorded from included
the right internal oblique (ROI), left internal oblique (LOI), right external oblique (ROE), left
external oblique (LOE), right multifidus (RMF), left multifidus (LMF), non- lifted gluteus

24
maximus (GM) and non-lifted bicep femoris (BF). Participants were positioned on a massage
table for sEMG preparation. Skin was prepared for electrode placement by shaving with a
razor, lightly abrading with fine sandpaper and cleaning with alcohol swipes. Two disposable
Ag/AgCl electrodes (Ambu® Blue Sensor N) 30 x 20 mm in size were placed 20mm apart on
each muscle belly. Placement of electrodes for the MF, GM and BF followed
recommendations by the European commission (Freriks, & Hermens, 2000). The electrode
placement for the OI was horizontal, inferior and medial to the ASIS, while the placement for
the OE as outlined by McGill, Juker, & Kropf, (1996). Prior to data collection, skin
impedance was verified using an Ohmeter (GRASS F-EZM5, Astro-Med Inc, USA) and a
reading of 3kΩ or less was considered acceptable. The sEMG was recorded using a Noraxon
Telemyo 900 (Noraxon inc., USA) telemetry EMG system sampling at 1000 Hz, the signals
were bandpass filtered, using a cut-off frequency of 16 and 500Hz, and amplified with an
overall gain of 1000. The ASLR was performed on a firm surface ground, with a small foam
mat for comfort. A sonar sensor, and tone emitter (School of Physical Education, NZ) was
placed above the lifted leg, and was set to emit a tone when the lifted leg reaches a height of
20 cm from the ground.
Procedure
Participants completed both the consent form and the personal information
questionnaire. Height was measured bare footed, standing, looking straight with a horizontal
Frankfort plane (ear to eye). Weight was measured with shoes removed and in light weight
clothing. Hip range of motion was measured for external rotation, internal rotation, hip
abduction and hip adduction using a goniometer. External and internal hip range of motion
was measured in a sitting position, while hip abduction and adduction was conducted in a
supine position.

25
HABER test
The HABER test was conducted using the custom made table and leg supports.
Participants were positioned in a prone position, with the participant’s legs fixed into the leg
supports. Innominate kinematics was measured using a Polhemus FastrackTM
electromagnetic
tracking system (Polhemus Incorporated, Colchester, USA). A local sensor was attached to
the spinal process of L3 using adhesive tape, in order to form a local coordinate system. The
local system allowed movements of the pelvis to be tracked relative to the spinal column, and
negate any effect of lateral lumber flexion or axial rotation. The participant began in a neutral
pelvic and hip position (0°), before each hip (left and right) was moved individually through
10° angle increments up to 50° (Adhia et al., 2015). At 0° and each 10° increment following,
the left and right anterior superior iliac spine (ASIS) and left and right posterior superior iliac
spine (PSIS) landmarks were palpated and digitized at 240hz using the stylus (Adhia et al.,
2015).
Figure 8. Digitizing the anterior superioer iliac supine (ASIS) using the stylus, while
participant lies prone, with right leg strapped into the support and the hip is abducted and
externally rotated (Adhia et al., 2015).

26
Sub MVC
Two tests were conducted to record a sub-maximum voluntary contraction (MVC) for
each muscle being measured in the ASLR, in order to establish a normative sample of muscle
activity. The two sub MVC tests included a prone double leg raise test (prone test) and a
crook double leg raise test (crook test). The prone test required participants to lie in a prone
position with their knees at a 90 degree angle. In this position they were told to lift their legs
off the group, while keeping both legs together and maintaining a 90° flexion at the knee. An
ultrasonic senor and auditory ‘beep’ sound was used to signal to the participant when they had
achieved a height of 5cm from the ground. This position would be held for a 3 second time
period in which they received a continuous tone to ensure the height of the legs was
maintained. The prone test was used to measure the activity in the posterior muscles (MF,
GM and BF) using sEMG. The test was performed 2-3 times to ensure the correct movement
was performed and an isometric contraction is held at the correct height for the correct
amount of time. The crook test required participants to lie in a supine position, with their
knees flexed to 90°, hips flexed to 45° and feet flat on the ground. They were asked to lift
their feet off the ground by 1cm and hold this position for 3s. The crook test was used to
measure the muscle activity of the OE and OI muscles using sEMG. The test was performed
2-3 times to ensure the correct movement is performed.
Both sub MVC’s were conducted to collect a sample of isometric EMG data, in which
to normalise the ASLR EMG. Normalised EMG data allows comparisons to be made across
individuals. A sub MVC was chosen to normalise EMG data based on evidence that it may be
a more reliable measure compared to a maximal voluntary contraction in some situations
(Dankaerts, O’Sullivan, Burnett, Straker, & Danneels, 2004). A sub MVC may be more
appropriate, as it allows multiple muscles to be recorded from at once, allowing more

27
functional movements to be used. The sub MVC might also be more appropriate for those
with acute or chronic pain, as it less stressful on the body.
Active straight leg raise test
The ASLR was conducted by having the participant lying in a relaxed, supine position
with hips in a naturally externally rotated position and feet 20 cm apart. Participants were
instructed to raise a single leg 20 cm above the ground, without bending the knee. Once this
height was reached the sonar emitted a tone to indicate to the participant to hold the leg still,
to ensure the minimum height threshold was maintained. The ASLR was conducted twice
with the left leg and twice with the right leg. Which leg was lifted first was randomly
determined and instructed to the participant by experimenters. EMG was recorded from the
right and left OE, OI, MF and the BF and GM of the non lifted leg. The ASLR test was
selected for use in this study as the supine position decreases form closure, placing emphasis
on the muscles to stabilise the lumbopelvic region. The ASLR has also been shown to have
high validity and reliability for assessing functional load transfer and mobility of the pelvis
(Mens et al., 1999).
Figure 9. Participant performing a left active straight leg raise (ASLR) underneath the sonar,
located in the top right corner. Note: Markers located on lower leg not applicable to current
study.

28
Data Analysis
Microsoft Excel (Microsoft, 2007, USA) was used to produce mean and standard
deviations for participant characteristics, including age, height, weight, BMI, physical activity
per week, intense physical activity per week, left and right hip internal rotation, external
rotation, abduction and adduction
HABER
Innominate movement pattern was modelled using three vectors, one to represent the
sacrum (left PSIS to right PSIS) and two representing each innominate (left PSIS to left ASIS
and right PSIS to left ASIS) (Bussey et al., 2004). The angle created between vectors in the
transverse plane, gives an indication of the position of the two innominate relative to the
sacrum and was calculated using the following formula:
Pelvic movement was calculate using the difference in angle from the neutral position (hip
0°) to the maximum HABER position (hip 50°), giving a change in angle from unloaded to
loaded. An increase in angle indicated an outward innominate movement relative to the
sacrum when loaded, while a decrease in angle indicated an inward innominate movement
relative to the sacrum when loaded. The possible combinations of innominate movement
therefore included both innominate bones moving inward (reciprocal inwards), moving
outwards (reciprocal outwards) or both moving in the same direction (unilateral left of
unilateral right) (figure 10). The HABER test was conducted on both sides (Left hip and right
hip), with each side tested producing a movement pattern outlined in figure 7. Each
participant therefore had two movement patterns, one for the right HABER and one for the
left HABER. Participants of interest in this study were those that demonstrated the reciprocal

29
outwards pattern in the right HABER test and the ones who demonstrated the right or left
unilateral pattern for both the left and right HABER.
Unilateral right and left Reciprocal Opening Reciprocal Closing
Figure 10. Innominate movement patterns observed during the passive loading of the hip
from the neutral position to the maximum HABER position.
Muscle EMG
EMG signals were processed using computer program Matlab (Mathworks Inc., MA,
USA, versions 2013b) using custom made scripts. Electrical noise (50Hz) was removed with
a 49.5-50.5 Hz band stop filter, while low and high frequency noise was removed with a 30-
300 Hz band pass filter (4th
order, dual pass, zero lag Butterworth). The EMG signals were
bias corrected and full wave rectified, before further filtered through a low pass filter with a
cut off frequency of 2.5 Hz to create a linear envelope (Winter et al, 2005). Linear envelopes
were further processed using Matlab codes which allowed the desired data to be manually
marked and then extracted for further analysis. This included marking start and ending points
of muscle activation during the sub MVC tests, to ensure an accurate representation of mean
activation was being extracted and used for normalisation. The data from the ASLR was also
manually processed based on sonar information, which indicated when the leg had reached a
height of 20cm. This ensured that the data extracted is during isometric muscle activation, to
best represent coactivation patterns.

30
Co-activation coefficient
In order to quantify and determine the extent of co-activation between muscle pairs, a
coactivation coefficient (CCI) was used. The CCI determines the magnitude of co-activation
based on a direct comparison of muscle activation between each pairing at each time point
(Nelson-Wong, & Callaghan, 2010). The co-activation coefficient was calculated using the
following equation:
Where EMG low refers to the activity of the muscle with the lowest amount of activation and
EMG high refers to the activity of the muscle with the higher amount of activation (Nelson-
Wong, & Callaghan, 2010). This ensures that the formula will not produce any calculation
errors due to dividing by 0. The CCI is a summation of EMG, therefore giving a sense of the
overall magnitude of coactivation. EMG data is sampled over a specific time period, and
therefore N refers to the number of time points being analysed. However each muscle activity
linear envelope was different in length of time, due to individuals holding the ASLR position
for slightly longer than others. Therefore the CCI for each muscle pairing was also normalised
to a 10 minute time period.
Statistical analysis
Microsoft Excel (Microsoft, 2007, USA) was also used to determine statistical
differences between groups, using a two-tail independent t test, at a significance level of 0.05.
Statistical analysis was conducted using SPSS software (version 22, IBM Corporation, USA).
A Tukey (1977) method of identifying outliers was used on each muscle pairing. The Tukey
(1977) method was altered to use k = 2.2 as per Hoaglin & Iglewicz (1987) recommendations
giving the formula:
Lower value = FL - 2.2(FU – FL) and upper value = FU + 2.2(FU – FL)

31
Where FL is the lower 25% quartile and FU is the upper 25% quartile of a normal distribution.
Outliers are defined as numbers that do not fit within the lower and upper range value.
SPSS (version 22, IBM Corporation, USA) was used to conduct four univariate
ANOVA tests on the CCI for four muscle pairings (ROE_LOE, ROI_LOI, RMF_LMF and
GM_BF). Muscle pairings were chosen based on previous literature and anatomical
significance to pelvic stability (Bealse et al., 2009a; Hu et al., 2012; Shadmehr et al., 2012).
The univariate model was formed using muscle pairing as the dependent variable and group
(reciprocal or unilateral) as the fixed factor independent variable. The ASLR was performed
using both the left and right legs, referred to as ‘side’. Side was included in the univariate
model as a random variable because which leg the participant performed the ASLR on first
was randomly decided. Sex was included in the univariate model as a co-variant, because of
the known biomechanical and anatomical differences between males and females (Chenot et
al., 2008). A univariate ANOVA was selected over a multivariate ANOVA, as the data
collected did not provide sufficient power due to a low sample size. A univariate test for each
muscle pairing assumes that each variable is independent; however this is not the case, as
each of the four muscle pairings belongs to the same individual. A Bonferroni corrective was
used to account for an increase in type 1 error, due to family wise error rate. This was done by
adjusting the alpha level from 0.05 to 0.0125.

32
CHAPTER IV
Results
Participant characteristics
The reciprocal pelvic movement group consisted of thirteen participants, including
seven males & six females. The unilateral pelvic movement group consisted of twelve
participants, including five males & seven females. The effect of sex was significant for
coactivation of the MF muscle (p = 0.029). Therefore sex was included in the univariate
analysis as a covariant, to remove male and female differences from the statistical analysis.
Participant descriptive characteristics are summarised in table 1, including the group mean
and standard deviation (SD) for participant demographics, physical activity status and hip
range of motion. All participants were right leg dominant and all but one were right hand
dominant. Participant characteristics including age, height, physical activity and hip range of
motion was not significant between groups (table 1). BMI (p = 0.08) and mass (p = 0.09) were
both close to being significantly different between the reciprocal group and the unilateral
group (table 1).

33
Table 1. Participant mean ± SD demographic information for reciprocal and unilateral
groups
Reciprocal
(n=13)
Unilateral
(n=12)
Mean SD Mean SD P value
Age (Years) 29 ±7 28 ±7 0.66
Height (cm) 169.12 ±8.67 172.45 ±12.62 0.47
BMI (kg/m2
) 23.84 ±2.01 25.81 ±3.15 0.08
Mass (kg) 68.55 ±10.49 76.68 ±12.03 0.09
PA (Hours/week) 12 ±13 9 ±6 0.45
Intense PA (Hours/week) 7 ±7 4 ±3 0.21
Left Hip INT ROT (°) 38.46 ±5.43 40.33 ±5.63 0.41
Right Hip INT ROT (°) 39.31 ±6.63 40.08 ±3.94 0.72
Left Hip EXT ROT (°) 35.85 ±6.97 34.67 ±5.61 0.64
Right Hip EXT ROT (°) 34.15 ±6.50 34.83 ±6.12 0.79
Left Hip ABDUCT (°) 37.77 ±4.07 39.00 ±6.16 0.57
Right Hip ADDUCT (°) 38.15 ±3.18 41.25 ±5.58 0.11
Ethnicity (n)
NZ European 6 5
Indian 5 2
European 1 2
Other 1 3
Sex (n)
Male 7 5
Female 6 7
NOTE: PA = physical activity, INT = internal, EXT = external, ROT = rotation

34
Muscle coactivation
Table 2. Mean (± 95% confidence interval) coactivation coefficient index (CCI) for the
ROE_LOE, ROI_LOI, RMF_LMF and GM_BF muscle pairings during a left and right ASLR.
Muscle Co-activation Index p-value
Muscle Pair Side Reciprocal Unilateral Group Side
Group
x Side
ROE_LOE L 669 (450, 889) 699 (470,928) .758 .799 .658
R 735 (515,955) 666 (437, 895)
ROI_LOI L 682 (390, 974) 563 (258, 867) .000* .012* .991
R 768 (476,1061) 652 (348, 957)
RMF_LMF L 219 (118, 321) 303 (197, 408) .003* .676 .888
R 216 (115,318) 314 (208, 420)
GM_BF L 194 (101, 288) 168 (71, 266 ) .003* .067 .950
R 163 (69, 256) 143 (46, 240)
ROE = Right external oblique, LOE = Left external oblique, ROI = Right internal oblique, LOI = Left
internal oblique, RMF = Right multifidus, LMF = Left multifidus, GM = Gluteus maximus, BF =
Bicep femoris

35
Right and left external oblique
The amount of coactivation between the ROE and LOE muscles was greatest for the
right ASLR in the reciprocal group, while the unilateral group showed greater coactivation in
the left ASLR. This is represented by a crossing pattern depicted in figure 11. The ROE and
LOE muscle pair showed no significant difference in coactivation between the reciprocal
group and the unilateral group (p = 0.758), in the left ASLR compared to the right ASLR (p =
0.799) or an interaction effect between group and side (p = 0.658) (table 2). In terms of
absolute coactivation, the OE muscle pairing produced the greatest CCI values regardless of
side or group. The OE CCI values were often ~4-5 times higher than the lowest muscle pair
coactivation observed.
Figure 11. Mean muscle co-activation index (CCI) ± 95% confidence interval, for the right
external oblique (ROE) and left external oblique (LOE), during a left and right leg active
straight leg raise test (ASLR), for participants in the reciprocal and unilateral groups.
400
500
600
700
800
900
1000
Reciprocal Unilateral
Co-activation
coefficient
(CCI)
Pelvic movement pattern
Left ASLR
Right ASLR

36
Right and left internal oblique
The reciprocal group demonstrated significantly greater coactivation between the ROI
and LOI muscles than the unilateral group (p = 0.00) for both sides, with a large effect size
(η2
= 0.478). There was also significantly greater ROI and LOI muscle coactivation during a
right ASLR than a left ASLR (p = 0.12). There was no significant interaction affect between
group and side for ROI and LOI coactivation (p = 0.991). The OI muscle pair achieved
similar values of CCI to the OE, which were ~4 times greater than the muscle pair with the
lowest coactivation, regardless of side or group.
Figure 12. Mean muscle coactivation coefficient index (CCI) ± 95% confidence interval, for
the right internal oblique (ROI) and the left internal oblique (LOI), during a left and right leg
active straight leg raise test(ASLR), for participants in the reciprocal and unilateral groups.
300
400
500
600
700
800
900
1000
1100
Co-activation
coefficient
(CCI)
Left ASLR
Right ASLR

37
Right and left multifidus
The amount of coactivation between the RMF and LMF was significantly less in the
reciprocal group compared to the unilateral group for both sides (p = 0.03), with a large effect
size (η2
= 0.967). The side on which the ASLR was performed had very little effect on RMF
and LMF coactivation (Figure 13). The left ASLR was very similar to the right ASLR in the
reciprocal group (219, 216 respectively) and in the unilateral group (303, 314 respectively).
There was no significant difference in RMF and LMF coactivation for side of the ASLR (p =
0.676), or an interaction effect between group and side (p = 0.888). Coactivation in the MF
was considerably lower (~2-3 times less) than the OE and OI muscles during the ASLR,
regardless of side or group.
Figure 13. Mean muscle coactivation coefficient index (CCI) ± 95% confidence interval for
the right multifidus (RMF) and the left multifidus (LMF), during a left and right leg active
50
100
150
200
250
300
350
400
450
500
550
Co-activation
coefficient
(CCI)
Left ASLR
Right ASLR

38
Gluteus maximus and bicep femoris
Coactivation of GM and BF was significantly greater for the reciprocal group
compared to the unilateral group (p = 0.03), with a large effect size (η2
= 0.415). The left
ASLR had a greater amount of GM and BF coactivation than the right ASLR for the
reciprocal group (left = 94, right = 168) and the unilateral group (left =163, right = 143).
However the difference in side of the ALSR was not significant (p = 0.67). The interaction
affect of group and side for coactivation of the GM and BF was also not significant (p =
0.950) (table 2). The GM and BF produced the smallest CCI values during the ASLR, which
were often ~4-5 times lower than the OE and OI muscles.
Figure 14. Mean muscle co-activation coefficient index (CCI) ) ± 95% confidence interval,
for the gluteus maximus (GM) and the biceps femoris (BF), during a left and right leg active
0
50
100
150
200
250
300
Co-activation
coefficient
(CCI)
Left ASLR
Right ASLR

39
CHAPTER V
Discussion
The high prevalence and debilitating nature of chronic LBP makes it a common and
expensive condition (Vleeming et al., 2008). The lack of knowledge surrounding the cause of
LBP leads to most cases being labelled as non-specific LBP (O’sullivan & Beales, 2007).
Treatment effectiveness is hindered by the lack of identifiable aetiology. One research path
that may reveal useful information about the condition is the analysis of muscle control and
pelvic mobility. The aim of this study was to investigate muscle coactivation in stabilising
muscles of the lumbopelvic region, during the ASLR test, among pain free individuals who
demonstrate a reciprocal opening pelvic movement compared to unilateral movement, during
the HABER test. The major findings of this study were that mobility of the pelvic joints is
associated with greater coactivation of the OI. This result highlights that having more
flexibility at the SIJ requires greater force closure during load transfer. Greater mobility was
also associated with greater activation of both the GM and BF muscles, which may reflect an
effort to control pelvic tilt and increase pelvic stability. While having a more rigid pelvis was
associated with greater MF coactivation and may reflect different strategies of force
attenuation relating to the mobility of the pelvis.
The two innominate patterns observed in the HABER test form the major focus point
of this study. The different movement patterns that occur under hip loading give an insight
into the structure and biomechanical nature of an individual’s pelvis. The reciprocal opening
movement pattern is the reflection of more mobile pelvic joints compared to the unilateral
movement pattern (Adhia et al., 2015). The pelvic girdle is a closed ring structure which is
jointed posteriorly at the SIJ and anteriorly at the pubic symphysis. The reciprocal opening
pelvic movement is caused by both innominate bones rotating laterally from the sacrum.
Lateral movement is allowed by greater sacroiliac mobility and laxity of the ligaments

40
supporting the SIJ (Bussey et al., 2009). In conjunction with movement at the SIJ, the opening
of the innominate bones must also be facilitated by separation at the pubic symphysis (Bussey
et al, 2009). The opening group therefore has greater mobility not only at the SIJ but at the
pubic symphysis.
The unilateral movement pattern is associated with rigidness of the pelvic joints
(Adhia et al., 2015). Applying a load to the hip causes both innominate bones to move in the
same direction. Movement in the same direction indicates that the SIJ is less flexible, and the
ligaments apply greater restriction to range of motion. The pubic symphysis may also be more
rigid, and not allow the pubic bones to separate (Bussey et al., 2009). Lack of mobility of the
SIJ and the pubic symphysis causes the pelvis to move as a single unit, as seen in the
unilateral movement pattern (Adhia et al., 2015). The unilateral pattern either favoured
movement to the left side or the right side, depending on the individual. Whichever pattern the
participant demonstrated was consistent when testing the right leg or the left leg. This meant
the pelvis would move towards the load in one HABER test, and move away from the load in
the other HABER test. This is an unusual observation as you would expect the type of loading
applied in the HABER test to cause the innominate to move towards the pulling force being
applied to the hip. The unilateral pattern was found to be more common in patients with SIJ
pain by Adhia et al., 2015, which may reflect restrictions of the pelvic joints, which is
associated with PGP.
Muscle coactivation
External oblique
Coactivation of the OE was observed to have no significant difference between the
reciprocal pelvic group and the unilateral pelvic group (p = 0.758). This suggests that OE
activity is very similar regardless of pelvic mobility. This result is reflective of the role of the

41
OE as a more global muscle that provides trunk movement, rather than a stabilising muscle of
the core (Richardson et al., 2002). The OE is more longitudinally aligned and more superficial
than the OI and transverse abdominis, therefore it does not provide as much stability to the
SIJ (Richardson et al., 2002). The high amount of activation in the OE is generated in an
attempt to counter act the anterior rotation and pelvic tilt, generated by the lifted leg and
tension of the hip flexor muscles (Park et al., 2013). The OE controls movement of the lower
trunk required when performing the ASLR, and is therefore very similar across groups,
regardless of pelvic stability needs.
It is interesting however that the OE has such high amounts of coactivation, given the
low load task. Beales et al., (2009a) reported that pain free subjects require ipslateral
activation only of the OE during the ASLR. Ipslateral activation local to the lifted leg assists
in preventing rotation of the trunk as load is transferred asymmetrically through the pelvis.
Ipslateral activation would be indicated by a low CCI value, but instead the high value
indicates bilateral activation of the muscles. This indicates that both groups were using a high
load strategy and a muscle control pattern similar to that observed in PGP individuals (de
Groot et al., 2008). The opposing findings of this study and the one of Beales et al (2009a)
may be caused by the differences in samples. This study carefully selected a certain sub group
of healthy individuals who showed a reciprocal or unilateral pattern, compared to the sample
of all women and unknown pelvic flexibility used by Beales (2009a).
Internal oblique
Coactivation between the OI muscles was significantly greater for the reciprocal
pelvic group, than it was for the unilateral pelvic group (p = 0.000). The OI along with the
transverse abdominis are important in providing force closure to the SIJ (Richardson et al.,
2002). The transverse alignment of the OI and the direct attachments to the ilium, generates

42
compression of the SIJ during bilateral activation (Richardson et al., 2002). Greater
compression helps increase friction and stability of the SIJ, to prevent shear stress in response
to loading (Pel, Spoor, Pool-Goudzwaard, van Dijke, & Snijders, 2008). However OI activity
during the ASLR in pain free individuals has previously been found to require ipslateral
activation only to stabilise the pelvis (Beales et al., 2009a). Therefore it seems that
participants of the reciprocal group require a greater amount of force closure than observed in
other pain free individuals, putatively due to the increased flexibility in their pelvic ring.
Side also had significant effect on the amount of OI coactivation during the ASLR.
The right ASLR had significantly more coactivation than the left ASLR (p = 0.012),
regardless of pelvic movement group. For the reciprocal group this corresponded with the
same leg that induced an opening reciprocal pattern during the HABER test. Reciprocal group
participants all demonstrated reciprocal opening patterns during a right HABER test and a
unilateral pattern during the left HABER test. This may indicate structural or functional
asymmetries within the pelvis that allows greater pelvic mobility during a right leg lift, while
decreasing pelvic mobility during a left leg lift (Bussey et al., 2009). This is supported by the
findings of Bussey et al (2009) who also found greater range of motion in the right SIJ during
the HABER test. Greater flexibility may be due to differences in the articulate surfaces of the
SIJ and may relate to hand/leg dominance (Bussey et al., 2009). Therefore greater OI
coactivation is observed during a right leg lift for both groups in order to stabilise the more
flexible right SIJ.
Multifidus
The coactivation between the bilateral MF was significantly greater for the unilateral
group compared to the reciprocal group (p = 0.03). Considering the role of coactivation to
stabilise joints, this result suggests that the unilateral group require greater spinal stability

43
than the reciprocal group during the ASLR (Hides et al., 2011). This may reflect the
difference in loading of the lumbopelvic region that occurs in each group due to the
differences in pelvic mobility (Adhia et al., 2015). The unilateral group have more rigid pelvic
joints which shunts load across the pelvis, transferring greater load to the lumbar spine. The
unilateral group may require greater MF coactivation to stabilise the lumbar spinal column,
under the load generated by the ASLR. The reciprocal group have greater flexibility in the
joints of the pelvis, which may allow them to attenuate forces more effectively through the
pelvis. The reciprocal group require greater activation of pelvic stabilising muscles such as
the transverse abdominis muscle and the OI to suitably stabilise the pelvis in response to
loading. The reciprocal group therefore attenuate more force in the pelvis and experiences less
load on the on the spinal column, requiring less MF activation to stabilise the vertebral joints
than the unilateral group (Hides et al., 2011).
The results of this study are comparable to studies which emphasises the importance
of MF activation in LBP, which originates from the lumbar spine (Hides et al., 2011; Hides,
Stanton, McMahon, Sims, & Richardson, 2008). Studies have commonly reported muscle
atrophy, inability to activate MF and delayed activation patterns, in individuals who
experience LBP (Wallwork et al., 2009; Hungerford et al., 2003). Lack of MF activation may
result in spinal instability and greater loading of ligaments and soft tissue of the spine,
generating pain. With the use of specific training targeting MF activation, lumbar stability is
likely to improve, which may improve pain for some individuals (Hides et al., 2008). This
intervention may be more important for those with a more rigid pelvis, as they require greater
lumbar spine stability than those with more pelvic flexibility.

44
Gluteus maximus & bicep femoris
Co activation between the GM and the BF of the non lifted leg was significantly
higher for the reciprocal group compared to the unilateral group (p = 0.03). The CCI provides
a measure of the amount of activation between the muscles analysed. Unlike the other muscle
pairings analysed, the GM and BF are located unilaterally on the body and do not share an
agonist - antagonist relationship or bilateral location such as the trunk muscles. Therefore the
interpretation of the CCI does not give information into the individual role of these muscles,
but the use of them together.
The GM and BF are both capable of providing a hip extension moment due to their
attachment to the innominate and femur (Hu et al., 2012). During the ASLR the non-lifted leg
assumes a stabilising role by contacting the surface of the floor and remains in fixed position.
The fixed position of the supporting leg means activation of the GM and BF generates a
posterior rotation of the innominate (Hu et al., 2012). Therefore it is observed through the
CCI that the reciprocal group produce a greater posterior rotation moment of the innominate
during the ASLR than the unilateral group. Posterior rotation is generated in the contralateral
BF in order to counteract the anterior rotation of the ipslateral innominate to the lifted leg,
which is caused by the activation of the hip flexors and weight of the leg (Hu et al., 2012). For
ipslateral anterior rotation to be effectively counteracted with contralateral posterior rotation
during the ASLR, the two sides of the pelvis must move as a single unit (Hu et al., 2012). The
reciprocal group have greater flexibility of the pubis symphysis joint than the unilateral group,
which may cause separation of the pubic bones during the ASLR. Mens et al (1999) termed
this occurrence ‘stepping’ as the innominate ipslateral to the lifted leg rotates anteriorly,
causing the pubic bone to be displaced posteriorly (figure 15). The reciprocal group is
therefore likely to endure more stepping of the pubic bones, than the unilateral group who
have less flexibility of the pubic symphysis joint. Greater stepping may decouple the sides of

45
the pelvis and decrease the effectiveness of controlling anterior-posterior pelvic tilt using
posterior rotation generated by the GM and BF. Greater muscle activation of the GM and BF
is then required to counteract movement of the opposite side of pelvis. Therefore greater GM
and BF activity is a reflection of a less effective load transfer mechanism across the pelvis
during an ASLR which occurs with a stepping of the pubic bones (Mens et al., 1999).
Figure 15. Stepping of the pubic symphysis during the active straight leg raise (ASLR) (Mens
et al., 1999).
The greater activity of the GM and BF in the reciprocal group may also be a reflection
of an attempt to produce greater SIJ stability during the ASLR. The lying position assumed
during the ASLR flattens the lumbar spine and counter-nutates the sacrum (Hungerford et al.,
2004). During counter nutation the sacrotuberous ligament becomes lax and reduces the
amount of support provided to sacrum stability (Vleeming et al., 1996). The GM and BF share
direct connections to the sacrotuberous and sacrospinous ligaments, and therefore are capable
of increasing tension during contraction (van Wingerden, Vleeming, Buyruk, & Raissadat,
2004). Therefore greater activity of the GM and BF may be required to assist in stabilizing the
sacrum during the ASLR, via tension of the sacrotuberous and sacrospinous ligaments. The
GM may also contribute to force closure of the pelvis, via attachments that cross the SIJ,
thereby increasing stiffness during contraction (van windergarden et al., 2004).

46
Clinical relevance
At present these individuals are able to sufficiently stabilise the pelvis during low load
transfer. However this group may be predisposed to developing pelvic girdle pain, if the
muscular control of the pelvis is altered. This may be brought on by an inciting event that
causes changes to the pelvic structures, or changes in the control of pelvic stability. Such an
event may include an injury to the region or more commonly be brought on by pregnancy
(Wu et al., 2004). Painful events have been shown to have significant effects on the central
nervous system and the motor control centres of the brain (Hodges & Moseley, 2003). As a
result the pre planned motor strategy to control pelvic stability maybe altered in order to avoid
loading of painful structures, regardless of load demands of the task (Hungerford et al., 2003).
This becomes problematic long term as motor strategies become permanent despite the
inciting incident being resolved (Hodges & Moseley, 2003). The aberrant motor strategy that
remains may be the source of inappropriate load transfer through the pelvis and increase stress
on certain tissue, resulting in an ongoing condition of chronic pelvic girdle pain (Hungerford
et al., 2003; O’Sullivan et al., 2002).
Based on a large amount of the literature surrounding LBP, instability of the lumbar
spine or SIJ is considered to be key contributors to pain (Pool-Goudzwaard et al., 1998;
Snijders et al., 1993a). This has lead to the development of exercises prescribed by
practitioners, which focus on increasing the amount of MF, transverse abdominis and other
abdominal muscles activation (Hides et al., 2011; Richardson & Jull, 1995). The rationale
behind this is to increase muscle coactivation, to stiffen the lumbopelvic joints, in order to
increase stability for load transfer (Richardson & Jull, 1995). Although the argument for this
is compelling, instability has not been proven as being the cause of the pain in many who
suffer from chronic LBP (O’Sullivan, 2012). Therefore treatments which attempt to increase
muscle activation to increase stability are only effective for a small number of specific

47
individuals (O’Sullivan et al., 2012). For example the findings of Hides et al (2008) who
reported MF strengthening exercises to improve LBP in elite young cricketers. This sub group
of LBP suffers undergo regular high load movements during their sport, and therefore require
high amounts of spinal stabilisation. But a large number of chronic LBP suffers only perform
low load tasks throughout the day, yet are treated with the general approach that more
stability is needed. This stance has been reinforced by studies which report coactivation in
those who suffer from pain, leading to the assumption that an increase in stability will resolve
pain (de Groot et al., 2008; Hodges, 1999; Richardson & Jull 1995). However greater stiffness
of the spine and pelvis may cause movement compensation and aberrant motor control,
leading to ongoing pain and weakness (Beales et al., 2009b).
The presence of coactivation as an indication of instability may be flawed, which
would explain why many individuals do not see improvement in LBP with increasing muscle
stabilisation (Jarvik et al., 2005). As observed in this study high coactivation was observed in
the OI, OE and MF muscle groups in healthy participants. These individuals were capable of
performing the ASLR without any pain, or heaviness of the leg, indicating suitable pelvic and
spinal stability. Therefore coactivation has occurred in the absence of instability and also pain,
which has been shown to alter muscle recruitment strategies (Lund et al., 1991). This
highlights that coactivation may not be indication of lack of stability, but may reflect more
individual dependent factors such as pelvic mobility, and muscle requirements during
movement.
Limitations
A possible limitation to the study may be the small and specific sample of participants. The
total number of participants was 25, with the majority associated with Otago University,
School of Physical Education. Participants were of a specific sub group in the population who

48
are possibly more active and have different body compositions compared to the average
person. Therefore the sample used may not truly reflect the overall population and the results
may not be able to be generalised to others. The two groups being compared in this study
were not perfectly matched in participant characteristics. The reciprocal group had a greater
number of males (7) to females (6), while the unilateral group had a greater number of
females (5) to males (7). This was accounted for in the statistical analysis by including
females as a covariant, to avoid the risk of a type 1 error, of a difference that is attributed to
sex rather than group differences. Participant characteristics of mass and BMI were also quite
different and close to statistical significance (Two tail t-test, p = 0.08 and p = 0.09
respectively). BMI and mass may reflect differences between groups in limb weight or muscle
mass, which may affect the ASLR test. The use of the CCI simplifies the muscle activity of
muscle pairs to a single value which represents the sum of the relative agonist to antagonist
muscle activity per unit of time. The CCI does not provide information regarding timing of
coactivation or the fluctuations in coactivation during the ASLR, rather is a sum of total
muscle coactivation. Using sEMG to measure data from an isolated muscle in the trunk such
as the OI, OE and the MF can sometimes provide an inaccurate reflection of muscle
activation. The signal has the potential to be contaminated with cross talk from surrounding
muscles, especially when the muscle is located deep (De Luca, 1997). This is minimised by
using recommended electrode placement that is positioned over the desired muscle belly only.
Practical implications
The findings of this study may help inform practices surrounding the identification
and treatment in those with non specific LBP. The HABER test may have practical
significance in classifying individuals in order to prescribe treatment, specific to the
mechanics of their pelvic joints. The use of the HABER test has previously been shown to

49
provide information on pelvic movement during passive loading, which is used to classify
pelvis flexibility (Bussey et al., 2009). It is therefore of interests to clinicians how flexibility
may influence the mechanics of the pelvis and what the implications of this are. This study
has provided an analysis of the effect of pelvic flexibility on stability demands during load
transfer. In this study greater pelvic mobility was shown to be associated with an increase in
requirements of pelvic stability, while low pelvic mobility was associated with loading of the
lumbar spine. This may assist in recognising pain emitting structures and therefore help
determine a course of action. However as mentioned earlier, treatment options need to exceed
the simple approach of improving bracing of structures, and consider the psychosocial aspects
that may underlie pain in many individuals.
Future studies
The findings of this study indicate that pelvic mobility affects the type and amount of
muscle activation used to stabilise the pelvis. Previous studies have often generalised
participants into groups such as pain negative or pain positive to analyse muscle activity.
Therefore pelvic flexibility is not identified or recognised as having an effect on stability,
when in certain studies it may be beneficial to control for. This is especially important in load
bearing situations, in which different amounts of pelvic flexibility will cause different
requirements in muscle activation patterns to achieve stability. Future research may wish to
look at the implications pelvic mobility may have on those with pain, opposed to those
without pain. Isolating differences between these groups may help reveal the deficits between
the interacting sub systems of stability.
The control of coactivation also requires more research to determine the control
mechanisms and significance to LBP. For those with chronic LBP more research is required
to provide causal relationships between mechanical factors and pain. For many individuals
instability is likely not the direct cause, and therefore prescribing exercises to increase

50
coactivation is ineffective. A greater understanding of coactivation, bracing and pain will
enable treatment plans to be better directed at the source of the issue rather than applying a
general solution. The future of LBP research must also adopt a more complex psychosocial
approach that recognizes the interaction between muscle control, sensory information and the
mechanics of the pelvis.
Conclusion
This study has provided an insight into the amount of coactivation observed in
individuals of differing pelvic mobility, during the ASLR test. The results of this study show
that healthy individuals with greater pelvic mobility require greater coactivation of the
muscles which stabilize the pelvic joints, such as the IO, EO and GM, BF. This highlights the
need to increase force closure of the SIJ during load transfer and a possible separation of the
pubis synthesis joint, which decouples the sides of the pelvis. It was also found that rigidness
of the pelvis may place more load on the lumbar spine, requiring coactivation of the MF to
increase bracing. Finally the right ASLR was found to require more stabilisation from the
coactivation of the OI muscles, compared to a left ASLR. This finding was attributed to the
greater flexibility of the right SIJ that is inherent to individuals, or may relate to hand/leg
dominance. The results of this study help improve understanding of how flexibility of the
pelvis may play a role in affecting load transfer and requirements in stability. Identifying
areas of greater loading may be useful in determining the source of pain in non specific LBP
suffers and aid in developing treatment options.

51
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