Strength duration curve (SDC)

STRENGTH DURATION
CURVE
-Dr. Aniruddha Barot (PT)
B.P.T.,
M.P.T.(Musculoskeletal),C.K.T.T.
Civil Hospital, Ahmedabad

OUTLINE
Definition
Advantages & disadvantages
of SD curve plotting
Optimum timing for SD
curve
Electrical parameters of SDC
Method of SD curve plotting
Characteristics of the curves
Rheobase, Chronaxie & Pulse
ratio
Rise time & Utilization time
Factors that affects the
accuracy of the SD curves
Practical uses of SD curve in
diagnosis
References

 Definition
• “This curve indicates the strength of
impulses of various duration required
to produce contraction in a muscle, is
obtained by joining the points that
graphically represent the threshold
values along the ordinate, for the
various duration of stimulus
displayed along the abscissa.”
• The normal curve has a characteristic
shape, which shifts towards “right” in
denervation & shows a “kink” in case
of partial denervation.
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15 14 13 13 13 13
0
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100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Miliamp
Millisec
Constant current
stimulation

 Advantages & Disadvantages of SD
curve plotting
• ADVANTAGES:
1. Simple
2. Reliable
3. Indicates the proportion of
denervation
• DISADVANTAGES:
1. In a large muscle only a
proportion of fibers may
respond, there for a full
picture is not obtained for
the whole muscle.
2. It also does not tell about
the site of lesion.

 Optimum timing for SD curve
• The patients with peripheral nerve injuries can have their SD
curve done 10-14 days after the onset of the lesion, when the
motor end plate is no longer functioning & Wallerian
degeneration, if present would have occurred.
• For other lesions of the motor unit, if there is a paralysis/paresis,
present, following acute onset or slow insidious onset, then after
considering relevant pathology for prognosis, the SDC, may be
utilized to assess the extent of the lesion & monitor progress.
It is best done weekly under the same conditions until there is
recovery or decision has been reached on the final state of the
muscle.
Once recovery takes place, there will be changes expected in the
curve every fortnight or so.

 Electrical parameters of SDC
• Rectangular, monophasic pulsed current.
• Frequency of 1 Hz.
• Pulse duration ranges from 0.01 to 300 ms.
• Constant current [CC] [more accurate] or constant voltage [CV] [more
comfortable] stimulator
• Cathode: at the motor point of the muscle.
• Anode: over the muscle few cm away from the motor point or other convenient
areas.
• Response: Minimally detectable (visible) twitch contraction.
Response of usually, two muscles supplied by the nerve distal to damaged area
are examined, one proximal & one distal usually starting 7-10 days after
injury.
Tests repeated every 2 weeks (depending on site of injury & muscle tested).

 Method of SDC plotting
1. Patient is explained about the procedure & placed in comfortable
position.
2. The part to be tested is exposed & kept well supported.
3. The room should be well lighted.
4. Check contraindications if any.
5. Cover the abrasions, decrease the skin resistance by cleansing
the area with soap & water & moisten the part with warm 1%
saline. [use tap water in the absence of warm 1% saline]
6. Therapist should also be in a comfortable position with hand
stable throughout the procedure.

7. Active electrode should be as small as possible to avoid spreading
of the current.
8. Set the parameters as described above with intensity on zero,
pulse duration at 300 ms & electrodes in position.
9. Stimulate with an active electrode at or around the motor point
area to get a minimal, visible contraction.
Find the best point & note the intensity of current needed.
Successively decrease the pulse durations, increase the intensity
for the particular pulse duration.
Repeat for all pulse durations up to 0.01 ms.

10. Maintain the same position,
angle & pressure for the active
electrode.
11. Plot the graph of the strength of
the current [Y axis] versus the
pulse duration [X axis].
12. Note the name, age, gender, date
of injury & date of performing the
test along with the diagnosis &
interpretation.
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40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Miliamp
Millisec
Constant current
stimulation

 Characteristics of the curve
[1] Normal Innervations:
• When all the nerve fibers supplying the muscles are intact, the
curve obtained has a characteristic shape as seen in figure 1.
• The curve is of this typical shape because the same strength of
stimulus is required to produce a response with all the impulses of
longer duration.
• While those with a shorter duration require an increase in the
strength of stimulus each time the duration is reduced.
• The point of rise of the curve with a constant current stimulator is
around 1 ms, whereas that with the constant voltage stimulator is
at 0.1 ms.

100
50
20
15 14 13 13 13 13
0
20
40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Miliamp
Millisec
Constant current
stimulation
20
15
10 10 10 10 10 10 10
0
20
40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Volt Millisec
Constant voltage
stimulation
FIGURE 1

[2] Complete denervation:
• When all the nerve fibers supplying a muscle have degenerated,
the SDC produced, is characteristic of complete denervation.
[figure 2]
• In this curve as the response is from the muscles which lacks
nerve continuity, only the long duration pulses will elicit the
response & there will be a need to increase the current intensity
from about 10 ms duration.
• The curve is no longer horizontal on the right hand side but is
converted into a distinctive steeply rising parabola, which is
displaced towards the right.

105
50
20
10
5
0
20
40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Miliamp
Millisec
Constant current
stimulation
70
50
30
20
15
10 10
0
20
40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Volt Millisec
Constant voltage
stimulation
FIGURE 2

[3] Partial denervation:
• When some of the nerve fibers supplying a muscle have
degenerated, while the others are intact, the curve takes a
typical shape as shown in figure 3.
• The impulses of longer duration stimulate both the innervated &
denervated muscle fibers, so a contraction is obtained with a
stimulus of low intensity.
• As the impulses are shortened, the denervated fibers respond
less readily, so that a stronger stimulus is required to a produce
a perceptible contraction & the curve rises steeply, like that of
denervated muscle.

• With the impulses of shorter durations, the innervated fibers respond to a
weaker stimulus than that required for the denervated fibers, so a contraction
of the denervated fibers is not obtained & this part of the curve is similar to
that of innervated muscles.
• Thus, the right hand part of the curve resembles that of denervated muscle &
left hand part that of the innervated muscle & a “kink” is seen at the point
where the two sections meet.
• The appearance of a kink or discontinuity in the curve is a reliable early sign of
denervation & in progressive involvement of nerve lesions, it provides slightly
later evidence of denervation than electromyography.
• The extent of denervation can be picked up by the shape of the curve and kinks.
• If a larger part of the muscle is denervated, the greater part of the curve
resembles that of denervated curve with the kink but if only a part of the fibers
are denervated & the majority innervated, then the curve resembles a normal
curve with a kink in it.

50
45
42
40
20
10 9
0
10
20
30
40
50
60
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Miliamp
Millisec
Constant current
stimulation
30
25 22 20
15 12 9
5
90
64
49
45
40
20
5
0
20
40
60
80
100
120
0.01 0.03 0.1 0.3 1 3 10 30 100 300
Volt Millisec
Constant voltage
stimulation
FIGURE 3
KINK

PULSE DURATION (ms)
INTENSITY
(mA
or
mV)
KINK

 Rheobase, chronaxie and pulse ratio
[1] Rheobase:
• It is the intensity of current required to produce a minimal perceptible &
palpable contraction using a pulse of infinite duration.
• Generally pulses of 100 or 300 ms duration are used to record rheobase.
• The shape of pulse is always rectangular.
• It is measured in milliampere or volt, depending upon whether a constant
current or constant voltage stimulator is used.
• Rheobase is usually measured by placing the cathode on the motor point of the
nerve.
• Normal values: 2-10 mA or 10-20 volts.
• In denervation, the value of rheobase may be less than that of the innervated
muscles & it often rises as re-innervations starts.

[2] Chronaxie:
• It is the duration of the shortest impulse, that will produce a response
with a current double that of the rheobase.
• It is the index of excitability & is the time in milliseconds, that is
necessary to induce minimal visible contraction, with a stimulus of twice
the strength of rheobase.
• Normal values: < 1 ms.
• In denervation, the value of chronaxie is more than that of innervated
muscles.
• The denervated muscle values for chronaxie are around 10 ms or more.

[3] Pulse ratio:
• It is the ratio of intensity of the current needed to produce a
muscle contraction with 1 ms duration to that required if the
duration of the pulse is 100 ms.
• It is a crude test.
• A value greater than 2.2 indicates abnormality.
• The extent of partial denervation can not be assessed from this
test.

 Rise time & Utilization time
Rise time: it is the pulse duration at which one need to increase
the intensity of current more than the rheobase intensity.
• It is late in normal innervation & early in denervation.
Utilization time: it is the shortest pulse duration at which you
get a minimal stimulation with intensity of current equal to the
rheobase current.

 Factors that affect the accuracy of
SDC:
1. Skin temperature: this alters the value of rheobase. An
increase in temperature lower the rheobase value, whereas a
decrease in temperature will raise the rheobase.
2. Humidity: high humidity decreases the value of rheobase.
3. Location of muscles: deeply placed muscles can not accurately
be located.
4. Oedema
5. Superficial fat: large amount of fat will cause increased
resistance to current flow, which leads to incorrect results.

6. Ischaemia: causes a rise in the threshold values, therefore a
greater intensity of current will be required.
7. Electrode position: the electrode, if not positioned properly may
cover some of the fibers of the adjacent muscle, therefore gving
inaccurate results.
8. Pressure variations: the variations of pressure of the hand held
electrode during testing procedure will give faulty results.
 Factors that affect the accuracy of
SDC:

 Practical uses of SDC in diagnosis
1. To detect the presence or absence of the excitable nerve fibers in a
muscle.
2. It also assesses the extent of denervation/innervation.
3. It detects the signs of re-innervations in a muscle. [in most cases, the
onset of regeneration is evident well in advance of the clinical return
of the voluntary contraction]
4. It monitors the progress of the lesion & denotes whether the lesion is
recovering/regressing.
5. The values of rheobase & chronaxie which also indicate about the
status of innervation, is also measured from the SDC.

 References
1. John Low and Ann Reed. Electrotherapy Explained. 4th
edition. Elsevier publications.
2. B Nanda. Electrotherapy simplified. 1st edition. Jaypee
publications.
3. N Vyas. Principles & practice of rehabilitation. 1st edition.
Jaypee publications.

Strength duration curve (SDC)

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