Strength- duration curve ppt presentation

Ravi Nair Physiotherapy College
STRENGTH-DURATION CURVE
Dr.Swapna Jawade
Datta Meghe college of Physiotherapy,Nagpur

Objectives for the class
At the end of the lecture, the learner will be able to
1. Definition of S-D Curve.
2. Techniques of plotting of S-D Curve.
3. Characteristics of S-D Curve.
4. Rheobase and chronaxie

DEFINITION
•Strength duration/Intensity duration curve shows the relationship between the magnitude
of the change of stimulus and the duration of the stimulus.
•The curve provides valuable information regarding the state of excitability of nerve lesion.
•It should be done only after 21 days following nerve injury.
•The plotting of S-D Curve indicates that the strength of impulses of various duration that
will produce muscle contraction.

• The purpose of S-D curve plotting is to know whether the stimulated
muscle is innervated, denervated or partially denervated.
• There are also other method for this purpose like EMG and NCV.
• S-D curve should be plotted after 20th day of injury/lesion.
• After 21st/22nd day, regeneration of nerve will start, generally it take
about 270 days to regenerate.

• Strength duration curve is a graph between electrical stimuli of different
intensities and recording the time needed by each stimulus to start the
response.
• he apparatus used for plotting S-D Curve supplies rectangular impulses
of different duration.
• Impulse with duration of 0.01, 0.03, 0.1, 0.3, 10, 30, 100, 300 ms are
required.

• The stimulator may be of either the constant current or
constant voltage type.
• The constant current stimulator was thought to produce the
more accurate result but constant voltage stimulator is
rather more comfortable for patient

• The patient must be warm, fully supported and in sufficient light.
• The indifferent electrode may be applied over some convenient
area usually on the midline of the body or the origin of the muscle
group.
• Active electrode placed over fleshy part of muscle. (Sometime two
small electrode may be used, one over each end of muscle belly).

• Current is applied using the longest stimulation first and increased until a
minimal contraction is obtained.
• Intensity of current (or voltage) is noted and impulse is shortened with
constant voltage than with the constant current stimulator.
• The shape of the curve is the essential feature.
• This procedure is repeated with each stimulation in turn, the intensity of
current being increased as required.

•  A minimal contraction is used, as this make it easy to
detect any change in strength, and electrode should be
placed in same point over the muscle throughout the test.
•  The S-D Curve is plotted from the result of the test, and
although it will be further to the left

Techniques of Plotting:
I. PART:
1. Receiving the patient (as in proforma)
2. Knowledge of condition
3. Preparation of trays (as described earlier)
4. Preparation of apparatus—Diagnostic electrical stimulator
to be used.

• Check whether all the knobs are at Zero.
• Checking the pins of the plug and check whether the switch is
turned off.
• Check the insulation of the wire.
• Check whether the switch in the stimulator is working.
• Check whether fuse is present in the apparatus; see that it is not
blown out.
• Check whether hand switch for patients use is intact and is working.

II. PART:
1. Positioning of the patient
2. Position of Physiotherapist
3. Checking for local contraindication
4. Reducing skin resistance.
III. PART:
1. Checking of apparatus
2. Correct placing of pads and electrodes (depending upon the nerve)

Instructions to the patient:
•Feel of current
•Instruction to inform if any burning occurs
•Warning not to touch anything
•Regulating current—Interrupted galvanic current
• Palpating tendon
• Winding up.

Other special points:
 Diagnostic stimulator to be used
 Interrupted galvanic current indicated
 Start with longer duration (from 100/300 ms)
 Select small muscle or select a muscle, which has
distinguished action, compare with 3 muscles.

• S-D Curve is of this typical shape because the impulses of longer duration all
produce a response with same strength of stimulus, irrespective of their
duration, while those of shorter duration, require an increase in the strength
of the stimulus each time the duration is reduced.
• . The point at which the curve begin to rise is variable, but is usually at a
duration of impulse of 1 ms with constant current and 0.1 ms with constant
voltage stimulator

• S-D Curve of complete denervation is when duration of
impulse is 100 ms or less, the strength of the stimulus must
be increased each time the duration the duration is reduced
and no response is obtained to the impulse of very short
duration.

• So the curve rises steeply and is further to the right than of
Normally innervated muscle.
• With the impulses of shorter durations, the innervated fibers
responds to a weaker stimulus than that required for the
denervated fibers.

• S-D Curve of partial denervation is the impulses of longer
duration can stimulate both innervated and denervated muscle
fibers, so a contraction is obtained with a stimulus of low
intensity.
• As impulse are shortened, the denervation fibers responds less
readily, a stronger stimulus is required to produce a perceptible
contraction and the curve rises steeply like that of denervated
muscle.

• Kink in S-D Curve is seen at the point where two section meet.
• The shape of curve indicates the proportion of denervation.
• A kink appears in the curve and as reinnervation progresses.
• Progressive denervation is indicated by the appearance of a kink,
increase in the slope and shift of the curve to the right.

• When stimulus is given using the maximum pulse width available
on the stimulator, the intensity of the current required to produce
a twitch is called rheobase of the muscle.
• Mainly 100 to 300 ms duration are used to record rheobase.
• The pulse is always rectangular measured in miliamperes or volts.

• Rheobase is measured using the cathode on the motor point
of the nerve or by using bipolar technique.
• Normal values of rheobase are 2 to 18 mA or 5 to 35 volts.
• Deltoid 14 volts, 5mA Triceps 18 volts, 5mA Abductor digiti
minimi 30volts, 8mA Frontalis 14volts,4mA

• Resistance of skin and subcutaneous tissue
• Edema and inflamation
• Ischemia and underlying pain
• Temperature variation
• Position of electrode
• Amount of subcutaneous tissue

• Degeneration
• Deneravtion
• Partial denervation generally produce no changes in rheobase.
• Re-innervation can show a sharp rise in rheobase which clinical
recovery

Motor Point
 It is a point where the nerve pierces deep
fascia.
 On this point there will be maximum
contraction of muscles take place on
minimum intensity of Electrical muscle
stimulator.

Characteristics/Interpretation of Curve:
1. Normal innervation:
When all the nerve fibers supplying the muscles are
intact, the strength duration curve has a shape
characteristic of normally innervated muscle.

• 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 of shorter duration require an increase in the
strengths of the stimulus each time the duration is reduced.

Fig.: Normally innervated muscle: (A) In constant current;
(B) In constant voltage

2. Complete Denervation:
When all the nerve fibers supplying a muscle have degenerated, the strength duration
produced is characteristic of complete denervation.
For all impulses with duration of 100 ms or less the strength of the stimulus must be
increased each time the duration is reduced and no response is obtained to impulses of
very short duration.
So that the curve rises steeply and is further to the right than that of a normally
innervated muscle

Fig.: Complete denervated muscle: (A) In constant current;
(B) In constant voltage

3. Partial Denervation
• As impulses shortened—denervated fibers respond less readily.
• So that a stronger stimulation is required.
• With impulse of shorter duration—innervated fibers responses.
• When some of the nerve fibers supplying a muscle have degenerated while others are
intact, the characteristic curve obtained clearly indicates partial denervation.
• The right hand part of the curve clearly resembles that of denervated muscle, the left
hand part that of innervated muscle, and a kink is seen at the point where the two parts
meet.

Fig.: Partially denervated muscle

Complete Denervation:
When all the nerve fibers supplying a muscle have
degenerated, the strength duration produced is
characteristic of complete denervation.

Question
1. Define sd curve
2. Characteristics/Interpretation of Curve
3. Explain Faradic galvanic test
4. Explain the Factor influencing rate of regeneration

REFERENCES
1. Forster A., Palastanga N.: Clayton’s Electrotherapy. Theory & Practice.9th
Edition.
2. Low J., Reed A.: Electrotherapy Explained. Principles & Practice. 3rd
Edition.
3. Starkey C.: Therapeutic modalities. 3rd
Edition.
4. Cameron, Mitchelle H.: Physical Agents in Rehabilitation. 4th
Edition.
5. Jagmohan Singh: Textbook of Electrotherapy. 2nd
Edition.

THANK YOU

Strength- duration curve ppt presentation

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Strength- duration curve ppt presentation