STRENGTH DURATION CURVE
KEY WORDS
Sd curve,
Denervation
Innervation
Wallerian degeneration
Introduction
The strength-duration curve is a graphical representation of the relationship between the
intensity of an electrical stimulus at the motor point of a muscle and the length of time
taken to elicit a minimal contraction in that muscle. Here, strength refers to the stimulus
intensity on the vertical axis while duration refers to the pulse duration on the horizontal
axis. It is used to determine nerve damage in lower motor neuron lesions
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.
Wallerian degeneration: Nerve degenerates proximally to nearest node of Ranvier and
distally throughout whole length. Debris is cleared by macrophagic activity. Process takes
up to 21 days to complete and is a preparation for regeneration.
Nerve regeneration
1. Regeneration of axons send out many branches one of which becomes myelinated and
continues to grow down the neural tube.
2. Growth rate approximately 1 mm per day.
It occurs unevenly throughout the regeneration period being initially faster.
Factor influencing rate of regeneration
1. Age of the patient – Faster in younger age group
2. Site of lesion – Faster when lesion is more proximal to spinal cord.
3. Nature of lesion – Faster following spontaneous regeneration than following
nerve suture.
Types of Injury
Seddon’s classification of injury
Neuropraxia:
– Loss of conduction without degeneration
– Nerve conduction possible below lesion
– Sensory part frequently least affected than motor.
Axonotmesis:
– Disruption of axon, but nerve sheath intact
– Wallerian degeneration is followed by axons regrowing to own end organs.
Neurotmesis:
– Disruption of axon and nerve sheath.
– Surgery required approximating nerve sheaths and enabling growing axon to
reach correct end organ.
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.
II. PART:
1. Positioning of the patient
2. Position of Physiotherapist
3. Checking for local contraindication (as described earlier)
4. Reducing skin resistance (as described earlier).
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.
For Radial Nerve - Extensor indicis
For Median Nerve - Abductor pollicis brevis
For Ulnar Nerve - Abductor pollicis
For Lateral Popliteal Nerve - Peroneus longus
Shape of the Curve
Normal Innervation
This is also called a "nerve curve". All nerve fibers supplying the muscle are intact. The
shape of the curve is a continuous rectangular hyperbola. The same intensity is required
to produce a response at longer durations. The intensity increases steadily for shorter
durations. The curve is usually seen rising at the 1ms mark. [1]
This curve depicts a normal, innervated muscle.
Complete Denervation
This is also called a "muscle curve". All nerve fibers supplying the muscle have
degenerated. The curve is characteristically steep and shifted to the right. The intensity
keeps increasing when lowering the duration below 100ms. There is no response seen at
very short durations. [1]
This curve depicts a completely denervated muscle.
Partial Denervation
Some of the nerve fibers supplying the muscles have degenerated while others are intact.
A characteristic kink is present in the curve. The right side of the curve represents the
denervated part of the muscle while the left side represents the innervated fibers of the
muscle.[1]
This curve depicts a partially denervated muscle.
Rheobase
It is the minimum intensity of current required to stimulate a muscle at infinite duration.
Its normal value ranges between 2 and 18 mA. The rheobase is greater for denervated
muscles.
The 'R' on the vertical axis denotes the rheobase value in mA
Chronaxie
It is the minimum time required for a current of double the intensity of rheobase to
stimulate a muscle. Its normal value is below 1ms. Chronaxie is inversely proportional to
excitability. Thus, its value is greater for denervated muscles.
The 'C' on the horizontal axis denotes the chronaxie value in ms
Utilization time
It is the time taken by a stimulus of rheobasic strength to excite the nerve and produce a
muscle contraction. Below this value, there will be no muscle contraction.
Factors affecting the Strength-Duration Curve
Skin resistance
Subcutaneous tissue like fat
Temperature
Electrode size, material and placement
Age of the subject
Fatigue
Advantages of the Strength-Duration Curve
It is quick and easy to perform. It requires minimal training. It is economical in
comparison to other clinical tests. [1]
Disadvantages of the Strength-Duration Curve
It only provides qualitative data in relation to the degree of denervation. It cannot locate
the site of the lesion. In large muscles, only a few fibres can be studied due to the limits
of the method
EXPECTED QUESTIONS
1.Disadvantages of the Strength-Duration Curve/ Advantages of the Strength-Duration
Curve
2.Nerve injury classification
3.uses of sd curve
4.SD curve plotting
