Strength-Duration curve

Strength-Duration curve

Depending upon the excitability of the nerve and muscle a graph is prepared by plotting the minimal voltage required for the muscle to contract against the duration of the stimulus in milliseconds.

The Status of a muscle as regards innervations, denervation, or reinnervations can be indicated by these curves

Stimulation of the nerve requires a polarized membrane (Between inside and outside the nerve)

Strength – Duration Curve is a graph between electric stimuli of different intensities and recording the time needed by each stimulus to start the response.

SD curve is plotted after the 20^th day of injury / Lesion

After 21^st / 22^nd day: 270 days to regenerate

The purpose of the SD curve is to know whether the stimulated muscle is innervated, denervated, or partially denervated.

Apparatus:

• 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
• SD curve:- Representation of quantities nonlinear relationship between intensity and duration of current to determine whether the muscle is innervated or denervated.

Normal Innervation:

SD curve is of typical shape because the impulse of longer duration all produces a response with the same strength of the stimulus, irrespective of their duration, while those of shorter duration require an increase in strength of the stimulus each time the duration is reduced.

Complete Denervation:

The duration of impulse is 100ms or less, the strength of stimulus must be increased each time the duration is reduced and no response is obtained to impulse of short duration.

So, the curve rises steeply and is further to the right than normally innervated muscle.

Partial Denervation:

The impulse of longer duration can stimulate both innervated and denervated muscle fibers, so a contraction is obtained with a stimulus of low intensity

As impulses are shortened, the denervation fibers respond less readily, a shorter stimulus is required to produce a perceptible contraction and the curve rises steeply like denervated muscle

With the stimulus of shorter duration, the innervated fibers respond to a weaker stimulus than denervated fibers

The kink in SD Curve is seen at the point where 2 sectors meet

Kink appears in the curve and as reinnervated progress.