It shows the interdependence between stimulus strength and the time required in activating the muscles. It indicates the strength of impulses of various durations required to produce muscle contraction by joining the points that graphically represent the threshold value along the ordinate for various durations.

Advantages of SDC:

  • This is a simple, reliable and shows a proportion of denervation.

Disadvantages of SDC:

  • In large muscles it can not shows the full pictures but only a proportion of muscle fibers can be stimulated.
  • It can not show the site of lesion.

Optimum timing of SDC:

SDC test can be done 10 – 14 days after the lesion has occurred. The degeneration of nerve from the proximal to distal is called Wallerian degeneration. When the motor end plate is no longer functioning, it is done weekly under the same condition until there is recovery and decision has been reached on the eventual final state of the muscle. SDC is used to identify denervation, partial innervation, and compression.

Methods of SDC:

Take a neuromuscular stimulator (TENS, DL­2­ stimulator) having rectangular duration i.e. 0.3, 0.1, 1, 3, 10, 30, 100, 300 ms and constant current. Put the passive electrode over the midline of the body or near the origin of the muscle. Put the active electrode over the fleshy belly of the muscle.
Alternately both the electrodes are placed on both ends of the muscle. First apply current having longest duration and look for minimum perceptible contraction, gradually shorten the impulse duration and note the corresponding increase in current strength. The electrode placement should not be changed through out the test. Plot a SD graph from the results of the test.

Characteristics of SDC:

     i.        Innervated muscles:

When all the nerve fibers supplying the muscles are intact, the strength duration curve has a shape characteristic of normally innervated muscles as shown in the figure.
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 strengths of the stimulus each time the duration is reduced.

   ii.        Denervated muscles:

When all the nerve fibers supplying a muscle have degenerated, the strength duration produced is characteristic of complete denervation as shown in the figure.
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. The curve rises steeply and is shifted to the right than that of normally innervated muscle.

  iii.        Partial denervated muscles:

For the stimulation of denervated fibers impulses of longer duration are required while for the stimulation of innervated fibers impulses of shorter duration are required. The kink produce show the partial denervation which disappear after 10 -20 days or month.

·        Rheobase (Rheo means intensity/ stimulus strength and base means foundation):

The minimum stimulus strength that produces a response is called Rheobase. 

·        Chronaxies (Chron means time and axie means axis):

The stimulus duration that produces a response when the stimulus strength is set to exactly at double rheobase.