Effects of asynchronous stimulation on the human quadriceps femoris muscle

Objective: Examine the effects of asynchronously activating motor units of the human quadriceps femoris muscle during electrical stimulation. Methods: Two stimulation channels were used to activate the muscle. One pair of electrodes was placed over the vastus medialis (Channel 1) and another pair was placed over the vastus lateralis (Channel 2). All contractions were isometric. The stimulation frequency was 10Hz for each channel and the effect of varying the delay between the two channels was tested. Results: Synchronous activation of both channels produced the greatest forces and stimulating the muscle 180 degrees out of phase (ie, with a 50-msec delay) produced the least forces. Conclusions: Activating motor units asynchronously under certain conditions will be less efficient than synchronous activation. This study, therefore, helps to define the boundary conditions when asynchronous stimulation may be helpful. (C) 1997 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.