Orderly Motor Unit Activation Using Sinusoidal Low Frequency Alternating Current Stimulation

Traditionally, electrical stimulation uses short duration (<1ms) charge balanced rectangular impulses to initiate action potentials. Although pulse stimulation provides an efficient means to activate nerve fibers, the fibers are activated synchronously to the depolarizing edge of the pulse and favors large caliber fibers resulting in an inverse order of recruitment. This study describes the use of low frequency alternating current (LFAC) stimulation using a pure tone continuous sinusoidal waveform to evoke motor nerve activation. In-silico and in-vivo experiments were conducted where LFAC was applied to the rat sciatic nerve via bipolar cuff electrode. The in-vivo model responses were quantified by measuring the electromyogram (EMG) responses of the lateral gastrocnemius and soleus muscles and the combined twitch force. These measures were made in comparison to standard rectangular pulse stimulation. These preliminary results indicate that the response to LFAC were en masse phase locked to the sinusoidal cycle, but of two different modes: 1) Burst mode, and 2) Unitary mode. These results are in agreement with the in-silico predictions. The LFAC activation threshold of the soleus muscle was lower than the lateral gastrocnemius's, suggesting a normal (small-large caliber) physiological order of recruitment. In contrast, the rectangular pulse stimulation produced an inverted order of recruitment.