A method to effect physiological recruitment order in electrically activated muscle

A new stimulation method has been utilized to achieve physiological recruitment order of small-to-large motor units in electrically activated muscles. The use of quasitrapezoidal-shaped pulses and a tripolar cuff electrode made selective activation of small motor axons possible, thus recruiting slow-twitch, fatigue-resistant muscle units before fast-twitch, fatigable units in a heterogeneous muscle. Isometric contraction force from the medial gastrocnemius muscle was measured in five cats. The physiological recruitment order was evidenced by larger twitch widths at lower force levels and smaller twitch widths at higher force levels in the muscles tested. In addition, force modulation process was more gradual and fused contractions were obtained at lower stimulation frequencies when the new stimulation method was employed. Furthermore, muscles activated by the new method were more fatigue-resistant under repetitive activation at low force levels. This stimulation method is simpler to implement and has fewer adverse effects on the neuromuscular system than previous blocking methods. Therefore, it may have applications in future functional neuromuscular stimulation systems.