A method to effect physiological recruitment order in electrically activated muscle

A stimulation is used to achieve physiological recruitment order of small-to-large motor units in electrically activated muscles. The use of quasitrapezoidal pulses and a tripolar cuff electrode make 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 small twitch widths at higher force levels. The force modulation process was more gradual and fused contractions were obtained at lower stimulation frequencies when the proposed stimulation method was used. Muscles activated by the 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. It may therefore have applications in future functional neuromuscular stimulation systems.