The authors are grateful to Dr.Vahid Nejati for provid- ing the n-back task and Mehrshad Golesorkhi for the
helpful revision of our manuscript.
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1. Introduction
electrical stimulation (FES) is a poten- tially useful technique to restore motor functions in individuals with spinal cord
injury, head injury, stroke, and multiple sclerosis (Agarwal et al., 2003; Hardin et al., 2007). Currently, FES systems utilize
peripheral nerve stimulation or direct muscle stimulation
to restore motor functions. Though some advances have been achieved by traditional FES systems, there are still unsolved problems. The challenge of peripheral nerve
stimulation or direct muscle stimulation is the physi-
ological recruitment order of motor units (Tai & Jiang, 1994) and the muscle fatigue. Larger motor units are
activated physiologically after smaller ones have been
recruited according to the “size principle” (Fang & Mor-
timer, 1991; Karu, Durfee, & Durfee, 1995). However,