Force and Fatigue Development Following Electrical Stimulation: The Effect of Frequency and Fiber Type

Functional electrical stimulation (FES) has been used broadly during rehabilitation and sport injuries to expedite tissue and muscle recovery. More recently, attention has been given to the use of electrical stimulation at the cellular level for muscle regeneration. One of the challenges, however, is understanding the optimal FES characteristics to stimulate a muscle without causing fatigue or adverse injury to the muscle. The purpose of this research is to evaluate the effect of different FES frequencies (10, 35, and 50Hz) on the onset of skeletal muscle fatigue, defined by a drop in muscle force. Further, to evaluate the electrical activity generated during FES application using surface electromyography and its relation to time to fatigue (TTF) at each frequency. Ten healthy individuals between the ages of 18-30 years were consented according to the IRB approved protocol. Two muscles with different fiber type composition were evaluated; the abductor pollicis brevis (APB) with high composition of type I fibers and vastus lateralis (VL) with high composition of type II fibers. Results indicated that the VL required a greater stimulation amplitude to generate the FES-induced initial force and generated quicker TTF than the APB. Findings support recommendations that stimulation at 50Hz may be most optimal for small muscles of predominantly type I fiber composition while stimulation at 10Hz may be most optimal for large muscles of predominantly type II fiber composition to prolong TTF.