Model of activation dynamics for an FES-induced muscle fatigue

A computer model of activation dynamics for an FES-induced muscle fatigue, which is one of the major problems in functional electrical stimulation (FES) has developed. Model consists of activation dynamics and Hill-type musculotendon dynamics. The authors modified the conventional activation dynamics to represent reduction and recovery of muscle force. By considering that FES can cause the shortening velocity of muscle to decline the authors modified the activation dynamics to affect the muscle shortening velocity. To model muscle fatigue the authors introduced the fatigue variable and the fatigue admittance assuming that the activation changes with the electrical admittance of muscle fibers. The fatigue variable relates the stimulation time to the applying pulses. The fatigue admittance changes the Ca²⁺ uptake rate and thus the muscle shortening velocity as a function of the fatigue variable. The computer simulation results showed that the proposed admittance-type model match well with the experimental observations reported in literatures in terms of force reduction and recovery as well