A musculotendon model of the fatigue profiles of paralyzed quadriceps muscle under FES

A musculotendon model of the quadriceps muscle of the activated leg of a paraplegic patient incorporating fatigue was developed. The right quadriceps of a paraplegic patient who was engaged in a functional electrical stimulation (FES) training program was used for the measurements. The muscle studied was considered trained, both relating to strength and fatigue resistance. Extended stimulation was applied with an adjustable electrical stimulator, providing monophasic rectangular pulse trains with a frequency of 20 Hz, pulse width of 0.2 ms, and an intensity of up to 220 mA. The intensity used corresponded to the intensity required for the tested patient to stand up. This intensity was selected to deliberately encourage fatigue, and the result was a gradual and steady decay of the muscle force due to fatigue. The model was able to predict the decaying force during continuous electrical stimulation, as well as to indicate the muscle parameters which yield the best fit between the model prediction and the previously obtained experimental force profiles.