The effect of functional electrical stimulation (FES) on paraplegic muscle fatigue

Stimulation trains of different combination of frequency and pulse-width can be used to generate the muscle force required to perform a functional task during functional electrical stimulation (FES). However, with repetitive activation, the muscle will fatigue and an increase in either the frequency or the pulse-width of stimulation will be required to enable the targeted muscle force to be maintained. This study compares isometric performance and paraplegic muscle fatigue using two different protocols: protocol 1 uses 5 different stimulation frequencies varying from 10Hz to 50Hz with other parameters fixed; and protocol 2 uses 5 different stimulation pulse-widths varying from 200μs to 400μs with other parameters fixed. This range is selected based on a suitable frequency and a pulse width for paraplegic. Muscle performance is assessed by measuring percent decline in peak force and maximum muscle force for different stimulation frequencies and pulse-widths. A simple rule is introduced to avoid spasm or injury to the leg during FES application. The results from this study show that higher frequency gives faster muscle fatigue and the selection of force required to perform a functional task is important for obtaining the optimum stimulation parameters. Stimulation pulse-width has no significant effect on the muscle fatigue but highly affects the maximum muscle force. The rule proposed is important and is found to be useful to avoid leg injury, spasm or uncomfortable feeling during FES application. This rule also can be used to choose optimum stimulation parameters.