Changing stimulation patterns improves performance during electrically elicited contractions

The clinical efficacy of functional electrical stimulation (FES) is limited by the rapid onset of fatigue. FES applications use electrical stimuli separated by regular interpulse intervals (constant-frequency trains or CFTs) to activate muscles. However, doublet-frequency trains (DFTs) may produce greater forces than CFTs, but also produce more fatigue. DFTs contain a series of doublets, two pulses separated by a short (5-ms) interpulse interval. We hypothesized that a combination of CFTs followed by DFTs would improve performance compared to either train type alone. Quadriceps muscles of 15 normal subjects were fatigued with either 150 DFTs, or initially fatigued with CFTs until the targeted isometric force was no longer produced and then switched to DFTs. The combination reached the targeted isometric force (mean +/- SEM) more times (59.80 +/- 4.03) than either the CFTs alone (51.20 +/- 3.50) or DFTs alone (55.33 +/- 3.81). This finding suggests that combining train types may be a useful strategy to offset the rapid fatigue that persons with neurological dysfunction, such as spinal cord injury, experience when using FES.