Effects of current amplitude and pulse duration modulation on neuromuscular fatigue during repetitive electrical stimulation

Functional electrical stimulation (FES) has been used to enhance function and prevent muscle atrophy in people with spinal cord injury (SCI). In this study, we examined the effect of two different stimulation parameter sets on FES-induced muscle fatigue in 10 healthy individuals. Methods: 3 FES protocols were applied to the knee extensor muscle group of participants. The experimental protocols were as follows: practice session, test session 1 and test session 2. Subjects were trained to perform the MVIC test, and their MVIC was measured in the practice session. A long pulse duration (1000μs) and a current amplitude set to evoke 25% MVIC at 30 Hz was applied on subject's thigh for 2 minutes in test session 1. The protocol for test session 2 was identical to the test session 1 with the exception of a different pulse shape. A short pulse duration (200μs) and a current amplitude set to evoke 25% MVIC was used in test session 2. The percentage decline in peak force, the recovery rate during rest-periods, and self-reported pain were compared between the two FES parameter sets in both test sessions. Results: Percent muscle fatigue was significantly lower for the parameter set with long pulse duration (1000μs) and low current amplitude (LL) than for the stimulation parameter set with short pulse duration (200μs) and high current amplitude (SH). The reduction of peak force between the first and last peaks was significantly lower in the LL condition than in the SH condition. Pain scores were significantly lower for the LL than for the SH. Conclusion: The use of LL reduced the occurrence of muscle fatigue and pain compared to the use of SH. These results suggest that stimulation with LL may help reduce muscle fatigue during FES application.