Contraction fatigue, strength adaptations, and discomfort during conventional versus wide-pulse, high-frequency, neuromuscular electrical stimulation: a systematic review.

Neuromuscular electrical stimulation (NMES) can be delivered in a conventional form (CONV) and using relatively wide-pulses and high-frequencies (WPHF). WPHF is proposed to reduce contraction fatigability and generate larger contractions with less discomfort than CONV; however, there are no systematic reviews to guide the selection of NMES types. This systematic review compared the effects of CONV versus WPHF on contraction fatigability, strength adaptations, and perceived discomfort in clinical and non-clinical populations. Eight studies were included. When averaged across all non-clinical participants in individual short- and long-term studies, there was either no difference between CONV and WPHF for all outcomes or WPHF produced more fatigability. In a subset of non-clinical participants ("responders"), however, WPHF reduced contraction fatigability during a single session. Long-term studies found no differences between protocols for strength adaptations in non-clinical participants and those with multiple sclerosis. We concluded that WPHF reduces contraction fatigability only in the short-term studies and in non-clinical responder participants and may exacerbate fatigability in non-responders. This review was registered in the prospective international registry of systematic reviews/PROSPERO (Registration Number: CRD42020153907). WPHF NMES may reduce fatigue in some participants and exacerbate fatigue in others. There were no differences in long-term studies between WPHF and CONV NMES on strength adaptations. Future high-quality research is needed to optimize outcomes of NMES-based programs.