Contraction fatigue, strength adaptations, and discomfort during conventional versus wide-pulse, high-frequency,...

Neuromuscular electrical stimulation (NMES) can be delivered in a conventional form (CONV_NMES) and using relatively wide-pulses and high-frequencies (WPHF_NMES). WPHF_NMES is proposed to reduce contraction fatigability and generate larger contractions with less discomfort than CONV_NMES; however, there are no systematic reviews to guide the selection of NMES types. This systematic review compared the effects of CONV_NMES versus WPHF_NMES 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_NMES and WPHF_NMES for all outcomes or WPHF_NMES produced more fatigability. In a subset of non-clinical participants ("responders"), however, WPHF_NMES 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_NMES 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). Novelty: 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.