The inclusion of interstimulus interval variability does not mitigate electrically-evoked fatigue of the knee extensors
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ORIGINAL ARTICLE
The inclusion of interstimulus interval variability does not mitigate electrically‑evoked fatigue of the knee extensors Alexandra F. Yacyshyn1,2 · Robin B. Huculak1 · Chris J. McNeil1,2 Received: 14 March 2020 / Accepted: 27 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Purpose Transcutaneous electrical stimulation (TES) is used to activate muscles when volitional capacity is impaired but potential benefits are limited by rapid force loss (fatigue). Most TES fatigue protocols employ constant-frequency trains, with stimuli at a fixed interstimulus interval (ISI); however, a brief ISI between the first two pulses (variable-frequency train, VFT) to maximize the catchlike property of muscle can attenuate fatigue development. The purpose of this study was to investigate if a VFT that simulates intrinsic variability of voluntary motor unit discharge rates would also mitigate fatigue, owing to the sensitivity of muscle to acute activation history. Methods On two visits, 24 healthy adults (25.3 ± 3.7 years; 12 females) received 3 min of intermittent TES to the quadriceps of the dominant leg. Trains of eight pulses at 10 Hz were delivered with a constant (100 ms) or variable ISI (80–120 ms). Contractile impulse, rate of force development (RFD), and rate of relaxation (RFR) were determined for each tetanus Results During fatigue and recovery, contractile impulse did not differ between protocols (p ≥ 0.796) and sexes (p ≥ 0.493), with values of 77 ± 17% control at task end and 125 ± 19% control 2 min later. RFD and RFR also showed no effect of the protocol (p ≥ 0.310) or participant sex (p ≥ 0.119). Both measures slowed (38 ± 23% and 33 ± 22%, respectively) but dissociated during recovery as RFD remained 16 ± 18% below control at 5 min, whereas RFR recovered to control by 30 s (101 ± 22%). Conclusion Contrary to expectations, the VFT protocol did not attenuate fatigue development, which suggests no benefit to mimicking the inherent variability of motor unit discharge rates. Keywords Constant-frequency train · Peripheral fatigue · Quadriceps · Sex-related differences · Transcutaneous electrical stimulation · Variable-frequency train Abbreviations CFT Constant-frequency train ISI Interstimulus interval MVC Maximal voluntary contraction RFD Rate of force development RFR Rate of force relaxation TES Transcutaneous electrical stimulation
Communicated by Nicolas Place. * Chris J. McNeil [email protected] 1
School of Health and Exercise Sciences, Faculty of Health and Social Development, The University of British Columbia, Okanagan Campus, 133‑1147 Research Road, Kelowna, BC V1V 1V7, Canada
Centre for Heart, Lung and Vascular Health, Faculty of Health and Social Development, University of British Columbia, Kelowna, BC, Canada
2
VA Voluntary activation VFT Variable-frequency train
Introduction Transcutaneous electrical stimulation has been used for nearly 60 years as a means to maintain skeletal muscle mass and/or restore motor function for populatio
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