Day to Day Consistency and Inter-subject Variability of Neuromuscular Responses and Performance Fatigability as the Resu
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ORIGINAL ARTICLE
Day to Day Consistency and Inter‑subject Variability of Neuromuscular Responses and Performance Fatigability as the Result of Maximal, Bilateral, Dynamic Leg Extensions Joshua L. Keller1 · Terry J. Housh1 · John Paul V. Anders1 · Cory M. Smith2 · Ethan C. Hill3 · Richard J. Schmidt1 · Glen O. Johnson1 Received: 10 February 2020 / Accepted: 4 August 2020 © Beijing Sport University 2020
Abstract Purpose The purpose of the present study was to examine the day-to-day consistency and inter-subject variability of composite and individual, neuromuscular and torque patterns of responses as a result of a fatiguing bout of maximal, bilateral, isokinetic leg extension muscle actions. Methods Ten men (Mean ± SD = 21.6 ± 2.5 years) completed two testing visits consisting of pretest and posttest maximal voluntary isometric contractions (MVIC) as well as 25 maximal, bilateral, isokinetic (180°/s), leg extension muscle actions. Electromyographic (EMG) and mechanomyographic (MMG) signals were collected from the vastus lateralis and normalized to the MVIC. Performance fatigability was defined as the percent decline in torque between the average of repetitions 1, 2, and 3 and repetitions 23, 24, and 25. Polynomial regression was used to examine the individual and composite relationships between EMG amplitude (AMP), EMG mean power frequency (MPF), MMG AMP, MMG MPF, and torque versus repetitions. Results There was no significant difference (d = 0.80; P = 0.17) in the performance fatigability between Day 1 (19.3% ± 7.1%) and Day 2 (26.9% ± 11.3%). Polynomial regression indicated substantial inter-subject variability, but high day-to-day consistency. Conclusion The composite EMG AMP, MMG AMP, and MMG MPF patterns of responses demonstrated consistency across days. The composite EMG MPF patterns of responses, however, were not consistent between Day 1 and Day 2. In addition, based on the high inter-subject variability, future studies should present both composite patterns of neuromuscular responses and the patterns of responses on a subject-by-subject basis to make inferences regarding fatigue-induced changes in motor unit activation strategies. Keywords Performance fatigability · Electromyography · Mechanomyography · Bilateral · Dynamic
Introduction
* Joshua L. Keller [email protected] 1
Exercise Physiology Laboratory, University of NebraskaLincoln, 1740 Vine Street, Lincoln, NE 68508, USA
2
Department of Kinesiology, Human and Environmental Physiology Laboratory, University of Texas At El Paso, El Paso, TX 79968, USA
3
School of Kinesiology and Physical Therapy, Division of Kinesiology, University of Central Florida, Orlando, FL 32816, USA
Performance fatigability has been described as “… the magnitude or rate of change in a performance criterion relative to a reference value over a given time of task performance or measure of mechanical output,” [22], p.411. Recent investigations [16, 27] have examined the inter-day consistency of performance fatigability (i.e., percent decline in p
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