Day to Day Consistency and Inter-subject Variability of Neuromuscular Responses and Performance Fatigability as the Result of Maximal, Bilateral, Dynamic Leg Extensions

Abstract

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. 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. 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. 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.