The effects of muscle quality and continuous cycling on motor unit behavior of the vastus lateralis

Abstract

Introduction: Recent advancements in surface electromyographic signal decomposition and the inclusion of ultrasonography have presented the opportunity for a deeper understanding of motor unit behavior. However, the influence of sex and muscle composition on motor unit recruitment patterns has yet to be investigated. In addition, little is known regarding the neuromuscular adaptions from endurance training on motor unit behavior during submaximal tasks and long duration activities. Methods: Three distinct experiments were conducted to expand the current literature on motor unit behavior. The first project investigated the influence of sex and muscle quality on motor unit action potential sizes for 10 males and 10 females. The second project longitudinally investigated the effects of 10-weeks of continuous cycling on motor unit firing rate behavior and recruitment patterns for 23 sedentary individuals. The third project longitudinally investigated the effects of 5-weeks of continuous cycling on motor unit firing rate behavior and recruitment patterns during consecutive, long duration contractions for 25 sedentary individuals. Conclusions: For project one, the greater muscle quality and motor unit action potential sizes exhibited by males suggests greater muscle fiber sizes of the higher-threshold motor units. For project two, participants displayed increases in maximal aerobic capacity and decreases in maximal strength. The decreases in maximal strength were associated with alterations in motor unit firing rate patterns and increases in muscle activation to match pre-training absolute torques. For project three, participants displayed increases in maximal aerobic capacity and decreases in maximal strength. Following 5 weeks of training, there were time and repetition dependent change in motor unit firing rate and likely recruitment patterns to compensate for losses in strength.