Assessment of muscle activation capacity: methodological considerations of muscle mechanics and implications for testing

Bampouras, Theodoros M. (2016) Assessment of muscle activation capacity: methodological considerations of muscle mechanics and implications for testing. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Muscle activation capacity assessment could be affected by the mechanical behaviour of the muscle, but this aspect has received little attention. Understanding the effect the muscle’s mechanical behaviour has on muscle activation capacity assessment can be paramount in achieving a better understanding of muscle function. The aim of the present Thesis was to examine the methodological implications of the mechanical behaviour of the muscle during muscle activation capacity assessment. Four studies were designed to examine the effect of the muscle-tendon unit on a) muscle activation capacity calculation methods and number of stimuli used, by manipulating quadriceps muscle length and consequently stiffness, b) stimulation intensity required and associated discomfort, by examining whether a lower than supramaximal stimulation intensity threshold, sufficient to stretch the muscle-tendon, exists, and c) the interplay between muscle mechanics and activation, by manipulating the testing position on the dynamometer, stabilisation and concurrent activation of remote muscles. Isometric knee extensions were used for all studies, and electrical stimuli was delivered to the muscle to quantify muscle activation capacity or induce muscular contractions by circumventing the voluntary neural drive. The results showed that a) altered muscle stiffness affects muscle activation values depending on the calculation method and number of stimuli used, suggesting caution to testing where muscle stiffness is likely to change, b) a lower stimulation intensity exists that can reduce subject discomfort while obtaining valid activation capacity results, widening the application of electrical muscle stimulation, and c) muscle activation must be considered in musculoskeletal models for more accurate predictions but the level of activation will ultimately depend on how stabilised the muscle is. Collectively, these results demonstrate the considerable effect muscle mechanics have on muscle activation capacity and that muscle strength assessment must take into account this aspect for more accurate inferences on muscle function.