The effects of muscle tissue mass on contractile performance

Skeletal muscles are the motors that drive human and animal locomotion. Yet despite their fundamental importance, our understanding of whole muscle behaviour is relatively limited due to practical and ethical considerations that hinder accurate in vivo measures. To estimate the behaviour of whole muscles, measures of single fibres or fibre bundles are often extrapolated to larger sizes without considering the consequences of the greater muscle mass. The goal of this thesis was to determine the effects of muscle mass on the contractile performance of whole skeletal muscles. In my first study, I developed a novel modelling framework to test different Hill-type model formulations under a range of cyclic contractile conditions. I then used this framework in my second study to examine the effects of distributed muscle mass on mass-specific mechanical work per cycle during cyclic contractions. I found that when the mass-enhanced muscle model was geometrically scaled from the size of a fibre bundle up to a whole human plantarflexor muscle, the mass-specific work per cycle decreased. In my third study, I examined the effects of muscle mass on the contractile behaviour of in situ rat plantaris muscle to validate the mass-enhanced Hill-type muscle model in my second study. In the fourth study of my thesis, I simulated cyclic contractions of a 3D continuum muscle model that accounts for tissue mass across a range of muscle sizes. I additionally compared the effects of greater muscle mass on tissue accelerations of the 3D muscle model to that of the in situ rat plantaris muscle from my third study to qualitatively validate the model simulations. I found that increasing the mass of the 3D muscle increased its volume-specific kinetic energy and was associated with lower mass-specific mechanical work per cycle. In my fifth study, I examined the effects of muscle mass on the metabolic cost and efficiency of muscle during cyclic contractions and how tendons of different stiffnesses alter these relationships. I found that larger muscles with greater mass are less efficient, primarily due to lower mass-specific mechanical work, and that the work and efficiency penalty of larger muscles can be offset to a certain extent by a tendon of optimal stiffness. Taken together, the results of these studies highlight that muscle mass is an important determinant of whole skeletal muscle behaviour.