Mechanisms that relate transverse loading of muscle to changes in contractile performance

Research has shown that factors such as the contraction of surrounding muscle, resistance to radial expansion, and external transverse loading have an effect on the way that muscle performs. Such factors are often ignored when doing muscle experiments, yet they can have significant effects on the force and power that a muscle is able to produce. The aim of this thesis is to determine to what degree external loading affects muscle force and to study the changes in muscle architecture due to external loading. The purpose of this thesis was to determine whether external transverse loading causes force reduction in humans, how external transverse loading affects muscle architecture, and describe the mechanisms involved. We applied an external transverse load to the medial gastrocnemius of prone participants as a point loaded mass. Transverse loading reduced ankle torque with higher loads resulting in greater reductions. Passive transverse loading caused a decrease in the resting pennation angle and muscle thickness, with higher loads leading to greater decreases. During activation of the muscle the pennation angle, muscle thickness, and fascicle thickness increase transiently relative to the amount of transverse loading. Alterations to the design of the experiment were made by applying a multi-directional external transverse load and changing the position of the participants from prone to seated, where sitting changed the resting length of the muscle. Our altered experiments showed no change in ankle torque. Minor differences were shown for pennation angle and muscle thickness both at passive resting values and peak active values, but not for fascicle thickness and fascicle. Fascicle thickness decrease transiently, and fascicle length increased. We replicated the external transverse loading experiments using a three-dimensional finite element model of a fibre-reinforced, non-linearly-elastic transversely isotropic composite biomaterial. The model consists of active and passive force-length properties of muscle fibre as well as passive force-length properties of base material. Results show external transverse loading causes architecture changes and reductions in muscle force. Muscle force reduction is dependent on direction of loading, initial pennation, and initial muscle length.