Transverse expansion of muscle fascicles during dynamic tasks: experimental and modelling approaches to study human gastrocnemii

Force generation is influenced by transverse shape changes of a muscle. Recent studies suggest that muscle fascicles may bulge in an anisotropic manner, and this may affect the relation between internal architecture and muscle deformations; however, to date fascicle bulging has not been quantified during active contractions. Understanding the nature and extent of the transverse deformations is necessary to explore the mechanisms driving the changes in internal geometry of whole muscle during contraction. The goal of this thesis is to quantify transverse deformations in muscles and fascicles using novel modelling and experimental techniques. The first study tested the accuracy with which 1D, 2D or 3D structural models of muscle could predict the pennation and muscle thickness for the medial gastrocnemius (MG) and lateral gastrocnemius (LG) in man during ankle plantarflexions. The second study acquired images from the MG and LG during cyclic contractions, and the transverse fascicle strains were calculated from their wavelengths within B-mode ultrasound images. For the third study, fascicle deformations were measured from two orthogonal ultrasound scans to provide 3D information of muscle geometry for the MG and LG. The results from the modelling study showed that a 1D model established a good relation between fascicle length and pennation; however, 3D models are necessary to understand the mechanisms underlying 3D structural changes. The second study found increases in the transverse fascicle strain while the longitudinal fascicle length decreased, however, the extent of these strains was smaller than expected. In the third study, transverse deformations in the MG were similar for the two transverse directions. However, the data for the LG confirm that transverse anisotropy in strain can occur in the muscle fascicles: as the LG fascicle length shortened, the fascicles bulged transversally in one direction while thinned in the other orthogonal direction. These results highlight that muscle fascicles do not bulge uniformly during contraction, and the implications for this behaviour on muscle function remain largely unexplored. This thesis provides a novel 3D perspective to enhance our understanding of the deformations of muscle fascicles during contraction, which in turn affect contractile performance and muscle function.