Josephson and Edman (1988) examined the maximum shortening speed of different fibre-types and found that fast fibres contract slower when within a fibre bundle than when in isolation. Muscle mass is also important for understanding whole muscle behaviour in small animals, particularly when the muscle is contracting submaximally. As a consequence, muscle mass is not accounted for in estimates of larger whole muscle behaviour where the effects of mass are likely not negligible. Because the effects of mass in maximally active single fibres or fibre bundles are likely negligibly small, the effects of muscle mass are not accounted for in this common method of scaling. To estimate the behaviour of whole muscle, measures of single fibres, fibre bundles, or small whole muscles during controlled, maximal contractions are often extrapolated to larger sizes by scaling the forces with cross-sectional area and the lengths and velocities with optimal length ( Zajac, 1989). 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. Skeletal muscles are the motors that drive human and animal locomotion. Overall, the results of this study highlight that muscle mass is an important determinant of whole muscle behaviour. These comparable results for the simulated and in situ muscle also show that this modelling framework behaves in ways that are consistent with experimental muscle. When we compared the effects of mass on the model tissue behaviour to that of in situ muscle with added mass during comparable work-loop trials, we found that greater mass led to lower maximum and higher minimum acceleration in the longitudinal ( x) direction near the middle of the muscle compared to at the non-fixed end, which indicates that greater mass contributes to tissue non-uniformity in whole muscle. These energy changes in larger muscles due to greater mass were associated lower mass-specific mechanical work output per cycle, and this reduction in mass-specific work was greatest for smaller initial pennation angles. In addition to greater relative kinetic energy per cycle, relatively more energy was also stored in the aponeurosis, and less was stored in the base material, which represented the intra and extracellular tissue components apart from the myofibrils. We found that increasing muscle size, and therefore mass, increased the kinetic energy per unit volume of the muscle. In this study we conducted simulations of a 3D continuum muscle model that accounts for tissue mass, as well as force-velocity effects, in which the muscle underwent sinusoidal work-loop contractions coupled with bursts of excitation. We previously showed how energy is distributed through contracting muscle during fixed-end contractions however, it is not clear how the distribution of tissue energy is altered by the kinetic energy of muscle mass during dynamic contractions. This mechanical energy is then distributed and stored in the tissue as the muscle deforms or is used to perform external work.
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