The anatomical arrangement of muscle and tendon enhances limb versatility and locomotor performance

The arrangement of muscles and tendons has been studied in detail by anatomists, surgeons and biomechanists for over a century, and the energetics and mechanics of muscle contraction for almost as long. Investigation of how muscles function during locomotion and the relative length change in muscle fibres and the associated elastic tendon has, however, been more challenging. In recent years, novel in vivo measurement methods such as ultrasound and sonomicrometry have contributed to our understanding of the dynamics of the muscle tendon unit during locomotion. Here, we examine both published and new data to explore how muscles are arranged to deliver the wide repertoire of locomotor function and the trade-offs between performance and economy that result.

Local movement in stimulated frog sartorius muscle.

Local movement was recorded in tetanically contracting frog sartorius muscle to estimate the nonuniformity in the distribution of compliance in the muscle preparation and the compliance that resides in the attachments of the preparation to the measuring apparatus. The stimulated muscle was also subjected to rapid length changes, and the local movements and tension responses were recorded. The results indicate that during tension development at resting length the central region of the muscle shortens at the expense of the ends. After stimulation the "shoulder" in the tension, which divided the relaxation into a slow decline and a subsequent, rather exponential decay toward zero, was accompanied by an abrupt increase in local movement. We also examined the temperature sensitivity of the two phases of relaxation. The results are consistent with the view that the decrease in tension during relaxation depends on mechanical conditions. The local movement brought about by the imposed length changes indicates that the peak value of the relative length change of the uniformly acting part was approximately 20% less than the relative length change of the whole preparation. From these observations, corrections were obtained for the compliance data derived from the tension responses. These corrections allow a comparison with data in the literature obtained from single fiber preparations. The implications for the stiffness measured during the tension responses are discussed.

Cell wall elastic properties of Chara corallina

Transcellular osmosis measurements are combined with measurements of the kinetics of relative length change, when Chara corallina cells are transferred from artificial pond water to polyethylene glycol (mol wt 300 to 400) of higher osmotic pressure to yield the constant k, which relates relative volume change ΔV/V to relative length change Δl/l (ΔV/V = k Δl/l). The value of k is 3.5 and is temperature independent between 7 and 30 °C. Static and kinetic estimates of the bulk modulus, ε, indicate that ε is temperature independent and has a value between 500 to 650 bars at high turgor pressures. The value of ε declines as the turgor pressure declines. We show how ε and k are related to Young's modulus and to Poisson's ratio for anisotropic C. corallina cell walls and point out that the previous treatments of the problem are in error.