MEDLER, S.: Comparative Trends in Maximum Shortening Velocity and Tetanic Tension: Influence on Skeletal Muscle Diversity
Maximum shortening velocity (Vmax) and maximum tetanic tension (P0) are two determinants of a muscle’s contractile properties. Muscle power output, contractile efficiency, and ultimately the energetics of locomotion are all linked to Vmax. P0 defines a muscle’s force production and is generally considered to be constant, although significant differences have been found among some fiber types. In this study, published values of Vmax and P0 for more than 90 muscles representing three phyla and about 50 animal species were analyzed. P0 was found to be constant at about 150 kN/m2, being unrelated to body mass, phylogenic group, or shortening velocity. As such, P0 may represent a design constraint in the evolution of skeletal muscle diversity. In contrast, a significant inverse correlation (p < 0.0001) was found between body mass and Vmax in locomotory muscles, consistent with Hill’s observation that smaller animals possess faster muscles. Body mass alone accounts for almost 50% (r2 = 0.48) of the variability among the different shortening velocities, independent of either phylogenic group or mode of locomotion. Neither the ultimate cause nor the molecular mechanisms responsible for this scaling effect is fully understood. The mass exponent of -0.19 is about the same as described for quadrupedal mammals, but is derived from a much more diverse group of animals than previously studied. At a molecular level, differences in Vmax are generally associated with different myosin isoforms and this is probably the case here. How animals experiencing indeterminate growth sense mass changes and then appropriately switch their muscles’ contractile properties is unknown, but provides an interesting model of muscular plasticity.
