Minimal shortening in a high-frequency muscle

Reducing the cost of high-frequency muscle contractions can be accomplished by minimizing cross-bridge cycling or by recycling elastic strain energy. Energy saving by contractile minimization has very different implications for muscle strain and activation patterns than by elastic recoil. Minimal cross-bridge cycling will be reflected in minimal contractile strains and highly reduced force, work and power output, whereas elastic energy storage requires a period of active lengthening that increases mechanical output. In this study, we used sonomicrometry and electromyography to test the relative contributions of energy reduction and energy recycling strategies in the tailshaker muscles of western diamondback rattlesnakes (Crotalus atrox). We found that tailshaker muscle contractions produce a mean strain of 3%, which is among the lowest strains ever recorded in vertebrate muscle during movement. The relative shortening velocities (V/V_max) of 0.2-0.3 were in the optimal range for maximum power generation, indicating that the low power output reported previously for tailshaker muscle is due mainly to contractile minimization rather than to suboptimal V/V_max. In addition, the brief contractions (8-18 ms) had only limited periods of active lengthening (0.2-0.5 ms and 0.002-0.035%), indicating little potential for elastic energy storage and recoil. These features indicate that high-frequency muscles primarily reduce metabolic energy input rather than recycle mechanical energy output.