Determining the influence of muscle operating length on muscle performance during frog swimming using a bio-robotic model.

Frogs are capable of impressive feats of jumping and swimming. Recent work has shown that anuran hind limb muscles can operate at lengths longer than the 'optimal length'. To address the implications of muscle operating length on muscle power output and swimming mechanics, we built a robotic frog hind limb model based upon Xenopus laevis. The model simulated the force-length and force-velocity properties of vertebrate muscle, within the skeletal environment. We tested three muscle starting lengths, representing long, optimal and short starting lengths. Increasing starting length increased maximum muscle power output by 27% from 98.1 W kg(-1) when muscle begins shortening from the optimal length, to 125.1 W kg(-1) when the muscle begins at longer initial lengths. Therefore, longer starting lengths generated greater hydrodynamic force for extended durations, enabling faster swimming speeds of the robotic frog. These swimming speeds increased from 0.15 m s(-1) at short initial muscle lengths, to 0.39 m s(-1) for the longest initial lengths. Longer starting lengths were able to increase power as the muscle's force-length curve was better synchronized with the muscle's activation profile. We further dissected the underlying components of muscle force, separating force-length versus force-velocity effects, showing a transition from force-length limitations to force-velocity limitations as starting length increased.