Meeting Abstract
Springs in series with muscles store maximal energy when muscles undergo slow, forceful fixed-end contractions, such as the long preparation for a locust jump. However, some organisms, such as bullfrogs, load their springs quickly and, therefore, cannot achieve maximal force capacity due to the force-velocity tradeoff of muscle. Here we ask if the spring constants measured from time-limited organisms (bullfrog: Lithobates catesbeiana) are tuned to maximize energy storage for sub-maximal force production, whereas for non-time limited organisms (grasshopper: Schistocerca gregaria), they are tuned to maximal force production. By using a dynamic muscle-spring simulation, we identified the optimal spring stiffness of the time-limited jumper (bullfrog) and non time-limited jumper (grasshopper), and we compared these with actual measurements of spring stiffness. We found that when spring-loading is time-limited, optimal spring stiffness is lower than when it is not. We also found that the measured spring stiffness of tendons in bullfrogs more closely resembles the optimal spring constant at biologically-relevant loading times (50 ms) than at maximum force production (t > 300 ms). Conversely, the measured spring constant in grasshoppers matches the optimal spring constant at maximum force production (t > 300 ms). These findings demonstrate the importance of combining muscle dynamics and comparative analyses of spring mechanics when probing the limits of energy storage in muscle-spring systems and placing these dynamics in the context of how animals use these systems in their normal activities.
