PILARSKI, J.Q.; NISHIKAWA, K.C.; PIEROTTI, D.J.; Nothern Arizona Univ.; Northern Arizona Univ.; Northern Arizona Univ.: Power amplification by pre-activation of semimembranosus in Rana pipiens

In frogs capable of jumping long distances, power output during takeoff has been shown to be much greater than the hind limb musculature is capable of producing based on after-loaded isotonic muscle preparations. Even conservative estimates show a three-fold difference between in vivo and in vitro power outputs per kg muscle mass. Reasons for this discrepancy include biomechanical, musculoskeletal, and neural adaptations most likely operating in combination. Because jumping performance is highly dependent on the ability of the hind limb muscles to produce mechanical power, the goal of the present study is to examine how different neural strategies affect the power output of these muscles. We hypothesize that hind limb extensors store elastic strain energy prior to takeoff while the muscle is contracting isometrically against a load, i.e. inertial load or co-contraction of antagonists, and recover the strain energy during the shortening event. We selected the m. semimembranosus to test this hypothesis in situ because it is a major hip extensor with <10% of its length being tendonous connections at the origin and insertion combined. The muscle was isolated and subjected to a series of after-loaded (control) and load-clamp experiments in which the muscle was stimulated to contract isometrically for 50, 100, or 200 msec prior to an instantaneous drop to lower loads. Muscle shortening produced a biphasic length profile that was used to calculate the force-velocity relationship. By examining both the fast phase and the slow phase, we conclude that elastic strain energy is stored within the muscle and contributes to powering the jump by increasing velocities of shortening, and thus power output, to levels that more closely reflect whole animal performance.