Abstract
Frogs’ outstanding jumping ability has been associated with a high power output from the leg extensor muscles. Two main theories have emerged to explain the high power output of the frog leg extensor muscles, either (i) the contractile conditions of all leg extensor muscles are optimized in terms of muscle length and speed of shortening, or (ii) maximal power is achieved through a dynamic catch mechanism that uncouples fibre shortening from the corresponding muscle-tendon unit shortening. As in vivo instantaneous power generation in frog hind limb muscles during jumping has never been measured directly, it is hard to distinguish between the two theories. In this study, we determined the instantaneous variable power output of the plantaris longus (PL) of Lithobates pipiens (also known as Rana pipiens), by directly measuring the in vivo force, length change, and speed of muscle and fibre shortening in near maximal jumps. Fifteen near maximal jumps (> 50cm in horizontal distance) were analyzed. High instantaneous peak power in PL (536 ± 47 W/kg) was achieved by optimizing the contractile conditions in terms of the force-length but not the force-velocity relationship, and by a dynamic catch mechanism that decouples fascicle shortening from muscle-tendon unit shortening. We also found that the extra-muscular free tendon likely amplifies the peak power output of the PL by modulating fascicle shortening length and shortening velocity for optimum power output, but not by releasing stored energy through recoiling as the tendon only started recoiling after peak PL power had been achieved.
Highlights
IntroductionThey are capable of jumping a horizontal distance exceeding ~30 times of their body length [1] by accelerating from a stationary initial position to great speed at takeoff in a fraction of a second
Near maximal frog jumping (>9 times the frogs’ body length) was investigated through direct measurement of plantaris longus (PL) muscle forces using a novel force buckle transducer adapted from an earlier design and optimized for use in frogs [19,25]
Our results suggest that the high peak power output by PL is produced through a dynamic catch mechanism during the propulsive phase of jumping [9]
Summary
They are capable of jumping a horizontal distance exceeding ~30 times of their body length [1] by accelerating from a stationary initial position to great speed at takeoff in a fraction of a second. Such an explosive movement requires a high power output from the frog hind limb muscles. There are two main theories in the literature proposed for explaining the mechanism of the high power generation in frog hind limb muscles during jumping. It is proposed that the hind limb muscles are used under optimal contractile conditions, which include operating at lengths across the plateau region of the force-length relationship and shortening
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