Summary

We measured the electromyographic (EMG) activity of seven hindlimb muscles during jumping in the bullfrog Rana catesbeiana. The semimembranosus, gracilis major, gluteus magnus, adductor magnus, cruralis and plantaris longus were consistently active approximately 20-40 ms before any perceptible movement, as indicated by simultaneous video recordings. Activity ended before full extension of the hindlimb and take-off. Activity in the semitendinosus was variable among the jumps recorded. Simultaneous measurements of EMG activity and length changes (via sonomicrometry) in the semimembranosus (SM) and gluteus magnus (GM) muscles indicated that the performance characteristics of these two muscles differed. The SM muscle (a hip extensor) shortens and is activated in a manner consistent with its producing power during a significant fraction of the take-off phase. It shortened by a mean of 26.2% of the resting length during the propulsive phase of the two longest jumps for each frog. The delay between the onset of EMG activity and the beginning of shortening averaged 24 ms, which was brief compared with that found for the GM. The total strain and mean shortening velocity of the SM increased with jumping distance. Contrary to our initial expectations, the GM muscle does not shorten as one would expect of a muscle involved in powering the jump throughout take-off. This muscle has an extensor action at the knee, but also has a flexor action at the hip. A long delay existed between the onset of EMG activity and the beginning of shortening (46-116 ms among the individuals tested). Shortening during take-off by the GM (a mean of 16.7% for all jumps) was much less than by the SM, and in many jumps most of this shortening occurred late in the take-off period. Although the GM cannot contribute directly to power output early in take-off, it may contribute to powering the jump indirectly by transferring energy from the hip extensors to the knee joint. We conclude that muscles previously assumed (on the basis of anatomical criteria) by ourselves and others to be powering the jump may show considerable diversity of function. We hypothesize that elastic energy storage is used to help power jumping, and therefore suggest that muscles in series with major tendinous elements should be targeted for further study.