We used high-speed video and electromyography (EMG) to measure in vivo performance of the trunk muscles (external obliques) in two related species of North American gray tree frogs, Hyla versicolor and Hyla chrysoscelis. Both species produce trilled calls with high sound intensity, but the sound pulse frequency within calls in H. chrysoscelis is twice that in H. versicolor. In both species, sound pulse frequency is directly correlated with the active contractions of the trunk muscles. The length trajectory during contraction and relaxation displays a saw-tooth pattern with a longer shortening phase compared with the lengthening phase. The longer time spent shortening may enhance power production, because the shortening phase is the active part of the cycle during which the muscle produces positive work. A similar total strain (approximately 21 % and approximately 19 % in H. versicolor and H. chrysoscelis respectively) is achieved in the first few pulses, and during subsequent pulses the muscle cycles with a reduced pulse strain (approximately 12 % and approximately 7.3 % in H. versicolor and H. chrysoscelis respectively). The higher pulse frequencies of H. chrysoscelis are thus associated with lower pulse strains. The EMG pattern is different in the two species. A single EMG stimulus occurs for each cycle in H. chrysoscelis, but two stimuli per cycle are found in H. versicolor. Indirect evidence suggests that the initial phase of shortening during a pulse is partly due to elastic recoil of the trunk.

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