The weak link: do muscle properties determine locomotor performance in frogs?

Abstract

Muscles power movement, yet the conceptual link between muscle performance and locomotor performance is poorly developed. Frog jumping provides an ideal system to probe the relationship between muscle capacity and locomotor performance, because a jump is a single discrete event and mechanical power output is a critical determinant of jump distance. We tested the hypothesis that interspecific variation in jump performance could be explained by variability in available muscle power. We used force plate ergometry to measure power produced during jumping in Cuban tree frogs (Osteopilus septentrionalis), leopard frogs (Rana pipiens) and cane toads (Bufo marinus). We also measured peak isotonic power output in isolated plantaris muscles for each species. As expected, jump performance varied widely. Osteopilus septentrionalis developed peak power outputs of 1047.0 ± 119.7 W kg(-1) hindlimb muscle mass, about five times that of B. marinus (198.5 ± 54.5 W kg(-1)). Values for R. pipiens were intermediate (543.9 ± 96.2 W kg(-1)). These differences in jump power were not matched by differences in available muscle power, which were 312.7 ± 28.9, 321.8 ± 48.5 and 262.8 ± 23.2 W kg(-1) muscle mass for O. septentrionalis, R. pipiens and B. marinus, respectively. The lack of correlation between available muscle power and jump power suggests that non-muscular mechanisms (e.g. elastic energy storage) can obscure the link between muscle mechanical performance and locomotor performance.

Figure 1.

Instantaneous power output for representative jumps from B. marinus (dashed line), R. pipiens (dotted line) and O. septentrionalis (solid line). Osteopilus septentrionalis developed much greater power outputs compared with the other species on both a body-mass-specific basis (a) and when power outputs were normalized to total hindlimb muscle mass (b). Power outputs were measured by force plate ergometry.

Figure 2.

Peak instantaneous power output, measured per unit hindlimb muscle mass, varied among species, with the highest powers developed by O. septentrionalis and the lowest by B. marinus (a). These differences were explained in part by differences in the total work performed (b) and in part by differences in the duration of takeoff (c). Asterisks denote significant differences (p < 0.05) between pairs indicated.

Figure 3.

Isotonic force–velocity curves for m. plantaris longus for (a) O. septentrionalis, (b) R. pipiens and (c) B. marinus. Data for different individuals are denoted with different symbols and are fitted with a rectangular hyperbola [6].

Figure 4.

Peak power output per unit muscle mass developed during a jump (shaded bars) compared with peak power output per unit muscle mass developed in vitro (white bars). Differences in power developed during jumping were not explained by differences in the power-producing capacity of the hindlimb muscles.