The main objective of this proposal is to examine the neuro-muscular differences between closely related groups of animals that locomote differently. Anurans (frogs and toads) provide a group of closely related species that have evolved distinctly specialized locomotor modes: walking (Kassina senegalensis), hopping (Bufo americanus), and swimming (Xenopus laevis). Examining homologous leg muscles (semimembranosus and plantaris longus) in these different species that locomote differently will provide insight to understanding how and why animals use different modes or gaits. In the proposed study, I will test the hypothesis that neural control remains similar among behaviors, while differences in the contractile properties or operating ranges of the locomotory muscles contribute to their functional differences during walking, hopping, and swimming. My goal is to compare the in vivo performance of these locomotory muscles with their basic contractile properties by measuring their force-length, force-velocity properties, and in vitro mechanical work while simulating their in vivo patterns of strain and activation. Hence, this work represents one of the first attempts to link the dynamics of in vivo muscle performance to a muscle's intrinsic contractile properties. The findings obtained will also provide insight into the different demands placed on the musculoskeletal system by each behavior and the degree of plasticity in neural control relative to muscular contractile properties in closely related animals that use different forms of locomotion.