From Salamanders to Mammals: Continuity in Musculoskeletal Function during Locomotion

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Abstract
Modern salamanders are the preferred model for studies of early tetrapod locomotion. The original cinematographic analyses of Evans [1946] and several subsequent reports have suggested that forward propulsion of salamanders is accomplished by three means: girdle rotation, limb retraction, and humeral-femoral rotation. In contrast to advanced mammals, the proximal limb segment of salamanders is held parallel to the horizontal plane (''sprawled'' posture). Thus, it is generally thought that longitudinal rotation of the humerus and/or femur throughout the stance (propulsion) phase passively drives the lower limb as the spoke of a wheel. We have tested this hypothesis in the hindlimb of adult Ambystoma tigrinum through electromyography and cinematography. Two postfemoral multiple joint muscles, the caudofemoralis (CF) and the puboischiotibialis (PIT), were implanted and their electrical activity patterns monitored during stepping. A marker pin in the distal femur allowed for simultaneous measures of the bone''s rotation. Contrary to the hypothesis (but strikingly similar to equivalent data for mammals), early stance is characterized by active knee flexion (CF silent; PIT active), hip stabilization, and no femoral rotation. Hip retraction and femoral rotation occur in the last half of the stance phase (CF active; PIT active). The electromyographic data supplement recent reports that have suggested a conservatism in the evolution of the neural control program for limb movement among tetrapods. We conclude that a continuity of bone-muscle function during stepping is maintained around the hip and knee joints from early tetrapods to mammals. Secondly, the multiple joint muscles of early vertebrates, with their mechanical and physiological advantages for bifunctionality may have preadapted early tetrapods for efficient limb movement.

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