Integration of Axial and Appendicular Function in a Model Mammalian System

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Vertebrate biologists have often studied the axial and appendicular musculoskeletal systems as though they were independent entities. Increasingly, however, evidence is emerging that provides a clear illustration that an atomistic approach to the musculoskeletal system can be misleading. For example, recent attempts to 1) document and understand a possible mechanical constraint on simultaneous running and breathing in ectothermic tetrapods; 2) document the extent to which locomotion interferes with venous return to the heart; 3) unravel the nature and causes of back injury and pain in humans; and 4) identify and test possible locomotor and ventilatory functions of the epaxial and hypaxial muscles have been limited by our understanding of how the extrinsic appendicular muscles load the trunk. If we seek to understand the function and evolution of the vertebrate musculoskeletal system, it is clear that we cannot examine the axial and appendicular systems separately.The work outlined in this proposal will monitor muscle activity under controlled manipulations of locomotor forces to determine: 1) how the extrinsic muscles of the pectoral and pelvic limbs load the axial musculoskeletal system during steady state locomotion, and 2) how the axial muscles stabilize the trunk against the locomotor loads imposed by the extrinsic appendicular muscles. The rationale of the method is that changes in a particular aspect of the locomotor forces must be met by correlated changes in the recruitment of the muscles responsible for the locomotor force. Hence, correlated changes in locomotor force and muscle recruitment are interpreted to reflect a functional role for the muscle being examined. At a fundamental level, the information we gain from this work on dogs will be applicable to tetrapods in general and to mammals. First, the basal running gait of tetrapods was a trot. Although there is much variation in body configuration, level of work produced by the axial musculoskeletal system, and integration of running and breathing, the basic mechanics of trotting appear to be largely uniform across tetrapods. Hence, an understanding of the interaction and function of the axial and appendicular muscles of trotting dogs should also provide insight to that of other groups of tetrapods as well. Second, bounding gaits such as the gallop characterize the locomotion of mammals. Improving our understanding of how locomotor forces are transferred between the limbs and trunk and how the trunk is stabilized during galloping in dogs can be expected to provide insight to mammalian body design and locomotion.