Mechanics of In Vivo Muscle Function During Locomotion

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9306793 Biewener The main objective of this research is to record directly how limb muscles function during the locomotion of animals. Despite the considerable interest and extensive research that has been carried out on the physiology, biochemistry and mechanical properties of vertebrate skeletal muscles, little direct data exist concerning the in vivo mechanics of muscle of function during normal locomotor activity. In particular, studies of human limb muscles necessarily rely on the indirect assessments of muscle force and lengthening . By making direct in vivo measurements of the forces exerted by select muscles and length changes that they undergone different animal species, we can better understand how muscles function dynamically to support and power the movement of an animal. Measurements of muscle force will be made by using either implanted tendon force tranducers or by making force-calibrated in vivo skeletal strain recordings at the muscle's attachment site on the bone. In vivo measurements of muscle fiber length change will be made using ultrasound emitting sonomicrometry electrodes implanted within the muscle. In this proposal, we will examine the in vivo performance of muscles in two terrestrial and two flying species. In studies of ankle extensor muscles of wallabies, these measurements will enable us to explore how requirements for elastic energy storage in muscle-tendon 'springs' to conserve energy expenditure affect how the nervous system activates these muscles to power hopping versus the animal's need to accelerate and change direction. In studies of the medial(MG) and lateral(LG) gastrocnemius muscles in white leghorn chickens, we will study the recruitment of muscle force generation in relation to shortening during walking and running. The differing fiber characteristics of these two muscles (LG: mainly fast and MG: mainly slow), combined with independent recordings of force output, provides a unique opportunity to study recr uitment within a single muscle. I a third set of experiments we will obtain direct measurements of force generation and fiber shortening in the pectoralis muscle of pigeons and crows during flight. These measurements will provide the first direct measurement of muscle mechanical power, as well as whole animal power, during locomotion in a vertebrate species. These studies will extend our previous work, which developed the skeletal strain approach to measure in vivo force generation directly by the pectoralis during flight i starlings and pigeons. In summary, most studies of skeletal muscle mechanics have relied either on experimentally controlled simulations of shortening and force development isolated whole muscles or muscle fiber bundles under quasi-static conditions that are unlikely to apply to muscles performing normal movements. The results obtained from the proposed studies, therefore, will provide valuable information concerning the dynamic force-velocity properties of muscles under normal functional conditions. The results obtained from these studies will help to better establish general principles underlying muscle function in animal locomotion, as well as have relevance for motor control and robotics research. ***