The performance of a skeletal muscle during movement is determined by the interaction of its intrinsic properties with the mechanical properties of the system to which it is linked. These interactions are complex and predictive equations are limited in some respects by the lack of empirical data on the performance of muscle under loading conditions that replicate those found during movement. Muscles serve three major mechanical functions during movement: producing force, producing work, and providing stability. These different functions are linked to differences in the length trajectory (sequence of length change) in relation to the motor activity of the muscles. Although we know that all three of these functions are performed by humans in walking and running, we know little of the conditions under which individual muscles operate while performing each function. Further we have no empirical data on the quantitative importance of each function to the cost of locomotion. The specific aims of this project are to: 1) Examine the in vivo contractile parameters (operational lengths, length trajectories, and amounts of series elasticity) for muscles that are active only while performing positive work in running and jumping; 2) Examine the prediction that during running and jumping actively lengthening muscles function to help stabilize the movement; 3) Quantify the relative energetic importance of the different mechanical functions served by muscles during running; 4) Measure the efficiency of fast and slow muscles under conditions of varying power output; 5) Quantify the influence of velocity dependent activation and deactivation on mechanical function of fast and slow muscles. The mechanical function of muscles used in running and jumping will be assessed in vivo using sonomicrometry and electromyography. Blood flow measurements using fluorescently labeled microspheres will be used in conjunction with other measures to estimate the relative contribution of the different limb muscles to the energy cost of running. In vitro work with the muscles used in jumping and running and computer modeling will examine the optimum conditions for accelerating inertial loads. Mouse muscles will be used to examine the influence of length trajectory and cycle frequency on mechanical performance and efficiency. This project is predicated on the assumption that examining how muscles are used in animals during movement allows us to better predict the design parameters important in human movement and will improve our understanding of both normal and dysfunctional human movement. Such studies will eventually assist in designing rehabilitative strategies that require an understanding of the diverse roles of muscles during movement.