Mechanical function of muscles and tendons during running

Just what is it that muscles do to keep a runner moving? It is only recently that new technologies allow us to go inside the “black box” of running animals’ muscle to measure key parameters of muscle function. The length change, force and power output in individual muscles during movement can tell us how many of the basic mechanical properties of muscle as an actuator – the force-velocity relation, the length-tension relation – interact with the mechanics of moving bodies to define the limits to locomotion. Wild turkeys are a favorite lab animal for measurements of muscle function during running. The questions we are currently addressing with this model include: How do muscles change mechanical function when the demands for mechanical power change, such as during uphill running? How does the spring-like function of tendons affect muscle behavior? Do muscles operate over the regions of their length-tension and force-velocity relations that are most favorable for force production during steady-speed running? Results from these studies suggest that muscles operate primarily as effective force producers during steady speed running, and they modulate length changes to increase power output for uphill running and acceleration.

Physiological limits to acceleration

The vast majority of studies of terrestrial locomotor mechanics and energetics have focused on steady-speed locomotion, while in nature it may be non-steady movements – accelerations, decelerations, turns – that constitute the majority of an animals locomotor activity. Studies of non-steady locomotion may provide insight into some of the most important mechanical influences shaping the evolution of the musculoskeletal system. As most sports-related injuries involve non-steady movements, studies of acceleration and ballistic movements may also be potentially useful to sports medicine. We are currently studying acceleration to determine the muscle properties that may limit maximal acceleration. We are using a variety of approaches, from classic biomechanical techniques such as force plate measurements, to isolated muscle experiments, to study performance during acceleration. Experiments with wild turkeys and other animals have suggested a surprisingly important role for tendon springs in powering accelerations.

The physiological basis for the energy cost of locomotion

An ongoing interest in the lab is the physiological basis for the energy cost of locomotion. Very clear trends in the energy cost of movement have been identified empirically in running animals, but the explanation for these trends is not complete. For example, a linear increase in rate of metabolic energy consumption with running speed has been observed for all running animals, but it has not been adequately explained from principles of muscle physiology. We have recently begun to examine the energy cost of generating muscular force in isolated muscles to test hypotheses about the energy cost of generating muscular force during running.

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