Newman, D.J., "Tuning Muscle Stiffness to Accomplish Neuromuscular Control in Hypogravity," 2nd World Congress of Biomechanics, Amsterdam, the Netherlands, July 10-15, 1994.
Human neuromuscular motor control is investigated for locomotion over the continuum of gravitational acceleration from Earth-normal gravity (1 G) through lunar gravity (1/6 G). Gravity plays a crucial, but poorly understood role in human locomotion. Human evolution in an Earth-normal 1 G environment and the development of our 1 G musculoskeletal system have presumably optimized performance for terrestrial locomotion. Theoretical and experimental studies in the literature suggest that locomotion varies in altered gravity, perhaps due to compensating mechanisms taking place in hypogravity locomotion. This study investigates the muscle mechanics for generating forces as well as the nature of muscle stiffness, or mechanical impedance, for locomotion across the continuum of gravity. Using an instrumented treadmill while flying Keplerian trajectories on NASA's KC-135 aircraft, four subjects performed ambulatory tasks under conditions of hypogravity, 1 G, and hypergravity. Results indicate that peak vertical force and stride frequency significantly decrease (p<0.05) as the gravity level is reduced while ground contact time is independent of gravity level. Subjects tended to lope over a wide range of speeds (~1.5 m/s to ~2.3 m/s) suggesting a change in the mechanics for hypogravity locomotion as compared to typical 1 G locomotion, and gait transition occurs at lower velocities for hypogravity locomotion compared to terrestrial locomotion. After exposure to a hypogravity-hypergravity-hypogravity protocol, subjects' neuromuscular control was significantly altered, resulting in adaptive muscle tuning. Adaptive tuning of the neuromuscular system is depicted by a constant baseline stiffness level for hypogravity locomotion, then an appropriate increase in stiffness for hypergravity ambulation (@ 1.8 G) that is followed by a significant increase in stiffness for the second exposure to hypogravity that exponentially decays to the baseline level. This adaptive tuning strategy suggests an unique ability to accommodate to altered gravitational fields as well as intervention from higher levels in the central nervous system.
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