Interaction of leg stiffness and surface stiffness during human hopping

Daniel P. Ferris and Claire T. Farley Department of Human Biodynamics, University of California, Berkeley, California 94720-4480 Ferris, Daniel P., and Claire T. Farley. Interaction of leg stiffness and surface stiffness during human hopping. J. Appl. Physiol. 82(1): 15-22, 1997. When mammals run, the overall musculoskeletal system behaves as a single linear "leg spring." We used force platform and kinematic measurements to determine whether leg spring stiffness ( k leg ) is adjusted to accommodate changes in surface stiffness ( k surf ) when humans hop in place, a good experimental model for examining adjustments to k leg in bouncing gaits. We found that k leg was greatly increased to accommodate surfaces of lower stiffnesses. The series combination of k leg and k surf [total stiffness ( k tot )] was independent of k surf at a given hopping frequency. For example, when humans hopped at a frequency of 2 Hz, they tripled their k leg on the least stiff surface ( k surf = 26.1 kN/m; k leg = 53.3 kN/m) compared with the most stiff surface ( k surf = 35,000 kN/m; k leg = 17.8 kN/m). Values for k tot were not significantly different on the least stiff surface (16.7 kN/m) and the most stiff surface (17.8 kN/m). Because of the k leg adjustment, many aspects of the hopping mechanics (e.g., ground-contact time and center of mass vertical displacement) remained remarkably similar despite a >1,000-fold change in k surf . This study provides insight into how k leg adjustments can allow similar locomotion mechanics on the variety of terrains encountered by runners in the natural world. running; spring-mass model; biomechanics; motor control 0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society