Journal of Applied Physiology
Vol. 82, No. 1, pp. 15-22, January 1997
EXERCISE AND MUSCLE

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

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