Interaction of leg stiffness and surface stiffness during human hopping

Abstract

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.