Interaction of leg stiffness and surface stiffness during human hopping

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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 tok leg in bouncing gaits. We found thatk leg was greatly increased to accommodate surfaces of lower stiffnesses. The series combination ofk leg andk surf[total stiffness (k tot)] was independent ofk surf at a given hopping frequency. For example, when humans hopped at a frequency of 2 Hz, they tripled theirk 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 fork 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 thek 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 ink surf. This study provides insight into howk leg adjustments can allow similar locomotion mechanics on the variety of terrains encountered by runners in the natural world.