INTERACTION OF LEG STIFFNESS AND SURFACE STIFFNESS DURING HUMAN HOPPING

When mammals run, the overall musculoskeletal system behaves as a sing le linear ''leg spring.'' We used force platform and kinematic measure ments to determine whether leg spring stiffness (k(leg)) is adjusted t o accommodate changes in surface stiffness (k(surf)) when humans hop i n place, a good experimental model for examining adjustments to k(leg) in bouncing gaits. We found that k(leg) was greatly increased to acco mmodate 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 frequ ency 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 su rface (k(surf) = 35,000 kN/m; k(leg) = 17.8 kN/m). Values for k(tot) w ere 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) adjustm ent, 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 i nto how k(leg) adjustments can allow similar locomotion mechanics on t he variety of terrains encountered by runners in the natural world.