Leg stiffness and foot orientations during running

This study was done to determine whether leg stiffness (K-leg) during running was related to rearfoot-to-forefoot angle in standing (RFA(st)) and running (RFA(run)). Footprints obtained from 32 subjects were used to calculate RFA(st) and RFA(run), defined as positive when forefoot axis was abducted from rearfoot axis. A spring-mass model was used to calculate K-leg in running from ground reaction forces, measured by a force platform. The K-leg of runners (13.0 +/- 2.7 kN.m(-1)) was negatively correlated with RFA(st) (-8.4 degrees +/- 6.4 degrees) and RFA(run) (-0.4 degrees +/- 7.2 degrees). When runners were divided into opened foot (RFA(run) > 0; N = 19) and closed foot (RFA(run) < 0; N = 12) groups, the K-leg of opened foot runners was less than that of the closed runners. We suggest that foot structure is a factor responsible for the differences in leg stiffness observed in runners.