The effect of speed on leg stiffness and joint kinetics in human running

The goals of this study were to examine the following hypotheses: (a) there is a difference between the theoretically calculated (McMahon and Cheng, 1 990. Journal of Biomechanics 23, 65-78) and the kinematically measured leng th changes of the spring-mass model and (b) the leg spring stiffness, the a nkle spring stiffness and the knee spring stiffness are influenced by runni ng speed. Thirteen athletes took part in this study. Force was measured usi ng a "Kistler" force plate (1000 Hz). Kinematic data were recorded using tw o high-speed (120 Hz) video cameras. Each athlete completed trials running at five different velocities (approx. 2.5, 3.5, 4.5, 5.5 and 6.5 m/s). Runn ing velocity influences the leg spring stiffness, the effective vertical sp ring stiffness and the spring stiffness at the knee joint. The spring stiff ness at the ankle joint showed no statistical difference (p < 0.05) for the five velocities. The theoretically calculated length change of the spring- mass model significantly (p < 0.05) overestimated the actual length change. For running velocities up to 6.5 m/s the leg spring stiffness is influence d mostly by changes in stiffness at the knee joint. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.