Moment and power of lower limb joints in running

The aim of this study was to test the suitability of inverse dynamics method for instantaneous expression of joint kinetics and muscle function with various parts of the ground contact when running at different speeds. Nine male runners ran at speeds of 4.0 m x s(-1), 6.0 m x s(-1) and at their maximal speed. 2-D video analysis (200-frame x s(-1)) was synchronized with 3-D ground reaction force measurements (10 m-long force platform). Mechanical joint power was computed from 2-D segment dynamics associated with joint forces and net moments in multi-body movements. From these computations two successive functional phases during contact were found in the ankle and knee joints: 1) extensor negative and 2) extensor positive work. The hip joint showed three phases: 1) extensor positive, 2) flexor negative and 3) flexor positive work. Peak joint power increased in every investigated joint with increasing running speed. The highest changes were observed in the hip joint: from 327 +/- 203 W at a speed of 4.0 m x s(-1) to 1642 +/- 729 W (p < 0.01) at the maximal speed. The results may suggest that the role of the ankle and knee extensors is to create high joint stiffness before and during the contact phase, while the hip extensors are the prime forward movers of the body with increasing running speed. In conclusion, the inverse dynamics method may be of importance for use in specifying the joint kinetics and muscle function. However, the interpretation may become clearer when this method is used simultaneously with EMG measurements.