The objective of this study was to correlate the effects of muscle force on the movement of the center of pressure (COP) for increased clinical utility of the COP measurement. Five fresh frozen cadaveric specimens were used to apply a 49 N sinusoidal muscle force to isolated or grouped extrinsic ankle muscles, and a constant ankle joint reaction force at different tibial positions. The muscle force and the vertical ground reaction force (GRF) play the role of a mechanical lever system so that the differential COP movement can be interpreted as a moment arm for the vertical GRF.