The forces acting within and upon a limb are derived from three sources: postural (gravitational), inertial, and muscular. A method for decomposition has been established for free limb movements (Hoy & Zernicke, 1986); however, that method does not apply to kinematically constrained tasks whereby the limb exerts force on the environment. Presented here is a method for calculating the muscular and postural components for a quasi-static limb during a kinematically constrained task. It is a modified form of the inverse dynamic method reported by Kautz and Hull (1993) combined with the technique of Gruben and López-Ortiz (2000). This method stabilizes the limb against gravity with moments at each joint of the limb. Data from quasi-static lower limb extension efforts in one individual were analyzed to compare predictions of our method with those of the Kautz and Hull (1993) method. Differences in the postural component of foot force between the two methods increased with knee extension. The novelty of the method presented here was the use of an experimentally derived direction for the muscle component of foot force and the inclusion of a physiologically-based criterion for determining the support of the limb against gravity.
Dept. of Kinesiology
Dept. of Mechanical Engineering
Dept. of Biomedical Engineering, University of Wisconsin-Madison, Madison, WI 53706-1189.
Note: The authors have no financial interest in this research.