Development of a modeling technique for the investigation of muscle activity and its effect on bone stresses in the human leg during an isometric exercise

The study of the stress distributions within a bone can provide significant insight into its function and the adaptations that may take place under a set of loading conditions. Because the individual muscle forces acting on a bone are directly related to its behavior, bone stress analyses depend on the accurate estimation of muscle forces. The development of an integrated modeling method to determine such muscle forces and to apply them to the stress analysis of a system of bones is described. Firstly, a numerical optimization method was used in the development and validation of a model to predict the individual muscle forces generated in a leg that produce a known net resultant load. Next, this model was integrated into a finite element simulation of a system of leg bones so that the calculated isometric muscle forces could automatically be applied and the bone stresses calculated. The geometric, material, and joint contact conditions needed to appropriately depict the system's behavior were established. Finally, the developed modeling technique was implemented in parametric studies to reveal significant changes in muscle activity and bone stress magnitudes and distributions due to variations in the direction of the net resultant load. The smallest muscle forces occurred in loading directions most typical of daily activities and resulted in bone stresses that were an order of magnitude smaller than those resulting from more uncommon loading directions. The developed modeling technique was shown to provide a controlled means of investigating the relationships between muscle forces and bone stresses.