INFLUENCE OF TWO-JOINT MUSCLES ON JOINT FORCES AND JOINT FORCE ANALYSES.

In predicting muscle forces in humans by using mathematical models of the musculoskeletal system, there are explicit or implicit assumptions about how the musculoskeletal system works or is controlled. The purpose of this study is to question these assumptions by presenting experimental evidence that in most cases the presence of two-joint muscles prevents the creation of a free body diagram in which conditions only within the diagram influence joint forces, which must be the case if a unique solution is to be obtained from the free body formulation. This in turn raises questions with the conventional procedures of joint force analysis and accuracy of muscle force predictions. It also raises questions about the concept of neuromuscular redundancy of a given kinematic and force state, since the presence of that redundancy is derived from the equilibrium requirements of the free body segment.