Mixed forward and inverse solutions in movement biomechanics

There are many more muscles crossing any given joint than there are degrees of freedom prescribing joint movement. One consequence of this arrangement is that muscle forces cannot be determined uniquely. This is the indeterminate problem in biomechanics, and virtually all attempts to solve it are based on either the inverse- or the forward-dynamics method in mechanics. The inverse method uses measurements of body motions and external forces to determine the net muscle torques acting about each joint. The forward approach uses net muscle torques (or muscle forces) as inputs to the model equations to calculate the corresponding body motions. Although both methods have their strong points, there are instances when neither is suitable if applied alone. One possibility is to combine the two methods, thereby exploiting the advantages of each. An example is given in which forward dynamics is used to calculate muscle forces in normal walking, and knee-ligament forces are then found by applying the inverse approach.