The projection of the ground reaction force as a predictor of internal joint moments

The projection of the ground reaction force vector is commonly used in clinical and rehabilitation settings to predict internal moments generated at the hip, knee, and ankle during walking. The results of using this convenient estimate are compared with those of a more complete measure (link segment method) in order to illustrate the differences quantitatively. Though both have intrinsic potential for errors, the author considers the link segment method much more accurate-and thus takes it as a standard against which the projection method may be judged. Thus, he defines differences between values obtained by the two methods as "errors" of the projection method. It is found that while the errors at the ankle are negligible, the errors at the knee and especially at the hip (and especially around the times of pushoff and heel contact) can give moments of incorrect magnitude or moments of the wrong polarity, especially at higher speeds.