Inverse dynamic calculations are often performed in the analysis of human ambulation. However, assumptions are made in the construction of the models that may not be valid for some prosthetic componentry, specifically polycentric knees used by transfemoral amputees. Data were collected for one subject using a polycentric knee. Moments and forces were calculated when ignoring the extra linkages of the knee and when including the extra linkages of the knee. Significant differences were seen in the hip moment and power between the two cases and indicate that care must be taken when comparing componentry that violates the basic assumptions of the rigid link model.