In order to perform an inverse dynamics analysis it is necessary to have 9 variables to define the reaction force completely (e.g. Force Vector (Fx, Fy, Fz), Center of Pressure (CPx, CPy, CPz), and Free Moments (Tx, Ty, Tz)). Most force platforms measure only 6 signals of the 9 required. This is not a limitation in gait laboratories because for studies of walking it is possible to make valid assumptions about 3 of the signals, thus providing all of the signals necessary for an Inverse Dynamics analysis of walking on the force platform (The assumptions are: CPz=top surface of the force platform, and Tx=Ty=0).

Note: The assumption that the two free moments are equal to zero are not valid for handles instrumented with 6 degree of freedom load cells, wheel chair rims, or any circumstance in which the person can wrench the force platform, or pull up on it.