The relationships between rotations across the knee joint in the varus-valgus and longitudinal directions and the externally applied moments at the foot are investigated in laboratory and in skiing field tests. Auto regressive moving average exogenous (ARMAX) models of rotation across the knee resulting from moments applied to the foot are constructed using methods of system identification. The predictions of rotations by ARMAX models are compared with measurements. The ARMAX models identified from moment-rotation data recorded in quasi-static laboratory tests are highly dissipative for varus-valgus and for longitudinal rotations. Models identified using the field data show components of vibration of the instrumentation on the test subject in addition to the rotation component. Predictions of rotation across the knee by models obtained from field test data compare favorably to those predicted with models obtained from the laboratory test data.