Left side: Testing apparatus with a lower (small) frame and an upper (large) frame. The upper frame can move in X, Y and Z directions, while the lower frame can move in X, Y, Z directions and rotate around the Y axis. Right side: An illustration of the system axes according to the ISB (International Society of Biomechanics) standards. The Z axis represents the medio-lateral axis while Y represents the supero-inferior axis and the X axis the antero-posterior axis. (Wu et al. ).