An exercise apparatus is disclosed. The exercise apparatus comprises: a three-dimensional coordinate system having an x-axis, a y-axis, and a z-axis; lower limb segments; a moving arm; a multi-axis force sensor; at least one footplate attached to said multi-axis force sensor for supporting a human foot or shoe; a compact real-time ankle motion measurement device, said motion measurement device attached between the leg and foot and capable of measuring the ankle joint motion in at least one degree of freedom; and a central processor, said central processor capable of calculating lower-limb joint moments in real time. The central processor calculates lower-limb joint moments with a modified inverse dynamic method, which does not involve computation of location of center of pressure.