Internal work estimation in three-dimensional gait analysis.

Many studies concerning the internal work of human motion have used two-dimensional kinematic models to estimate kinetic energy of the segments. The generalised co-ordinate concept has been applied here to a simultaneous and bilateral gait analysis. A three-dimensional kinematic model based on quaternions has been developed. To estimate the kinetic energy of a multi-body system, only two constants and two variables are needed: the segment body mass, the inertia tensor, the position and the orientation of the local co-ordinate system with respect to the inertial co-ordinate system. The variation of the kinetic energy is used in the calculation of the internal work for an able-bodies subject during the gait cycle. Both the internal work and the instantaneous energy correlation coefficient enable the determination of a conservative phase delimited by the beginning of the single support phase until the flat-foot phase, and a non-conservative phase corresponding to the period from heel-off to contralateral heel-strike including the double support phase.