1. A method for estimating and tracking joint angles of a human multi-segment linkage, the method comprising: a) measuring inertial data as a function of time using an inertial measurement unit positioned at a distal end of the human multi-segment linkage; and b) computing from the measured inertial data estimated joint angles of the human multi-segment linkage as a function of time using a nonlinear state space estimator including an observation model incorporating an acceleration measurement vector comprising both translational accelerations and gravitational effects.