A system for automatic accumulation and reduction of three-dimensional, human kinematic data has been designed and implemented which avoids the questionable and non-physiologic gait kinematics and forces often produced by contemporary techniques. The new system introduces three-dimensional, linkage analysis methods to describe completely the kinematics and estimate total moments and net forces between links, including a momentary axis-of-rotation solution approach to eliminate subjective location of "joint centers." Kinematic data acquisition is accomplished by an infra-red, opto-electronic device with thirty LED markers sampled by two cameras. A calibration scheme produces 10-bit accuracy. The three-dimensional point reconstruction is accomplished photogrammetrically. Sets of markers are grouped in segment-oriented, rigid arrays each with an imbedded body coordinate system (BCS). A least-squares, best fit is used to resolve the rotation matrix for each BCS. A piezo-electric, multi-component force platform synchronously acquires a force reference in the lower extremity, multi-segmented, kinematic chain. Kinematic data are significantly improved over conventional gait analysis techniques by a 315-Hz sampling rate, 1-mm and 20-milliradian resolution and accuracy and an improved marker-array mounting system employing stable anatomical mounting locations. The system also operates in real-time at 100 Hz. The frequency domain response of the overall system, as well as several classes of human motion, is well characterized. providing for intelligent selection of low-pass filter cutoff frequencies for noise elimination.