This paper describes a biomechanical investigation of the human hip during level walking, while climbing and descending stairs, and when rising from a sitting position. Triads of flashing, light-emitting diodes attached to the pelvis, thigh, shank, and foot are photographed by a biplanar technique to generate kinematic data. Kinetic data is collected using a piezoelectric force platform. The inverse dynamics problem associated with the three lower extremity segments is solved for the time variations of the intersegmental force and moment resultants at the hip, knee, and ankle. An optimization technique is used to distribute these resultants to the load-carrying structures in the neighborhood of the hip, and to the two-joint muscles that flex and extend the knee and ankle. Typical results for a group of normal volunteers are presented and discussed.