M any studies describe hip biomechanics during gait (3,4,10,14, 15,18,21-23,33,35). Detailed investigation of temporally simultaneous acetabular contact pressures, ground reaction force, kinematics, joint torques, and electromyographic (EMG) activity during aided and unaided gait, however, are not found in a single literature source. Detailed biomechanical data are often the basis for therapeutic recommendations, such as specific lower limb transverse rotations ("turn out") or extension, to relieve weight bearing in a worn or necrotic portion of the femoral head or acetabulum. Because the variable of interest, hip stress, cannot be observed directly, but limb motions and assistive device use can be observed and controlled, the relationship of acetabular contact pressures to observable clinical phenomena is important to guide gait training for patients with hip pathology. We critically examined the magnitude and timing of hip biomechanical variables during the gait cycle of a subject with a pressure-instrumented femoral head prosthesis. Kinematics The hip rotates approximately 40" in the sagittal plane during a normal stride (7,31). Maximum hip flexion of 30-35" occurs in late swing phase at about 85% of the gait cycle; maximum extension of 10" is The literature is devoid of complete descriptions of hip biomechanics during gait. We present for the first time simultaneously acquired in vivo acetabular contact pressures, ground reaction forces, kinematics, hip torques, and electromyographic (EMGI activity during gait with and without a cane from an 85-year-old male with a left instrumented femoral head prosthesis. Highest acetabular contact pressures occurred in all gait trials at the posterosuperior acetabulum, just prior to peak EMG, adductor torque, and ground reaction force during late stance phase. Contralateral cane use reduced both peak acetabular contact pressure and gluteus medius EMG but not adductor torque or ground reaction force. These data identify a small area of high acetabular and femoral head stress that could occur during each of a human's millions of gait cycles annually and indicate that muscle activity, rather than solely body weight, drives hip loading. Clinicians who desire to limit hip loads should reduce both hip muscle contraction and weight bearing in late stance.