The pressures on human articular cartilage have been measured in vivo. An instrumented femoral head prosthesis that telemeters interarticular pressure at 10 discrete locations 253 times per second was implanted in apposition to natural acetabular cartilage. Data were acquired during surgery, recovery, rehabilitation, and normal activity, for longer than 1 year after surgery. Pressure magnitudes were synchronized with body-segment kinematic data and foot-floor force measurements so as to locate transduced pressure areas on the natural acetabulum and correlate movement kinematics and dynamics with local cartilage pressures. The data reveal very high local (up to 18 MPa) and nonuniform pressures, with abrupt spatial and temporal gradients, that correlate well both in magnitude and distribution with in vitro data and computer simulations of synovial joint mechanics. Peak pressures in vivo are, however, considerably higher than pressures measured in vitro under the putative forces experienced by the joint in life, particularly in normal movements where cocontraction occurs in agonist and antagonist muscles across the hip joint. Thus, extant gait-analysis studies which apply inverse Newtonian calculations to infer joint forces establish the lower limit on such forces, since such analyses include only the net muscular torques about the joint and cannot account for the contribution of the increment in joint force due to muscular cocontraction. Our data also contribute to the understanding of normal synovial joint tribology and the possible role of mechanical factors in the deterioration evident in osteoarthritis. Further, design criteria for both partial and total hip replacement prostheses and specific aspects of rehabilitation protocols following hip surgery (e.g., the extent to which crutches and canes unload the hip joint) warrant reconsideration in light of the extraordinary high pressures measured during the activities of daily living.The maximum resultant force across the human hip joint during walking and running had been first estimated (1-3) and then measured in vivo (4-6) at from 2.5 to 5.8 times body weight, but the local pressures and pressure distribution between opposing cartilage layers in life have not been reported. Such knowledge is essential to understanding the physiology and pathology of human articular cartilage. How the opposing thin (1-3 mm) layers of articular cartilage in synovialjoints distribute high loads across matingjoint areas is essential to understanding the remarkable tribology (7) of normaljoints-high load carriage, low coefficients of friction (0.01-0.002), and long life-and may illuminate the role of mechanical factors in the etiology of osteoarthritis. Current opinion on the pathogenesis of this widely prevalent [estimated to afflict more than forty-million persons in the United States in 1962-64 (8)] and frequently incapacitating chronic disease (9) implicates focal mechanical stress on or in

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