HIP STRESS DURING LIFTING WITH BENT AND STRAIGHT KNEES

''Correct'' body mechanics during lifting are believed to protect the back by employing knee and hip flexion while keeping the back straight . Lower limb joint stress, however, has been largely ignored. We compa red hip cartilage contact stress during ''leg lifting'' with ''back li fting'' (lifting with bent or straight knees, respectively) in a subje ct fitted with a pressure instrumented hip endoprosthesis. Synchronize d pressure data and whole-body kinematics and kinetics were collected simultaneously while the,subject lifted an 11.8 kg mass from the floor to waist level. The highest pressure, 13.7 MPa, occurred during leg l ifting at the antero-lateral femoral head transducers opposed at maxim um hip flexion by the postero-superior quadrant of acetabular cartilag e. In back lifting, the highest pressure, 11.5 MPa occurred in the sup ero-lateral aspect of the head, which during hip extension was opposed by the posterior quadrant of the acetabulum. Maximum pressures and hi p torques occurred simultaneously with peak hip flexion, during the in itial lifting of the burden from the floor. Acetabular contact pressur es during leg lifting were on average twice as great as during back li fting, and both techniques generate much greater hip stress than gait (typically 4-6 MPa). Degenerative changes in the articular surface of the acetabulum occur primarily on the postero-superior aspect, corresp onding to the locations of peak contact pressures in the present in vi vo data. Thus leg lifting puts more stress on the postero-superior reg ion, and probably contributes to more hip cartilage degeneration, than does back lifting. We conclude that although leg lifting may mechanic ally protect the back, it substantially increases hip cartilage stress .