On hip and lumbar biomechanics. A study of joint load and muscular activity.

Loading moment of force about the hip and lumbo-sacral joints during straight and flexed knee lifting was calculated at regular intervals during the lifts, using a computerized sagittal plane model. Lift with flexed knees and burden moved close to the body was found to induce lower load. Important parameters influencing the induced joint load are discussed. EMG levels from three trunk and seven hip muscles were recorded with surface electrodes and normalized, i.e. expressed as a percentage of the recorded level of each muscle during an isometric voluntary contraction. The erector spinae, gluteus maximus and adductor magnus muscles were more activated initially in the flexed knee lifts than in the straight knee lift, while the hamstrings were more and earlier activated in the flexed knee lifts. In order to estimate what proportion of the maximum hip extensor muscle moment was used to counteract the induced load moments about the hip, assessments of maximum hip extensor muscle moments were made: the highest extensor muscle moment was exerted at 90 degrees hip flexion, decreasing with decreasing hip angle. The knee angle did not significantly influence the muscle moments exerted by the hip extensor muscles. To calculate the magnitude and direction of the hip compressive load from known load moments, the length of the moment arms of three hip extensors was determined at every fifth degree of flexion at hip flexion angles between 0 degrees and 90 degrees by combining data from autopsy specimens and patients, the latter examined by computed tomography. There were significant differences in moment arm length between men and women but there was low correlation between subject's height and length of muscular moment arms. The calculated compressive force during the lifts was about three times body weight and the direction of the compressive force passed through the area described by others as having the most severe osteoarthrotic lesions on the femoral head. Hip joint load moments and normalized EMGs from five hip muscles were assessed during rising exercises for the lower limb in order to determine to what extent joint load and EMG is influenced by adaptive adjustments of the exercise. A multicomponent force-measuring platform and pictures on cine-film were used in the calculations of load moments. Posterior foot position lowered the load moment and slightly increased the level of muscular activity in the hip extensors. The design and use of a force prediction diagram for estimating induced hip extensor muscular forces is described.(ABSTRACT TRUNCATED AT 400 WORDS)