Measured and modelled static and dynamic axial trunk torsion during twisting in males and females.

Study of the mechanics of trunk twisting is of special interest given epidemiological evidence linking occupational twisting to increased incidence of low back pain. An anatomically detailed, three-dimensional model of the trunk (rib cage, pelvis, five lumbar vertebrae and 50 muscles), was used to predict maximum axial trunk torque. Predicted axial torques were compared with measured torques. Thirty-one (10 male and 21 female) subjects performed maximum effort isometric twisting exertions, at 0 degrees of twist and +/- 30 degrees of twist together with dynamic exertions, at 30 degrees s-1 and 60 degrees s-1. Females were able to generate approximately two-thirds of the torque of males (males, 97Nm; females 60Nm, isometric at 0 degrees). When the trunk was prerotated to 30 degrees, subjects were able to generate greater torque when the effort was toward the 0 degree position (approximately 105Nm by males and 68Nm by females). Experimental data indicated that velocity of rotation and amount of twist are important modulators of axial torque. Changes in muscle length were demonstrated to be minimal from model output as most muscle length changes during a twist from 0 degrees to 30 degrees, measured between the pelvis and the shoulder harness, were less than 1%, although some portions of the abdominal obliques underwent a length excursion of 5%. The small changes in the individual muscle force components that contribute to twist, i.e. the muscle unit vector about the axial twist axis and its moment arm that change as a function of twisted position, do not entirely account for the measured differences in torque, suggesting that additional mechanisms influence axial torque generation.