Ten subjects performed isometric weight-holding and force-resisting work tasks while standing upright, both with and without a twist of the trunk;​ with the trunk bent laterally;​ and in postures involving combinations of bending and twisting. The lumbar trunk muscle contraction forces and the lumbar spine compression and shear forces imposed by these tasks were predicted using a biomechanical model. Myoelectric activity was recorded quantitatively at eight locations over the back muscles and at four locations over the abdominal wall muscles. Correlation coefficients from 0.67 to 0.88 were found between the predicted muscle contraction forces and the measured myoelectric activities when the predictions were made so as to minimize muscle contraction force per unit area. Trunk twisting and lateral bending were found to load the spine and trunk muscles less than trunk flexion or holding of weights in front of the body.