Trunk and lower extremity muscle activity in seated weight-moving tasks: Three-dimensional analyses of intersubject and intertask differences

A three-dimensional musculoskeletal model was used to predict the trunk and lower extremity muscle activity required to stabilize the body while performing dynamic tasks in the seated posture. We studied seven subjects performing four tasks consisting of cyclic or single-directional movements of a hand-held weight in the sagittal plane. Five different optimization schemes involving the minimization of muscle forces, muscle stresses, or joint force components were used to predict the 64 muscle force-time histories. A quantitative method was used to compare prediction schemes by correlating predicted muscle forces with measured myoelectric data and ranking the efficacy of each scheme for different tasks and subjects. The results showed that (1) the trunk and lower extremity muscles play an important role in stabilizing the seated posture in these tasks. (2) the most successful muscle force prediction scheme was not the same for all seven subjects performing a given task, or for a given subject performing all four tasks, and (3) these linear optimization techniques successfully predicted activity in up to 10 of the 15 muscles whose myoelectric signals were actually monitored.