Biomechanical analyses of weight-moving tasks in the seated position.

A three-dimensional biomechanical model of a human was developed to give a better understanding of the mechanical requirements for performing four dynamic tasks in the seated position. The tasks consisted of cyclic or single-directional movements of a hand-held weight in the sagittal plane. Each task was studied using a combination of mathematical model simulations and experimental measurements in seven subjects. An inverse dynamics formulation was used to calculate reactions at each major body articulation. Five linear optimization schemes were used to predict forces in 64 postural muscles crossing the trunk and lower extremity joints. These schemes involved the minimization of muscle forces, muscle stresses, or joint reaction components. A novel correlation method was used to compare the predicted muscle forces with measured electromyographic data over a complete task cycle. This method was also used to assess the efficacy of each prediction scheme in different tasks and subjects. The calculated joint loadings and observed muscular activities demonstrated that the lower extremities were important in maintaining a stable seated posture. Joint moments reached 49, 43, 15, and 42% of measured maximum voluntary strength at the trunk (third lumbar level), hip, knee, and ankle joints, respectively. Furthermore, considerable trunk extensor, hip flexor and extensor, knee flexor, and ankle plantar-flexor activities were required for all tasks. When judged by the number of statistically significant correlation coefficients, the model successfully predicted the time history of muscle activity in up to two-thirds of the 15 unilateral muscles whose activity was measured experimentally. A major finding was that 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. The significance of this study is that it provides a theoretical basis for understanding how seated posture is stabilized in static and dynamic tasks. This, in turn, will lead to improvements in the management of patients with trunk, hip, knee, or ankle musculature deficits.Ph.D.Applied SciencesBiomedical engineeringMechanical engineeringMechanicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128187/2/8812995.pd