Modeling human regulation of momentum while interacting with the environment

Unrestricted Many activities of daily life require continual interaction with the environment. Improving understanding of how humans control interaction forces during goal-directed tasks may aid in improving performance and reducing injury risk. Modeling allows us to explore causal mechanisms and test hypotheses regarding control and dynamics of human movement. In this thesis, experimentally validated dynamic models were used to study how modifications in control affect performance and mechanical loading in different contexts including wheelchair propulsion, landings, and springboard diving. In both 2D and 3D models of wheelchair propulsion, redirection of the reaction force (RF) was found to be an effective mechanism for redistributing load across the elbow and shoulder without decrements in task performance. How RF affects load distribution was found to be subject specific and dependent on time in push cycle and the configuration of the user in the wheelchair. In land-and-stop tasks, increases in knee and hip joint flexion at contact reduced peak impact force and impulse generated during the impact phase, increased ability to generate impulse during the post impact phase (PIP), and increased either the mechanical demand or time required to reduce CM momentum to zero during PIP. Simulations of a forward somersault dive performed on a springboard revealed that increasing forward tip of the body during flight decreased CM vertical velocity but increased CM horizontal velocity and angular momentum at departure. Increase in downward CM velocity at board contact increased all momenta at departure. Increased squat depth at contact increased vertical velocity but decrease horizontal velocity and angular momentum of the CM at departure. Increasing hip flexion and knee extension excursions during board recoil increased angular impulse generation at departure. Overall, we found that there exists multiple ways to achieve the mechanical objectives of each task.; Results show that modifications in strategy include regulation of RF and segment kinematics of the upper / lower extremities, which alter the magnitude and distribution of mechanical demand experienced. Subject specific, experimentally validated models provide knowledge that may aid making decisions to improve performance and reduce risk of injury in both clinical and athletic environments.