Biomechanical models have been used in designing human work environments to evaluate potential risks to workers before a work environment is constructed. In order for work environments to be modeled correctly, most biomechanical models require as input, an accurate body posture of the worker. This information can be obtained by, either measuring the posture of workers for the task of interest, or estimating the posture. This research explores methods to estimate working postures by developing a model that can predict a worker's posture. The model in this thesis represents the body of the worker with ten links: neck, left and right forearms, left and right upper arms, body, left and right thighs, and left and right calves. The work task inputs consist of the magnitude and direction of the force applied to the hands, and the distances between the hands and the floor. By using these inputs, the model can predict a posture by optimizing an objective function of two criteria: Total Squared Moment and Balance. Model constraints also ensure that a predicted posture is feasible for human. The output of the model is the predicted posture in terms of ten body joint angles: neck, left and right elbows, left and right shoulders, hip, left and right knees, left and right ankles. These joint angles are defined as angles relative to horizontal. The prediction posture can be used as a base reference when inputting into other biomechanical models. By predicting posture from the model, one can obtain postures of the workers without direct measurement of postures from the workers, which can be expensive and time consuming. / Graduation date: 2002

[Truncated abstract] The study of biomechanical system dynamics consists of research to obtain an accurate model of biomechanical systems and to find appropriate torques or forces that reproduce motions of a biomechanical subject. In the first part of this study, specific computational models are developed to maintain relative angle constraints for 2-dimensional segmented bodies. This is motivated by the fact that there is a possibility of models of segmented bodies, moving under gravitational acceleration and joint torques, for its segments to move past the natural relative angle limits. Three models to maintain angle constraints between segments are proposed and compared. These models are: all-time angle constraints, a restoring torque in the state equations and an exponential penalty model. The models are applied to a 2-D three segment body to test the behaviour of each model when optimizing torques to minimize an objective. The optimization is run to find torques so that the end effector of the body follows the trajectory of a half circle. The result shows the behavior of each model in maintaining the angle constraints. The all-time constraints case exhibits a behaviour of not allowing torques (at a solution) which make segments move past the constraints, while the other two show a flexibility in handling the angle constraints more similar to a real biomechanical system. With three computational methods to represent the angle contraint, a workable set of initial torques for the motion of a segmented body can be obtained without causing integration failure in the ordinary differential equation (ODE) solver and without the need to use the “blind man method” that restarts the optimal control many times. ... With one layer of penalty weight balancing between trajectory compliance penalty and other optimal control objectives (minimizing torque/smoothing torque) already difficult to obtain (as explained by the L-curve phenomena), adding the second layer penalty weight for the closeness of fit for each of the body segments will further complicate the weight balancing and too much trial and error computation may be needed to get a reasonably good set of weighting values. Second order regularization is also added to the optimal control objective and the optimization has managed to obtain smoother torques for all body joints. To make the current approach more competitive with the inverse dynamic, an algorithm to speed up the computation of the optimal control is required as a potential future work.