This dissertation presents two predictive biomechanics studies, fatigue-based posture and motion predictions. For the fatigue-based posture prediction study, the three-compartment controller fatigue model is integrated with an inverse dynamics optimization routine to predict the optimal posture, joint fatigue, and endurance time for a box carrying task. A two-dimensional human model with 10 degrees of freedom is used. For the box carrying task, the feet are stationary on the ground, and the hand location and box weight are given. The joint fatigue-based posture prediction formulation considers joint angles, three-compartment control values, and total box carrying duration (endurance time) as design variables. The objective is to maximize the total time while adhering to task and fatigue constraints, including compartment unity constraint, residual capacity constraint, and a novel coupled failure constraint. The optimization predicts the optimal posture, joint torque, endurance time, joint fatigue progression, and joint failure conditions. The novel joint fatigue-based formulation suggests the optimal posture to maximize endurance time with a given box weight. The simulation is efficient and provides optimal results in about 5 seconds of CPU time on a regular computer.

The fatigue-based motion prediction study investigates the progression of fatigue and forecasts the optimal motion trajectory in a repetitive lifting task. The lifting problem is mathematically formulated as an optimization problem to minimize dynamic effort and joint acceleration subject to physical and task-specific constraints. The design variables include control points that determine joint angle profiles using quartic B-splines. Additionally, profiles of the dimensions of the three and four compartments for spinal, shoulder, elbow, hip, and knee joints are treated as additional design variables. The study involves numerical simulations and experiments, using a 20 kg box as an external load for repetitive lifting. Simulation outcomes include detailed joint angle profiles, joint torques, and the progression of joint fatigue. The profiles of joint angles and torques follow distinct periodic patterns. Simulation results suggest a maximum of 11 (3CC) and 13 (4CCr) lifting cycles before the repetitive lifting task with a 20 kg box becomes unfeasible. Notably, these projected outcomes match observations from the experiments (predicted 13 cycles). Fatigue-based posture and motion predictions have significant contributions for ergonomic design and injury prevention in workplace.