Book excerpt: Lower extremity powered exoskeletons (LEPE) are powered orthoses that enable persons with spinal cord injury (SCI) to ambulate independently. Since locomotor therapy must be specific and resemble natural gait patterns, to promote motor recovery, current LEPE control architectures may be inappropriate since they typically use able-bodied, pre-recorded reference position and force data, at normal walking speeds, to define exoskeleton motion and predict torque assistance. This thesis explored two aspects: a) able-bodied walking dynamics between 0.2 m/s and the person's self-paced speed to provide a biomimetic basis for LEPE control and b) musculoskeletal modelling of LEPE-human dynamics. For walking dynamics, appropriate regression equations were developed for stride, kinematic, and kinetic parameters. These equations can be used by LEPE designers when constructing angular trajectories and forces for LEPE control at any given speed. An inflection point at 0.5 m/s was identified for temporal stride parameters; therefore, different walking strategies should be considered for walking above and below this point. The full body musculoskeletal model (Anybody) of persons with SCI using the ARKE LEPE incorporated all external contact forces and inertial properties (exoskeleton and person) and was driven using real LEPE SCI user kinematics and kinetics. For the lower extremity, large dorsiflexion range of motion, large device anterior tilt, incomplete knee extension, and uncontrolled center of pressure forward progression lifted the heel during stance. This triggered step termination before trajectory tracking at the knee and hip was complete, thereby reducing hip extension, increasing knee flexion through stance, increasing knee and hip support moments, and increasing thigh and shank strap reaction forces. This also shortened effective participant limb length, further shortening step-length and LEPE walking speed. For the upper-limbs, LEPE users walked with more anterior trunk tilt and twice the shoulder flexion angle, compared with persons with incomplete SCI. This increased forces and moments at the crutch, shoulder, and elbow. Crutch floor contact periods were 30-40% longer, resulting in upper-extremity joint impulses 5 to 12 times greater than previously reported. Improved step-completion and upright posture would reduce support loads on the crutches and upper-limbs, and would further improve LEPE-human lower limb interaction forces. Improved upright posture and LEPE-human interaction forces would enhance mobility and quality of movement for people with SCI.