Wearable assistive devices, such as lower-limb exoskeletons and prostheses, can be used to augment human locomotion and may allow users to perform tasks with reduced metabolic effort, or promote physical activity and improve overall health. The goal of this dissertation is to examine the biomechanics and energetics of bouncing gaits (i.e., hopping and running) and cycling, and determine how assistive devices influence performance. First, I determined how the major biomechanical determinants of metabolic power in hopping and running change across step frequency (Chapter 1), and how hopping biomechanics are influenced by use of a passive, full-leg exoskeleton with linear and non-linear stiffness springs (Chapter 2). Then, I examined how altering bicycle and prosthesis configurations affect the biomechanics and metabolic costs of cyclists with a unilateral transtibial (below-knee) amputation (Chapters 3 & 4).

In Chapter 1, I found that active muscle volume per step decreases as step frequency increases in hopping and running, with the largest reductions attributed to the muscles surrounding the knee joint. I also found that accounting for changes in active muscle volume improves estimates of metabolic power using the ‘cost of generating force’ framework. These results support the general hypothesis that the metabolic cost of bouncing gaits is related to the magnitude of active muscle volume recruited to generate force and the rate at which force is produced.

In Chapter 2, I found that hopping in place using a passive, full-leg exoskeleton primarily assists the muscles surrounding the ankle, followed by the knee and hip, due to the exoskeleton’s average moment arm about each joint. Moreover, use of degressive stiffness springs within the exoskeleton provides the greatest reduction in the muscle-tendon unit’s contribution to overall ankle and knee joint moment and power, likely due to greater elastic energy stored in the spring for a given displacement and the length of the exoskeleton moment arms. These findings suggest that use of a passive full-leg exoskeleton for bouncing gaits may assist multiple joints and provide valuable information for the development of future assistive devices.

In Chapters 3 and 4, I found that in cyclists with a transtibial amputation, shortening the affected side crank arm length and increasing the prosthetic effective leg length while using a daily-use prosthesis did not affect biomechanical asymmetries between legs or influence efficiency. However, shortening the affected side crank arm length when cyclists used a cyclingspecific prosthesis (CSP) provided small reductions in hip joint power and hip transfer power asymmetry. Use of a CSP also reduced knee joint angle asymmetry and improved efficiency compared to a daily-use prosthesis. Thus, bicycle and prosthesis configurations influence rider mechanics and performance, and optimizing these configurations could promote cycling as exercise and improve the overall health of athletes with a lower-limb amputation.

Together, these studies improve our understanding of the biomechanical and metabolic effects of integrating assistive devices in human locomotion and may inform the development of future devices aimed at improving human performance.