Maintaining appropriate lower-limb joint stiffness is critical for walking performance, as it facilitates tasks such as absorbing impacts, maintaining balance, and providing body support and propulsion. Quasi-stiffness is often used to assess joint stiffness during dynamic tasks such as walking, as it accounts for passive soft tissue stiffness and active muscle force generation. Quasi-stiffness is characterized by the joint moment-angle relationship within discrete phases of the gait cycle.

Previous studies note that healthy individuals modulate sagittal-plane quasistiffness in response to different walking conditions. However, frontal-plane quasistiffness remains largely unexplored but has implications for understanding balance control in challenging mediolateral tasks. In Chapter 2, we established methods for characterizing frontal-plane hip and ankle quasi-stiffness during walking and explored how quasi-stiffness changes in both planes when walking at different step widths. We found that at wider step widths, there was increased sagittal-plane ankle quasi-stiffness and decreased hip quasi-stiffness in both planes along with changes in gait kinematics that suggest a decrease in balance control.

To further understand the neuromuscular control of quasi-stiffness, we used musculoskeletal modeling and simulation in Chapter 3 to identify individual muscle contributions to sagittal-plane quasi-stiffness during healthy walking. In addition to the primary ankle dorsiflexors/plantarflexors, knee flexors/extensors and hip flexors/extensors, we found that distant and contralateral muscles were major contributors to sagittal-plane ankle, knee and hip quasi-stiffness through dynamic coupling and joint angle modulation.

Unlike biological joints, passive prostheses cannot modulate their ankle-foot stiffness in response to different walking conditions. As a result, in tasks such as load carriage, prescribed prosthetic ankle-foot stiffness may not be optimal for the task demands. In Chapter 4, we determined how the presence and placement of added loads result in quasi-stiffness compensations from biological joints when wearing passive prostheses. Furthermore, we evaluated how load carriage affects intact limb compensations when wearing a passive or powered prosthesis, since unlike passive prostheses, powered devices can mimic muscle force generation. A tradeoff in intact hip and ankle demand suggested a shift towards cautious gait when carrying an anterior load, and improved ankle work symmetry using the powered prosthesis was encouraging for reducing intact limb compensations.