Individuals post-stroke are commonly prescribed passive-dynamic ankle-foot orthoses (PD-AFOs) to aid in the control of shank forward rotation during mid-to-terminal stance due to plantar flexor weakness. PD-AFO bending stiffness is a key orthotic characteristic that can assist the plantar flexor muscles by providing the resistance needed to control shank forward rotation. Controlling shank forward rotation can store and return mechanical energy that can potentially increase gait velocity and decrease total mechanical cost-of-transport (COT). We developed a prescription model that can quantitatively prescribe PD-AFO bending stiffness based on an individual’s level of plantar flexor function deficit. The overall purpose of this dissertation was to quantify and compare post-stroke COT and gait velocity along with underlying joint-level mechanisms through a range of conditions over time while the participant wears their quantitatively prescribed PD-AFO and examine the relationships between COT and common clinical assessments. Ten individuals with chronic stroke (> six months post-stroke) participated and underwent five visits. A clinical evaluation and the data needed to prescribe and manufacture the PD-AFOs were captured at the first visit. The PD-AFOs were manufactured in carbon fiber material and then fitted to the participants. Data was collected as the participants walked on an instrumented treadmill while wearing the PD-AFO and then returned two weeks and four weeks later to complete the same analyses. Simulation modeling analysis was conducted to statistically test Aims 1-3. Results showed that the quantitatively prescribed PD-AFOs significantly decreased total mechanical COT and increased gait velocity, by at least one MDC, compared to not wearing an orthosis. When comparing wearing the PD-AFO to wearing the SOC AFO, seven out of the nine participants significantly decreased total mechanical COT and although all participants increased gait velocity, only two increased by at least one MDC. Despite these improvements, inconsistent results were seen in the joint-level parameters across the three orthoses conditions. While wearing the PD-AFO across a range of gait velocities, changes in total mechanical COT were seen. While wearing the PD-AFO over time, minimal to no changes were seen in the global and joint-level parameters. Lastly, three of the clinical assessments (Fugl-Meyer assessment, dorsiflexion ROM with the knee flexed to 90°, and gait velocity) showed potential relationships to changes in total mechanical COT magnitudes while wearing the PD-AFO. In general, these results improved our understanding of how quantitatively prescribing PD-AFO bending stiffness effected gait biomechanics and energetics over a range of conditions.