Ankle-foot orthoses are assistive devices regularly prescribed to aid individuals with plantar flexor weakness and help improve gait function. Passive-dynamic ankle-foot orthoses (PD-AFOs) constitute a class of ankle braces that rely on material properties and design features to dictate orthosis function. A variety of fit and functional characteristics can be tuned to customize PD-AFOs to meet an individual’s needs. PD-AFO stiffness is a key functional characteristic that drives the resistance to bending and therefore the amount of assistance provided by the orthosis. Thus, PDAFO stiffness may enable orthoses to substitute for lost plantar flexor function. While it is agreed that PD-AFO stiffness must be personalized to meet an individual’s needs in order to achieve optimal performance outcomes, customizing PD-AFO stiffness is currently complicated by the limited understanding of how these assistive devices interact with the complex dynamics of the lower extremity musculoskeletal system. Furthermore, current craft-based, manual orthosis design and fabrication methods inhibit the objective tuning of PD-AFO stiffness.

The purpose of this dissertation was to systematically identify the influence of customized PD-AFO stiffness on gait function at both the joint and muscle levels. This goal was achieved by first developing an innovative method to objectively customize and rapidly manufacture PD-AFOs with precise and predictable characteristics. Using this method, a series of PD-AFOs with controlled stiffness levels were customized and fabricated for two healthy individuals. Experimental movement analysis data were recorded as each subject walked with the customized PD-AFO perturbations and the resulting joint level compensatory strategies were examined. Finally, the characteristics of the customized PD-AFOs were integrated into a healthy musculoskeletal model, and simulation tools were used to identify the muscular mechanisms underlying the joint level compensatory strategies.

The findings from this dissertation revealed complex compensatory strategies arising from orthosis use. The results demonstrated that added plantar flexor assistance provided by the PD-AFO stiffness could substitute for some of a subject’s net ankle moment via down regulation of soleus activity, supporting the theory that PD-AFO stiffness can be used to help substitute for insufficient plantar flexor strength. However, this substitution presented as a complex compensatory strategy as the healthy individuals compensated to the PD-AFO perturbations by generating a premature increase in the net plantar flexion moment. This joint level compensation was attributed to normal or elevated gastrocnemius activity, a complex compensation likely needed to counteract the disadvantageous influence of the PD-AFO stiffness at the knee early in stance, but not available in patient populations with plantar flexor weakness. Additionally, the compensatory strategy was characterized by preservation of natural shank progression in all conditions despite dramatic changes in ankle kinematics with PD-AFO use, a compensation likely aided by the footplate design. This finding suggested that maintaining natural shank progression may be important for functional gait and also demonstrated that orthosis footplate design can influence the compensatory strategies. All of these findings suggested that PD-AFO stiffness may be used to aid individuals with impaired plantar flexor strength, but additional orthosis design characteristics may also need to be customized in order to achieve enhanced function with orthosis use.