Many individuals with cerebral palsy (CP) and stroke are prescribed ankle foot orthoses (AFOs) for use during daily life. AFOs have been shown to improve pathologic gait and walking speed in CP and stroke by providing support and alignment. There are many different types of AFOs available such as posterior leaf spring AFOs, rigid AFOs, and articulated AFOs. Further, there are many parameters that can be customized or tuned for each type of AFO, such as stiffness, heel height, shank to vertical angle, and foot plate length. However, how different types of AFOs and the customization of specific parameters impact muscle function remains unclear.

The goals of this dissertation were to evaluate how different types of AFOs and different tuning parameters impact gait kinematics and muscle function. Of particular interest is the gastrocnemius, a key muscle that crosses the knee and ankle joints and is commonly tight among individuals with CP or stroke. Gastrocnemius operating length, defined as the total muscle and tendon length during a functional activity, influences ankle and knee kinematics during gait. Therefore, understanding and potentially controlling gastrocnemius operating length may help to properly select and optimize the design of AFOs to improve gait. To understand the impact of AFOs on gastrocnemius function, this dissertation includes four primary aims.

The first aim was to evaluate how current methods for tuning, or adjusting patientspecific properties of AFOs, impact gastrocnemius operating length and gait kinematics using musculoskeletal modeling. We performed a case study of an adult stroke survivor who received three AFOs that were tuned by a trained orthotist. The orthotist observed the individual’s shank to vertical angle (SVA, the angle of the tibia with respect to vertical, which is commonly assessed by orthotists during gait analysis) and adjusted the AFO’s fixed plantarflexion angle and heel height to make the SVA more similar to unimpaired individuals. For this case study, we found that tuning the AFOs based on SVA resulted in a decrease in gastrocnemius operating length and increased knee flexion angle during swing. These results demonstrated how musculoskeletal modeling can be used to evaluate muscle function during walking and the impacts of adjusting SVA for stroke survivors.

The second aim was to evaluate how different types of AFOs, representing the current standard of care, alter gastrocnemius operating length and gait kinematics in children with CP. We evaluated gastrocnemius operating length for eleven children with CP who each received two types of AFOs. Since individuals wear AFOs all day, we sought to evaluate if the gastrocnemius operated in a stretched or shortened position. For individuals with CP with more mild involvement, we found both types of AFOs stretched the gastrocnemius during walking for the majority of individuals. However, for individuals with CP and more severe involvement, we found only the more solid, Cascade AFOs stretched the gastrocnemius in some individuals. This study suggested AFOs can potentially be a rehabilitation modality by providing dynamic stretching exercise for a short and tight gastrocnemius during gait.

The third aim was to create a flexible platform for fabricating AFOs with 3D printing and scanning technology, which could support research on how AFO stiffness impacts joint and musculotendon function. The current standard AFO designs are not only cost and time ineffective, but also provide limited control of AFO stiffness. This study demonstrated a novel method for fabricating a variable stiffness AFO using a 3D scanner and printer. To adjust the stiffness, elastic polymer bands were fabricated with varying stiffness. The 3D printed AFO is cost and labor effective compared to current AFO fabrication methods. 3D printed AFOs can also can provide adjustable AFO stiffness, as well as versatility to combine with various measurement tools such as ultrasound and electromyography.

The forth aim was to evaluate how AFO stiffness and walking speed impact joint kinematics, gastrocnemius muscle length, and Achilles tendon (AT) length in unimpaired individuals. In this study we used the 3D printed AFO created in Aim 3. We found that as AFO stiffness increased, peak AT length, peak gastrocnemius activation level, and peak ankle dorsiflexion angle significantly decreased. However, peak gastrocnemius muscle length and peak AFO dorsiflexion moment increased with increasing AFO stiffness. Gastrocnemius muscle length and lengthening velocity significantly decreased with slower walking speeds. This study illustrated how human musculoskeletal system interplays with different stiffness AFOs. Building on these methods, future research can inform AFO prescription for individuals with neurologic injuries to maximize stretching or other rehabilitation or performance goals during walking.

This dissertation provides important evidence for how humans adapt to various AFO properties and suggests important implications for the design and prescription of AFOs. This work provides a quantitative evaluation of how AFOs impact musculotendon dynamics among individuals with stroke (Aim 1) and cerebral palsy (Aim 2). The fabrication methods in Aim 3 creates a powerful and flexible research platform for evaluating AFO design, which may be extended to fabrication of AFOs for daily use with further improvements in additive manufacturing materials and methods. The final study (Aim 4), provides the first experimental evidence combining ultrasound and musculoskeletal modeling to understand how muscle and tendon length are impacted by AFO design. These evaluations provide guidance for future AFO design and prescription that can not only augment human mobility for unimpaired individuals, but also provide improve metrics for improving function and guiding rehabilitation for individuals with neurologic impairments.