The Effects of a 12-Week Custom Foot Orthotic Intervention on the Intrinsic Muscles of the Foot, and Dynamic Stability During Unexpected Gait Termination in Healthy Young Adults

Introduction: Custom-made foot orthotics (CFO’s) are a commonly prescribed intervention to help individuals that are suffering from foot pain and foot disorders. However, the mechanisms of CFO’s are still poorly understood and are not well known. With the plantar intrinsic muscles of the foot being in direct contact with the CFO, it puts these structures at risk for disuse muscle atrophy as a result of being offloaded. Therefore, the purpose of the current study was to determine the effect of a 12-week custom-made foot orthotic intervention on the intrinsic muscles of the foot and dynamic stability during unexpected gait termination. Methods: Eighteen healthy young adults participated in the study. Participants were allocated by stratified sampling into either the: (a) orthotic group (n= 9) or (b) control group (n= 9). Beginning of each testing session, participants’ right foot was assessed by diagnostic ultrasound to measure the cross-sectional area (CSA) of the flexor digitorum brevis (FDB), abductor digiti minimi (Abd DM), and abductor hallucis (Abd H). Subsequently, participants completed an unexpected gait termination protocol and data was collected using force plates, motion capture, and electromyography (EMG) to assess dynamic stability. A total of 50 walking trials were completed at baseline, 6-weeks and 12-weeks, where 25% of the trials were unexpected gait termination. The variables used to measure dynamic stability were M/L COM-BOS and A/P COM-COP. Additionally, the amount of muscle activity was determined by average EMG magnitude and integrated EMG. The secondary outcome measures of interest were vertical force rate of loading (ROL), step width, step length and gait velocity. Results: At the end of the 12-week intervention, the participants in the OG had significantly smaller CSA of the FDB (9.6%) (p\u3c0.001), Abd DM (17.1%) (p\u3c0.001) and Abd H (17.4%) (p\u3c0.001) plantar intrinsic muscles. Despite muscle atrophy, individuals in the orthotic group showed an improvement of 1.1 cm in M/L COM-BOS (p\u3c0.001) at 12-weeks and were as stable as the CG during gait termination. Additionally, there were significant differences of ROL between the groups during first (p\u3c0.001) and second single stances (p\u3c0.001) at the end of 12-weeks. Lastly, there was no significant difference in average EMG magnitude of the intrinsic muscles between the groups. Discussion: The short-term use of CFO’s created a decrease in CSA of the FDB, Abd DM and Abd H plantar intrinsic muscles. These findings help understand the adaptations that are occurring when you offload specific structures such as the plantar intrinsic muscles. Although both groups were similar in creating stability when exposed to the mechanical perturbation, the participants in the OG adapted a compensatory strategy to recover their balance. Therefore, these findings along with future research can help develop guidelines to enhance the use of CFO’s by adding rehabilitative exercises to prevent disuse atrophy from occurring