Foot Muscle Size and Balance in Stroke: Evidence of Structural-Functional Dissociation.

PURPOSE: This study investigated foot muscle and plantar fascia size measurements and their relationship with static balance dysfunction in stroke patients. METHODS: This cross-sectional study included 90 stroke patients (3-12 months post-stroke) and 95 healthy controls. Ultrasound imaging quantified thickness and cross-sectional area (CSA) of intrinsic foot muscles (e.g., flexor hallucis brevis), extrinsic foot muscles (e.g., tibialis anterior, peroneus longus), and plantar fascia segments. Force platform measured center-of-pressure (CoP) parameters during 30-s static standing under eyes-open and eyes-closed conditions. RESULTS: Stroke patients exhibited significantly reduced peroneus longus (d = 1.14, p < 0.001) and tibialis anterior (d = 0.70, p < 0.001) muscle sizes, reduced flexor hallucis brevis muscle thickness (d = 0.76, p < 0.001), and increased plantar fascia thickness in middle and posterior segments (d = 0.74, p < 0.001; d = 0.71, p < 0.001). In healthy controls, foot muscle parameters strongly correlated with balance metrics, with flexor hallucis brevis thickness showing significant correlation with mediolateral displacement under eyes-closed conditions (r = -0.52, p < 0.001). However, no significant correlations between structural parameters and balance metrics were observed in stroke patients after false discovery rate correction, indicating structural-functional dissociation. CONCLUSION: Although stroke patients exhibited mixed size changes in foot-ankle structures, the dissociation between structural changes and balance function suggests that dysfunction is primarily attributable to central nervous system damage rather than to peripheral structural alterations. This implies that rehabilitation should focus on neuromuscular control training rather than on isolated muscle strengthening.