Effects of Transcranial Direct Current Stimulation Combined with Foot Core Exercise on Brain Structural Plasticity and Ankle Functional Performance in Individuals with Chronic Ankle Instability

Objective: Chronic ankle instability (CAI) is characterized not only by peripheral sensorimotor deficits but also by maladaptive alterations in brain structure and function. This study aims to investigate the effects of a 4-week individualized transcranial direct current stimulation (tDCS) with foot core exercise (FCE) on brain structural, functional characteristics, and ankle function performance in individuals with CAI. Methods: A double-blind, randomized, sham-controlled trial was conducted. A total of 30 participants with CAI completed the intervention and were included in the final analysis, with 15 participants in the tDCS + FCE group and 15 in the control group (sham stimulation + FCE). Both of two groups conducted a 4-week FCE program (3 sessions per week, 20 min per session, 12 sessions in total). During training, the tDCS＋FCE group received concurrent tDCS with individualized current intensity targeting the sensorimotor cortex, based on individualized head model, whereas the control group received sham stimulation. Magnetic resonance imaging was used to assess the brain structural plasticity gray matter volume (GMV) and cortical thickness (CT) and brain functional characteristics (brain complexity) of participants. Balance control performance and ankle force sense were also assessed. Two-way ANOVA with repeated-measures was used to examine group × time interactions, and linear regression analysis was conducted to explore associations between changes in brain structural and functional characteristics and performance improvements. Results: 1) Compared with the control group, the tDCS＋FCE group induced significant increases in GMV and CT in sensorimotor-related regions (P＜0.05), brain complexity also enhances significantly (F ≥ 4.342, P＜0.05, \eta _p^2 ≥ 0.148); 2) compared with the control group, the tDCS＋FCE group induced significantly reduced center-of-pressure sway velocity and area during single-leg stance with eyes closed, as well as significantly reduced force sense errors in ankle dorsiflexion and inversion (F ≥ 4.599, P＜0.05, \eta _p^2 ≥ 0.150); 3) in individuals with CAI, improvements in brain structure plasticity were significantly associated with increases in brain complexity and improvements in ankle function performance (P＜0.05). Conclusions: Four weeks of individualized tDCS combined with FCE can enhance the structural plasticity in the sensorimotor cortex of individuals with CAI. This enhanced structural plasticity is accompanied by increased brain complexity and are closely associated with improvements in ankle function performance. These findings suggest that a central-peripheral integrated rehabilitation strategy may be an effective approach for improving functional deficits in individuals with CAI.