Therapeutic Exercises Induce Distinct Regional Activation Patterns of the Fibularis Longus Muscle in People With Chronic Ankle Instability

Background: Chronic ankle instability (CAI) is common in athletic populations and is associated with recurrent sprains and impaired performance. Recent evidence shows that fibularis longus has distinct anterior and posterior regions and that, during isometric eversion, people with CAI display a reduced relative contribution of the posterior region. This muscle is typically trained with band-resisted eversion in supine (BES) and BOSU-based exercises (seated and single-leg eversion, BOSU-Sit and BOSU-SL, respectively), yet it is unknown whether these specific tasks correct or perpetuate regional activation deficits. Hypothesis: As exercise demands increased from BES to BOSU-based tasks, fibularis longus activation would be differentially modulated and people with CAI would exhibit a greater anterior shift of the activation barycenter together with a reduced relative contribution of the posterior region, particularly during the BOSU-based tasks. Study Design: Descriptive laboratory study. Level of Evidence: Level 5. Methods: A total of 40 physically active adults (CAI, n = 20; no-CAI, n = 20) performed 3 exercises: BES, BOSU-Sit, and BOSU-SL. High-density surface electromyography recorded fibularis longus activation using a 64-electrode grid. Root mean square was computed for anterior and posterior regions, and the activation barycenter was calculated along the x -axis (anteroposterior) and y -axis (cephalocaudal). Results: In CAI, both BOSU exercises shifted the x -axis barycenter anteriorly relative to BES (BOSU-Sit, P = 0.01; BOSU-SL, P = 0.01). BES was the only exercise in which people with CAI exhibited greater posterior-region activation than controls ( P = 0.003). In contrast, BOSU-SL produced an anterior-dominant pattern, with greater anterior-region activation than the posterior region ( P = 0.04) and lower posterior-region activation than BES ( P = 0.008). Conclusion: Therapeutic exercises commonly grouped as “fibular strengthening” are not neuromuscularly equivalent. In this acute task comparison, BES was associated with a more posterior barycenter and greater posterior-region activation in CAI, whereas BOSU-based tasks primarily activated the anterior region.