Delayed reaction time and altered spatial activation of Fibularis longus in chronic ankle instability: A high-density surface electromyography study

Chronic ankle instability (CAI) is characterized by recurrent episodes of sprain and delayed activation of the fibularis longus, which compromises dynamic ankle stabilization. However, whether delays in muscle reaction time occur uniformly across its neuromuscular compartments or follow a region-specific pattern remains unclear. This study aimed to compare global reaction time and determine whether this temporal response differed between the anterior and posterior regions, while also characterizing the spatial distribution of fibularis longus activation during sudden ankle inversion in individuals with CAI and healthy controls. Fifteen individuals with CAI and fifteen healthy controls (No-CAI) were recruited. Participants performed a sudden 30° ankle inversion task on a custom-built platform, during which high-density surface electromyography (HD-sEMG) was used to assess regional reaction time and spatial activation of the fibularis longus through the displacement of the activation barycenter. No significant interaction was observed between region and group (p = 0.7826). However, the CAI group displayed a significantly longer reaction time (MD = −41.49 ms; 95% CI = −50.15 to −32.83; p = 0.0001) and a significant anterior shift of the barycenter during destabilization (MD = −0.80 mm; 95% CI = −1.46 to −0.13; p = 0.0209) compared to controls. These findings suggest that CAI is characterized by a global delay in fibularis longus activation and a task-dependent spatial redistribution. The anterior shift in activation may reflect a compensatory strategy to maintain eversion torque in mechanically disadvantageous positions. These insights may inform neuromuscular rehabilitation strategies targeting both temporal and spatial aspects of muscle activation.