Distinct patterns in neuromuscular adaptation to repeated perturbations in chronic ankle instability.

BACKGROUND: Neuromuscular impairments following ankle sprains are central to chronic ankle instability (CAI), potentially leading to compensatory muscle co-contraction to regulate ankle stiffness, particularly in response to external perturbations. The acute effect of practice on muscle activation and postural responses reveal initial deficits in motor control and indicate the capacity of individuals with CAI to overcome these deficits within their specific constraints. This study aimed to examine adaptive changes in lower limb muscle co-contraction between CAI and healthy control (HC) participants during repeated perturbations and assess whether practice influences postural control and strategy. METHODS: Twenty-three CAI and 23 HC participants performed a single-leg balance task involving repetitive mediolateral perturbations. Surface electromyography, ground reaction force and whole-body kinematics were recorded, and margin of stability (MoS) and the coupling between MoS and ankle-/ hip-joint torques were calculated. RESULTS: Individuals with CAI demonstrated different adaptive changes in muscle co-contraction for Tibialis Anterior (TA) - Peroneus Longus (PL) and TA - Soleus compared to HC. In healthy controls, TA-PL co-contraction decreased significantly with practice, while no changes were observed in the CAI group. Repeated perturbations improved MoS and MoS-Hip torque coordination in CAI, suggesting improved postural control and hip strategy. CONCLUSIONS: Rehabilitation for CAI should target both the whole-body coordination and ankle adaptation exercise since ankle joint appears refractory to change in CAI individuals. Future research should explore whether co-contraction patterns influence risk of ankle sprain in CAI populations, linking lab-based performance to real-world injury risks.