Brain regulation of muscle tone in healthy and functionally unstable ankles.

CONTEXT: Current research into the etiology of joint instability has yielded inconsistent results, limiting our understanding of how to prevent and treat ligamentous injury effectively. Recently, cortical reorganization was demonstrated in patients with ligamentous injury; however, these neural changes have not been assessed relative to joint laxity. OBJECTIVE: The purpose of the current study was to determine if changes in cortical excitability and inhibition occur in subjects with functional ankle instability, as well as to investigate the relationship between these measures and joint laxity. DESIGN: Posttest only with control group. SETTING: University laboratory. SUBJECTS: 12 subjects with no history of ankle sprain (CON) and 12 subjects with a history of unilateral functional ankle instability (UNS). INTERVENTIONS: Subjects were tested for joint laxity using an instrumented ankle arthrometer. Cortical excitability and inhibition were assessed using transcranial magnetic stimulation (TMS) to obtain motor-evoked potentials and the cortical silent period from the lower leg muscles. MAIN OUTCOME MEASURES: Joint laxity was quantified as peak anterior displacement and inversion rotation. Active motor threshold, slope, and intensity at 50% of peak slope of TMS-derived recruitment curves were used to quantify cortical excitability from lower leg muscles, while the cortical silent period from the peroneus longus was used to represent intracortical inhibition. RESULTS: No significant differences were observed between groups for laxity or cortical measures. CON demonstrated a significant relationship between laxity and tibialis anterior excitability, as well as laxity and silent period, while UNS ankles demonstrated significant relationships between peroneal and soleus excitability and laxity measures. CONCLUSION: Our results support relationships between laxity and measures of excitability and inhibition that differ between healthy and unstable subjects. Future research should further investigate the mechanisms behind these findings and consider cortical influences when investigating altered joint laxity.