Quadratic association between corticomotor and spinal-reflexive excitability and self-reported disability in participants with chronic ankle instability

Context: Spinal-reflexive and corticomotor excitability may have a critical role in altering muscle function needed to stabilize the ankle in people with chronic ankle instability (CAI). Objective: To determine the association between self-reported disability and both spinal-reflexive and corticomotor excitability in people with CAI. Design: Descriptive laboratory study. Setting: Research laboratory. Participants: 30 participants with CAI. Main Outcome Measures: Soleus spinal-reflexive excitability was measured with normalized Hoffmann reflexes (H:M ratio), and corticomotor excitability was measured with transcranial magnetic stimulation and quantified by normalized motor-evoked-potential (MEP) amplitudes at 120% of active motor threshold (120%MEP). Self-reported disability was quantified with the activities-of-daily-living and sport subscales of the Foot and Ankle Ability Measure (FAAM-ADL and FAAM-S). Separate linear Pearson product-moment correlations and nonlinear quadratic correlations were used to determine associations between the neural-excitability and disability variables. Results: Thirty participants were included in the spinal-reflexive-excitability analysis, while only 19 were included in the corticomotor analysis. There was a significant, weak linear association between H:M ratio and FAAM-ADL (R = .39, P = .03) and a nonsignificant, weak linear association between H:M ratio and FAAM-S (R = .36, P = .06). There were significant, moderate quadratic associations between H:M ratio and both FAAM-ADL (R = .48, P = .03) and FAAM-S (R = .50, P = .02). There was a significant, moderate linear association between 120%MEP and FAAM-ADL (R = -.48, P = .04) and a nonsignificant, moderate negative linear association between FAAM-S (R = -.42, P = .07). There was a significant, moderate quadratic association between 120%MEP and FAAM-ADL (R = .57, P = .046) and a significant, strong quadratic correlation between 120%MEP and FAAM-S (R = .71, P = .004). Conclusions: There are significant quadratic associations between self-reported disability and both spinal-reflexive and corticomotor excitability of the soleus. CAI participants with low or high neural excitability present with lower function.