Spinal Reflex Excitability of Lower Leg Muscles Following Acute Lateral Ankle Sprain: Bilateral Inhibition of Soleus Spinal Reflex Excitability

Abstract

Neural changes in the ankle stabilizing muscles following ankle sprains are thought to be one contributing factor to persistent ankle dysfunction. However, empirical evidence is limited. Therefore, we aimed to examine spinal reflex excitability of lower leg muscles following acute ankle sprains (AAS). We performed a case-control study with 2 groups consisting of 30 young adults with AAS and 30 aged-matched uninjured controls. Hoffmann reflex (H-reflex) testing was performed to estimate spinal reflex excitability of lower leg muscles: soleus, fibularis longus (FL), tibialis anterior (TA). Maximal H-reflex (H_max) and motor responses (M_max) were determined by delivering a series of electrical stimuli at the sciatic nerve. H_max/M_max ratios were calculated to represent normalized spinal reflex excitability. Separate group-by-limb analyses of variance (ANOVA) with repeated measures found there were no significant interactions for any of the muscles (SL: F_1,56 = 0.95, p = 0.33, FL: F_1,51 = 0.65, p = 0.42, TA: F_1,51 = 1.87, p = 0.18), but there was a significant main effect of group in the soleus (F_1,56 = 6.56, p = 0.013), indicating the H_max/M_max ratio of soleus in the AAS group was significantly lower bilaterally (AAS = 0.56 ± 0.19, control = 0.68 ± 0.17, p = 0.013), with no significant group differences in the other muscles (FL: F_1,51 = 0.26, p = 0.61, TA: F_1,51 = 0.93, p = 0.34). The bilateral inhibition of the soleus spinal reflex excitability following AAS may be significant in that it may explain bilateral sensorimotor deficits (postural control deficits) following unilateral injury, and provide insights into additional therapies aimed at the neural change.

Keywords: ankle injuries; arthrogenic muscle inhibition; hoffmann reflex; neuroplasticity.