Impaired antagonist inhibition may contribute to akinesia and bradykinesia in Huntingtonʹs disease

Objective: To test the hypothesis that besides impaired agonist facilitation, impaired antagonist inhibition also contributes to delayed initiation (akinesia) and slow execution (bradykinesia) of voluntary movements in Huntingtonʹs disease. Methods: Fifteen patients with Huntingtonʹs disease and 11 age-matched controls participated in the study. The amount of agonist facilitation was measured as the increase in soleus H-reflex amplitude prior to ballistic voluntary plantar flexion (soleus contraction). Antagonist inhibition was measured as the decrease in soleus H-reflex prior to ballisitic dorsiflexion (tibialis anterior (TA) contraction). The amount of agonist facilitation and antagonist inhibition was correlated with the time needed for motor initiation (reaction time) and movement execution (movement time). Results: Starting 50 ms prior to soleus contraction, soleus H-reflex increased in control subjects but less so in patients. Soleus H-reflexes decreased in controls 25 ms prior to TA contraction, while this antagonist inhibition was completely lacking in patients. Thus, patients with Huntingtonʹs disease not only displayed reduced agonist facilitation, but impaired antagonist inhibition as well. Moreover, more impairment of antagonist inhibition correlated significantly with more severe akinesia and bradykinesia. Conclusions: Antagonist inhibition prior to and during agonist contractions is markedly impaired in Huntingtonʹs disease. This impairment might contribute to motor slowness in these patients.