Paradoxical modulation of tendon tap reflex during voluntary contraction in Parkinsonʹs disease

Objective Inadequate supraspinal modulation of spinal motor control mechanisms such as α–γ coactivation is supposed to cause difficulty in maintaining proper voluntary contraction in Parkinsonʹs disease (PD). Methods Subjects were 42 patients with PD and 20 normal volunteers. Soleus H-reflex and tendon tap reflex (T-reflex) were recorded. The maximal reflexes (Hmax and Tmax) at rest were recorded first. Next, the stimulus intensities were fixed to obtain a reflex size of around 25% of Mmax at rest for both H- and T-reflexes, and the reflexes were recorded at rest, during tonic plantarflexion (TPF), and at the onset of plantarflexion. Results Hmax at rest was 55% and Tmax 30% in normal subjects, while they were 36 and 31%, respectively, in PD. The size ratio of Tmax and Hmax at rest in PD was larger than normal. In PD, the size of H-reflex increased with TPF as in normal subjects, but T-reflex decreased. These changes in T-reflex were correlated with the grade of rigidity, bradykinesia, and time for 10 m gait. H-reflex had no such correlations. Conclusions T-reflex was abnormally modulated in PD especially during tonic contraction. Significance Inappropriate supraspinal modulation of the spinal reflex pathways disturbs motor performance in PD.