Paradoxical modulation of tendon tap reflex during voluntary contraction in Parkinson's disease

Inadequate supraspinal modulation of spinal motor control mechanisms such as alpha-gamma coactivation is supposed to cause difficulty in maintaining proper voluntary contraction in Parkinson's disease (PD).

Subjects were 42 patients with PD and 20 normal volunteers. Soleus H-reflex and tendon tap reflex (T-reflex) were recorded. The maximal reflexes (H(max) and T(max)) at rest were recorded first. Next, the stimulus intensities were fixed to obtain a reflex size of around 25% of M(max) 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.

H(max) at rest was 55% and T(max) 30% in normal subjects, while they were 36 and 31%, respectively, in PD. The size ratio of T(max) and H(max) 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.

T-reflex was abnormally modulated in PD especially during tonic contraction.

Inappropriate supraspinal modulation of the spinal reflex pathways disturbs motor performance in PD.