Objective: Inadequate supraspinal modulation of spinal motor control mechanism s such as α-γcoactivation is supposed to cause difficulty in maintaining prop er voluntary contraction in Parkinson’s disease (PD). Methods: Subjects were 42 patients with PD and 20 normal volunteers. Soleus H-reflex and tendon tap refl ex (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 siz e of around 25%of Mmax at rest for both H-and T-reflexes, and the reflexes we re recorded at rest, during tonic plantarflexion (TPF), and at the onset of plan tarflexion. 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 r est in PD was larger than normal. In PD, the size of H-reflex increased with TP F as in normal subjects, but T-reflex decreased. These changes in T-reflex wer e correlated with the grade of rigidity, bradykinesia,and time for 10 m gait. H -reflex had no such correlations.Conclusions: T-reflex was abnormally modulate d in PD especially during tonic contraction. Significance: Inappropriate suprasp inal modulation of the spinal reflex pathways disturbs motor performance in PD. Objective: Inadequate supraspinal modulation of spinal motor control mechanism s such as α-γ coactivation is supposed to cause difficulty in maintaining er er contractive in Parkinson’s disease (PD). Methods: Subjects were 42 patients with PD and 20 normal volunteers. Soleus H- The maximal reflexes (Hmax and Tmax) at rest were recorded first. Next, the stimulus intensities were fixed to obtain a reflex siz e of around 25% of Mmax at rest for both H-and T-reflexes, and the reflexes we re recorded at rest, during tonic plantarflexion (TPF), and at the onset of plan tarflexion. Results: Hmax at rest was 55% and Tmax 30% in normal subjects, while The size ratio of Tmax and Hmax at est in PD was larger than normal. In PD, the size of H-reflex increased with TP F as in normal subjects, but T-reflex These changes in T-reflex wer e correlated with the grade of rigidity, brady Kinesia, and time for 10 m gait. H-reflex had no such correlations.Conclusions: T-reflex was abnormally programmed in d in PD especially during tonic contraction. Significance: Inappropriate suprasp inal modulation of the spinal reflex pathways disturbs motor performance in PD.