Impaired modulation of tonic muscle activities and H-reflexes in the soleus muscle during standing in patients with Parkinson's disease

The presence of postural disturbance in patients with Parkinson's disease (PD) was assessed by the displacement of the centre of foot pressure (CFP) and by changes in the amplitude of the soleus H-reflex when patients maintained an upright standing posture, followed by a forward-leaning posture. Thirteen patients and 13 age-matched normal controls (N) were studied. PD patients showed the following differences when compared to normal subjects: (1) the range of displacement of the CFP associated with forward leaning was significantly smaller (P<0.01); (2) the ratio of the increase in the soleus EMG activity to the CFP displacement (Delta EMG/Delta CFP) was larger (P<0.01), and the value of the Delta EMG/Delta CFP increased significantly in relation to the scale of clinical severity (P<0.01); and (3) the ratio of the increase in the amplitude of the soleus H-reflex to the soleus muscle EMG activity (Delta H-reflex/Delta EMG) was significantly lower in PD patients (P<0.05). The value of the Delta H-reflex/Delta EMG decreased significantly with the scale of clinical severity among the patients (P<0.05). These results suggest that the modulation of both the tonic stretch reflex and the phasic stretch reflex in the soleus muscle during standing are impaired in PD patients, and these impairments may partly cause their disability in the maintenance of a standing posture. Abnormalities in Ib inhibition and presynaptic inhibition are considered to be possible mechanisms in the disturbed modulation of the tonic stretch reflex and the phasic stretch reflex in PD patients during standing. (C) 1997 Elsevier Science B.V.