It has been reported that the soleus H-reflex is smaller during standing than sitting. This could be the result of modulation the stretch reflex gain of soleus by descending pathways in these different postures. The gain of the soleus stretch reflex is also modulated during locomotion. The neural mechanism responsible for these changes is thought to involve modulation of presynaptic inhibition of Ia terminals on spinal motoneurons. Although the soleus H-reflex is less during standing than during sitting in normal subjects, in patients with spasticity, the arm muscle tone is sometimes seen clinically to increase during standing. This observation suggests that different mechanisms underlie the posture-related modulation of the stretch reflex in arm compared with leg muscles. To explore this hypothesis, we examined the modulation of H-reflexes evoked in both soleus and flexor carpi radialis (FCR) during sitting and standing respectively. Seventeen normal subjects and ten hemiplegic patien
ts participated in this study. H-reflexes were without background EMG in order to minimise muscle afferent feedback. H-reflexes were also recorded during standing in the soleus with the accompanying background EMG. The amplitudes of H-refelxes were expressed as their size to the maximal M-response in each posture.
In the normal subjects, soleus H-reflex were decreased during standing compared with sitting when no background EMG. When recorded during standing with the accompanying background muscle activity, unstable changes were observed in the soleus H-reflex. The H-reflex in FCR changed in a variable manner between sitting and standing in normals. However, in eight out of ten hemiplegic patients, the FCR H-reflexes recorded from the spastic side were increased during standing compared with sitting. Taking into account the responses in the absence of background EMG, the differences in posture-related modulations of H-reflex between soleus and FCR probably depend on differences in descending control of their respective spinal motoneuro pools. In the presence of background EMG, i.e., with increased muscle afferent feedback, both the muscle afferents and the descending patheway that affect motoneuron excitability caused unstable changes in soleus H-reflexes between sitting and standing. On the spastic side of the hemiplegic patients, the descending excitation to FCR is probably increased during standing which leads the increases in these H-reflexes compared with normal subjects. And this modulation is achieved by the presynaptic inhibition of Ia synapses, assessed by D1 method. These results suggest that posture-related modulation of stretch reflex gain in arm and leg muscles is subject to different descending control onto the presynaptic inhibition of Ia synapses. Less