Presynaptic inhibition of muscle spindle Ia afferents by group I afferents from the same and other muscles has been studied in the lower limbs of subjects with upper motoneuron lesions. The experiments utilised conditioning of soleus test monosynaptic reflexes during controlled voluntary contraction. The protocol was designed to isolate presynaptic inhibition from postsynaptic components. The relation between estimate of inhibition and test reflex amplitude was examined. The subjects showed less inhibition than controls at all levels of voluntary torque investigated (less than or equal 15 Nm). Two thirds had weak inhibition which did not show the decrease during muscle contraction characteristic of controls. The degree of difference from the normal situation correlated with severity of the clinical sign (weakness of voluntary ankle flexion).
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