Transmission in a pathway mediating presynaptic inhibition of soleus Ia afferent terminals was assessed from the reduction of soleus muscle H reflex amplitude brought about by a short conditioning vibration pulse applied to the anterior tibialis muscle. In a group of young subjects (aged between 21 and 37 years) the mean reduction in reflex size with the limb relaxed was to 29.2% of control, whereas in a group of older subjects (aged between 59 and 74 years) the reduction was significantly less, to 55.0% of control (P < 0.001, Student's t test). In the younger group the amount of presynaptic inhibition elicited was inversely related to the plantar-flexor torque produced by tonic contraction of gastrocnemius-soleus; often at higher torques no reduction in reflex size was seen. In contrast the older group showed only a slight reduction in presynaptic inhibition with increasing torque, so that at torques of 5 and 7 N m the amount of presynaptic inhibition elicited was similar in the younger and older groups. The older subjects were not able to sustain the higher torques produced by the younger subjects. These results suggest that the control of transmission in this spinal pathway changes with increasing age.

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