To evaluate functional change in the spinal reflex pathway with ageing, we studied heteronymous Ia facilitation from the quadriceps to soleus muscle in 30 normal volunteers (aged 24-68 years). The size of the test H-reflex of the soleus muscle was adjusted to 25% that of the maximal M-response. The conditioning stimulus was adjusted to 1.5-fold the motor threshold to stimulate all the Ia fibres in the femoral nerve. Facilitation was quantified as the slope of the very early part of facilitation, within 0.8 ms of onset. This procedure enabled us to evaluate the extent of monosynaptic Ia facilitation without contamination by other effects. The extent of facilitation decreased linearly with age. This decrease in facilitation could reflect a decrease in the number of Ia fibres and in their conduction velocities, and an increase in presynaptic inhibition on Ia terminals. The increase in presynaptic inhibition may be an adaptive phenomenon in the ageing of the neuromuscular system or, alternatively, a deteriorating process with decreasing flexible supraspinal modulation.
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