Compound group I excitatory input is differentially distributed to motoneurons of the human tibialis anterior

The distribution of the compound group I excitatory input to various-sized motoneurones in the human tibialis anterior muscle was studied, using low-intensity electrical stimulation of the common peroneal nerve. The stimulation initiated the H-reflex response in all motor units with a latency of approximately 40 ms (range 30-45 ms). In each experiment, the amplitude of the H-reflex responses in a pair of simultaneously active motor units were assessed. It was shown that, although the stimulus intensity was identical, the amplitude of the H-reflex response was bigger in the motor unit with the higher recruitment threshold of the pair compared with the size of the reflex in the unit that had a relatively lower recruitment threshold. The present results are compared and contrasted with the findings in animals and one human study that suggested that the smaller-sized motoneurones receive larger group I excitatory input.