F responses studied with single fibre EMG in normal subjects and spastic patients.

Recurrent responses of single human motor neurones after antidromic activation, the so-called F response, have been studied in the abductor digiti minimi muscle in normal subjects and in spastic patients with the single fibre EMG technique. The F response occurs rarely and in groups. Half of all motor neurones did not produce any F response within an arbitrarily chosen time interval of 200 successive stimuli at 1 and 2 Hz. Spastic patients had as many silent neurones as the normal subjects but the responding neurones more frequently produced F responses (P less than 0.001). When a neurone has few responses at rest the frequency increases under activation of the contralateral or ipsilateral muscle (P less than 0.05). When there are many responses in the relaxed state there is a decrease (P less than 0.001) under activation. The behaviour of an individual neurone is consistent in different activation periods.