Evidence for further recruitment of group I fibres with high stimulus intensities when using surface electrodes in man.

Changes in the firing probability of voluntarily activated motor units in the human lower limb were recorded in response to stimuli applied to peripheral nerves of the lower limb using surface electrodes. In each instance the monosynaptic Ia excitation produced by heteronymous afferent volleys was studied. This peak of excitation was found to increase with the intensity of the stimulation up to 2.5-4 x the motor threshold (MT), i.e., 4-7 x the threshold of Ia afferents. It is concluded that, when using surface electrodes in human subjects, group Ia effects may not be maximal with stimulus intensities below 4 x MT, and the effects evoked by different afferent populations (e.g., Ia and II) cannot be clearly separated solely on the basis of the electrical excitability of these fibres.