EVIDENCE FOR FURTHER RECRUITMENT OF GROUP-I FIBERS WITH HIGH STIMULUSINTENSITIES 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 ea ch instance the monosynaptic Ia excitation produced by heteronymous af ferent volleys was studied. This peak of excitation was found to incre ase with the intensity of the stimulation up to 2.5-4x the motor thres hold (MT), i.e., 4-7 X the threshold of Ia afferents. It is concluded that, when using surface electrodes in human subjects, group Ia effect s may not be maximal with stimulus intensities below 4XMT, and the eff ects evoked by different afferent populations (e.g., Ia and II) cannot be clearly separated solely on the basis of the electrical excitabili ty of these fibres.