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Department of Medical Physiology, Panum Institute, University of Copenhagen, Denmark.

1. The soleus (Sol) H reflex was conditioned by magnetic stimulation of the contralateral motor cortex at rest and during voluntary contraction in healthy human subjects. The intensity of the magnetic stimulus was adjusted so as to have no effect on the H reflex at rest. During tonic voluntary contraction the same magnetic stimulus produced a facilitation with a short latency and a long duration, thus reflecting an increased excitation of Sol motoneurones by the magnetic stimulus during voluntary contraction. 2. The amount of reflex facilitation produced by brain stimulation within the initial 0.5-1 ms after its onset was investigated at different times during dynamic ramp-and-hold plantar flexion. The facilitation was largest at the onset of voluntary activity in the Sol muscle. It then decreased abruptly within 100 ms after the onset of the voluntary contraction. Neither the voluntary Sol activity nor the control H reflex decreased at this time. 3. Electrical stimulation of the brain with the anode placed lateral to the vertex produced a facilitation of the H reflex, which preceded the facilitation evoked by magnetic stimulation by 1-2 ms. The facilitation produced by the magnetic stimulus occurred or increased at the onset of contraction in relation to rest in all experiments. However, this was the case in only two out of eight experiments, when the brain was stimulated electrically. 4. The size of the reflex facilitation measured at the onset of contraction was larger the faster the contraction. Positive correlations were found between the size of the facilitation and the peak of the first and second derivative of the torque and the peak Sol EMG activity. 5. It is suggested that the observed changes in the size of the short-latency reflex facilitation produced by magnetic brain stimulation mainly reflects changes in the excitability of corticospinal cells, since similar changes were not observed in the size of the unconditioned Sol H reflex or in the short-latency reflex facilitation produced by electrical brain stimulation. The data support the hypothesis that fast conducting corticospinal fibres with monosynaptic projections to spinal motoneurones are involved in the initiation of voluntary movement in man.