Electrical stimulation over muscle tendons in humans: Evidence favouring presynaptic inhibition of Ia fibres due to the activation of group III tendon afferents

Electrical stimulation over muscle tendons produces a transient suppression of voluntary EMG activity; its onset latency is approximately 55 ms in the forearm extensor muscles. This phenomenon has been attributed to the activation of a polysynaptic inhibitory pathway originating from Ib afferent fibres. To clarify its origin we conducted several experiments in 10 normal healthy subjects. The EMG silence after tendon stimulation appeared at relatively high stimulus intensities (> 50 mA); conditioning cutaneous stimulation left it unchanged, and the inhibition had a short recovery cycle (50 ms). Tendon stimulation still evoked EMG suppression during an ischaemic block of fast-conducting afferents. The motor potentials evoked by transcranial magnetic stimulation of the motor cortex during the EMG silence remained almost unchanged, whereas the H reflex was strongly inhibited. Hence we conclude that tendon stimulation activates slow-conducting tendon afferents, possibly group III fibres, connected not through a polysynaptic pathway originating from Ib afferents but through an oligo- or disynaptic inhibitory circuit. The EMG suppression after tendon stimulation probably represents a dysfacilitation of the alpha-motor neurons due to presynaptic inhibition of Ia fibres produced by tendon afferent input to the spinal cord.