This study examines the effects of electrical stimulation of hindlimb flexor nerves on the fictive locomotion pattern. Locomotion was initiated by stimulation of the mesencephalic locomotor region in the decerebrate paralysed cat and monitored by recording the electroneurogram from selected hindlimb flexor and extensor muscle nerves. Flexor nerves were stimulated using short trains (20-50 stimuli at 100 Hz) during either the flexor or the extensor phase of the fictive locomotor cycle. 2. Stimulation of tibialis anterior (TA), posterior biceps and semitendinosus (PBSt) or sartorius (Sart) nerves at 5 times threshold (T) during the flexor phase of the fictive locomotor cycle terminated on-going activity in flexor nerves and initiated activity in extensors. Thus, flexor nerve stimulation during flexion shortened the locomotor cycle by resetting to extension. The failure of lower intensity (2T) stimulation of PBSt or Sart nerves to reset the step cycle to extension suggests that group II afferents are responsible for these actions. Resetting evoked by 2T stimulation of the TA nerve may be due to a high proportion of group II afferents with low electrical threshold. 3. During extension, stimulation of TA and PBSt nerves at 5T did not perturb the locomotor rhythm whereas Sart stimulation prolonged the locomotor cycle. 4. Stimulation of cutaneous or knee joint afferents failed to produce effects similar to those evoked by stimulation of flexor muscle nerves at group II strength. These findings are at odds with those obtained elsewhere in the acute spinal, DOPA fictive locomotion preparation. The possibility that group II resetting during fictive locomotion is not mediated by flexion reflex pathways but by previously unknown pathways released in the present preparation is discussed. 5. Since many of the flexor afferents recruited by 5T electrical stimulation are the length-sensitive group II fibres, spindle secondaries may act to regulate the duration and onset of flexor and extensor activity during real locomotion. The resetting from flexion to extension also suggests that unexpected or enhanced activity of flexor secondaries during swing would promote a switch of the step cycle to stance.
