Title: the Spinal Control of Locomotion and Step-to-step Variability in Left-right Symmetry from Slow to Moderate Speeds Authors: 1 Affiliation: Running Head: Spinal Control of Locomotion at Slow Speeds Address for Correspondence: 12

26 When changing speed during locomotion, both temporal and spatial parameters of the pattern 27 must adjust. Moreover, at slow speeds the step-to-step pattern becomes increasingly variable. The 28 objectives of the present study were to assess if the spinal locomotor network adjusts both temporal and 29 spatial parameters from slow to moderate stepping speeds and to determine if it contributes to step-to30 step variability in left-right symmetry observed at slow speeds. To determine the role of the spinal 31 locomotor network, six adult cats were spinal-transected (spinalized) at low thoracic levels and trained 32 to recover hindlimb locomotion. Cats were implanted with electrodes to chronically record 33 electromyography (EMG) in several hindlimb muscles. Experiments began once a stable hindlimb 34 locomotor pattern emerged. During experiments, EMG and bilateral video recordings were made 35 during treadmill locomotion from 0.1 m/s to 0.4 m/s in 0.05 m/s increments. Cycle and stance durations 36 significantly decreased with increasing speed while swing duration remained unaffected. Extensor burst 37 duration significantly decreased with increasing speed while sartorius burst duration remained 38 unchanged. Stride length, step length and the relative distance of the paw at stance offset significantly 39 increased with increasing speed while the relative distance at stance onset and both the temporal and 40 spatial phasing between hindlimbs were unaffected. Both temporal and spatial step-to-step left-right 41 symmetry decreased with increasing speed. Therefore, the spinal cord is capable of adjusting both 42 temporal and spatial parameters during treadmill locomotion and it is responsible, at least partly, for the 43 step-to-step variability in left-right symmetry observed at slow speeds. 44 45