The basic rhythmic activity that underlies stepping is generated by a neural network, situated in the spinal cord, known as the locomotor central pattern generator (CPG). While a series of lesion experiments have demonstrated that the mammalian locomotor CPG is distributed throughout the ventral portion of the caudal spinal cord, the specific transverse distribution of this neural network is unclear. Here we evoke fictive locomotor activity of various frequencies in upright spinal cords prepared from male and female neonatal mice. This preparation enables us to use an imaging approach to identify locomotor-related cells across the transverse plane of the spinal cord. Results indicate that there is a clear shift in the recruitment of cells toward the ventromedial, and away from the ventrolateral, spinal cord as the frequency of fictive locomotion increases. Surprisingly, the analysis of multiple frequencies of fictive locomotion in the same spinal cord indicates that few neurons are involved in locomotor ou...