In humans, studies of back muscle activity have mainly addressed the functioning of lumbar muscles during postural adjustments or rhythmic activity, including locomotor tasks. The present study investigated how back muscles are activated along the spine during rhythmical activities in order to gain insights into spinal neuronal organization. Electromyographic recordings of back muscles were performed at various trunk levels, and changes occurring in burst amplitudes and phase relationships were analyzed. Subjects performed several rhythmic behaviors: forward walking (FW), backward walking (BW), amble walking (where the subjects moved their arms in phase with the ipsilateral leg), walking on hands and knees (HK) and walking on hands with the knees on the edge of a treadmill (Hand). In a final task, the subjects were standing and were asked to swing (Swing) only their arms as if they were walking. It was found that axial trunk muscles are sequentially activated by a motor command running along the spinal cord (which we term "motor waves") during various types of locomotion or other rhythmic motor tasks. The bursting pattern recorded under these conditions can be classified into three categories: (1) double-burst rhythmic activity in a descending (i.e., with a rostro-caudal propagation) motor wave during FW, BW and HK conditions; (2) double-burst rhythmic activity with a stationary motor wave (i.e., occurring in a single phase along the trunk) during the 'amble' walk condition; (3) monophasic rhythmic activity with an ascending (i.e., with a caudo-rostral propagation) motor wave during the Swing and Hands conditions. Our results suggest that the networks responsible for the axial propagation of motor activity during locomotion may correspond to those observed in invertebrates or lower vertebrates, and thus may have been partly phylogenetically conserved. Such an organization could support the dynamic control of posture by ensuring fluent movement during locomotion.