What can two-directional walk tell about central pattern generators?

We performed an experimental-theoretical study to investigate the various features of EMG activity of hind limb muscles (m. iliopsoas (IP), m. tibialis anterior (TA), m. gastrocnemius medialis (GM)) of the decerebrate cat whose walking in different locomotor modes (bilateral forward (FW), bilateral backward (BW) and bidirectional (BIDIW)) was induced by epidural stimulation of the spinal cord. An attempt was made to explain the obtained data in terms of various models of the central pattern generator (CPG). During BIDIW, the activity of TA and GM in oppositely directed hind limbs differs from each other while maintaining the reciprocity of the activity of the antagonist muscles. The activity of TA and GM for the hind limb going backward in BIDIW is similar to the activity of these muscles in bilateral BW, and the activity of TA and GM for hind limb, going forward in BIDIW, is similar to the activity of these muscles in bilateral FW. The patterns of IP activity in both hind limbs in BIDIW are similar, but IP activity is lower than for bilateral FW and higher than for bilateral BW. The stability of IP muscle activity is more variable than that of TA and GM. The data obtained can be explained from the standpoint of the two-level models of the CPG, as well as a complex scheme for coordinating the CPG of different limbs, including reciprocal relationships between flexion half-centers and the influence of excitatory and inhibitory afferent signals on each level of the system in different phases of the locomotor cycle.