What Can Bidirectional Walking Tell Us about Central Pattern Generators?

We report here an experimental-theoretical study of the features of the EMG activity of the iliopsoas (IP), tibialis anterior (TA), and gastrocnemius medialis (GM) muscles of the hindlimbs of decerebrate cats during walking induced by epidural spinal cord stimulation in various locomotor modes determined by the direction of movement of the treadmill belts: unidirectional walking in the forward (FW) and backward (BW) directions and bidirectional walking (BDW); an attempt was made to explain the data obtained in these experiments from the point of view of various central pattern generator (CPG) models. While maintaining the reciprocity of the operation of antagonist muscles during BDW, the activity of the TA and GM in BDW in oppositely directed limbs differed from each other: the activity of the TA and GM for the limb going backwards in BDW was similar to the activity of these muscles in unidirectional BW, and the activity of the TA and GM for limb moving forward during BDW was similar to the activity of these muscles during unidirectional FW. The patterns of IP activity of both limbs in BDW were similar, but IP activity was lower than for unidirectional FW and higher than for unidirectional BW. The stability of IP muscle activity was more variable than that of TA and GM. These data can be explained from the point of view of the operation of two-level CPG models, as well as a scheme for coordinating the CPG of different limbs, including reciprocal relationships between flexion half-centers and the influences of excitatory and inhibitory afferent signals on each component in the system in different phases of the locomotor cycle.