Chemically induced independent activation of the elbow flexor and extensor centers. A, Rhythmic flexor EMG bursts developed first after bath application of NMDA (60 μm) while the extensor was still tonically active (Induction). Alternation of flexor and extensor bursts then emerged with well coordinated rhythmic limb movement (Walking). The vertical dotted linesindicate the transition of activation from the flexor to the extensor muscles. The flexor turned to tonic activation while rhythmic extensor bursts were still evident when the walking-like rhythm started to deteriorate after several hours of walking-like movement (Waning). The flexor and extensor bursts became synchronized for a short period during washout of NMDA (Washout). The EMG signals have been low-pass filtered at 20 Hz and rectified. B, Cross-correlation analysis shows the antiphase relationship (negative peak value at 0) between the flexor and extensor bursts during walking. C, During washout, the flexor and extensor bursts were synchronized, as shown by the in-phase relationship in cross-correlation (positive peak value at 0).