Interactions between locomotor, respiratory, and cardiac rhythms were investigated in human subjects (n = 11) walking on a treadmill. Investigation of the phase relationship between heart rate and gait signals revealed that cardiac rhythms were entrained to locomotor rhythms when both frequencies were close to an integer ratio. Coherence spectra were estimated between heartbeat fluctuation, respiratory, and gait signals, and their magnitudes were evaluated. The results suggest that the respiratory-induced fluctuation in heartbeat would vary depending on the strength of the cardiolocomotor coupling. The synchronization tends to occur for one or two specific phases in an individual subject, but there was some variation among subjects. When the subjects voluntarily synchronized their cadence with the cardiac rhythm, the heart rate and blood pressure varied depending on the phase lag within a cardiac cycle. The coordination of locomotor and cardiac rhythms is discussed.