Regulatory model for the emergence of cardiorespiratory-locomotor coupling during exercise in humans

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The objective of the present study was whether a phase synchronization between cardiac, respiratory, and locomotor rhythms relates to certain biological functions, such as minimization of the energy costs of cardiac and respiratory work. To this end, we investigated whether Vo_2, Vco_2, and ventilatory equivalents are altered with the emergence of cardiorespiratory-locomotor coupling (CRLC) during walking, as well as whether respiratory rhythm can modulate the strength of cardiolocomotor coupling. Eleven healthy subjects participated in a 20 min session of walking on a motorized treadmill. The treadmill speed was adjusted until a target heart rate of 120 beats/min. The subjects were required to walk synchronizing their locomotion pace with their mean heart rate at a ratio of 1:2, during which they controlled their respiratory rhythm either to their locomotor rhythm or to computer generated waveform of 0.5Hz. The strength of phase synchronization between cardiac and locomotor rhythms wa … More s evaluated by phase coherence. We found in all subjects that when breathing rhythm was voluntarily synchronized with locomotor rhythm, phase coherence became high (P<0.01) compared with that observed when breathing rhythm was paced constant. Spectral analysis on fluctuation of heart period revealed that the respiratory modulation of heartbeats decreased when phase coherence increased. We also found that averaged ventilatory equivalent for 0_2 tended to decrease slightly but significantly with the emergence of CRLC. We then developed a coupled oscillator model using FitzHugh-Nagumo equation that could reproduce the synchronization between two rhythms, I.e. cardiac and locomotor rhythms. We included phase dependent property of cardiac oscillator, which was assumed to exhibit phase advance during systolic phase. Results of the simulation demonstrated that synchronization occurred at just before the diastolic phase of cardiac cycle, which was comparable with the experimental data obtained. Less