Respiratory and Cardiac Phase Coupling With Voluntary Actions Across Motor Tasks.

Bodily rhythms such as breathing and heartbeat influence perception and motor processes. Recent studies have indicated that breathing phases, particularly exhalation, synchronize with voluntary actions, potentially reflecting a general influence on motor intention. However, this effect might depend on the specific effector and movement direction. This study aimed to investigate (i) respiratory synchronization across different voluntary motor tasks, (ii) the interaction between stimulus-locked and action-locked respiratory coupling, and (iii) cardiac synchronization with voluntary actions. A total of 32 healthy participants performed two voluntary motor tasks: a modified Libet clock task and an elbow flexion-extension task. In the Libet clock task, the participants monitored a rotating dot on a clock face and either pressed a key at a self-chosen time (key-press condition) or released the key after holding it pressed (key-release condition). In the elbow flexion-extension task, the participants spontaneously pushed (elbow extension) or pulled (elbow flexion) a joystick. Across tasks, voluntary actions showed an overall tendency to occur during exhalation across multiple effectors (finger and elbow) and movement directions (extension and flexion). Furthermore, stimulus-locked respiratory phase was associated with subsequent action timing, suggesting that trial structure can shape respiration-action coupling. We found no robust evidence for systematic cardiac-phase modulation of voluntary action timing, although a weak condition-specific trend was observed. Collectively, these findings support respiration-action coupling across diverse actions and highlight a potential contribution of stimulus-locked respiratory dynamics to voluntary action timing.