Respiration–action coupling during self-initiated action: Evidence for action alignment with ongoing breathing

Voluntary actions are coupled with the respiratory cycle, yet whether this coupling primarily reflects action timing relative to ongoing breathing or respiratory adjustment to anticipated action remains unclear. This pre-registered study compared respiration-action coupling across three conditions using a modified Libet clock task. Thirty participants completed the task, and data from 29 participants were included in the final analysis. Participants performed key presses under Self-initiated (freely chosen timing), Delayed (predictable timing after one full clock rotation), and Immediate (speeded response to stimulus onset) conditions. Using the preregistered Hilbert-based analysis, circular uniformity tests revealed significant respiratory phase concentration at key press in the Self-initiated condition, but not in the Delayed or Immediate conditions. Time-resolved phase consistency analysis further showed a significant cluster including pre-keypress time points only in the Self-initiated condition. These findings suggest that self-chosen action timing is preferentially coupled with ongoing breathing rhythms, beyond the effects of preparation time or predictability alone. Additionally, actions performed during inhalation were followed by shorter and deeper subsequent breaths across all conditions, indicating greater perturbation of ongoing breathing when acting during inhalation. These results suggest that respiration-action coupling is most clearly expressed when action timing is self-chosen, and that acting during inhalation may perturb ongoing respiration more strongly than acting during exhalation.