Respiration-action coupling generalizes across distinct finger movement configurations

The respiratory rhythm actively shapes when we initiate voluntary actions, yet it remains unclear whether respiratory phase influences a general preparatory state for action onset or varies with movement-specific parameters. We addressed this by recording electroencephalography (EEG), respiration, and self-paced actions during a Libet-style task in which participants initiated four finger movements that varied in muscle recruitment (flexion vs extension) and movement direction (upward vs downward). Movement onsets were non-uniformly distributed across the respiratory cycle and consistently clustered during mid-to-late expiration across all conditions, indicating that respiration-action coupling generalizes across these finger movement variants. In addition, the readiness potential (RP) preceding movement did not significantly differ between movement types or directions. Furthermore, a circular-linear correlation analysis indicated that RP amplitude varied with respiratory phase, with larger amplitudes during expiration than inspiration. Together, these findings suggest that respiration provides a temporal frame that biases when self-initiated actions occur and modulates the amplitude of preparatory activity, whereas the overall RP waveform is largely conserved across finger movement variants.