["Psychophysiology, Volume 63, Issue 5, May 2026. ", "\nABSTRACT\nFast‐paced breathing (FPB) has emerged as a technique to purposefully increase physiological activation to facilitate motor‐cognitive performance. Empirical evidence of prominent physiological processes associated with FPB, cardiac changes however, remain scarce. To address this gap, the present study sought to systematically quantify the changes in heart rate variability and cardiac contractility during FPB at 35 and 55 cycles/min (CPM), compared to spontaneous and paced breathing (6 and 15 CPM), while accounting for breathing discomfort. Healthy, physically active participants (N = 38) performed each breathing exercise for 2.5 min, with electrocardiography and impedance cardiography signals recorded throughout. Heart rate variability was indexed through RMSSD, and the pre‐ejection period (PEP) served as an index of cardiac contractility. Results showed that FPB at 35 and 55 CPM led to reduced heart rate variability, whereas only the latter condition elicited a significant increase in cardiac contractility, suggesting a more pronounced stimulation. Breathing discomfort level increased as the breathing frequency deviated from the spontaneous range, reaching a moderate level at 55 CPM. These findings elucidate the frequency dependence in reciprocal and independent cardiac patterns, informing conceptual models of paced breathing.\n"]