BACKGROUND: Despite the positive effect of early post-cardiac surgery exercises, the concern of safety issues is high, indicating the need for standardized and quantitative mobility indicators to enhance early mobilization monitoring in the intensive care unit while ensuring patient safety. This study aimed to compare the safety and efficacy of quantitative phase I exercise versus usual care in patients who have undergone cardiac surgery. METHODS: In this non-randomized, single-blind trial, patients following on-pump cardiac surgery were allocated to either quantitative exercise (intervention group, n=114) or usual care (control group, n=114) based on their willingness. Continuous cycling ergometer was used for intervention, and the peak power was recorded for setting next exercise intensity. Noninvasive cardiac output monitoring during exercise was used for safety management. Patients received one time per day, seven sessions in total. RESULTS: Two-way repeated-measure analysis of variance demonstrated significant group and test time difference for forced expiratory volume in 1 s (FEV1) (P=0.01), maximum expiratory pressure (MEP) (P=0.02), peak expiratory flow (PEF) rate (P=0.045), and 6-minute walking distance (6MWD) (P=0.001); more improvement of 6MWD in intervention group (P<0.001). Intervention group showed significant post-test improvements except FEV1/forced vital capacity (FVC). Post-test differences between groups were observed in FEV1 (P=0.02), FVC (P=0.02), MEP (P=0.02), 6MWD (P<0.001), and △6MWD (P<0.001). Analysis of covariance with smoking as covariate showed consistent results. Postoperative stay in intensive-care unit in the intervention group [mean ± standard deviation (SD), 3.0±1.3] was shorter than that in the control group (mean ± SD, 3.5±2.2) (P=0.03). CONCLUSIONS: Quantitative phase I exercise applied to patients following cardiopulmonary bypass is safe and recovers respiratory and physical capacity more quickly.