Exercise training requires the careful application of training dose to maximize adaptation while minimizing the risk of illness and injury. High-intensity interval training (HIIT) is a potent method for improving health and fitness but generates substantial autonomic imbalance. Assuming a supine posture between intervals is a novel strategy that could enhance physiological readiness and training adaptations. This study aimed to establish the safety and feasibility of supine recovery within a HIIT session and explore its acute effects. Fifteen healthy, active males (18-34years) underwent assessment of cardiopulmonary fitness. Participants completed two identical HIIT treadmill sessions (4 x [3min at 95% VO2max, 3min recovery]) employing passive recovery in standing (STANDard) or supine (SUPER) posture between intervals. Heart rate variability (HRV), HRV recovery (HRVrec; lnRMSSD) and heart rate recovery at 1min (HRrec) were assessed using submaximal constant speed running tests (CST) completed prior to, immediately after and 24h following HIIT. No severe adverse events occurred with SUPER, and compliance was similar between conditions (100 ± 0%). The change in HRVrec from the CST pre-to-post-HIIT was not different between conditions (p = 0.38); however, HRrec was faster following SUPER (39 ± 7bpm) vs. STANDard (36 ± 5bpm). HRV 24h post-SUPER was also greater (3.56 ± 0.57ms) compared to STANDard (3.37 ± 0.42ms). Despite no differences in perceived exertion (p = 0.23) and blood lactate levels (p = 0.35) between SUPER and STANDard, average running HRs were lower (p = 0.04) with SUPER (174 ± 7bpm) vs. STANDard (176 ± 7bpm). Supine recovery within HIIT attenuates acute cardioautonomic perturbation and accelerates post-exercise vagal reactivation. SUPER enhances recovery of vagal modulation, potentially improving physiological preparedness 24h post-HIIT. Further research exploring the chronic effects of SUPER are now warranted.