Modified Stress Score and Sympathetic–Parasympathetic Ratio Using Ultra-Short-Term HRV in Athletes: A Novel Approach to Autonomic Monitoring

<b>Background:</b> Monitoring autonomic balance provides valuable insights into recovery status and physiological readiness, both of which are essential for performance optimization in athletes. The Stress Score (SS) and Sympathetic-Parasympathetic Ratio (SPS), derived from Poincaré plot heart rate variability (HRV) indices, have been proposed as practical markers of sympathetic activity and overall autonomic balance. However, these traditional calculations often require lengthy recordings and specialized software, limiting their feasibility in field settings. This study introduces modified versions of these metrics derived from ultra-short-term (1 min) time-domain HRV recordings: the Modified Stress Score (MSS) and Modified Sympathetic-Parasympathetic Ratio (MSPS). <b>Methods:</b> Competitive male athletes (<i>n</i> = 20, age = 21.2 ± 2.1 year, height = 183.6 ± 8.9 cm, weight = 79.2 ± 10.3 kg) completed a maximal exercise test with HRV recorded before and after exercise. <b>Results:</b> Following natural log-transformation, MSS and MSPS demonstrated strong correlations with SS and SPS across all time points (<i>r</i> = 0.87-0.94, all <i>p</i> < 0.01) and displayed the expected physiological responses to exercise and recovery. <b>Conclusions:</b> These findings suggest that MSS and MSPS are practical, accessible tools for assessing autonomic balance in athletes. Their application may enhance our ability to monitor recovery status, guide individualized training strategies, and optimize performance in applied sport settings.