An optimization study of the ultra-short period for HRV analysis at rest and post-exercise

Ultra-short-term heart rate variability (HRV) analysis (< 5 min) has been extensively growing in the field of exercise performance for autonomic assessment. However, the validation of ultra-short-term HRV was unclear in the recovery period of exercise. This study aimed to elucidate the agreement between ultra-short-term HRV (0–30 s, 0–1 min, 0–2 min, 0–3 min, 0–4 min) and standard short-term HRV (5 min) and to explore the optimal recording duration under rest and post-exercise conditions. 69 participants were recruited to perform physical exercise on a treadmill with an intensity of 6 km/h, 9 km/h and 12 km/h, independently. The standard deviation of RR-interval (SDNN) and root mean square of successive differences of RR-intervals (RMSSD) were calculated by using ultra-short periods and standard period at rest condition (Pre-E) and three post-exercise trials, i.e., Post-E1, Post-E2 and Post-E3, respectively. One-way ANOVA with repeated-measures and Cohen's d statistics were conducted, and Bland-Altman analysis and interclass correlation coefficients (ICC) were used to assess the levels of agreement. For SDNN and RMSSD, the results of agreement analysis at rest condition were different from those at post-exercise. At Pre-E, SDNN and RMSSD were reliable for ultra-short-term HRV analysis at all ultra-short periods, i.e., 0–30 s, 0–1 min, 0–2 min, 0–3 min and 0–4 min, with most ICCs greater than 0.9 and Cohen's d showing trivial differences (Cohen's d = 0.024–0.117). However, at post-exercise, SDNN0–30s, SDNN0-1min, RMSSD0–30s and RMSSD0-1min showed significant differences with SDNN5min and RMSSD5min, respectively (p < 0.01), and the ICCs was not perfect (< 0.9). HRV analysis with time duration longer than 2 min showed nearly perfect reliability in all post-exercise trials, with trivial differences (Cohen's d = −0.003–0.110) and perfect ICCs (ICCs = 0.916–0.998). Furthermore, the limits of the agreement became tighter as the period duration increased in Bland-Altman plots. This study demonstrated that ultra-short-term HRV analysis was a good surrogate of standard HRV time-domain measures to reflect the autonomic regulation at rest and post-exercise. Specifically, ultra-short-term HRV0–30s or HRV0-1min was recommended at rest condition, whereas longer than 2 min recording period was reliable to obtain SDNN and RMSSD for the accuracy of HRV analysis.