Agreement of ultra-short-term heart rate variability measure after different repeated bouts of sprint ability tests.

This study aims to explore: (1) the validity of post-exercise ultra-short-term heart rate variability (HRV_ust) after two different bouts of repeated sprint ability test (RSA), and (2) the relationship between HRV_ust measure and RSA performance. Twenty adolescent male futsal players voluntarily participated in this study (age: 17.65 ± 1.81 years, body height: 170.88 ± 4.98 cm, body weight: 61.78 ± 4.67 kg). The participants performed a standard RSA test (RSA_standard) and an RSA test with a 10% decrement of the best sprint time test (RSA_10%decrement) on two separate occasions within a week. On both occasions, a 5-min resting electrocardiography was administered pre- and post-RSA exercise protocols. The first 30-s (HRV_ust30s), 60-s (HRV_ust60s), and 60-120-s (HRV_ust1-2min) were extracted and used to compare with the standard of 5-min HRV recording (HRV_criterion). The natural logarithm (ln) of the standard deviation of normal-to-normal intervals (SDNN) and root mean square of successive normal-to-normal interval differences (RMSSD) HRV indices were utilised to establish intraclass correlation coefficient (ICC_2,1), coefficient of variation (%CV), and Pearson product-moment correlation (r). Results revealed the ICC values of HRV_ust lnSDNN (RSA_standard = 0.77-0.88; RSA_10%decrement = 0.41-0.71) and lnRMSSD (RSA_standard = 0.81-0.86; RSA_10%decrement = 0.57-0.82). Furthermore, significantly positive correlations between best sprint time and post-exercise HRV_ust indices were found in lnSDNN (r = 0.47-0.62; p < 0.05) and lnRMSSD (r = 0.45; p < 0.05). Additionally, a large CV of lnSDNN (RSA_standard = 32%-45%; RSA_10%decrement = 29%-39%), lnRMSSD (RSA_standard = 50%-66%; RSA_10%decrement = 48%-52%), and ratio (RSA_standard = 45%-126%; RSA_10%decrement = 27%-45%) was found after the RSA protocols. In conclusion, the number of bouts of RSA exercise potentially influences the agreement of post-exercise time-domain HRV_ust indices to standard HRV measure.