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.