Nocturnal Heart Rate Variability and Morning Orthostatic Test as Tools to Monitor Training Load

Abstract

Both the nocturnal heart rate variability (HRV) and the morning Orthostatic Test are widely used to monitor training load and recovery status of athletes. Both measurements have been observed to be associated with cardiac autonomic regulation. But less is known whether the measurements show similar responses to training load. PURPOSE: This study investigated the effects of easy and hard training days on the nocturnal HRV and the Orthostatic Test. In addition, the aim was to investigate relationships between nocturnal and morning HRV variables. METHODS: Thirty-four recreational endurance runners performed nocturnal R-R interval recordings and the morning Orthostatic test (5 min supine + 3 min standing) after an easy and a hard training day (moderate- or high-intensity endurance training). Nocturnal HRV was analyzed as four-hour period starting 30 min after going to bed for sleep. Morning HRV was analyzed over 4 min supine (00:30-04:30) and 2 min standing position (06:00-08:00). In addition, peak HR after stand-up and Orthostatic heart rate (HR) (HRstanding - HRsupine) were analyzed. RESULTS: Training load (session RPE) of hard training day (638 ± 638) was significantly (P < 0.001) greater compared to easy training day (50 ± 100). Nocturnal HR was lower (52 ± 6 vs. 57 ± 9, P < 0.001) and all nocturnal HRV variables were higher (Total power: 8.86 ± 0.63 vs, 8.41 ± 0.79, P < 0.001) after easy training day. HR in supine position was lower (53 ± 7 vs. 56 ± 7, P = 0.011) and high frequency power in supine position was higher (8.06 ± 0.89 vs. 7.74 ± 0.93, P = 0.027) after easy day. Other Orthostatic test variables were not different between easy and hard training days. Nocturnal HR and HRV variables showed moderate correlations (r = 0.62 - 0.78, P < 0.001) to the morning supine variables, but trivial and small correlations (0.19 - 0.39) to standing variables. CONCLUSIONS: Different training load after easy and hard training days can be observed in both nocturnal HRV and Orthostatic Test. However, the differences are greater in nocturnal HRV recordings compared the morning Orthostatic Test. In addition, it seems that standing variables are not able to evaluate the training load of the previous day. The results suggest that both nocturnal HRV and selected, supine variables of the Orthostatic test can be used in monitoring training load.