Relationships between training, nocturnal heart rate variability and morning cortisol saliva levels in young endurance athletes

Abstract

Introduction. In order to reach the peak levels of performance, athletes must find a way to combine intensive training with sufficient amount of recovery. This is especially important in endurance sports where both training volume and intensity are relatively high. It is not always easy for the athletes to tell the difference between normal training-induced fatigue and non-functional overreaching which is why researchers have pursued to develop methods to assess the recovery from training. There are several ways to evaluate physiological stress and recovery of which HRV and cortisol secretion are very different, but both commonly used. The aim of this study was to search what changes and trends occur in stress and recovery state markers HRV and salivary cortisol in young endurance athletes and what are the relationships between these two stress markers. Methods. Seven well-trained junior athletes (age 16-18), who trained for cross-country skiing year-round, participated in the study. The study protocol included a seven weeks long measurement period where the HRV and morning salivary cortisol measurements were made every other week to allow for four testing weeks. The nocturnal HRV was measured with a contact-free sleep tracking Emfit QS-device that reported the magnitude of HRV with a time-domain variable rMSSD. The free cortisol levels were measured from the saliva that was collected by participants themselves immediately after waking up using the Passive Droolmethod. The study protocol did not alter the athletes’ training programmes but the training characteristics (volume, intensity, load) from each athlete were followed during the measurements, using electronic training diaries. Results. The only significant changes in the training characteristics occurred in the weekly training volume and weekly training load (p<0.05). The nocturnal HRV values decreased during the three testing weeks and then increased to the last one, while the morning salivary cortisol level showed a reverse effect by decreasing during the first three test weeks and then increased to the last one. However, the changes within the stress markers were also not considered significant. The average cortisol values varied between 58 and 72 nmol/l. The changes in the morning salivary cortisol level and nocturnal HRV, showed a high negative correlation when the HRV levels were low and the cortisol levels high (-0.893, p<0.01; -0.857, p<0.005). The significant correlation was found also between the salivary cortisol level and the three-day training intensity (0.811, p<0.05). No other correlations between the training characteristics or the stress markers were considered significant, even thought the correlation coefficients were often relatively high. Conclusions. The results of the present study authenticate the assumption that ANS and circulating hormones work together and are both responsible from the regulation of stress reactions and that the co-operation of these two stress systems is stronger in more stressful conditions. Significant connections between the morning salivary cortisol levels and the training intensity were also found denoting that salivary cortisol could serve important information about the recovery state of body after hard training. The participants’ absolute cortisol values were notably higher that they should on average person (reference interval for morning salivary cortisol levels in adults is 4–28 nmol/l (Association for Clinical Biochemistry 2012) which can be considered as a finding that endurance training causes hypercortisolism for young endurance athletes or as a sign of non-functional overreaching or overtraining. HRV did not show any significant associations with any of the training characteristics. In further research, it would be important to find more subjects to do this type of study in order to get more significant and valid findings.