This study was aimed at examining the effects of HRV-BF Training on aerobic exercise recovery. Participants in the experimental group underwent ten sessions of HRV-BF training. The Astrand rhyming test was applied before (pre-test) and after (post-test) 10 sessions of HRV-BF training for aerobic exercise performed in the resting period for 5 minutes and during the recovery periods (post-exercise, 2-minute, and 3-minute periods). Results show SDDN values were significantly higher in the experimental group for the within-group and between-group 2min and 3min comparisons (see Table 3). SDNN represents the standard deviation of the normal R-R intervals, while R is the peak of a QRS complex (heartbeat) (Wang & Huang, 2012) and is the most used time domain measure of HRV (Kleiger et al., 2005). SDNN is a reliable and explanatory measure for assessing HRV (Schipke et al., 1999).
A significant difference was observed in favor of the experimental group in rMSSD (post) and rMSSD (3min) within-group comparisons. There was also a significant between-group difference in favor of the experimental group in rMSSD (3min) regardless of time. Regarding the time*group interaction, a statistically significant difference was observed in favor of the experimental group in rMSSD (3 min) (see Table 4). rMSSD is a time domain measure used to predict vagal changes reflected in HRV (Shaffer et al., 2014; Routledge et al., 2010). Our study showed a noticeable improvement in rMSSD after aerobic performance in the experimental group. HRV-BF training enabled the parasympathetic system to be activated more quickly by affecting rMSSD in the recovery process following aerobic exercise.
Regarding LF/HF, a significant difference was observed in favor of the control group in LF/HF (post-test) and in favor of the experimental group in LF/HF (2min) in group comparisons (see Table 5). The LF/HF ratio assumes that HF power can be produced by the PNS (parasympathetic nervous system). In contrast, LF power can be produced by the SNS (sympathetic nervous system) (Shaffer & Ginsberg, 2017). In this model, a low LF/HF ratio reflects parasympathetic dominance, whereas a high LF/HF ratio indicates sympathetic dominance (Billman, 2013; Shaffer & Ginsberg, 2017). In the present study, although there was a significant difference in LF/HF in favor of the control group after the test, there was an increase in the parasympathetic activity of the experimental group and the sympathetic activity of the control group during the fast phase of recovery (LF/HF 2min). This indicates that parameters show noticeable improvement after aerobic performance with HRV-BF training, indicating an efficient parasympathetic system or sympathovagal balance (SDNN, rMSSD, LF/HF), one of the HRV parameters. That is, HRV-BF can positively affect cardiovascular recovery after aerobic exercise.
HRV parameters change during exercise, reflecting changes in the autonomic system. The primary factor influencing HRV is exercise intensity, with higher intensity eliciting lower HRV during exercise. As intensity increases, there is minimal change and a slower HRV gain (Michael et al., 2017). In addition, increasing exercise intensity increases sympathetic activation and more significant cardiac parasympathetic deprivation (Michael et al., 2017; Makivic et al., 2013). HRV responses have been primarily measured during and after exercise, while the regulation of these responses after exercise using HRV-BF has not been examined. Michael et al. (2016) examined the difference in short-term HRV in rapid recovery after three different exercise intensities in which 12 men cycled for 8 minutes at low (40–45%), medium (75–80%), and high (90–95%) exercise intensities. HRV was measured during and 10 minutes after the end of the exercise (sitting position). HRV recovery was faster after low-intensity exercise and delayed at higher intensities. Esco et al. (2017) compared post-exercise HRV immediately after exercise performed at 65% of maximum oxygen consumption reserve (65% VO2R) using treadmill and bicycle exercises. After exercise at 65% VO2R, healthy men had slower HRV recovery after bicycle than treadmill exercise. Brown and Brown (2007) measured HRV in athletes before and after high-intensity exercise to examine cardiac autonomic regulation after exercise. Mean RR interval and SDNN were lower after exercise, with no difference in gender. In short, cardiovascular balance is impaired after aerobic exercise because of the sympathetic nervous system. This effect delays recovery for a short time after exercise.
Several studies have examined the effects of HRV-BF training on the autonomic nervous system. These generally focus on the psychological state/emotional effects of HRV-BF training (Lagos et al., 2008; Prinsloo et al., 2013; Goessl et al., 2017; Deschodt-Arsac et al., 2018) and physiological state. For example, Göçmen et al. (2023) examined 24 basketball players aged 18–24 by applying 10-week HRV-BF training to the experimental group. The results indicated that rMSSD, HF, SDNN, and respiratory values improved significantly for athletes in the experimental group compared to the control group. Lin et al. (2020) investigated the effects of HRV-BF and autogenic training on HRV parameters. The results revealed higher HRV indices and lower respiratory rates during and after administration in the HRV-BF group. Participants applying the HRV-BF protocol had reduced respiratory rates and increased cardiac outputs with baroreflex.
Perez-Gaido et al. (2021) examined the effects of HRV-BF while recovering from submaximal aerobic exercise. They reported improvements in MeanRR, SDNN, rMSSD, and LF values following HRV-BF after submaximal exercise and concluded that Heart Rate Variability Biofeedback during recovery improves cardiac variability and the time spent exercising, lowers recovery time and improves the perceived physical exertion and the recovery perception.
Firth et al. (2022) applied 12 sessions of HRV-BF training (20 minutes/session) to examine the effects of HRV-BF training on vagus tone. There were no significant differences in vagus tone between the experimental and control groups. Similarly, Laborde et al. (2021) compared the effects of only slow-paced breathing with slow-paced breathing and HRV-BF. There were no significant differences in the mood and HRV parameters of the two groups, although participants in the second group reported higher satisfaction in a questionnaire. Lehrer et al. (2003) evaluated HRV-BF to increase vagal baroreflex gain and improve lung function among healthy adults. They found acute increases in heart rate variability in the low-frequency and total spectrum associated with slow breathing during BF periods. In the BF group, independent of respiratory changes, basal baroreflex gain and flow increased at peak expiration, independent of cardiovascular changes. Overall, some studies report positive effects of HRV-BF training, and others report no change.
The emWave Pro + device used for HRV-BF training in the present study has been used in many previous studies (Scolnick et al., 2014; Low and Chan, 2021; Rose et al., 2021; Aritzeta et al., 2022; Nashiro et al., 2022; Göçmen et al., 2023; Nashiro et al., 2023). Sarwari and Wahab (2018) evaluated the effectiveness of self-regulation techniques by applying HRV BF technology to heart compliance levels among university students. They used the quick coherence technique (QCT) and HeartMath emWave device and software to collect data. HRV-BF technology and QCT helped increase the participants’ HRV scores and heart coherence.
Whited et al. (2014) used an emWave device to determine whether HRV-BF training affects physiological tone and stress responses. They found that the BF group exhibited higher parasympathetic responses after BF training. Rose et al. (2021) used emWave pro + and inner balance devices to examine the effects of short-term HRV-BF training (two sessions including familiarization) on undergraduate students’ mood, arousal, cognitive labor, and movement time. No significant differences were found in the pre-test and post-test comparison of the experimental and control groups from watching non-emotional (physical education/movement training) videos.
The strengths of our study include applying HRV-BF for five weeks (10 BF sessions; two sessions/week for five weeks) and a well-matched experimental and control group. However, there are also some limitations. This study was limited by the absence of a placebo group watching motivational videos, a small number of elite soccer players, and the absence of follow-up measurements after the BF training sessions. Recommendations for future studies: i) the effects of different HRV-BF training protocols (frequency and duration) on recovery can be compared, ii) the recovery effects of HRV-BF after different exercise protocols (anaerobic exercise and strength exercise) can be examined, and iii) gender differences on recovery effects of HRV-BF can be compared.