Effects of a Single Power Strength Training Session on Heart Rate Variability When Performed at Different Simulated Altitudes

Introduction

During the last decades, heart rate variability (HRV) has been utilized as a noninvasive method to assess the autonomic balance in clinical practice and in sport and exercise science (Melanson, 2000; Aubert et al., 2001; Carter et al., 2003; McNarry and Lewis, 2012). Reduced HRV is common in several pathologies such as diabetes, heart diseases, hypertension, asymptomatic left ventricular dysfunction, and myocardial infarction (Takase et al., 1992). Conversely, interventions such as regular physical activities that reduce the sympathetic activity and/or increase the parasympathetic activity, that is, increasing the variability of beat-to-beat intervals, have been shown to protect against lethal arrhythmias (Kendall et al., 1995).

Many studies have reported that regular moderate exercise training increases the HRV index (Al-Ani et al., 1996; Iellamo et al., 2000; Pichot et al., 2000; Hedelin et al., 2001; Tulppo et al., 2003; Garet et al., 2004; Hautala et al., 2004; Kiviniemi et al., 2006), whereas long-term and continued heavy exercise might cause fatigue and diminished HRV (Gladwell and Coote, 2002). Next to the changes occurring after regular exercise training, changes in HRV after single exercise bouts are supposed to indicate an imbalance between exercise stress and recovery (Sibony et al., 1995) and sustained sympathetic activity (Sandercock et al., 2005). It is important to recognize that the type and intensity of exercise may influence the HRV response both for aerobic and anaerobic exercises (Aubert et al., 2001; Berkoff et al., 2007).

Thus, HRV may represent an important tool to identify and monitor the autonomic nervous imbalance after short-term high-intensity exercise to denote overreaching (Kaikkonen et al., 2008). According to Mourot et al. (2004), anaerobic training may promote significant HRV changes shortly after the exercise bout (1 hour), but may return to baseline levels after 24 to 48 hours of recovery. Therefore, the measurement of HRV changes 24 hours (1 day) after maximal, but rather short, stimuli (including intermittent hypoxic exercise) might be indicative of the adequateness of the training stimulus and of adaptive physiological consequences.

In addition, during the last decades, athletes have adopted hypoxic exposure as a complementary training method to improve physical performance, including anaerobic performance (Álvarez-Herms et al., 2014, 2016). Studies on the effects of hypoxia on HRV showed that acute hypoxic exposure causes a decrease in the total spectral power, which is associated with an increased sympathetic tone accompanied by a decrease in parasympathetic activity (Perini et al., 1996; Sevre et al., 2001). However, to our knowledge, no study has assessed HRV 24 hours after one acute, maximal, anaerobic training session performed at different altitude levels. It might be expected that anaerobic exercise performed in more severe hypoxia will provoke greater physiological stress than in rather moderate hypoxia.

Thus, monitoring HRV response after a single exercise session performed at different altitude levels may be considered important to establish the hypoxia level by which the autonomic balance is disrupted 24 hours after maximal all-out training. The aim of this study was to analyze the impact of one high-intensity training session (maximal power strength training capacity) at different hypoxic levels (i.e., FiO₂ 20.9%, 16.5%, and 13.5%) on HRV changes after a 24-hour recovery period (1 day) and determine if more severe hypoxia alters the autonomic balance to a greater extent. We hypothesized that more severe hypoxia at high altitude induces additional stress that disrupts the balance of the autonomic nervous system to a greater extent compared with low and moderate altitudes and that HRV would not fully recover after 24 hours.

Materials and Methods

Results

Discussion

The presented findings contrast with our hypothesis, indicating that the autonomic balance is not affected 24 hours after high-intensity, short-term, power strength training even when performed at simulated high altitude conditions (i.e., 4000 m). The average power generated during the training sessions did not differ between conditions (3187 ± 46, 3184 ± 15, and 3285 ± 43 W for normoxia, moderate hypoxia, and severe hypoxia, respectively) (p < 0.05). Moreover, power output during the first and the last set did not differ between conditions (p > 0.05).

The HRV analysis after exercise training procedures (Kamath et al., 1991; Yamamoto et al., 1991; Casadei et al., 1996; Buchheit et al., 2004; Povea et al., 2005) has been considered a useful tool for the assessment of positive adaptation (Levy et al., 1998; Melanson, 2000; Tulppo et al., 2003) or maladaptation (Hedelin et al., 2001; Mourot et al., 2004) to physical load stimulus. Commonly, HRV has been assessed immediately or several hours after single exercise bouts or after an aerobic training phase, yet rarely after anaerobic training.

The present findings show that heavy exercise bouts do not influence HRV measured after 24 hours. This is in accordance with previous studies (Furlan et al., 1993; Bernardi et al., 2000; James et al., 2002), yet this study expands these findings by showing that this is even the case when anaerobic exercise is performed at hypoxic levels equivalent to 4000 m of altitude.

Although the impact of hypoxia on the autonomic nervous system seems markedly related with a predominance of sympathetic activity (Povea et al., 2005), the results of our study indicate a whole recovery after 24 hours. Noteworthy, previous studies by Schmitt et al. (2006, 2008, 2018) have reported that the training intensity during moderate altitude training camps directly increases the sympathetic dominance on the HRV pattern. However, as those studies were performed during permanent altitude exposure, the results do not contradict our findings.

The relatively short duration (∼20 minutes of anaerobic exercise) of the hypoxic stimulus and thus low total energy turnover of the training session might explain our finding. In this regard, the total physical work performed in hypoxia may play a greater role for long-term HRV recovery than the intensity of exercise (Mourot et al., 2004). Accordingly, Buchheit et al. (2004) observed that hypoxia added to a moderate exercise load had no additional effect on HRV indexes.

Power strength training is commonly applied as a basic training stimulus to improve physical performance. In anaerobic and intermittent sports, power strength exercises are considered a specific method to improve performance, while aerobic exercise could be used as a complementary activity (Mcguigan et al., 2012). In whatever case, power strength training may promote profound acute neuromuscular fatigue, requiring enough recovery time to repeat a similar stimulus (Linnamo et al., 1997). Moreover, elite athletes performing in intermittent and anaerobic sports may add hypoxia to increase the training stimulus and thus improve performance (Álvarez-Herms et al., 2014).

The HRV analysis could be an elegant and useful tool for monitoring the recovery between activities in which the volume is rather low, yet the training intensity is high and performed repetitively in hypoxia. Our results show that this type of training can be safely applied in a daily schedule even when performed at HA (FiO₂ 13.5%) with the aim to increase the training stimulus. This could be beneficial not only in altitude training camps for competitive athletes but may also be included in exercise training routines for physical fitness of workers at altitude, including employees and contractors for mining companies, astronomical observatories, or border and customs servants.

Limitations

In addition to the small sample size, a limitation of this study is the missing HRV monitoring during the immediate hours after the altitude training sessions. Hence, we are not able to state whether the different altitude levels affected the HRV kinetics during the early hours after the anaerobic training bouts. In addition, the low training load (20 minutes of hypoxic training stress in each condition) applied in this study could explain HRV recovery after 24 hours. Furthermore, day-to-day HRV must be considered when interpreting the present results (Al Haddad et al., 2011) to fully validate the HRV analysis as a long-term tool to determine individual adaptability to maximal anaerobic stimulus even when performed in hypoxia.

Conclusions

Although HRV alterations may occur immediately after a single training session, in the present study, we were not able to demonstrate differences in parameters of HRV 24 hours after intense, but short, anaerobic training at different altitude levels. Further research is needed to identify the influence of consecutive days of anaerobic training (possibly accompanied by aerobic exercise training sessions) performed in intermittent hypoxia on HRV, that is, applying the Training High–Living Low model.

Authors' Contributions

J.Á.-H., S.J.-S., and H.G. conceived of the presented idea, carried out the experiment, wrote the manuscript, developed the theoretical formalism, performed the analytic calculations, and contributed to the final version of the manuscript. M.B. supervised the project, verified the results, and contributed to the design of the research and writing of the manuscript. F.C. and G.V. provided critical feedback and helped shape the research, interpreting the results, and worked on the manuscript.