BLDE University Journal of Health Sciences

ORIGINAL ARTICLE
Year
: 2020  |  Volume : 5  |  Issue : 2  |  Page : 114--126

A systematic review on the effect of high.intensity training on heart rate variability in sports professionals and healthy young adults


Shweta Shenoy, Prachi Khandekar 
 MYAS- GNDU Department of Sports Sciences and Medicine, Guru Nanak Dev University, Amritsar, Punjab, India

Correspondence Address:
Prof. Shweta Shenoy
MYAS GNDU Department of Sports Sciences and Medicine, Guru Nanak Dev University, Amritsar - 143 005, Punjab
India

Abstract

BACKGROUND: Exercise plays an essential role in managing cardiovascular disorders, and high-intensity training (HIT) exercise is a potential, time-effective alternative to traditional aerobic exercise. OBJECTIVES: The objective of this study was to evaluate the effect of HIT program on the time and frequency domains of heart rate variability (HRV) in sports professionals and healthy young adults (18–30 years of age) through a systematic review. METHODS: Physiotherapy Evidence Database (PEDro), PubMed, ScienceDirect, Taylor and Francis Online, and Google Scholar were searched for relevant studies. PEDro scale was used to assess internal validity, exclude risk bias, and assess the methodological quality of studies. RESULTS: A total of 4430 search results were obtained by searching for specific keywords, out of which, only 7 studies met the inclusion criteria after removing all the duplicate articles. Our results showed that HIT programs have significant effect on HRV (root mean square of successive difference [RMSSD], mean difference [MD] =13.35, 95% confidence interval [CI] =1.73, 24.97), high-frequency (HF, MD = 381.7, 95% CI = 321.54, 441.86) power, and low- to high-frequency power (MD = -0.10, CI = -0.73, 0.53) ratio in young adult athletes and healthy young adults. No significant difference was found in other HRV variables. CONCLUSION: HIT is an effective exercise program that can cause improvement in parasympathetic and sympathetic cardiac modulation depending on the duration of the HIT program. HIT program can be given for 2–4 weeks to improve HRV in young adults.



How to cite this article:
Shenoy S, Khandekar P. A systematic review on the effect of high.intensity training on heart rate variability in sports professionals and healthy young adults.BLDE Univ J Health Sci 2020;5:114-126


How to cite this URL:
Shenoy S, Khandekar P. A systematic review on the effect of high.intensity training on heart rate variability in sports professionals and healthy young adults. BLDE Univ J Health Sci [serial online] 2020 [cited 2021 Apr 14 ];5:114-126
Available from: https://www.bldeujournalhs.in/text.asp?2020/5/2/114/303974


Full Text



Young adults in the Asian Pacific region are susceptible to cardiovascular disorders,[1] which are a cause of concern in the Indian population as well.[2] Exercise plays an essential role in managing cardiovascular disorders, and high-intensity training (HIT) exercise is a potential, time-effective alternative to traditional aerobic exercise in this context.[3] Many types of researches support the evidence that HIT is superior to continuous moderate exercise to improve fitness and cardiovascular health.[4] Recent reviews have given the hypotheses that HIT can be an effective and superior strategy to improve cardiovascular disorder and related risk factors such as poor cardiorespiratory fitness, vascular dysfunction, glycemic control, and insulin resistance.[5],[6] Studies have revealed that low-volume HIT is an exercise strategy that is used by several health consultants and fitness experts for performance gains.[7] The benefits of HIT are caused by the induction of capillary growth and a sudden proliferation of endothelial cells.[8] Previous researches have described that exercise with relatively high exercise intensity improves cardiac function in humans,[9] the mechanism of which lies behind the fact that HIT brings about adaptive changes in the left ventricle, thus improving VO2 max (maximal oxygen uptake).[10] Similar adaptations are required in sports, and professional sports are characterized by involvement of high-intensity exercise training that brings acute and chronic adaptations in heart rate (HR).[11],[12],[13],[14]

HIT includes intervals between the bouts, involves alternating short periods of intensive exercise with low-intensity recovery periods.[15] HIT protocols differ widely, with intense exercise sessions lasting from 20 s to 5 min for each repetition, with high-intensity volumes as low as 3 min/week.[16],[17] Studies reported evidence that low-volume, HIT also encourages physiological remodeling that helps to develop metabolic control in muscular and cardiovascular functions.[16] A minimum 2 weeks of HIT is required to elicit muscle oxidative capacity and maximal activity of mitochondrial protein enzymes.[18],[19],[20],[21] The HIT method consists of physical exercise, which includes both strength and endurance components, and these two components have been shown to have beneficial effects on cardiovascular parameters, mitochondrial function in skeletal muscle, endothelial functions, and aerobic work capacity by improving insulin sensitivity and recovering the lipid profile in adults.[22],[23],[24]

The cardiovascular system is regulated by the sympathetic and parasympathetic autonomic regulation path of the nervous system.[25] The sympathetic and parasympathetic nervous system remains in the balanced condition during the normal functioning of the heart, and any change in this balance during the heart beat cycle can be established by HR variability (HRV).[25],[26] The HRV measurement is a noninvasive method for the evaluation of autonomic changes about the heart,[25,27-29] which are measured during rest.[30] HRV is a marker of cardiovascular autonomic function, and it predicts cardiovascular health risk.[31] HRV measured after physical activity is used to assess the cardiac autonomic functions regarding the activity. There are two domains of measurements of HRV, the time domain (which includes standard deviation of N-N intervals [SDNN], SDSD, and root mean square of successive difference [RMSSD]) and the frequency domain (including high frequency [HF], low frequency [LF], ultra LF, and very LF).[32] HRV is affected by several factors including physiological and pathological factors (such as endocrine factors, respiratory factors, neurological factors, and cardiovascular disease), lifestyle factors (such as physical activity, alcohol, and tobacco and drug consumption), and nonmodifiable factors (such as age, gender, and ethnicity).[33] Along with the healthy and diseased population, HRV also has application in the field of sports.[34],[35],[36],[37],[38]

Previous systematic reviews summarized the beneficial effect of endurance training on HRV and autonomic functioning of the heart in older individuals.[39] Abreu et al.[40] in their systematic review explained that HIT can also improve cardiac autonomic functions. They included studies comprising healthy controls as well as patients having metabolic syndrome aged between 18 and 60 years. Though the HIT influences HRV, little is known about the duration–dose relationship of HIT in different populations. Specifically, although the literature has explained the influence of HIT on HRV in older population,[40] less is known about how HIT may influence the young population, specifically those involved in sports. Some studies have described that HIT can be used to improve cardiac modulation,[40] whereas other researches contradict this view.[41] To the best of our knowledge, no systematic review has explained the dose–response relationship to clear this controversial science. Previous researches also do not clear about the optimal training load of? high-intensity exercise to create the most favorable training adaptation while reducing its detrimental effect. Thus, the current systematic review is focused on studies that have monitored the effect of HIT on HRV in young adult professional athletes and healthy individuals.

 Methods



Data source and search strategy

Preferred Reporting Items for Systematic Review and Meta-Analysis recommendations were used to report the systematic review as shown in [Figure 1]. The data were manually extracted from electronic database search from PubMed, ScienceDirect, Taylor and Francis Online, and Google Scholar using keywords such as “HIT,” “high-intensity training,” “high- intensity interval training,” “HRV,” “heart rate variability,” “HRV in athletes,” “adult athletes,” or “Autonomic cardiac functioning.” The search was carried out from September 2018 to February 2019. To assess internal validity, exclude risk bias, and assess the methodological quality of studies and to make their results interpretable, a 11-point Physiotherapy Evidence Database (PEDro) scale was used.{Figure 1}

Selection criteria

The search was limited to young healthy and sports populations. Only English language articles were assessed. Types of the study included were randomized controlled trials, case studies, clinical trials, and comparative studies. The study criteria included patient/population/problem-intervention-comparison-outcome format which was used to determine eligibility criteria including the studies which had individuals those were professional adult athletes or healthy individuals (without any cardiovascular or systemic disorder) between the age of 18 and 30 years, and carried out HIT program at 80%–90% of HRmax or 70%–85% of VO2 max for at least 2 weeks. To be included, studies must have included either one frequency domain or time domain variable of HRV measurement. Studies were excluded if they did not include individuals of the selected age criteria, or the studies were in a non-English language. Studies also had to have data on mean and standard deviation.

Data extraction

Two authors (PKS and SS) were involved in the selection of articles independently. All the duplicate articles were removed. All the titles and abstracts were evaluated to exclude unrelated articles. Full texts of all the related articles were examined according to the inclusion and exclusion criteria. Information regarding the age of the participants, population characteristics, duration of HIT, variables of HRV, primary and secondary variables, and the results were assessed manually in the studies.

The primary variables selected were root mean square of successive RR interval difference (RMSSD), the standard deviation of RR (SDRR) intervals, low-frequency (LF) power, high-frequency (HF) power, LF/HF power ratio, and total power. Secondary variables included were VO2 max, HRmax, HRrest, HRrecovery, and RR means. The data were reported in the included studies in the form of pre- and post-mean and SD values of the following variables (with their units of measurement): RMSSD (ms2), SD of RR interval (ms), LF power (ms2), HF power (ms2), LF/HF ratio, HR (beats per minute), VO2 max (ml/kg/min), and? Heart rate reserve (HRR)-sec.

Risk-of-bias assessment

PEDro scale was used to assess internal validity, exclude risk bias, assess the methodological quality of studies, and make their results interpretable. Studies were independently rated for quality by two authors (PS and SS). If there was any disagreement on any score, it was re-assessed and resolved. Each criterion of the scale was rated either done “?” (score = 1) or not done “X” (score = 0). The total score for the methodological quality of each included study was calculated by summing all the responses (maximum score 10 as one item out of 11 relates to external validity). Studies were categorized as poor (score of < 4), fair (score of 4 or 5), good (score of 6–8), and excellent quality (score of > 8) based on the total scores obtained.[29]

Statistical analysis

The random-effects model method was used to perform a meta-analysis of different variables of time domain and frequency domain of HRV. The mean difference (MD) was calculated by subtracting mean prevalues from mean postvalues for primary variables before and after the intervention. Two studies[42],[43] had not explained their results in the tabulated form, so mean and standard deviation values were estimated from graphs of the results. In one study,[42] the standard deviation was estimated using a moderate correlation coefficient estimate (0.5) among pre- and post-values of the variables. Some studies evaluated HRV variables in natural log-transformed values, so to remove variability, we took antilog values of these variables. A meta-analysis was carried out in R (CRAN) software (R Studio, Inc. Software Company, Boston, Massachusetts, United States) using the metafor package. Data were presented as MD ± 95% of confidence interval (CI) with a statistical significance of P < 0.05. A separate meta-analysis was conducted for each primary variable (RMSSD, SD of RR interval, LF power, HF power, total power, and LF/HF ratio).

 Results



A total of 4430 initial types of research were identified after the removal of duplicate studies. Seven studies were systematically reviewed, which met the eligibility criteria. These seven articles were assessed qualitatively [Table 1] and quantitatively [Table 2] which included six randomized controlled trials and one nonrandomized trial. The included studies were published between the years 2003 and 2018. [Appendix Table 1] represents Variables assessed in the included studies and [Appendix Table 2] represents Detailed assessment of the studies based on physiotherapy evidence database scale.{Table 1}{Table 2}[INLINE:1[]INLINE:2]

Quality assessment

The quality of the included studies was assessed using a 11-point PEDro scale. The mean score of the PEDro scale was 6.71, ranging from 4 to 10. Two studies were of fair quality,[44],[45] four were of good quality,[42],[43],[46],[47] and one was of excellent quality.[48] Most frequently, the score points which were not found in the study were blinding of all the participants (item 6), blinding of all the therapists (item 7), and blinding of all the assessors (item 8).

It was found that there was a significant increase in RMSSD and HF power and a significant decrease in the LF/HF ratio. Overall, RMSSD increased after the interventions of HIT having MD = 6.13, 95% CI =. 1.74, 10.52. Four studies[42],[45],[47],[48] showed an increased RMSSD, while one study[46] showed a decrease in RMSSD. The results for SDRR showed no statistically significant difference after HIT program.[44],[46],[48] However, there was an increase in overall SDRR value (MD = 12.01, 95% CI = -3.41, 27.44). Results of the frequency domain suggested that there is an overall increase in LF power with MD = 166.78, 95% CI = -108.74, 442.31 (but no statistical significance was found in any study). Three studies[43],[44],[48] explained that there is a decrease in LF power, indicating a decrease in sympathetic modulation, while two studies[42],[45] indicated an increase in LF power. Regarding HF power, there were four studies,[42],[43],[44],[45] which showed an overall increase with MD = 108.02, 95% CI = -66.90, 282.93, whereas only one study[44] showed a significant increase in the variable. LF/HF ratio decreased overall with MD = -0.00, 95% CI = -0.40, 0.39. Three studies[42],[46],[48] showed an increase in LF/HF ratio, whereas the study of Shihang et al.[48] showed a reduction in LF/HF ratio.

Characteristics of subjects

Characteristics of the participants are described in [Table 1]. Overall, 87 participants were considered for the study as only 87 out of a total of 231 participants in the studies received intervention according to our eligibility criteria. The mean age of the participants was 24.79 years, ranging from 19.3 to 30 years. Four studies described their mean body mass index which ranged from 19.9 to 29.7 kg/m2. One study has not discussed it. One study has described body composition in the form of body weight 72.08 Kg and height 175.44 cm. One article presented VO2 max as 62 mL O2/min/kg. All the studies had different interventions based on their segregation into different groups.

Characteristics of interventions

Characteristics of the intervention are also explained in [Table 1]. Most of the studies used cycle ergometer for training, some used treadmill running, and two studies used terrain training and swimming as they have chosen precompetition training period in triathletes.[42],[46] Each study has involved training at a high intensity of 80%–90% of their maximal HR or at VO2 max > 70%. One study did not give a detailed assessment of intensity parameters, but they have explained the training as high intensity.[48] HIT intervention was given 2–6 times a week for the duration of 20–60 min in one session. The mean duration of the HIT session was 3.71 weeks.

The authors in the study described the performance of HRV measurement upon waking up in supine and standing positions during each week.[42],[44] Before and after swimming in the supine position;[34] in the morning, 24 h before training and 24 h after the training in supine position;[46] at rest during seated position before and after the first and last training sessions, respectively;[47] in laboratory condition in sitting position during morning hours,[43],[45] HRV was assessed by either electrocardiography or by a portable cardiac monitor.

 Secondary outcomes evaluated



The secondary outcomes [Table 1] which were observed in this study were HR, mean R-R interval, PNN50%, SDNN, PNS indicator, SNS indicator, total harmonic power, fractal power, SD1, SD2, and VO2 max. One study showed no significant changes in variables such as PNN50%,[46] mean HR, and mean R-R values,[46] whereas another study showed a significant increase in HR, PNS indicator, and fractal power, but no significant difference in total harmonic power and SNS indicator.[44] One study showed a likely decrease in HR.[42] A study done by da Silva et al. showed no significant changes in SD1, SD2, HR, and VO2 max.[45] In one study, no significant difference in SDNN was found. Another study done by de Souza et al. showed a significant decrease in HR, SD1, and SD2.[45] In one study, a significant improvement in mean RR interval was observed.[43]

 Discussion



The current systematic review explains the effect of the HIT training program on HRV variables in young adult sports players and healthy individuals. Five primary measures of HRV were included in the meta-analysis, out of which, two were of time domain and three were of the frequency domain. These variables were RMSSD, SDRR, LF power, HF power, and LF/HF ratio.

Our results showed that when training programs are incorporated with HIT, it causes positive adaptation in cardiac modulation depicted by changes in the time and frequency domains of HRV. The authors of the studies in our systematic review included a duration of 2–4 weeks of HIT session (Carter et al.[44] used two blocks of 4-week HIT sessions) and frequency of 3–4 sessions per week and found significant changes in HRV variables. These findings can be attributed to the increase in stroke volume due to increased blood volume following HIT for 2–4 weeks.[49]

The analysis of RMSSD is not affected by statistical heterogeneity, whereas all the other primary HRV variables were affected by statistical heterogeneity. The reasons for this heterogeneity can be the co-interventions which were given into different studies along with HIT. Thus, it can be stated that RMSSD is a consistent marker among all HRV variables. In the current study, we used MD and 95% CI to describe the effect of intervention as all the studies measured primary HRV variables on the same scale. Overall, RMSSD increased after the interventions of HIT. Four studies[42],[45],[47],[48] showed increased RMSSD, indicating improved parasympathetic modulation, whereas one study[46] showed a decrease in RMSSD following 4 weeks of HIT. This decrease can be seen due to the relatively HF of HIT (6 times/week) and a relative decrease in resting duration, causing central fatigue. This suggests that HIT with less recovery period is harmful for parasympathetic modulation in young adults. The results for SDRR showed no statistically significant difference after HIT program.[44],[46],[48] However, there was an increase in overall SDRR value (MD = 10.15, 95% CI = -0.25, 20.55), suggesting an increase in the parasympathetic outflow.

Results of the frequency domain suggest that there is an overall increase in LF power (but no statistical significance was found in any study). Three studies[43],[44],[48] explained that there is a decrease in LF power, indicating a decrease in sympathetic modulation, while two studies[42],[45] indicated an increase in LF power. Regarding HF power, there were four studies[42],[43],[44],[45] which showed an overall increase, with only one study[44] showing a significant increase in the variable. This shows that an overall increase in the parasympathetic cardiac modulation can be caused by two blocks of 4 weeks of training at the frequency of four times a week of HIT interspersed in it. These results are similar to the results of Nummela et al.[50] who showed that nocturnal increase in HF power is seen after two blocks of 4 weeks of HIT sessions. The ratio of LF to HF power is also an important component of the frequency domain as it depicts changes in sympatho-vagal modulation. The results of our study indicate an overall decrease in the LF/HF ratio. Three studies[42],[46],[48] showed an increase in LF/HF ratio. This increase can be attributed to enhanced functioning of heart due to increased load of HIT in response to the augmented levels of epinephrine and norepinephrine associated with sympathetic activity, while the study of Shihang et al.[48] showed reductions in the LF/HF ratio. These differences in the findings of the studies could be due to relatively larger frequencies of HIT (6 times/week). Altogether, the results showed that the HIT of more than 4 weeks causes an increased sympathetic functioning of the heart. These findings suggest that a block of training between 2 and 4 weeks with HIT is useful for improving the parasympathetic activity of the heart.

Overall, it was found that there is a significant increase in RMSSD and HF power,[44],[47] indicating an increase in the parasympathetic modulation and a significant decrease in LF/HF ratio, indicating a decrease in sympatho-vagal action. Similar results were found in the systematic review of Abreu et al.[40] They also explained that training with the high-intensity program can improve parasympathetic modulation at rest as shown by an increase in RMSSD and a decrease in the LF/HF ratio. Changes in central (stroke volume, cardiac output, and blood volume) and peripheral (mitochondrial content and capillary density in skeletal muscles) factors are dependent on exercise intensity, duration, and frequency.[51]

HIT program is superior to increase parasympathetic cardiac modulation over other training programs such as a moderate-intensity continuous training program.[52] The HIT is associated with increased breathing efforts. Previous studies have proposed that parasympathetic control is increased as a result of the mechanical effect of increased breathing on sinus node.[53] This could be seen as a mechanism as to how HIT might have improved parasympathetic control. The differences in the HRV outcomes are not evident when the HIT regimen was compared with strength training.[54] Thus, further researches are required to compare various programs to examine the benefits of HRV.

The results of the present study show a clear picture that HIT improves HRV variables, especially RMSSD, HF power, and LF/HF power ratio, in young adults and also in athletes of the same age. The meta-analysis showed that the duration and frequency affect HRV adaptations, and those training protocols which included continuous HIT program for more than 4 weeks showed attenuation in these adaptations. The frequency of training for more than four times a week also shows no significant improvement in HRV responses. Thus, a training that involves HIT must be planned accordingly to have beneficial adaptations. The included studies showed that not only athletic but also nonathletic healthy young adults can be benefited from this exercise program. Aeschbacher et al., 2015,[55] showed that a healthy lifestyle is related to a higher HRV and in line with the previous findings, the present study also confirms that the HIT can be recognized as an important training method to influence the autonomic control of the heart. The future implications may involve evaluating the benefits of HIT in the young population having disorders such as obesity, hypertension, and other systemic diseases.

To best of our knowledge, no systematic review has evaluated training adaptations through HRV when training periods are interspersed with the HIT program in the young population (healthy and athletic young adults). Guidance for dose–response relationship is provided in our study, which can guide sports scientists to implement it into their regular schedule of training.

Despite the detailed evaluation, there were several limitations in our systematic review including that this systematic review included a small number of studies, the effect of gender was not seen in the analysis, included studies were affected by heterogeneity, not all the studies were randomized controlled trials, and the participants of the included studies also were involved in training other than HIT. Hence, the response which we got is not solely due to HIT. Future researches can include studies that used only HIT as a training program to determine the effect.

 Conclusion



HIT is a unique program that increases parasympathetic and sympathetic cardiac modulation depending on the duration and frequency of the HIT program. The duration of 2–4 weeks of the HIT training is beneficial for improving the parasympathetic activity of the heart. Thus, HIT should be incorporated in regular exercise practice to gain benefits in the autonomic functioning of the heart.

Practical implication

HIT is a great tool to modify HRV. Sports scientists and trainers must choose a dose of HIT program judiciously to maximize benefits in autonomic cardiac modulations. This review indicates that HIT when performed for 2–4 weeks and 3–4 times a week maximizes the benefits on HRV variables, but durations more than that can cause overreaching and attenuation in cardiac modulation.

Acknowledgment

The study was supported by MYAS GNDU, Department of Sports Sciences and Medicine, Guru Nanak Dev University, Amritsar. This department is funded by the Ministry of Youth Affairs and Sports, Government of India. We acknowledge the support provided.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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