AUTONOMIC NERVOUS SYSTEM
HEART RATE VARIABILITY (HRV) is a well-established marker of autonomic nervous system function, both sympathetic and parasympathetic.
The SYMPATHETIC NERVOUS SYSTEM prepares the body for stressful or emergency situations, that is, for fight or flight. Therefore, the sympathetic system increases the heart rate and the force of heart muscle contractions and widens (dilates) the airways to make breathing easier. It causes the body to release stored energy. Muscle strength increases. This system also produces sweat in the palms of the hands, dilation of the pupils and erection of hair. It slows down body processes less important in emergency situations, such as digestion and urination.
However, the PARASYMPATHETIC NERVOUS SYSTEM controls body processes during ordinary situations. Typically, the parasympathetic system is dedicated to preserving and restoring. Slows heart rate and lowers blood pressure. Stimulates the digestive tract to process food and eliminate waste. The energy from the transformation of food is used to restore and form tissues (1).
In the current society of “I CAN DO EVERYTHING” the sympathetic nervous system is predominant over the parasympathetic nervous system. We are SELF-EXPLAINED BEINGS which makes us constantly on alert, preventing our body from being preserved and repaired. (I RECOMMEND READING THE FATIGUE SOCIETY BY AUTHOR BYUNG-CHUL HAN).
WHAT YOU NEED TO KNOW ABOUT HRV
HRV is the VARIABILITY BETWEEN HEARTBEAT AND BEAT, that is, when someone has a resting heart rate of 60 beats per minute, we would think that the heart beats at a frequency of 1 beat per second, but in reality, there is variation in time between beats, for example there could be a 0.8 second interval between 2 beats and then a 1.2 second interval between the next etc. This variability between heartbeats is important because it is indicative of the strength/input of the parasympathetic nervous system, the body’s rest and repair system. Therefore, HRV will allow us to assess and evaluate our sympathetic nervous system, allowing us to know how our health is. Healthy people will have scores of 40 to 70, and top endurance athletes may have numbers of 90 to 100 (or even higher). Conversely, less healthy people will have numbers in the lower 40 range, and anything below 20 may be cause for concern. It will always depend on age, that is, the older we are our HRV will always be lower (2).
High HRV states show good health, while REDUCED HRV, which reflects the imbalance of the sympathetic and parasympathetic system (i.e., increased sympathetic activity or reduced vagal activity), is associated with with cardiovascular risk factors such as physical inactivity, hypertension, diabetes and cardiovascular diseases (CVD) themselves.
Imbalance of the autonomic nervous system, with a shift towards increased sympathetic tone and decreased sympathetic tone, has been shown to be associated with an increased risk of CARDIAC MORTALITY. Therefore, HRV has become an important and well-recognized tool for identifying patients at risk of cardiovascular death (3).
WHAT IS HRV USEFUL FOR IN AN ATHLETE?
It has been proposed that training prescription according to the state of cardiac vagal activity, measured through HRV, may be a beneficial method to improve adaptation to resistance training, since it has been widely observed that vagal activity of the autonomic nervous system is related to individual adaptation to resistance training.
This means that a demanding workout will be better assimilated by the athlete with a normal HRV than a day that has, for various reasons, a low or extremely high HRV.
In RESISTANCE SPORTS, the deviation of HRV is a non-invasive marker of acute and chronic adaptation to resistance exercise. In the short term (e.g., within 48 hours of exercise), recovery of HRV to baseline is thought to coincide with restoration of thermoregulatory, metabolic, hemodynamic, and fluid balance-related processes. that are altered by physical effort (4.5).
Over the long term (e.g., weeks or months), HRV profiles that reflect higher and/or more stable resting values have been associated with greater improvements in post-intervention fitness outcomes among sedentary, moderately trained, highly trained and clinical populations.
Traditionally, HRV index has been measured with ECG, and quantified by RMSSD. Currently, the development and validation of new applications (i.e. smartphone applications: Kubios-HRV, Elite-HRV, Mobile Lab or HRV4Training) facilitate daily HRV measurements and their quantification and thus individual adaptation of training loads and recovery (6).
HOW TO IMPROVE HRV?
Exercise training improved HRV in patients with Type II Diabetes and hypertension (as we know that hypertension is strongly related to low HRV), with a decrease in sympathetic activity and an increase in parasympathetic activity. Resistance training demonstrated the strongest benefits on HRV parameters, but the benefit of aerobic training for increasing HRV has also been demonstrated.
In addition, improving sleep and correct eating habits by avoiding the consumption of alcohol, tobacco and drugs will also contribute to its improvement (7).
BIBLIOGRAPHY
1. Cygankiewicz I, Zareba W. Heart rate variability. En: Handbook of Clinical Neurology. Elsevier; 2013 p. 379-93. Disponible en: https://linkinghub.elsevier.com/retrieve/pii/B9780444534910000316
2. Tegegne BS, Man T, van Roon AM, Snieder H, Riese H. Reference values of heart rate variability from 10-second resting electrocardiograms: the Lifelines Cohort Study. Eur J Prev Cardiol. 2020;27(19):2191-4. Disponible en: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7734556/
3. Kleiger RE, Miller JP, Bigger JT, Moss AJ. Decreased heart rate variability and its association with increased mortality after acute myocardial infarction. Am J Cardiol. 1987;59(4):256-62. Disponible en: https://linkinghub.elsevier.com/retrieve/pii/0002914987907958
4. Vesterinen V, Nummela A, Heikura I, Laine T, Hynynen E, Botella J, et al. Individual Endurance Training Prescription with Heart Rate Variability. Med Sci Sports Exerc. julio de 2016;48(7):1347-54.
5. Javaloyes A, Sarabia JM, Lamberts RP, Moya-Ramon M. Training Prescription Guided by Heart-Rate Variability in Cycling. Int J Sports Physiol Perform. 2019;14(1):23-32.
6. Granero-Gallegos A, González-Quílez A, Plews D, Carrasco-Poyatos M. HRV-Based Training for Improving VO2max in Endurance Athletes. A Systematic Review with Meta-Analysis. Int J Environ Res Public Health. 2020;17(21):7999.
7. Picard M, Tauveron I, Magdasy S, Benichou T, Bagheri R, Ugbolue UC, et al. Effect of exercise training on heart rate variability in type 2 diabetes mellitus patients: A systematic review and meta-analysis. PLoS ONE. 2021;16(5):e0251863. Disponible en: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8128270/