During their sporting lives, athletes must face multiple difficulties and different kinds of stress, that can have consequences for their health and level of performance(1). When they experience changes or challenges (stressors), their body produces physical and mental responses. Stress is a normal human reaction and the human body is designed to experience stress and react to it (fight-or-flight response). But every player’s body reacts differently to physiological and psychological stressors, this is why discovering the ideal “stress-recovery balance” (2) for an individual has a significant impact on their ability to handle training loads effectively, perform optimally, develop quickly, and avoid certain injuries.
Among others, heart rate variability (HRV) is a method for assessing the effects of stress on our bodies. It is generated by heart-brain interactions and the autonomic nervous system (ANS) processes. A healthy heart is not a metronome, the oscillations of a healthy heart are complex and constantly changing, which allow the cardiovascular system to rapidly adjust to sudden physical and psychological challenges to homeostasis (3). These variations(4) among consecutive heartbeats (the R-R interval or “NN interval” to emphasize that the heartbeats are normal, beat intervals that differ more than 50ms) over time represent the HRV (Fig. 1).
This beat-to-beat variation in heart rhythm is considered normal and desirable, and research increasingly shows that higher HRV is associated with a better physical fitness(5) , with improved
quality of life(6), or good health and reduced mortality(7) , while decreased HRV is linked to stress, fatigue and even burnout. It reflects the impact of multiple sources of stress on the athlete, not just training, but also sleep, nutrition, mental, social problems and emotional stress.
The autonomic nervous system has two main divisions, the Sympathetic and Parasympathetic, it controls not just the heart but supplies many internal organs, including the blood vessels, stomach, intestine, liver, kidneys, bladder, genitals, lungs, pupils, sweat, salivary, and digestive glands. After the autonomic nervous system receives information about the body and external environment, it responds by stimulating body processes, usually through the sympathetic division, or inhibiting them, usually through the parasympathetic division. With some of the HRV parameters, we can try to understand if the stress response system can adjust better to internal and external stimuli.
There are several types of arithmetic manipulation of R-R intervals to represent HRV(8). One of the main divisions for how HRV is calculated is by either using time or frequency. The time-domain analysis represents basic mathematical and statistical measures such as RMSSD (root mean square of the differences) and SDNN standard deviation of adjacent NN (normal to normal) intervals. The frequency-domain or power spectral analysis represents the distribution of variation in HR as a function of frequency(9).
Any of these parameters are associated with ANS activities. Tracking them consistently and matching their modifications with daily activities can provide important information about sympathetic and parasympathetic modulation and the balance between them. Hereunder are the most used HRV measures in sports training (Fig. 2).
Time domain measures of HRV
Value Units Meaning Mainly associated with
SDNN ms Standard deviation of normal-to-normal RR-intervals Total Variability
RMSSD ms Root Mean Square of the Successive Differences Parasympathetic
Frequency domain measures of HRV
Total Power ms² Entire spectral area, < 0.40 Hz Total Variability
VLF ms² Very Low Frequency, 0.003 – 0.04 Hz Pre-Frontal Cortex
LF ms² Low Frequency, 0.04 – 0.15 Hz Sympathetic
HF ms² High Frequency, 0.15 – 0.4 Hz Parasympathetic
How to properly use HRV in sports
Nowadays the HRV is becoming one of the most used training and recovery monitoring tools in sports(10). The possibility of applying HRV in such variety is based on the fact that cardiovascular autonomic regulation is an important determinant of training adaptations, before also being responsive to training effects(11).
HRV can be easily assessed in athletes with different devices, some of them are portable, like wristwatch monitors, HR belts, smartphones with a camera and dedicated app, rings, or with more sophisticated technology like the PPG (photoplethysmography). A less practical HRV assessment can also be done ECG (electrocardiogram) device. Nowadays with improved analysis techniques, one of the biggest advantages of HRV analysis is that the day-to-day variability of bodily responses to continuous training processes can be assessed with minimum time and effort, with no expensive equipment or skilled lab specialists.
But for optimal and reliable results, it’s very important when this measurement is detected, and how is interpreted.
For this reason, it is very important to follow these suggestions:
1. Be consistent: use stable and repeatable conditions daily, without the influence of external factors. This is why one of the best moments to take your HRV measurement should be right after waking up, possibly while still in bed, in a comfortable supine or seated position. Another option is night-long measurements, but they are usually not practicable with any technology listed above.
2. Use the same device and protocol: there are different standard protocols in measuring the HRV, depending on which kind of technology is used and which kind of analyses are supposed to be done. From the ultra-short-term HRV measurement method, demanding only 1 min of data acquisition after the stabilization period, to the 3 min supine, 5 min seated or alternating supine, and standing position in a 10-min orthostatic test(12). Embrace the one that is more useful for you and applies to your needs and time availability.
3. Create your average baseline: scientific literature provides some ranges that could be used as indicators, but every person is different due to many factors, like age, gender, size, genetics, etc.(13). It takes at least 5-6 weeks to get a reliable baseline.
4. Analyze trends: Day-to-day guidance based on acute HRV changes can certainly be helpful to make small adjustments to your daily training plan or lifestyle, however, this kind of analysis cannot tell you much about the big picture. How is your overall condition? How are you adapting to a new training phase or change? Analyzing trends can help you answer these questions and provide a better understanding of your overall condition in that specific moment (14).
5. Tags your changes: HRV is a highly sensitive metric, and for a proper analysis it’s very important to associate daily fluctuations with some relevant information such as typology of training, health conditions, behaviors, traveling, weather changes, nutrition plan, or supplementation. Some devices already allow this possibility, and its contribution to a proper HRV interpretation is fundamental.
Monitoring what an athlete does during training sessions or games has been a major part of professional sports for years. Technology is providing more accurate information about external- internal load ratio, but the importance of what happens away from the practice facility should not be underestimated. Everything that occurs between one training session and the next, from what they eat, to how they sleep, impacts an athlete’s ability to recover and perform at the best level. HRV is a non-invasive method used to obtain valuable information concerning physiological changes that occur in the response to training and lifestyle. HRV metrics are relevant in the analysis of stress that the body experiences during training and they help increase insight into physiological recovery after training.
However, due to the high sensitivity of this metric, to have useful and reliable information, standard protocols and methods must be applied by precisely following all indications. Data reading should be done by considering acute and chronic stressors, which can then help to plan effective changes that can influence performance and quality of life.
Start knowing yourself also through the HRV.
Francesco Cuzzolin, Julio Calleja-Gonzalez, Igor Jukic, Baris Kocaoglu, Sergej M. Ostojic, Mar Rovira, Jaime Sampaio.
Cuzzolin, F., Calleja-Gonzalez, J., Jukic, I., Kocaoglu, B., Ostojic, S.M., Rovira, M., Sampaio, J. (2021). Heart Rate Variability (HRV) – the athlete’s health and performance “Black Box”. Euroleague Players Association (ELPA), Performance Advisory Board (PAB), Newsletter #5.
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