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Heart rate variability (HRV)

For more information:

HRV as an indicator of pre-competitive arousal

Changes in heart rate recovery after HIIT in well trained cyclists

Heart rate variability - Cycling Fitness Winter 2013/4 (coming soon!)

Dan's blog on his HRV PhD studies (coming soon!)

Heart rate has long been used to guide and inform endurance training. The price of today’s heart rate monitors (~£20 for the cheapest!) makes its measure and use for training very accessible. Although heart rate does vary with dehydration, heat or humidity, alongside daily variations, heart rate remains a very popular and very useful, objective measure of training intensity1.

While the response of the heart during exercise is now better understood by physiologists, the exploration of the physiological mechanisms underpinning its control is now a hot spot in sports science. A deeper insight into the way the heart is controlled by the central nervous system, via the measure of heart rate variability (HRV), offers a non invasive alternative for assessing physiological and psychological degrees of fatigue as well as preparedness for performance in athletes.

What is HRV?

Heart rate variability (HRV) refers to the fluctuations of heart rate around an average value. While the rhythmic beating of the heart at rest was once believed to be monotonously regular, we now know that it is actually surprisingly irregular (Figure 1).

HRV_explanation

Figure 1: Two individuals with the same resting heart rate of 60 beats.min-1 but with less (top) or more (bottom) beat-to-beat variability.

Heart rate variability (HRV) was first derived from electrocardiogram recording (ECG). The heart is a muscle capable of generating its own electrical impulse from its centre (the sinoatrial node) through its four chambers so that the muscle smoothly contracts for the delivery of deoxygenated blood to the lungs, and oxygenated blood to the main circulation. The “R spikes” on an electrocardiogram precedes the main beating of the heart and the expression “R-R intervals” is another term to refer to HRV (Figure 1). R-R intervals oscillate around a mean value and it is these changes between successive beats that are analyzed to provide an insight into the nervous control of the heart and possible state of fatigue.

Rest vs exercise?

As HRV will provide an insight into the way the nervous system is controlling the heart beat, any parameters affecting the nervous system will affect HRV (noise, light, emotions, etc). Therefore, it is better to record HRV in a rather relaxed state.

The analysis of HRV could offer a way for sports scientists to better understand how the heart is nervously controlled during exercise. But although the recording of HRV is easy during exercise (see equipment below), the editing and interpretation remains extremely complex1. This is the reason that only HRV at sub-maximal intensities (easy pace; below lactate threshold) has been studied so far2, 3. Methods3 that have been presented in the literature for editing and interpreting exercise R-R intervals are still under scrutiny although they have not received as much attention as resting HRV.

24-hour recording was traditionally used in HRV analysis4 (clinical situations) but segments of shorter periods, usually 5 minutes (the minimum), are now recognized as sufficiently long for a good insight into HRV5. Since the signal has to be stationary during the recording, it is highly recommended to control the environment (dark and quiet room), do the measure in the hour following waking with no breakfast (no digestion), and keep the breathing frequency1 constant at 12 breaths per minute (or one cycle – i.e. inhalation and exhalation – every 5 seconds). Some familiarization will be required to keep this rather high breathing frequency in athletes. Supine as well as upright position can be studied as long they are reproduced consistently6. As you can see, this procedure is not too strenuous (rest; 10 min in total; first thing in the morning) and is attractive for athletes HRV screening if the interpretation of the signal provides a valid quantitative and qualitative insights into states of nervous fatigue.

Heart rate variability application

Originally, HRV was analyzed from ECG recording. Recently, a practical and user-friendly system for recording R-R interval at a high sampling (1000 Hz) has been developed. It is a simple heart rate monitor from the Polar brand, which also provides software to download and edit (removing the ‘noise’) the signal (Figure 2). The validity of the polar when compared to ECG appears good enough for its use in resting supine position6, but not convincing in resting standing position6 or during exercise7 but further studies should implement these findings.

There aren’t many studies on the reliability of HRV measure8, especially with the polar monitors. This is surprising as reliability does need to be addressed to ensure the experimental procedure produces consistent HRV results from one day to the other. So far, it seems that healthy subjects display better reliability than clinical populations9 and paced breathing improves reliability8.

Analysis and meaningfulness of the HRV signal

HRV_analysis_steps

Figure 2: Steps to follow when investigating HRV from the recording to the interpretation of the signal

New software for the advanced analysis of HRV presents the use of polar with apparent ease (Figure 3; "processing”). Clear pre-set reports can be rapidly printed (pdf), which facilitates the presentation of results to athletes. “time-domain parameters” such as the mean and standard deviation of the overall R-R intervals values, as well as “frequency-domain parameters” can be easily obtained, the later being proposed to provide a good overview of the nervous control of the heart. But the interpretation of these parameters remains difficult.

HRV_frequency_analysisOut of the several parameters computed, it is possible to estimate the extent to which the two major and complementary parts of the nervous system are involved in the control of the heart: The parasympathetic system could be viewed as the “brake” or the “parachute”. It functions with actions that do not require immediate reaction. It is permanently active at rest to slow down the heart rate. Without its activation, our resting heart rate would actually be around 120 beats.min-1. When starting exercise, it becomes less active and even less so when intensity increases. Activated, this system makes the R-R intervals less stable, increasing the differences between subsequent R-R intervals, and likewise affecting the high frequencies of the HRV signal. It makes the R-R intervals more variable, the low frequencies of the signal being increased too (Figure right).

The sympathetic system is always active but becomes more active during times of stress (therefore during exercise too). For an analogy, it works as “an accelerator”. This system affects the low frequencies of the HRV signal (Figure right).

Heart rate variability and state of fatigue

One of the most pronounced cardiovascular adaptations to endurance training is a lowered resting heart rate, which has been proposed to occur through an increase in the have indeed concluded an enhancement of this activity with endurance training1. While resting heart rate has been suggested as monitoring under-recovery, changes of a few beats remain difficult to explain for physiologists. Alongside resting heart rate, analyzing HRV would enable the physiological mechanisms responsible for the changes in resting heart rate to be depicted. Even after a regenerating period, heavy training can induce changes in the overall nervous control that can result in under-performance and HRV could help screening the changes in the nervous control1.

HRV importance lies in its potential to identify overtraining syndrome. Indeed, the dysfunction or imbalance between the sympathetic and parasympathetic nervous system has been presented as one reason for the signs and symptoms of the overtraining syndrome. Recent studies have found an increase in sympathetic tone in endurance athlete at the end of a 6-9 week protocol10, and 3 weeks of heavy training shifted the cardiac nervous balance toward a predominance of the sympathetic over the parasympathetic system11. In the latter study, during a week following the heavy training, there was an abrupt ‘compensation’ (or ‘over’ recovery!) marked by a dramatic increase in HRV, associated with a relative increase in the parasympathetic drive and a decrease in the sympathetic drive. It has been hypothesized that during the early stage of the overtraining syndrome, the sympathetic system is continuously altered, whereas during advanced overtraining the activity of the sympathetic system is inhibited, resulting in a marked dominance of the parasympathetic system12. Monitoring HRV throughout a season would enable to depict an overtraining syndrome in its early stage.

REFERENCES

1.   Aubert et al. Sports Med 2003, 33, 889-919.

2.   Cottin et al. Med Sci Sports Exerc 2004, 36, 594-600.

3.   Perini et al. Eur J Appl Physiol Occup Physiol 1990, 61, 143-148.

4.   Myrtek. Int J Psychophysiol 1990, 10, 117-123.

5.   Eur Heart J 1996, 17, 354-381.

6.   Gamelin et al. Med Sci Sports Exerc 2006, 38, 887-893.

7.   Kingsley et al. Int J Sports Med 2005, 26, 39-44.

8.   Pinna et al. Clin Sci (Lond) 2007, 113, 131-140.

9.   Sandercock et al. Int J Cardiol 2005, 103, 238-247.

10. Uusitalo et al. Clin Physiol 1998, 18, 510-520.

11. Pichot et al. Med Sci Sports Exerc 2000, 32, 1729-1736.

12. Kuipers. Med Sci Sports Exerc 1998, 30, 1137-1139.