In previous articles we have explored the relationship between excessive stress, whether caused by training or other life factors, and the tendency towards Overtraining Syndrome, Overuse Injuries and Acquired Training Intolerance. In all cases we have seen that part of the solution to avoiding these problems is that of adopting the optimal amount of training stress, a sort of “Goldilocks Zone”, in which activity is sufficient to stimulate the required fitness improvements but not too much that it pushes the organism into chronic fatigue, where rest and recovery are enough to allow gains in strength, speed and endurance but not too much that there is a decrease in fitness.
Nowadays most athletes are aware that observing changes in heart frequency at rest and during exercise or stimulation is a way of judging rates of performance, recovery and health. The first written descriptions of heart rate (measured by the pulse) are found in the writings of the ancient Greek physician and scientist Herophilus, (ca. 335- ca. 280 BC).
These theories were later expanded by the Greco-Roman physician, Galen of Pergamon (131-200 AD), who wrote at least 18 books on the pulse, including 8 treatises that
described using pulse for the diagnosis and predicting the prognosis of disease. His teaching on pulse dominated medical practice for almost sixteen centuries, through the Middles Ages and the Renaissance till the dawn of the modern era. He was the first to report on the effects of exercise on pulse. For example, in “The Pulse for Beginners” he states:
“Exercise to begin with – and so long as it is practiced in moderation – renders the pulse vigorous large, quick, and frequent. Large amounts of exercise, which exceed the capacity of the individual, make it small, faint, quick and extremely frequent.”
This last sentence contains a truth, which most modern day athletes would do well to heed.
Towards the end of the 17th century more accurate measurement of time allowed for more quantitative evaluations of heart rate. John Floyer (1649–1734), an English physician, is credited with inventing what he called the “The Physician Pulse Watch,” a portable clock with a second hand and push-piece that could stop the watch. Using this device, he tabulated both pulse and respiration under a variety of conditions. He published his findings in two volumes and became an advocate of using the timing of the pulse so that “we may know the natural pulse and the excesses and defects from this in disease”.
With the increased availability of accurate time-pieces, periodic fluctuations in the arterial pulse were soon described. In 1733, the Rev. Stephen Hales (1677–1761) reported that the beat-to-beat interval and arterial pressure level varied during the respiratory cycle.
In more modern times and in particular towards the latter part of the twentieth century further developments, in both detection and measuring instruments, have led to more accurate observance and analysis of the heart frequency and variations within the frequency. This has lead to the creation and use of Heart Rate and Heart Rate Variance protocols as valid diagnostic devices in the risk assessment of cardio vascular disease and the treatment of patients post myocardial infarction.
Over the last few years such instruments which, until recently were relatively large, cumbersome and difficult to interpret have been made available to the larger public through the diffusion of inexpensive heart rate measuring devices, the use of Bluetooth technology and the creation of easy to use apps on smart phone and tablet devices.
But what exactly is Heart Rate Variability and why might it be an important monitoring tool for athletes?
From an athletic point of view, we know that the heart rate increases when we exercise, and this is because the heart needs to pump faster to deliver oxygen to our muscles, and we also know that once we have finished exercising the heart rate decreases. This increase and decrease of the heart rate is controlled by the autonomic nervous system (ANS). Within the ANS there are two branches, which directly influence the heart rate. These are the Sympathetic Nervous System (SNS), which accelerates heart rate. This system is also known as the “Fight or Flight” mechanism, which is activated when we have to defend ourselves (physically or mentally) or perform a substantial physical activity. The other is the Parasympathetic Nervous System (PNS), which is responsible for slowing the heart rate. This system is also known as the “Rest and Repair” mechanism, which is activated when we are relaxing, resting, sleeping or just generally “chilling out”. Meditation, deep breathing and gentle yoga exercises also stimulate PNS activity.
It is important to note that neither of these two systems is more important than the other, and that the ideal organic state is achieved when they are working in unison. The SNS should be getting us ready and prepared for important work meetings, psyching us up for an intense training session or contributing to our race performance. The PNS should be making sure that we get the correct rest, that our muscle and mental recovery is as complete as possible and that we are generally as “cool and hip” as possible. Things become complicated when one of these systems become predominant over the other for a prolonged period of time. The classic, and exaggerated, example within the field of endurance sports would be the continuous use of intense training sessions without allowing sufficient rest and recovery. In this case the SNS would continue to be in a state of predominance due to the excessive training stress, creating an imbalance. If this situation is continued for a long enough period the organism will rebel, a state of overreaching will be reached and ironically the PNS will become predominant. Lethargy and inability to train will be the consequence.
Therefore it is clear that an instrument, which can show us whether our athletic body is in balance or is tending towards domination by either of the systems, would be an optimal ally. In the past athletes have utilized the technique of measuring their resting heart rate first thing in the morning as an indicator of readiness for training, and certainly an abnormal increase in the resting heart rate may act as a valid warning sign. However a study made by French and Swiss universities (Relation between heart rate variability and training load in middle-distance runners. Med. & Sci. in Sports and Exercise: January 2000) showed that over a 3 week intense training period the runners experienced only a 3,74 (9%) change in beats per minute at rest, while Heart Rate Variability, which was measured contemporarily, showed a 40% difference in readings. Their conclusions were that “……heart rate variability appeared to be a better tool than resting heart rate to evaluate cumulated physical fatigue, as it magnified the induced changes in autonomic nervous system activity. These results could be of interest for optimizing individual training profiles”.
Heart Rate Variability refers to the very small almost imperceptible differences in time between each heart-beat. Contrary to what would seem logical the greater the differences in the regularity of the heart beat, the more healthy we may consider the body, the heart and the balance between SNS and PNS. The majority of HRV monitors measure these small differences over a period of between 2 to 3 minutes and convert the date into an HRV score through the use of an “RMSSD” (root mean square of successive (adjacent) differences) algorithm, which subsequently reports the value on a logarithmic scale. This gives a number on a scale that runs approximately to 100. There are two important numbers to consider, one is the daily HRV score and the other is the baseline score, which is created over time. The higher your baseline HRV score is, the better. The HRV baseline is not a fixed number; it will change gradually as long as adequate training stress and recovery are balanced, increasing over weeks and months toward your genetic maximum. Conversely, it can also decrease gradually over time, if training or recovery is not sufficient. On the other hand the daily HRV score will show far greater variance depending on the training and recovery cycle. This variance permits us to choose whether it is advisable to undertake an intense training session or whether it is better to take a more conservative approach or even if a rest day is necessary. The majority of apps do this by providing a colour-coded score. Green indicates that the body is ready to handle intense or voluminous training, orange is a warning sign that a more relaxed training approach such as an aerobic session may be preferable whereas red is a sign that rest and recovery is an absolute necessity. However for an athlete a constantly daily green is not necessarily a positive sign, as it would indicate that training stress is not sufficient to elicit improvements. It is better to have a majority of green which indicate that recovery is taking place, interspersed with orange that indicates that activity has been sufficiently intense as to elicit a training response. It is important to note that the HRV score is also affected by other life stressors, such as sleep, mental or emotional states, and even nutrition and hydration.
By tracking both the daily score and the gradual changes in the baseline value, we can be sure of a balanced and gradual improvement in athletic improvement without the risk of overtraining or undertraining. Similarly a stagnant baseline value or worse, a decreasing value, are signs that some issues should be addressed in either the training or life sphere.
HRV levels vary with age so although a score between 60 (not well trained) up to 90 (very well trained, elite, professional sports) on most apps is generally considered the norm, it is important to note that these values will change as we age. To give an example, within the Elite HRV app a baseline value of 55 would be considered a good score for a man in the 55-65 age range while it would be considered a very low value for someone in the 20-30 age range. There are also some variances, in some cases significant variances, in the scores and values produced by different HRV apps, however these are entirely due to the methods of calculation and transformation and do not reflect any difference in accuracy or diagnosis. The scores or values should not be considered as a means for evaluating personal fitness compared to peer groups, due to the fact that each one of us has our own distinct heart rates and patterns. The importance and utilisation lie in the observation of daily variations and on-going changes in the baseline value.
There are numerous apps available which range from freeware (Elite HRV (basic), HRV+) to a small fee (Bioforce HRV, Elite HRV (team), Inner Balance, ithlete, Sweet Beat, HRV4Training). Each app will have different requirements in terms of support technology (smart phone, tablet) and recording device (heart rate strap) so it is important to check the details before purchasing an HRV app.
Following recent developments, and a number of research papers (Extraction of Heart Rate Variability from Smartphone Photoplethysmograms: Rong-Chao Peng, Xiao-Lin Zhou, Wan-Hua Lin, Yuan-Ting Zhang. 2015), into the use of smartphone cameras for HRV monitoring through the use of photoplethysmograms, some newer apps are proposing this technology.
HRV4 Training is an example of this and the founder of this company, Dr. Marco Altini (photo), has just issued a new scientific paper, “Comparison of heart rate variability recording with smart phone photoplethysmographic, Polar H7 chest strap and electrocardiogram methods": D.J. Plews, B. Scott, M. Altini, M. Wood, A.E. Kilding and P.B. Laursen, International Journal of Sports Physiology and Performance, 2017.
Once a suitable app has been chosen it becomes important to choose an optimal timing for the daily reading. This would preferably be each morning, shortly after waking up. This is because circadian rhythms and hormonal fluctuations can cause HRV to change throughout the day, whereas a morning reading should give consistent conditions. It can be done in a standing position, sitting down or supine. The majority consensus is for a seated position, as the supine position may show stronger PNS signals while the standing position, except for exceptionally strong and well trained athletes, will show excessively strong SNS signals, however it is fundamental that the same position is used for all readings. The actual reading takes between 2 to 3 minutes, at the end of which some apps will request you to fill in other data such as duration and quality of sleep, as well as duration and intensity of exercise of the preceeding night and day. This can subsequently serve as useful information in order to understand the influence of sleeping patterns and training or rest on the HRV score.
Each system utilises different graphics, however the daily reading will provide a snapshot of the condition of that day. Here is one of my
daily readings with the EliteHRV app. At that time my baseline value was 54 so with a daily reading of 55 I was slightly in the PSN section of the chart but still clear to go for a training session. If my HRV had been higher, at say 57, the indication would have been in the orange, due to the body undertaking repair and recovery, and a recommendation to either rest or only do low intensity aerobic training. Similarly a low score, for example 51 would put me in the orange area of excessive SNS activity indicating stress and fatigue. Again the recommendation would be for rest or low intensity activity.
Over a period of 10 -15 days a graph can be formed showing the tendency of the HRV scores from which an idea of how training stress is affecting the organism is quite intuitive. Here is a 10 day graph showing variations (bar graph is daily readiness and line graph is HRV scores. The large differences in HRV scores over the days from 14th to 16th February are quite clear. The recommendation on each day was the same, light activity or rest. However looking at the HRV activity we can also see that while there was strong PNS activity on the 14th and 16th, this was separated by a day of stronger SNS activity on the 15th indicating stress and fatigue. Most likely the athlete (me) overindulged in aerobic activity on the 14th producing an uptake in SNS activity on the 15th rather than a return to homeostasis. The line is far more regular over the other days in which SNS and PNS activity are more evenly balanced.
For any athlete keen to get the most out of his training regime while, at the same time, maintaining a healthy profile, the use of HRV monitoring would seem to be a relatively cheap but efficient method. The short monitoring time and immediate biometric feedback are undoubtedly favourable considerations.
If you wish to know more about HRV training or, indeed, get underway with your own HRV monitoring here are a number of company websites, which can help you.
https://elitehrv.com
http://www.hrv4training.com
http://hrvplus.com
http://www.bioforcehrv.com
https://www.myithlete.com