Heart rate variability, or HRV for short, has become popularized by mainstream media and technologies in recent years. The most common example of a piece of technology that many athletes utilize that uses HRV is the Whoop strap; however, there are many other devices that will measure HRV as well. The Whoop strap in particular has gained more and more popularity of late, and I can’t help but wonder how many athletes and coaches really understand HRV and what the research behind it suggests? It’s not as straightforward as you might think. Before we dive into the research behind HRV, however, we first need to discuss some basic heart anatomy and physiology.
The heart has four main chambers (see Figure 1):
Right atrium – receives deoxygenated blood from the body’s periphery
Right ventricle – pumps deoxygenated blood to the lungs where it picks up oxygen
Left atrium – receives oxygenated blood from the lungs
Left ventricle – pumps oxygenated blood to the body’s periphery
Each chamber of the heart doesn’t contract in unison. The different chambers contract at slightly different time points during the contraction of the heart. When these chambers of the heart contract, an electrical signal is produced. An electrocardiogram (ECG or EKG for short) picks up on these electric signals and can depict the depolarization (electrical signal that leads to contraction) and repolarization (electrical signal that leads to relaxation) of each of the heart’s chambers. Figure 2 below depicts a typical and normal ECG.
Many might remember this standard ECG diagram from some of their high school or college anatomy and physiology courses. Essentially, the P-wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T-Wave represents ventricular repolarization. Atrial repolarization is hidden behind the QRS complex as they both occur at the same time. Keep in mind that the depolarization and repolarization of the ventricles and atria are not synonymous with muscular contraction and relaxation. The electrical activity picked up on an ECG is the electrical activity of the heart, not the mechanical activity of the heart. However, heart contraction shortly follows depolarization and relaxation shortly follows repolarization.
Now, why does this all matter you might be wondering? That is because HRV is essentially measuring the time between each R-R interval on an ECG, or in other words, the time interval between each ventricular depolarization of each heartbeat. In a healthy individual, there is a varying duration of time between ventricular depolarizations (or the QRS complex) that is influenced by the breathing cycle. In other words, there is variation in the duration of time between each beat of the heart throughout your normal breathing cycle, also known as respiration cycle.
Why is there variation throughout a respiration cycle? It has to do with the influence that respiration has on vagal tone, or vagus nerve stimulation (1). The vagus nerve controls sympathetic and parasympathetic activity of the body. A decrease in the stimulation of the vagus nerve leads to greater sympathetic activity and an increase in heart rate. An increase in the stimulation of the vagus nerve leads to greater parasympathetic activity and a lower heart rate. Think of sympathetic activity as stimulating a greater stress response and parasympathetic activity as stimulating a greater relaxation response. For example, someone exercising vigorously is in a very sympathetic state and someone doing meditation might be in a very parasympathetic state.
Inhalation causes an increase in pressure within the lungs, which is known to trigger inhibition of the vagus nerve (known as the lung inflation reflex) and an increase in heart rate. Exhalation, therefore, leads to a reduction in pressure within the lungs, a subsequent increase in vagus nerve stimulation, and a decrease in heart rate. This is a very long-winded way of stating that when you inhale, heart rate increases slightly, and when you exhale, heart rate decreases slightly. This is what is known as HRV, or the variability in time between each heart beat during normal respiration at rest.
It is important to understand the underlying anatomy and physiology of heart rate during the respiratory cycle at rest as, like I have mentioned, this is how we measure HRV. Typically, HRV is assessed by a device measuring the electrical activity of the heart or via detecting pulse rate somewhere along the surface of the skin. The former is less commonplace in the real world as it requires being hooked up to an ECG. The latter, however, is how must commercial devices measure HRV. A device is worn at the wrist and detects the pulse of the arteries running along the wrist. It is always measured when at rest and not during activity or exercise.
What is HRV Typically Used for in Athletes?
There has been a plethora of research documenting that greater HRV at rest in adults is associated with better health outcomes and lower risk of chronic disease in general (2). This is because a greater HRV is usually indicative of a greater parasympathetic (i.e., relaxation) state at rest, which is a good thing and is associated with things like lesser total body inflammation and healthier diurnal cortisol patterns. In athletes, however, HRV is used less for determining health outcomes and more in determining fatigue or stress levels. It is postulated that a greater HRV is indicative of a more rested state and a lower HRV is indicative of a state of greater fatigue. At a certain threshold, if HRV gets low enough for long enough, it is thought to be indicative of an overreached or overtrained state. Someone who does no training, or casually exercises for health, and leads a very relaxed lifestyle may carry minimal to no residual fatigue and physiological stress. A dedicated endurance athlete who trains regularly may be carrying around a healthy amount of physiological fatigue and stress on a daily basis. An athlete who pushes their body very hard for a brief period of time,but with adequate recovery between training sessions, and training weeks may begin to carry too much fatigue, putting them in a state of functional overreaching with noticeable performance decrements. This state is considered potentially positive, should the right amount of rest and de-loading occur in the realm of 5-7 days, as when the athlete recovers, they can usually achieve a state of higher overall performance and fitness. If the athlete, however, continues to push themselves, their fatigue and stress levels continue to build to the point where they are in a state of non-functional overreaching, performance starts to decline significantly, and health may be impacted. If the athlete continues to maintain their usual training habits while in a state of functional overreaching, they may eventually exceed their body’s capacity to handle stress and the body then breaks down. This is known as overtraining syndrome, and it carries with it significantly reduced performance and serious health consequences. Full-blown overtraining syndrome can take months or years to recover from! Figure 3 depicts the continuum of fatigue to better put these terms into context.
Figure 3. Continuum of fatigue
Because of the need to manage fatigue and training stress, it is postulated that by measuring HRV on a daily basis, an athlete can use HRV to inform training decisions, namely whether or not to do intense training on a given day based on HRV. For example, if an athlete has a very hard training session scheduled, but their HRV is really, really low, it could be suggested that the athlete do an easier session to promote recovery and attempt that hard training session the next day if their HRV recovers a bit. Many commercial products and devices on the market that athletes use take HRV into consideration along with sleep data gathered from the device to provide a “recovery score” of some sorts through which the athlete can use their best judgement to make training decisions. However, coaches and practitioners working with athletes might just have athletes they work with gather HRV and upload it into a training software so that the coach can make training-related decisions based on their HRV trends.
Keep in mind, this is the theory and general idea surrounding HRV. As we will see next, the research is a bit murkier and less clear cut than what is described above. Let’s dive into this research next.
What Does the Research Say About HRV in Athletes?
Fortunately, there has been an increasing number of studies utilizing HRV among untrained, well-trained, and elite athletes. There have been some studies that have compared the utilization of HRV to guide training and compared this to a “normal” training approach, with some of these studies showing promising results. For example, a 2016 study by Vesterinen and colleagues (4) divided 40 recreational runners into an HRV-guided experimental group and a traditional pre-defined training group. After a 4-week baseline training period, the traditional group trained according to the pre-defined schedule of workouts for 8 weeks, which included 2-3 moderate-to-high-intensity workouts each week. The HRV-guided group also followed an 8-week training plan; however, they utilized a 7-day rolling average of HRV values to determine what session to do on a given day. If HRV values were high and indicative of good recovery, they would go ahead with whatever workout was planned for that day, including if it was a high-intensity workout. If their HRV was lower and they had a high-intensity workout planned, they would do an easy session instead based on the feedback from the HRV measurement. Both VO2 max and 3,000 meter running performance were measured before and after the 12-week study period. The number of moderate and high-intensity training sessions completed was lower in the HRV-guided group (13.2 +/- 6.2 sessions in total) compared to the traditional group (17.7 +/- 2.5 sessions in total), but the HRV-guided group saw statistically significant improvements in 3,000 meter running performance while the traditional group did not. VO2 max significantly improved in both groups. This study concluded that there is potential to use HRV to guide and inform training decisions, namely when to do higher-intensity sessions or when to scratch it for an easier session. Other studies with similar approaches have found similar results (6,7).
These studies, however, were all done on untrained or moderately-trained endurance athletes. There has been research to document that the HRV responses to training vary considerably between recreationally-trained athletes and elite athletes. In elite athletes, HRV has been documented to have no apparent pattern in relation to training load, with some athletes seeing increases in HRV, others seeing decreases in HRV, all despite none of these athletes being in a non-functional overreaching state or overtrained state (3). Even among elite athletes that did achieve a non-functional overreaching state or overtrained state, HRV data remained equivocal, with HRV being increased, decreased, or not changing at all (3).
Finally, it has also been shown that in some recreationally-trained and elite athletes, HRV may actually decrease during training, rebound to be higher than normal pre-training levels during a taper or pre-race recovery period (possibly signaling positive adaptation to training), and then decrease back to normal levels a few days out from a major competition (3).
This may all sound a bit confusing, and it is, as the research is still relatively new and not as clear-cut as we may like it to be yet. There is not necessarily enough sound evidence to definitively say that we should all be using HRV to guide our training decisions, but this is not to say that it cannot still be a useful tool that can give us information that we take into consideration. It is just how you use this information that matters.
So, if you already use a device or are wanting to get a device that takes HRV measurements for you and compiles that data into a graph by itself or combines it with other data to give you a “recovery” score, here is how I would suggest using that device and data:
For the first 2-4 weeks, just get a baseline of your norms, gather the data, don’t use it to inform training decisions, and just watch it. Get used to seeing how HRV or recovery scores change after easy days, hard day, easy weeks, hard weeks, races, etc. Get a feel for how you as an individual respond, as research is pretty clear that everyone has their own unique HRV responses to training (3).
After you have watched and waited for a few weeks, then look at trends in your data over time and start noticing patterns and relationships. If you notice that certain HRV values or recovery scores usually precede a bad string of workouts in which you feel flat or tired, well then this is a relationship to consider and utilize to make a training decision in the future. If you notice your HRV or recovery score drops to a certain level for a period of time before you get sick or start to notice injuries or niggles, this is a worthwhile relationship to take note of and utilize to inform training in the future.
Although there are multiple studies that may indicate HRV-guided training programs can elicit superior performance results to a non-HRV-guided approach, there are still a small number of these studies and some have been done on relatively untrained individuals (which is not the vast majority of endurance athletes). Therefore, I would refrain from skipping or modifying hard workouts on the basis of one single bad HRV value or recovery score value as you very well might have a great workout that day despite what the data tells you. Try not to get too caught up in day-to-day changes in HRV or recovery scores, as these daily changes can sometimes be meaningless. It is more the longer-term trends you would want to take note of. Again, if you start to notice trends over time, low HRV values below a certain point that usually always precede a bad workout, then this would be what you might want to consider and then maybe start to scratch hard workouts on the days in which you see a really low HRV value.
The research is still new and it does not offer us definitive answers yet. Much more research is needed before we go as far as prescribing training based on HRV, and to be honest, I do not know if we will ever, nor should we ever, prescribe training based entirely on a piece of data as there is so much more to how we feel than a single physiological value. However, HRV is a very promising tool for informing training, and it is definitely something worth exploring if you are interested. Just be sure to avoid getting too caught up in the daily fluctuations of the data and to take into account other pieces of data, such as how you actually feel! You never know, your HRV may say one thing, but if you feel great, then let it rip!
Chapleau MW, Sabharwal R. Methods of assessing vagus nerve activity and reflexes. Heart Fail Rev. 2011;16(2):109-127. doi:10.1007/s10741-010-9174-6
Singh N, Moneghetti KJ, Christle JW, Hadley D, Froelicher V, Plews D. Heart rate variability: an old metric with new meaning in the era of using mhealth technologies for health and exercise training guidance. part two: prognosis and training. Arrhythmia & electrophysiology review. 2018 Dec;7(4):247.
Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports medicine. 2013 Sep;43(9):773-81.
Vesterinen V, Nummela A, Heikura I, Laine T, Hynynen E, Botella J, Häkkinen K. Individual endurance training prescription with heart rate variability. Medicine and science in sports and exercise. 2016;48.
Kiviniemi AM, Hautala AJ, Kinnunen H, Nissilä J, Virtanen P, Karjalainen J, Tulppo MP. Daily exercise prescription on the basis of HR variability among men and women. Medicine and science in sports and exercise. 2010 Jul 1;42(7):1355-63.
Kiviniemi AM, Hautala AJ, Kinnunen H, Tulppo MP. Endurance training guided individually by daily heart rate variability measurements. European journal of applied physiology. 2007 Dec;101(6):743-51.
Happy training and racing!
-Ryan Eckert, MS, CSCS
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