In recent years, there have been a number of products on the market that claim to measure your “readiness for training” by tracking something called Heart Rate Variability (HRV). They claim that they’re backed by science, and can help you to optimize your recovery and avoid overtraining. Indeed, there are a lot of articles (often referenced in the marketing for these products, as well as on general fitness websites) that discuss the benefits of tracking HRV. These invariably link to research articles that tend to be technically dense and largely incomprehensible to the average reader.
 
It’s natural to assume that anything that is backed by this kind of complicated science must be legitimate. At a casual glance, it certainly seems a lot more scientific than waking up, deciding that you feel a bit too sore and achy to go training, and taking the day off; but let’s take a closer look and what the research actually says.
 
What is heart rate variability, and why should I track it?
 
Let’s start with a quick physiology primer. We know that heart rate isn’t a constant. It gets faster when we do something strenuous (or even think about doing something strenuous), and slows down at rest. If we do regular training to improve our cardiovascular endurance, we’ll probably find that our resting heart rate drops, and this is usually seen as a sign of improved fitness. These changes in heart rate are controlled by the two branches of our autonomic nervous system, which is the part of our nervous system that affects our physiology and internal organs, without our conscious control. The sympathetic nervous system controls our “fight or flight” reactions, whereas the parasympathetic nervous system puts our body in a state to “rest and digest”.
 
The interesting thing is that it turns out the relative balance between the sympathetic and parasympathetic activity can be measured, and one way of doing this is by looking at small differences in the intervals between heartbeats. For example, when you breathe in, your heart rate increases very slightly; as you breathe out, it decreases again. These changes can be analyzed using some fairly tricky math, and they can give an indication as to the relative dominance of each branch of the autonomic nervous system.
 
So far so good, but what does that have to do with overtraining?
 
Good health and performance both require the right balance between sympathetic and parasympathetic activity. It’s not your training that makes you fitter and stronger; it’s the rest afterwards that allows your body to adapt to the stress put on it (this process is known as supercompensation). If your body doesn’t get the chance to do this properly then your performance deteriorates, and you might become more vulnerable to infections or other health problems. Under-recovery can also lead to changes in hormonal levels and autonomic balance.
 
As an athlete training intensely for an important event, it would be really useful to have an objective and foolproof way to tell when you’re approaching a physiologically fatigued state and need to rest. You don’t want to skip training every time you feel a bit tired, but nor do you want to get sick or risk a decrease in performance at a crucial event because you’re doing too much.
 
That’s where heart rate variability comes in. The theory is that changes in measurements of HRV can be used to detect shifts in autonomic balance, and this can be used to determine whether an athlete is in the early stages of overtraining.
 
It turns out that the devil is in the detail, though, and the details here are devilishly complex.
 
First of all, the physiology of overtraining is still not completely understood. It appears that the characteristic autonomic imbalance associated with fatigue states can go in either direction; in some cases the sympathetic branch becomes more dominant, but other cases are accompanied by an increase in parasympathetic dominance. This means that it’s not as simple as “a higher HRV is better.” Although HRV generally tends to increase in athletes as they become fitter, it turns out that changes in HRV in either direction may accompany the early stages of overtraining.
 
Correlation is not causation, revisited
 
There’s an increasing body of research that shows a relationship between HRV changes and various other physiological markers of overtraining states. Many of the studies involve fewer subjects than we’d ideally like to see - remember that smaller studies tend to produce statistically less reliable results. (This tends to be a problem with a lot of sports science research in general.) What we do have suggests that HRV analysis may indeed provide a good early indicator of overtraining; but despite its growing popularity amongst athletes, this kind of relationship isn’t enough to prove its validity.
 
What’s missing from the evidence are large, well conducted scientific studies that compare athletes using one of these HRV trackers against a those following a standard training plan, or better still, against athletes using more traditional methods of measuring overtraining. If tracking HRV could be shown to give better training results, fewer cases of overtraining, or some other measure of improved health, then that would be well worth paying attention to. Although there have been a few studies that have attempted something along these lines, they have been small and ultimately not terribly conclusive - although they suggest that the approach may be promising.
 
And another thing…
 
When you buy an app that claims to measure “heart rate variability,” what is it that it’s actually measuring? HRV isn’t like measurements of heart rate, which should come out the same regardless of which tool you use; the math behind HRV is complicated and there isn’t one single measurement of HRV - there’s a whole stack of parameters that emerge from the math. In other words, the answer you get depends on exactly how you do the analysis. It’s not immediately obvious from the research which formula will give us the most useful answer to the question “should I go ahead and do my normal workout today?” - especially without prior knowledge of your individual training history and goals. The algorithms used by these apps to determine a measurement of heart rate variability are generally proprietary - meaning that it’s hard to be sure what you’re getting.
 
The verdict?
 
I’m going to go with “promising but unproven” for this one. The science makes sense, and there are some preliminary results that suggest that the idea has potential. I think it’s too soon to claim, though, that this is a scientifically proven method for tracking an athlete’s physiological readiness for training, especially if you’re talking about forking out for one of the commercial gadgets on the market.
 
A review from 2006 stated, “At present, available data does not allow for final conclusions with respect to the usefulness of traditional HRV indices in assessing an individual's exercise performance and monitoring training load. The discrepant results published so far are due to several factors including insufficient study size and design, and different HRV methods. Large-sized and prospectively designed studies are necessary for clarification.”
 
Although more research has been accumulated since then, a recent review article in 2016 still offers a cautious conclusion: “Referring to athletes, changes in the patterns of their ANS reflected by altered HRV may serve as useful parameters for managing their physical fatigue and establishing their exercise intensity… However, standard protocols and methods for research with athletes should be established considering the intensity, the duration of exercise…”
 
If you want to try out an HRV tracker, then go ahead - but there’s no guarantee that it’ll work any better than more traditional methods of measuring overtraining (such as those described here). The real training geeks amongst you may want to cross check your HRV results with those of another method, for example, the recovery stress questionnaire for athletes. I’d love to know how the results turn out!