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You’re scheduled to do a big workout today – maybe it’s a long run, maybe it’s Leg Day – but you don’t have the usual bounce in your step. Maybe you should just take it easy, let your body recover and go hard tomorrow. Would that be a smart move, or just lazy?

That’s a tricky decision even for elite athletes, and sports scientists have long sought some sort of impartial biomarker that would reveal whether you’re sufficiently recovered to train hard again. After experiments with options ranging from brain waves to body temperature, the hottest candidate these days is a parameter called “heart-rate variability,” or HRV – and a new review in the Journal of Science and Medicine in Sport gives it a cautious endorsement.

Even at rest, your heart never has a perfectly regular beat. If your pulse is 60 beats per minute, you’ll sometimes have 0.99 seconds between beats, other times have 1.01 seconds, and so on. HRV is a measure of how much your pulse is fluctuating from its average value.

HRV is interesting to sports scientists because it depends on the balance between two components of your autonomic nervous system. When the sympathetic branch of the nervous system, associated with the fight-or-flight response, is activated, HRV decreases. When the parasympathetic (or rest-and-digest) branch takes over, HRV rises. As a result, a high HRV when you wake up in the morning suggests that you’re recovered and ready to train, whereas a low value suggests you might need a break – in theory, at least.

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For the new review, a research team led by Peter Düking of the University of Würzburg in Germany, working with colleagues from several institutions around the world including the University of Ottawa, pooled the results of eight studies of HRV-guided training. Subjects typically monitored their HRV every morning, and adjusted their planned training intensity based on whether their HRV was higher or lower than the previous day.

Compared to ordinary pre-planned training, this approach produced clear improvements in physiological parameters such as lactate threshold, but no statistically significant gain in actual performance in cycling and running time-trials.

There was a key detail, though: the number of “non-responders” who didn’t see any increase in fitness (or in some cases lost fitness) was lower in the HRV group. In other words, HRV-guided workouts didn’t necessarily make a successful training block any better, but it reduced the chances of an unsuccessful training block thrown off the rails by inadequate recovery.

So how do you implement HRV-guided training? First, you need an accurate measurement, which you can get from a chest-mounted heart-rate monitor, a smart watch or an app like HRV4Training that uses your phone’s camera to detect the pulse in your finger.

“Context is king when interpreting HRV values,” Düking says. Work stress, a cup of coffee, exercise and other factors can push your values up or down, so it’s best to take a measurement as soon as you wake up in the morning.

Given all these sources of variability, it’s also important not to overreact to a single day’s measurement. Marco Altini, the data scientist who developed HRV4Training, suggests comparing your reading to a moving 60-day average (which he calls your “normal”) and a moving 7-day average (your “baseline”). If a single day’s reading is below normal, take it as note of caution and see how you feel. If your 7-day baseline drops below normal, take it as a stronger signal to take an easy day.

The sensitivity of the HRV measurement to a wide variety of stimuli can also be considered a strength, Altini notes. If you’re stressed out at work or up late with a sick child, it will show up in your HRV reading. “Training is hardly the only stressor in anyone’s life,” he says, “and it all affects us physiologically.”

Still, neither Altini nor Düking suggests letting the data make all your decisions. It’s still important to have a sensible training plan, to pay attention to how you feel subjectively and to consider what else is going on in your life. “HRV is only one out of many parameters which can give information to coaches and athletes,” Düking says. “We still need human intelligence in the training procedure.”

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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