If, like a third of Britons, you own a fitness tracker, you may already count your steps, track your sleep quality and keep a tally of calories consumed and expended.
And, reasonably, you might also think that your daily data is comprehensive enough to keep ill health at bay.
But those ‘in the know’ are adding another measurement to their DIY wellness assessment: their heart rate variability, or HRV. Unlike plain heart rate – the number of beats per minute made by your heart – HRV determines the time between each beat, and how that varies.
This is given as an overall HRV ‘score’, which is a calculation based on just how much variation there is.
A higher score, meaning greater variability, is the most desirable. A lower score, meaning less variability, may mean you’ve been exercising too hard – and you need a break.
HRV has long been a parameter used by elite athletes seeking that extra physiological insight to give them the edge over rivals – allowing them to push their bodies to limits, without risking damage from over-training, which can harm the immune system and lead to injury.
But now the trend is creeping into the wider world of fitness.
Ellie Goulding, the fitness-conscious singer, has hinted that HRV is an important aspect of her own regime: ‘I spent lots of time running as a teenager and in my early 20s – 10k a day at least,’ said the 33-year-old.
‘And that gave me a very good heart rate variability.’
There are certainly no shortage of ways in which you can monitor your HRV. It can be recorded on an Apple Watch, some versions of FitBit (the Charge 3, Ionic and Versa), Polar chest-band heart-rate monitors, the Whoop wristband, the Oura ring (spotted on the fingers of Prince Harry and Will Smith), and through apps such as Wattson Blue, which allows you to record your HRV using the camera on your smartphone, and ithlete, which uses a finger sensor to take an HRV reading.
But what does HRV really mean, and is it really worth adding to the ever-growing list of metrics we track?
John Brewer, a professor of sport and exercise science at the University of Suffolk, explains that heart rate is not constant, pointing out that just because your resting heart rate reads as 60 beats per minute, that doesn’t mean your heart beats once every second.
He says: ‘In a nutshell, your heart rate variability is a measure of your heart’s electrical pattern, and the milliseconds between heart beats.
‘And it is prone to fluctuations due to a range of factors including your age, fitness levels, work stress, time of day, diet, the amount of alcohol or caffeine you consume and your general health.’
There is no normal HRV level and no point in comparing yours with anyone else’s.
As a rough guide, Whoop, which makes the fitness tracker of the same name, says that 20 to 25-year-olds usually have an average HRV in the 55 to 105 range, while 60 to 65-year-olds tend to be between 25 and 45. But what really matters is how your figure fluctuates on a daily basis – and the higher it stays, the better. Experts advise measuring in the morning.
If you’ve have overdone it at the gym or if there have been too many ‘stressors’ in your life, your HRV will drop – an early warning sign that you need time to recover.
If, on the other hand, your HRV remains high, it means you have sufficient energy reserves to continue attacking any workout plan with gusto. Jim Pate, senior exercise physiologist at the Centre for Health and Human Performance, says: ‘Stress affects the nervous system, which in turn causes the variability in the time between heartbeats to decrease.
‘It is like having your foot constantly on the accelerator to maintain speed, rather than applying a little and then letting it off to coast.
‘So, in simple terms, lower HRV signals higher levels of mental and physical stress.’ Studies have suggested exercising when HRV is high leads to greater gains in fitness, compared to exercising when HRV is lower.
And elite cyclists who tailored their training to their HRV improved performance significantly during training.
But other researchers have found HRV makes no difference to exercise or sports performance, and Prof Brewer says: ‘The jury is still out. Scientists are still undecided whether it is worth tracking.’
Pate adds: ‘It is one tool to help monitor your workout load, but there are a lot of other factors to consider.’
It should not, says Prof Brewer, become an exercise obsession. ‘Don’t read too much into HRV as it doesn’t tell the whole story,’ he says. Often, the most reliable – if boring – way to tell if you are overdoing things is simply to listen to your body.
Excessive fatigue, poor sleep, constantly struggling in classes or on runs and cycle rides despite no increases in workload are a sure sign as any that your body needs a rest.
‘These classic complaints are often overlooked,’ says Pate.
‘Monitoring your heart rate and HRV can be helpful to some people, but in the end if you are still thinking, “Well, you know… I’m just not feeling right”, then you should give yourself a break.’