Q+A: I can’t do threshold runs. Is my HRM wrong?

Q I’ve been trying threshold running, using a heart rate monitor and keeping my mile pace at 10 seconds below my 10K race pace. However, I struggle to maintain this speed, even though my HRM says I’m working at just 60 per cent of my predicted maximum of 170 (220-50, my age). That’s well below the 80-90 per cent suggested in RW. Am I doing something wrong?

A Heart rate monitors have had a positive impact upon runners’ ability to train ‘smart’. But what many users don’t fully appreciate is the enormous variation between individuals, especially in factors such as maximum heart rate (MHR). I assume that you are using the term ‘threshold’ to refer to the lactate accumulation threshold. Using heart rate to control threshold sessions carries two big assumptions: firstly, that you have correctly estimated your MHR; secondly, that your lactate threshold does actually occur at 80-90 per cent of your MHR.

Maximum heart rate is normally calculated by using an equation such as 2202age, or 2142(0.83age) for men and 2092(0.93age) for women. There’s a great deal of individual variation in MHR, though. For example, the age-predicted MHR for a 25-year-old may be 195, but in reality it can range from 175 to 215. The rate of age-related decline of MHR also varies, which implies that the variation seen at the age of 25 may increase as one gets older. It’s possible that your MHR is much lower than the 170 you state. If, for example, your actual MHR were only 150, 60 per cent of your age-predicted 170 becomes 68 per cent of your actual maximum; this could make a big difference to how the pace feels.

The second assumption is that your lactate threshold occurs at 80-90 per cent of your MHR. If you’re not used to anaerobic training, your lactate threshold could actually be lower than 60 per cent. In my own laboratory we’ve measured heart rates at the lactate threshold ranging between 140 and 185 in a group of male athletes in their 20s; this ranged from 70 per cent to 90 per cent of their MHR.

I suspect that the reason that you’re struggling is that you have either a lower MHR than your age-predicted value, and/or your lactate threshold is relatively low. You could measure your true MHR by doing a treadmill test, although I would only advise this for those accustomed to maximal exertion. Alternatively, Polar ‘M-series’ heart rate monitors offer a facility called ‘OwnZone’, which predicts maximum heart rate from sub-maximal data using a technique based around assessing the beat-to-beat variability of your heart rate.

Dr Alison McConnell, sport and exercise physiologist at Brunel University