Training with a heart rate monitor: max heart rate, zones and the Karvonen method

In 2011 I wrote a post in Italian about training with a heart rate monitor. Fifteen years later it’s still the most commented post on this site, so here it is in English. I also fixed a few things I got wrong back then.

What a heart rate monitor does

A heart rate monitor counts your heartbeats per minute (bpm) while you exercise. In 2011 that meant a chest strap sending its signal to a watch. Today most sports watches read it at the wrist with an optical sensor.

Wrist sensors are fine for steady running or cycling. During intervals and gym work they lag behind or jump around. If the numbers matter to you, a chest strap paired to your watch over Bluetooth Low Energy or ANT+ is cheap and still more accurate.

Why bother at all? Heart rate tells you how hard your body is working, no matter how you feel that day. Francesco Moser built his training around it in the early 1980s. In 1984 he broke the hour record and won the Giro d’Italia.

Maximum heart rate

Training zones are percentages of your maximum heart rate (HRmax). The best way to find yours is to measure it: a supervised test, or the highest value your monitor records over a few all-out efforts. Most people estimate it instead.

My 2011 post called 220 - age the “Cooper formula”. That was wrong. It’s usually credited to Fox and Haskell (1971), and it was never meant to be precise. A 2001 meta-analysis by Tanaka, Monahan and Seals found a better fit:

HRmax = 208 - 0.7 * age

For a 22-year-old, 220 - age gives 198 and Tanaka gives 193. Either one can be off by 10 bpm or more for a given person. A measured value beats any formula.

Zones as a percentage of HRmax

The simple approach multiplies HRmax by a percentage. These are the zones from the original post, for an HRmax of 198:

Easy is where long, steady sessions live. Aerobic work builds endurance. The anaerobic zone is for short, hard intervals, and you can’t stay there for long.

The Karvonen method, done right

The Karvonen method works on heart rate reserve (HRR): the gap between your maximum and resting heart rate. Your resting heart rate drops as you get fitter, so the zones move with you.

HRR    = HRmax - HRrest
target = HRrest + HRR * intensity

In 2011 I wrote it as 220 - age - resting HR, which isn’t the Karvonen formula, and then used age 20 instead of 22 in the example. Here’s the same example done properly, for a 22-year-old with a resting heart rate of 70:

HRmax = 220 - 22   = 198
HRR   = 198 - 70   = 128
65%   = 70 + 128 * 0.65 = 153
75%   = 70 + 128 * 0.75 = 166

So the easy zone for this person is 153-166 bpm, higher than the 129-149 from plain percentages. That’s expected: Karvonen accounts for the heart rate you already have at rest.

To measure your resting heart rate, check it in the morning before getting out of bed, a few days in a row, and use the lowest value.

About the fat-burning zone

This is the part I’d rewrite the most. In 2011 I wrote that running hard doesn’t burn fat and eats into your muscles instead. That’s an overstatement.

At low intensity a larger share of your energy does come from fat. At higher intensity you burn more energy per minute in total, though, and weight loss depends mostly on your energy balance over days and weeks, not on which fuel you burned during one session.

Easy-zone training is still worth doing. You can do a lot of it, it builds your aerobic base, and you recover from it quickly. Just don’t pick it because of the label.

Disclaimer

I’m not a doctor or a coach; this is what I’ve learned from reading and training. Talk to your doctor before starting, especially if you’re new to exercise or have a heart condition.