Cardio TrainingSports Science

Are heart rate monitor watches accurate?

By November 24, 2017 May 23rd, 2019 No Comments

Which heart rate monitor should I use? Should I buy a heart rate monitor watch? If so, what’s the best type of monitor to track my exercise?

Heart Rate Monitor

These are some great questions but before answering them let’s take a step back and work out why we should monitor heart rate at all during exercise.

The heart beats blood around the body to provide oxygen for muscles to do work, so if the muscles are working harder, more oxygen is required and therefore the heart has to beat more times per minute to match the demand.

In fact, at submaximal exercise intensity you will see a linear relationship between exercise intensity and heart rate. For example, if you are running on a treadmill you’re heart rate will rise in the same proportion as your running speed. See the example below.

Heart Rate Work Chart

If you establish this linear relationship for your own exercise (the relationship will be specific to the mode e.g. running different to biking) you can use it to more accurately set the intensity you should be working at.

If you’re not sure how to do this book in with one of our trainers for a submaximal heart rate test.

So the next question is what is the best way to measure heart rate? Well the gold-standard measure is to use ECG (electrocardiogram) which picks up the electrical firing signals of your heart via electrodes stuck to the skin on your chest.

However, an ECG isn’t that practical for those working out in the gym. These days there are a number of alternative options available, including:

  • Heart rate chest belts that connect to your watch/phone via Bluetooth. These are generally the most accurate alternative to an ECG but you may find it frustrating and/or uncomfortable fitting a strap around your chest every time you exercise.
  • Hand-gripped heart rate monitor (e.g. as seen on the handles of our treadmills, cross-trainers and bike) which will pick up the electrical signals carried across the skins’ surface each time the heart beats.
  • Optical heart rate monitoring technology as used in watches. These watches work on the principal that there are small changes in the colour of the skin every time your heart beats and blood pumps through arteries. Their popularity has grown over the last years mainly because they are convenient as there is not a need to wear a strap.

Now you understand the theory and the technology it’s time to work out which approach is most reliable and accurate. Reliability means that, under the same exercise conditions, the device will give you the same heart rate from day to day.

Accuracy means that the device will give you a true score. So a device can be reliable but not accurate (e.g. one device may always measure 15 beats lower than what the heart rate actually is) but if the heart rate monitor is not reliable it will never be accurate (e.g one day the device may be 5 beats lower and the next day 10 beats lower than actual heart rate).

Typically heart rate chest belts are seen as reliable and accurate. But are optical heart rate watches reliable and accurate?

In the table below you will see some anecdotal testing done at Warwick Workout comparing heart rates collected with Fitbit Surge (optical heart rate) and Polar heart rate belt.

Fitbit Optical mean heart rate Polar chest belt mean heart rate Difference
Intermittent shuttle running on court (17 min) 127 137 7.30%
Seated rest (10 min) 62 66 6.06%
Stationary bike (5 min) 111 115 3.48%

From these results you will see that the biggest difference between Fitbit and Polar occurred with the intermittent shuttle running which involved change of direction, change of speed and was stop-start in nature.

However, on a stationary bike the heart rates were similar and within 5 beats per minute between the watches.

This observation is backed up with research that suggests optical heart rate monitors like Fitbit are ok when the exercise is submaximal and at a steady intensity e.g. riding/walking/biking at a constant speed during aerobic training.

However, the research suggests that heart rates collected with an optical heart rate watch can be unreliable and inaccurate when performing intermittent activity or exercise with changing intensities (e.g. weights, team sport or interval training) as the watch will lag behind and not keep up with the change (see some latest research from Medicine Science in Sports and Exercise, Vol 49, Issue 12).

In summary, here’s the take home points:

  • Monitoring your heart rate during exercise is a great way to determine what intensity you should be working at.
  • There are different heart rate technologies out there: hand-grip, chest strap, arm strap, finger, optical watches. Remember each heart rate technology will have different accuracy and reliability, so don’t mix and match between sessions.
  • Typically optical heart rate watches will underestimate your heart rate.
  • Optical heart rate is ok to use in continuous, constant intensity training (e.g. a 20 min treadmill run) but cannot be relied on when the exercise is stop-start with changes of speed and intensity (e.g. interval training, weight training).

by Dr Laurence Houghton, High Performance Coach at Warwick Workout