Can we kiss the chest strap goodbye?

Products used:

  • Garmin Fenix 5S (wrist-based heart rate)
  • Garmin Forerunner 920XT (strap heart rate)

In the last edition (Volume 24.7), I wrote a Tech Talk Road Test feature reviewing the latest offerings in multisport watch technology – the Suunto Spartan Sport Wrist HR and the Garmin Fenix 5S. Having access to the newest technology my favourite feature of these watches was the wrist-based heart rate.

I quickly fell in love with it. It’s no secret that chest strap heart rate monitors can be annoying, and that’s putting it mildly. So, as you can imagine, I was beyond excited to do away with the old, smelly, sweaty strap because now I had access to wrist-based heart rate technology. In fact, I was that excited about this new technology that I told anyone that would listen about it. Anyone that feigned even the slightest interest: “Margs, that’s a cool new watch,” was soon privy to my spiel about the new technology that I got to play with and test. But not everyone that I encountered was as enthusiastic about this as I was, and soon questions around the accuracy of wrist-based heart rate technology started to surface.

Intrigued by this I decided to look into the accuracy of wrist heart rate technology and to compare wrist heart rate with the old strap heart rate monitor. I got my Garmin Forerunner 920XT out, dug the strap back out from the back of my training draw, changed its battery and started my not super scientific, n=1 study to see how chest strap heart rate monitors compare to the wrist-based heart rate technology of the Garmin Fenix 5S. I do have to add here that I’m not a coach or an expert in this area – this is purely what I’ve discovered and the questions I have attempted to explore.   

GREEN OPTICAL SENSORS: When the watch is placed on your wrist, the optical sensors shine green, and this green low-intensity light shines through your skin and is then able to determine your heart rate.

First up, why bother measuring and monitoring heart rate?

If you’ve been around triathlon for a while, and you work with a coach, you’ll know that coaches often prescribe training sessions based on particular heart rate zones. This is typically written as T1 to T5 in your program, where T1 is the recovery zone (60-70% of your max heart rate), while T5 is referred to as lactate tolerance or maximum intensity (95-100% of your max heart rate). Hitting these particular heart rate targets or zones in training is important if you want to get the most out of your training. Also, in an iron distance event, coaches may suggest sticking to a particular heart rate zone say on the bike (this is typically T2 or T3 – the aerobic zones) to ensure better performance during the marathon. So, measuring and monitoring your heart rate can bevery useful, and for those that don’t have access to a power meter, heart rate is a more accurate metric (compared to just relying on Rate of Perceived Exertion or RPE) to measure effort.       

How does wrist-based heart rate work compare to strap heart rate?

The way wrist-based heart rate technology works is as follows: underneath the watch are green optical sensors, which, when placed on your skin, detect your heart rate. When placed on your wrist the optical sensors turn on and shine green, and this green low-intensity light shines through your skin. The optical sensor is then able to determine your heart rate based on the changing light refracted by the blood flowing through your veins. What’s more, the optical sensors shine green as it is said green sensors better detect heart rate across a broader range of skin tones – interesting.

On the other hand, chest strap heart rate monitors detect the electrical activity that is transmitted through the heart muscle to make it contract. In other words, strap heart rate measure the small electrical impulses given off by your heart as the muscle contracts.

Both methods of measuring heart rate (wrist and strap) require a good connection with your skin to get the most accurate reading. With wrist-based heart rate monitors you need to make sure the watch is on firmly (maybe even on a little tighter compared to what you might be used to) and above the wrist bone, while with the chest strap you need to make sure you have the fit right for a more accurate reading.

Can we do away with the strap?

Don’t be too hasty. While kissing the strap goodbye would be amazing – I mean, see ya later chaffing and scars – the accuracy of wrist-based heart rate is still up for debate. While wrist-based heart rate technology is improving (for example the Fenix 5S uses Garmin’s signature Elevate™ optical wrist-based HR technology), it has been said that the accuracy of wrist-based heart rate is still uncertain, while the accuracy of chest strap heart rate monitors has been confirmed through various studies over the years. A study testing various wrist-based heart rate monitors, published in JAMA Cardiology (the Journal of the American Medical Association), found that wrist-based heart was variable – best at rest, with accuracy falling with increasing exercise intensity. In fact, the authors of the study recommend ‘using a strap heart rate when accurate heart measurement was important’. Well, that’s disappointing (*sigh*) – chest strap, I need you back.

My n=1 study

With all of the above in mind and having access to both the Fenix 5S (wrist-based heart rate) and the Forerunner 920XT (chest strap), I decided to play – to compare wrist-based heart rate with the chest strap during various workouts and training sessions.  

Attempt 1 – ride to work

My first attempt at comparing the two devices was during a ride to work (think: low intensity, very much aerobic – I’m not one of those commuters that feels the need to ‘race’ on my way to work). What I found, when I got to work and compared the data, was that the chest strap completely wigged-out at one point, affecting my overall data.

Looking at the Garmin website, they do state that high-voltage power lines can interrupt chest strap technology so maybe this is what happened here? My regular commute to work has tramlines, which possibly interfered with the chest strap. Interesting.

Attempt 1 – Heart Rate (HR) Readings:

Avg. HR – Fenix 5S 129

Avg. HR – Forerunner 920XT 163

Max. HR – Fenix 5S 163

Max. HR – Forerunner 920XT 210 – what the? *

Looking back at my TrainPeaks diary, my maximum heart rate (as calculated using an FTP test completed leading into Ironman Cairns last year, 2016) was 166 on the bike, so the 210 max HR above is definitely wrong – I would be vomiting at that heart rate.

Attempt 2 – easy ride home from run training, no tramlines

Due to the potential interference of the tramlines and therefore possibly inaccurate results, I decided to compare the heart rate methods during another aerobic ride – my easy ride home after run training. This is a different route compared to the usual route I take to work and doesn’t have tramlines. So, I was hoping to see some better results.

The results were better and more comparable, which is what I have seen in the readings I have done – other people who have compared wrist-based heart rate with chest strap have also found comparable results during easy, aerobic workouts.

Attempt 2 – Heart Rate (HR) Readings:

Avg. HR – Fenix 5S 122

Avg. HR – Forerunner 920XT 123

Max. HR – Fenix 5S 155

Max. HR – Forerunner 920XT 154

I have read that while wrist-based heart is relatively comparable with chest strap during easy sessions, it is during intensity that the wrist-based heart rate diminishes. Again, curious to see what readings I would get, I decided to test this out during an interval run session I did at training.

Attempt 3 – run training, intervals/speed 

During my run session, the Fenix 5S (wrist-based heart rate) was about 10bpm lower than the Forerunner 920XT (chest strap). So, given that chest strap accuracy is known, if you are training to heart rate and using wrist-based heart rate, then your run could be off due to the potential inaccuracy of wrist-based heart rate.

It has been said that one of the main reasons for the potentially inaccurate readings with the wrist-based heart rate monitor during harder intensity training is the possibility of the watch moving up and down your wrist, rather than staying in the one spot during harder workouts.

Attempt 3 – Heart Rate (HR) Readings:

Avg. HR – Fenix 5S 127

Avg. HR – Forerunner 920XT 134

Max. HR – Fenix 5S 165

Max. HR – Forerunner 920XT 175

Attempt 4 – long aerobic, ride, easy to moderate pace (Beach Road, no tramlines)

Again, it would seem that the heart rate readings obtained using wrist-based heart rate and the chest strap are similar during easier sessions. During my long, easy to moderate paced ride on the weekend both devices should the same results.

Attempt 4 – Heart Rate (HR) Readings:

Avg. HR – Fenix 5S 127

Avg. HR – Forerunner 920XT 127

Max. HR – Fenix 5S 160

Max. HR – Forerunner 920XT 159

To sum up

While easier sessions show similar heart rate readings between the wrist-based heart rate and the chest strap, I found that harder sessions or training at intensity is where there was a significant difference between the two heart rate methods. So, does this mean that if you’re using a watch for general activity, wrist-based heart rate is a fine option, but if you’re looking at doing more intense training, following prescribed heart rate zones and you’re after more reliable data then it’s better to stick to the trusty strap?   

So, what should I use – wrist-based heart rate or chest strap?

Well, this isn’t a straightforward answer.
Other things to consider include the following:

  • At the end of the day, heart rate is only a number.
    So, perhaps, whatever device you choose to use, use that one device rather than chopping and changing; to help with continuity. And remember that, particularly at intensity, wrist-based heart rate may show inaccurate readings – you might not be hitting targets that you could be hitting.
  • Know what might affect your heart rate readings, especially if you think your monitor may have wigged out (see my ‘Attempt 1’). Know your expected maximum heart rate (through an FTP test – speak to your coach about this) to then know what you should expect to see.
  • Realise that there are factors that affect heart rates irrespective of what device you use, such as the temperature, fatigue and hydration status. So, get to know your body, speak to your coach and in a race (and also perhaps in training) use a combination of heart rate and RPE (perceived exertion).   
  • Talk to your coach before you spend your cash on the latest technology. Your coach should be up to speed with the latest reviews and literature on what is most accurate. I spoke to two well-known tri coaches while researching this piece and both agreed that wrist-based heart rate isn’t as accurate as chest strap at this stage and in training and racing it’s best to use a combination of heart rate and RPE to monitor exertion and to help set the pace.   

As for me – while wrist-based heart rate is an exciting concept, for now, I think I’ll stick to my chest strap
(take me back chest strap!).    



Margaret Mielczarek

Margaret Mielczarek is the deputy editor at Australian Triathlete Magazine and writes the web series 'Shenanigans of a Deputy 2.0'. She is a passionate age-group triathlete and four-time Ironman finisher - currently in training for Ironman number five!

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