The fitness industry has become extremely innovative at getting their clientele to keep coming back for more. At first, all we needed was a very simple gym membership, basic equipment, and comfortable clothes. Over time, it seems the list of fitness essentials keeps growing and growing. These days we can buy a specific fiber from the Himalayas that will regulate your core temperature to achieve perfect performance. There is magic "glutes cream" to sculpt an ideal butt. Then we have wrist-worn monitors with all the features, measuring a variety of fitness markers promising to push you to the threshold of Gainsland (fictitious country). The multi million-dollar fitness industry has convinced many that if you want to reach your fitness goals a wrist-worn heart rate monitor is a must, with many people striving to hit calorie deficit goals and peak heart rate zones for their workouts. But before you spend your well-earned money or measure your success on these tools, let's review a study to determine how these trackers potentially affect you, the consumer, and what positive applications we can rely on.
This study, executed by Jo et al (2016), asked 'what is the accuracy of the Fitbit Charge HR (common wrist-worn fitness tracker) compared to the Electrocardiogram (ECG) monitor? During testing, participants were connected to a 12-lead ECG and wore a Fitbit Charge HR according to manufacturer instructions. Participants included 24 female subjects who performed the following training protocol for 77 minutes:
The exercises below were performed in the following order for 5 minutes each. A rest period of 5 minutes was implemented between each exercise, where heart rate data was collected.
1) Cycling at 60W
2) Cycling at 120W
3) Treadmill walk
4) Treadmill jog
5) Treadmill run
6) Dumbbell lateral arm raise 12 reps
7) Alternating resisted lunges 12 reps
8) Plank 60 second
According to the data, the lower the intensity or the effort level, the more accurate the Fitbit was in correlation with ECG. The range of deviation between tracker and ECG on low intensity bouts was from + 16 bpm to -26 bpm (moderately acceptable). The highest accuracy of the fitness tracker was observed on resting heart rate.
For the higher intensity exercises, starting with heart rate of 116 bpm or higher, the results were quite different. It was recorded that the deviation from ECG to fitness tracker was from +29 bpm to -54 bpm, indicating a large tendency for the Fitbit to underestimate heart rate.
To paint a picture the Fit bit tracker would record a heart rate of 120 bpm, but in reality it could upwards of 160 + bpm; quite a significant difference.
“...the performance of the Fitbit was exceptionally poor during cycling, resisted arm raises, resisted lunges, and abdominal plank with underestimations up to 21 bpm” (MASS Volume 2, Issue 4)
The author of the paper moves on to stipulate reasons why the inaccuracy occurs during high intensity output, when it would seem to have the most practical relevance. He stipulates that due to the sensor on the wrist being so close to the skin “may negatively influence the quality of the PPG signal (photoplethysmographic)”. This may explain the level of inaccuracy in arm involved contractions such as the lateral raise, lunges, griping bike bars while cycling. He proceeds to explain how skin pigmentation, sweat and location of sensor could be important variables in accurately measuring heart rate. His suggestion was to use a traditional device with a chest-strap sensor for more accurate results.
It may sound like a defeat for fitness trackers, but considering the variety of reasons why people may want to use them, there are positive applications after all. Today’s trackers provide features such as step counters, reminders to move, as well as sleep and resting heart rate monitors. So, although not ideal for measuring the intensity of your prescribed workouts, there are many benefits to being motivated to improve overall health and can therefore be a very useful tool.
References: MASS Volume 2, Issue 4
Study Reviewed: Validation of biofeedback wearables for photoplethysmographic heart rate tracking. Jo et al. (2016)