Last week, Tech Times reported a wearable fitness tracker developed by Fitbit ended up saving a woman's life by alerting her to a dangerous health condition she was suffering from.

The device showed an elevated heart rate and led to the discovery of a pulmonary embolism, which could have been fatal had it not been detected on time.

But a new study suggests fitness trackers may not be completely reliable when it comes to monitoring heart rates.

According to co-author Lisa Cadmus-Bertram, a kinesiologist at the University of Wisconsin, wearing a wristband tracker while exercising may alter the device's heart rate readings.

"Heart rate is easiest to measure during rest, but once you start exercising, more variables come into play including sweat, which may have an effect," explains Cadmus-Bertram.

Large Discrepancies From EKG Readings During Exercise

The research, published April 11 in the Annals of Internal Medicine, offers evidence that fitness trackers are less accurate than an electrocardiograph in measuring heart rate during exercise.

In an experiment, a joint team of scientists from the University of Wisconsin and Loras College compared four types of wristband trackers commonly used by fitness enthusiasts - the Fitbit Surge, the Basis Peak, the Fitbit Charge, and the Mio Fuse.

The researchers tested them against each other and also in comparison with EKG readings. They enlisted the help of 40 volunteers - all healthy adults between 30 and 65 years of age - to wear the trackers, two on each arm.

The four devices monitor heart rate with the help of a light-emitting diode that tracks light reflected by the wearer's skin to detect small changes in skin blood volume.

While study participants remained at rest, the Fitbit Surge gave the closest readings to the EKG - known as the most accurate method of tracking heart rate.

Readings taken by the Basis Peak - which coincidentally is the most expensive of the four devices and was voluntarily recalled last year after overheating issues caused burns in some users - were furthest off from the EKG, while measurements from the other two wrist-worn trackers fell in between.

However, when the volunteers did a moderate-intensity exercise test and hopped up on a treadmill for 10 minutes at 65 percent of their maximum heart rate, the data reported by the monitors on the fitness trackers was inconsistent with the EKG readings.

All the trackers showed a "relatively poor" performance, diverging from the EKG with as many as 41 beats per minute too slow and 39 beats per minute too fast. Moreover, the wristband trackers recorded discrepancies in measuring the same heart rate of the same person under the same conditions.

No Reason To Discontinue Use

Although the experiment found accuracy problems in the fitness trackers, Cadmus-Bertram says "they don't need to be perfectly accurate" to give wearers motivation for "a more active lifestyle."

While researchers agree further studies are needed in order to determine whether the monitoring feature in these devices is sufficiently reliable "to help clinicians advise their patients about health issues," Cadmus-Bertram specifies there is "no reason for the general public not to use it for feedback and motivation."

In response to the study results, Fitbit officials stated the company's products are not intended to be medical devices "but give other valuable wellness measurements not measured by an EKG, such as estimated maximum oxygen consumption during exercise, which can indicate cardiovascular fitness."

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