This the University of Wisconsin, 40 healthy,This the University of Wisconsin, 40 healthy,

This Medical Pearl continues the theme of things
having to do with physical activity—specifically, the accuracy of wrist-worn
activity/heart rate monitors.


Activity trackers are generally used to
motivate individuals to engage in healthy behaviors. Devices have emerged on
the market as alternatives to traditional heart rate trackers, which require separate
chest leads. One type of monitor is a wrist-worn tracker with a light-emitting
diode (LED). It measures the heart rate from tiny changes in skin blood volume
by using light re?ected from the skin. Such devices are unobtrusive and
appropriate for continuous, long-term wear. Although previous studies have
shown that they are potentially accurate for measuring the number of steps a
person takes during routine activities and heart rate at rest, less is known
about their accuracy when heart rate is measured while exercising. Two recent
studies have addressed this deficiency.

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In a report from the University of Wisconsin,
40 healthy, consenting adults aged 30 to 65 years without cardiovascular
conditions had heart rate trackers placed on each wrist along with equipment to
assess heart rate using standard electrocardiographic (ECG) leads.1
Measurements were taken at rest and at 1-minute intervals for 10 minutes and also
while the participant exercised on a treadmill at 65% of the maximum predicted heart
rate. Comparisons were made between the wrist heart rate trackers and the heart
rate directly measured by ECG. At rest, the agreement was best for the Fitbit
Surge (Fitbit), which had the narrowest limits of agreement (3.1 to 4.5
beats/min variation), worst for the Basis Peak (Basis) (17.1 to 22.6
beats/min), and intermediate for the Fitbit Charge (Fitbit) (9.5 to 10.2
beats/min) and Mio Fuse (Mio Global) (7.8 to 9.9 beats/min). When participants
exercised at 65% of the maximum heart rate, the limits of agreement were
relatively poor for all the activity trackers (Mio Fuse, 22.5 to 26.0
beats/min; Basis Peak, 27.1 to 29.2 beats/min; Fitbit Surge, 34.8 to 39.0
beats/min; and Fitbit Charge, 41.0 to 36.0 beats/min). All the trackers
performed better at rest than during moderately active exercise.


The investigators concluded that although
wrist-worn trackers may help monitor daily activity, more research is needed
before it can con?dently concluded that the monitoring feature for heart rate
is useful for other purposes such as the assessment of maximal heart rates
during exercise. Similar findings were also reported by Singh et al who did an
experiment with high school students.2 The students evaluated their heart rate at baseline
(resting), after briskly walking on a 300-m track (post-walk), and after
running on a 300-m track (post-run) comparing a Fitbit and a directly measured
heart rate using a stethoscope. The investigators found a more than a 5%
difference between data recorded by the Fitbit and manually obtained heart
rates for most variables. There were two instances in which participants’ directly
measured heart rates after vigorous running were greater than 140 beats/min but
in which the Fitbit failed to provide a reading. The heart rate data reappeared
only after the heart rates had decreased to less than 140 beats/min.


While wrist monitoring devices may be useful
for routine activities, both sets of investigators from the studies described
believed that physicians and consumers need to be aware of the limitations of
these devices.