July 10, 2017 / 5:02 PM / a month ago

Placement and speed affect step monitor accuracy

Activity monitors can help in meeting health goals, but how they’re worn can significantly affect their accuracy and reliability, an Australian study suggests.

In a test of several consumer wearable activity monitors and one used by researchers and doctors, the accuracy of step counts was better when the device was worn at the waist versus the wrist, but also varied widely depending on walking or running speed.

“Current world health recommendations are for people to complete 150 minutes of moderate-intensity physical activity per week to assist with improving overall health outcomes,” senior study author Belinda Parmenter told Reuters Health by email.

“Studies have indicated that a reference goal of 10,000 steps per day can also provide an appropriate level of activity to meet the minimum recommendations,” said Parmenter, an exercise physiologist at the University of New South Wales in Sydney.

She and her colleagues recruited 31 adults to complete three minutes on a treadmill at five different speeds ranging from about 3 miles per hour to about 7.5 miles per hour. The researchers videotaped each session to manually count steps for comparison to the monitors.

Each participant wore an Actigraph monitor - often used by researchers and clinicians for sleep or activity monitoring - on the waist and another Actigraph on the wrist. They also wore a Fitbit One on the waist, and on both wrists, a Fitbit Flex, Fitbit Charge HR and Jawbone UP24.

When the study team examined step counts for all the devices, they found errors ranging from about 41 percent for the wrist-worn Actigraph and less than 1 percent for both the waist-worn Fitbit One and Actigraph, according to the report in Gait and Posture.

For instance, in three-minute bouts at speeds ranging from 5 kilometers per hour to 12 kph, the Actigraph monitor worn at the waist had average step counts less than 1 percent more or less than the manual count by the researchers. But worn on the wrist, the Actigraph was off by an average 29 percent at slow speeds to 50 percent at the highest speed.

Fitbit One worn at the waist was always less than 2 percent off the manual count, while wrist-worn monitors ranged from less than 1 percent to about 12 percent difference, depending on speed.

“The Fitbit One worn around the waist performed the most accurate across all speeds when compared to all the devices worn around the wrist (Jawbone, Fitbit Charge, Fitbit Flex, Actigraph),” Parmenter said.

“During walking and running, the body’s center of mass is located at the pelvis. Movement of the center of mass provides the best indication of whole body movement.”

Since waist-worn devices are close to the pelvis, they can provide more accurate measurements of the body’s movement than wrist-worn devices, she added.

But among the wrist-worn devices, the Jawbone was the most accurate, performing well at both the slower walking speeds of the average adult, as well as the running speeds, Parmenter said.

“Other wrist-worn devices became more accurate as walking/running speed increased. For researchers, the Actigraph wGT3XBT worn on the waist matched the Fitbit One.”

People using step counts to meet the physical activity recommendations of 150 minutes per week of moderate-intensity exercise (or 30 minutes five times a week) can do so by completing at least 3,000 steps in 30 minutes, she added.

“These studies have shown that this equates to an intensity high enough to meet the current physical activity recommendations. So, for healthy individuals, as one way to meet the recommendations, we could be aiming to complete 3,000 steps in 30 minutes at least 5 times a week, as a part of our 10,000 steps a day goal,” Parmenter said.

The study also found that for the wrist-worn devices, there was no difference between the step counts measured on the dominant and non-dominant hand, Parmenter noted.

“However, the results are based purely on a treadmill test and did not include the effects of activities of daily living such as brushing teeth etc. More studies need to be done in a free-living environment,” she said.

SOURCE: bit.ly/2suafj5 Gait and Posture, online June 21, 2017.

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