De­vices can track your steps, not your heart rate

Study finds that fit­ness mon­i­tors of­ten give in­con­sis­tent car­diac data

Baltimore Sun Sunday - - NEWS - By Melissa Healey

Us­ing that nifty fit­ness mon­i­tor to keep track of your heart rate while you ex­er­cise?

If you ex­er­cise while re­main­ing still, it might work pretty well. The prob­lems start if you move while ex­er­cis­ing.

A new study pub­lished in the An­nals of In­ter­nal Medicine put four wear­able fit­ness track­ers to the test — both against one an­other and against the kind of elec­tro­car­dio­g­ra­phy mon­i­tor you’d en­counter while tak­ing a stress test in a doc­tor’s of­fice.

The re­sults show that the wrist­band fit­ness track­ers may be a fine gauge of how many steps you take. But when it comes to track­ing changes to your heart rate that come with movement, these mon­i­tors don’t stack up, the authors found.

The track­ers tested in the study range in price from $57 (for the Mio Fuse) to $500 (for the Ba­sis Peak). To mea­sure heart rate, at least one of them uses light re­flected from the skin to de­tect tiny changes in skin blood vol­ume.

Re­searchers from Univer­sity of Wis­con­sin-Madi­son and Lo­ras Col­lege in Dubuque, Iowa, had 40 healthy adults strap on four pop­u­lar ac­tiv­ity track­ers, two on each arm. Study par­tic­i­pants, ages 30 to 65, also were rigged up to an elec­tro­car­dio­graph, which uses leads in a chest strap to de­tect the wearer’s heart rate.

For sta­tion­ary sub­jects, the Fit­bit Surge di­verged least from the heart rate mea­sure­ment taken by the elec­tro­car­dio­graph. Read­ings taken by the Ba­sis Peak di­verged the most. The Fit­bit Charge and the Mio Fuse fell in be­tween.

But when the sub­jects were asked to ex­er­cise on a tread­mill for 10 min­utes at 65 per­cent of their max­i­mum heart rate (a moderately in­ten­sity pace), the per­for­mance of all the track­ers was deemed “rel­a­tively poor,” ac­cord­ing to the study. Com­pared with the elec­tro­car­dio­graph’s read­ings, the track­ers re­ported heart rates that were as many as 41 beats per minute too slow and as many as 39 beats per minute too fast.

The fit­ness mon­i­tors were also in­con­sis­tent in their per­for­mance.

When the re­searchers tested the “re­peata­bil­ity co­ef­fi­cient” — es­sen­tially, the de­gree to which a given tracker re­turned the same mea­sure­ments for the same par­tic­i­pant un­der the same con­di­tions — the Fit­bit Surge per­formed roughly as well as an elec­tro­car­dio­graph while sub­jects were at rest. But when study par­tic­i­pants ex­er­cised, all four track­ers re­turned heart rate read­ings that var­ied at least twice as much as did the elec­tro­car­dio­graph.

Wrist­band ac­tiv­ity track­ers are gain­ing ad­her­ents among re­searchers as well as among fit­ness en­thu­si­asts. But while the mon­i­tors may be good at mea­sur­ing strides and at en­cour­ag­ing users to take more of them, their use by physi­cians and re­searchers may be pre­ma­ture, the study authors con­cluded.

“More re­search is needed be­fore we can con­fi­dently con­clude that the monitoring fea­ture for heart rate is suf­fi­cient to help clin­i­cians ad­vise their pa­tients about health is­sues and con­duct clin­i­cal trails that re­quires a high level of ac­cu­racy and re­li­a­bil­ity for heart rate mea­sure­ment,” they wrote.

Lisa Cad­mus-Ber­tram, a Univer­sity of Wis­con­sin ki­ne­si­ol­o­gist who worked on the study, said the re­sults shouldn’t prompt any­one to toss their tracker.

“On the whole, fit­ness track­ers still pro­vide a tremen­dous amount of use­ful in­for­ma­tion to the av­er­age user who just wants some feed­back to help them in­crease their ex­er­cise level,” she said.

The new study is not the first to de­ter­mine that wear­able de­vices don’t pro­vide reli­able heart rate mea­sure­ments. Re­searchers at Cal­i­for­nia State Poly­tech­nic Univer­sity-Pomona reached the same con­clu­sion last year af­ter test­ing the heart rates of 43 healthy adults with Fit­bit’s PurePulse heart rate mon­i­tors.

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