Popular fitness trackers made for swimmers are accurate enough for recreational use to track and improve performance, but not for use in training by competitive swimmers, a recent study concludes.
Researchers tested two products, the Finis Swimsense and Garmin Swim, and found them to be comparable to each other and pretty good at tracking distance, laps and different types of swimming strokes.
Stroke counts were not as consistent, however. And the devices' ability to recognize different stroke types is likely to vary more widely with amateur swimmers than with competitive athletes, the researchers note.
"As part of the so-called 'quantified self' movement, individuals who swim as part of a recreational health and fitness program can benefit from monitoring some basic indices of their performance," said senior study author Gearoid O Laighin of the Center for Research in Medical Devices at the National University of Ireland in Galway.
"Swimming ranks amongst the most popular leisure activities worldwide," O Laighin told Reuters Health by email. "The general health benefits of regular swimming are well established and swimming is one of the few sports that can be enjoyed during all stages of life."
O Laighin and colleagues tested the two swim fitness trackers by following 10 national competitive swimmers - five men and five women - during 60 laps of swimming. The swimmers wore both activity monitors while doing intervals of the butterfly, backstroke, breaststroke and frontcrawl. The devices identified stroke type, swim distance, lap time, stroke count, stroke rate, stroke length and average speed. The data was compared against underwater video recordings for accuracy.
Both devices accurately identified the four swim strokes and accurately counted swim distance and laps during the middle of an interval. However, laps performed at the beginning and end of an interval weren't always accurate, and stroke count measurements were off by a few counts in all but two occasions. These small errors also affect the stroke rate, stroke length and average speed numbers since they're based on lap times and stroke counts, the study authors write in PLoS ONE.
The Garmin Swim correctly identified the four swimming strokes with 95.4 percent overall sensitivity, and the Finis Swimsense was slightly more sensitive at 96.4 percent. With total distance, the Garmin tracker was three laps short, all for the frontcrawl stroke. The Finis tracker registered the correct number of laps but showed variations within the strokes. Both devices showed errors in lap times at the beginning of lap intervals.
"It's great to see researchers dive into the details of the devices, and the numbers came out well across the board," said C.J. Fiala, spokesperson for Finis Swimsense in Livermore, California. "When creating the product, we wouldn't accept a large margin of error, and we believe this is the most accurate it can get."
In general, both devices overestimated stroke counts in all intervals except with the butterfly, which the trackers tended to underestimate. Overall, accuracy was lowest with the breaststroke.
The varying accuracy may be related to stroke technique and how individual swimmers perform, O Laighin said. The error margin may likely increase when the trackers are used by recreational swimmers rather than elite swimmers.
"Although activity trackers have been increasing in popularity, it has been difficult to measure water-based activities due to waterproofing issues," said James Sanders of the Movement Insights Lab at the National Center for Sport and Exercise Medicine in Leicestershire, UK, who wasn't involved with the study.
"These two swimming sensors are highly influenced by technique and stroke mechanics, so the results should be interpreted with caution by a recreational swimmer whose technique may be unrefined," Sanders told Reuters Health by email.
The Garmin Swim sells for $149.99 and the Finis Swimsense sells for $179.99. There was no financial assistance from either company for the study.
Garmin media representatives didn't return calls or emails requesting comment.
"Understanding what you're doing in the water gives you the opportunity to improve your skills," Fiala said. "If you have inaccurate results, you're not getting a real picture of what you're doing and won't know how to improve."