Most fitness trackers can measure a lot: steps taken, heart beats, sleep quality and workout performance.
That's a lot of data, but are they useful? It depends on what you're looking for. Those new to fitness might want something that's motivational, while those already active might want something sophisticated — but not necessarily easy to use.
Here's a look at fitness devices from four leading manufacturers. Microsoft's Band 2 and Motorola's Moto 360 Sport were excluded from consideration in part because battery life was poor — tough for marathons of more than four hours. There's a risk your tracker will end up in a drawer after a few months, especially if you get it for free as a gift or as part of a corporate wellness program. Know what you're getting and how you might use it before buying or adding one to your gift list.
The recommended devices all claim to be water resistant, so they can withstand sweat or rain. But only Garmin's are designed for swimming and showering. And for Apple Watch, the protection doesn't extend to leather bands.
For the Basics
Fitbit is the market leader in wearable devices, thanks to its simplicity and wide range of offerings. At $130, the Fitbit Alta offers basic step tracking. The $200 Fitbit Blaze adds heart-rate monitoring and a larger screen. The $250 Fitbit Surge adds GPS for better pace and distance readings during outdoor workouts (though all-day step tracking doesn't use GPS to conserve battery).
For running and cycling, the Blaze doesn't have GPS but borrows the one on your phone if you carry it.
All three models come with automatic sleep monitoring. Just check Fitbit's companion app to see how long and how well you've slept. A new feature reminds you to go to bed — perhaps too gently, as it's easy to overlook or ignore.
All Fitbits have amazing battery life and can go for several days without a recharge. Their screens aren't as vivid as what rivals offer, though the Blaze has color, a first for a Fitbit.
One of Fitbit's strengths lies in its community. If a friend has one, you can use Fitbit's app to compete and nudge each other. As a result, Fitbits are good motivational device for those new to physical activity. Fitbits also sync with just about every device out there — iPhones, Android, Windows and Macs.
For the Budget
Samsung's Gear Fit2 offers high-end features for $180, or $70 less than a comparable Fitbit. This includes GPS for better distance monitoring during outdoor workouts. There's automatic sleep tracking, too.
The screen is more vivid than on Fitbits, but it can be tough to read in direct sunlight. You'll need to crank up the brightness, which is fine, as the battery should still last six or seven hours with GPS use (and a few days without). The Fit2 also tracks squats, lunges and other strength exercises — rare in a tracker.
While the Fitbit Blaze and the Surge notify you of missed calls and messages on your phone, the capabilities are poor compared with smartwatches. The Fit2 does better by offering limited message replies from the device, but it doesn't offer the app choices that smartwatches do.
An Android phone — not necessarily Samsung's — is required for full functionality. No iPhones or PCs. You can compete with other users of Samsung's S Health app — not just Fit2 owners — but the community isn't as vibrant as Fitbit's.
Unfortunately, a bike frame or a backpack can easily knock the device off your wrist. The Fit2's price is nice — but not if you need to buy a replacement.
For the Performance
Most fitness trackers are designed for general fitness needs and offer little customization for power users.
Garmin's Vivoactive is the exception, at $220 for the base model and $250 for the newer "HR" version with heart-rate monitoring. Both have GPS, automatic sleep tracking and some phone notifications. They sync with a variety of devices, as with Fitbits.
Both Vivoactive models combine the best of Garmin's stand-alone gadgets for running, golfing and other sports. Runners are able to record times after each lap on a track, while other fitness devices typically limit you to auto-intervals every mile or so.
There are still times you'll need a dedicated device. Swimming on the Vivoactive, for instance, is limited to the pool, while the $450 Forerunner 735XT allows open-water tracking.
Neither Vivoactive model has a fancy screen, but that improves readability in sunlight and extends battery life — plenty even for super-slow marathons. The devices are bulky for all-day wearing. Garmin is about performance, not fashion. And in another tradeoff for added functionality, Garmin's app isn't as easy to navigate as Fitbit's.
For the Lifestyle
At $300 or more, the Apple Watch is for those who want more than fitness on their wrist — such as turn-by-turn navigation, or the latest weather. You need to carry an iPhone for full functionality.
Apple Watch doesn't have GPS, though it will borrow the phone's GPS readings if you run, walk or bike with it. This also calibrates the watch so that you can leave the phone home after a few workouts (sorry, calibration doesn't work for cycling). Unlike the Blaze, the watch won't record your specific route without a third-party app.
You're given goals on minutes to exercise and calories to burn — customized based on your sex, age, weight, height and current fitness level. Apple Watch will prompt you to adjust goals that you consistently meet or miss. On rival devices, you typically have to take the initiative to go through those settings.
Although Apple Watch isn't alone in offering reminders to take breaks and walk around, it's more effective than rivals by turning it into a game. Do it 12 times throughout the day to complete a blue circle.
Apple Watch requires a nightly recharge and isn't designed for tracking sleep, though third-party sleep apps are available. The battery runs out after 4.5 hours to five hours in a marathon, but you can extend its life by disabling heart-rate monitoring during workouts.
This fall, Apple plans to release an app designed to reduce stress by guiding you through deep-breathing exercises — offering a new approach to fitness.
Get the best of KQED's science coverage in your inbox weekly.