With so many activity trackers on the market, it's hard to know which one is best for which crowds. The Withings Pulse O2 ($119) seems most suited to fairly inactive people who want to know how many steps they take in a day as well as their resting heart rate, and perhaps those who are already deep users of the Withings family of products. Withings sells everything from smart Wi-Fi scales to at-home blood pressure cuffs, and so if you're already in that ecosystem, the Pulse O2 is a fine addition. But it isn't ideal for runners, cyclists, swimmers, or anyone with a moderately active lifestyle who cares about their heart rate during activity.

The Pulse O2 has one unique feature not seen in other big-name activity trackers: an oximeter, which measures the saturation of oxygen in your blood. Whether that measurement is one you need to know is another matter. I for one had to look up what my blood oxygen score meant, and it isn't a data point I feel I've been missing. The addition of the oximeter is the only real difference between the O2 and it predecessor, called simply Withings Pulse, which we rated highly at the time it was first released. In the year since then, however, the market has matured rapidly, particularly among devices offering a heart rate monitor. The Pulse O2 just hasn't kept pace.

The Pulse O2 packs a lot of features into a tiny, power-efficient device that you can wear on your wrist, belt loop, bra strap, or anywhere else you can clip it. But in testing, not all the features held up.

Design and Features

The Withings Pulse O2 looks like an unassuming plastic rectangle with two rounded sides. It measures a scant 1.69 by 0.87 by 0.31 inches (HWD) and weighs only 0.28 ounces. It has an OLED touch screen, but you can't even see it until it flashes a message. One tiny, flush button on the top lets you turn on the Pulse O2, cycle through the screens, and change modes. On the reverse edge is a micro USB port for charging. The back of the device holds the optical sensor that's used for oximeter and heart rate readings.

You set up the device with an Android or iOS device (more on the compatibility requirements in a bit) and the Withings Healthmate mobile app. After entering your age, height, weight, sex, and a few other optional pieces of information about yourself, you're ready to wear the Pulse O2 all day and night long. It measures steps, distance, calories burned, elevation, sleep, heart rate, and blood-oxygen level. It also has a stopwatch/timer feature if you want to clock an activity.

The Pulse O2 comes in black or blue and ships with a wristband, clip, and USB cable. The clip is the same one that came with the previous Withings Pulse, which I find a little clunky when compared with the super sleek and comfortable Fitbit One. The Fitbit One, which is an Editors' Choice, has a more elongated shape, making it more comfortable to wear on the front of a bra, which is where a lot of women discreetly hide their fitness trackers.

If you prefer to wear the Pulse O2 like a watch, you have to slide it into a holder attached to the wristband. It's awkward, and once it's in there, the heart rate monitor/oximeter is completely unavailable. You have to slide out the tracker to use it.

Swappable wristbands for the Pulse O2 let you change the color, with more bands on the horizon. 

In the Field

I've been wearing the Withings Pulse O2 for a little less than a week. The count for my miles traveled seems close to my known averages, but it doesn't record bicycling, which I've been doing a lot lately. The sleep monitor recognizes the difference between light and deep sleep, which is a more advanced feature among fitness trackers. If you have trouble sleeping through the nights, it's very helpful to be able to see whether you're waking up at natural points in your sleep cycle, or whether something in your environment is disrupting you from deep sleep.

Compared with the few other activity trackers that monitor deep and light sleep, the Pulse O2 is less automated than others. You have to manually enable and disable sleep mode when you go to bed and wake up. The Basis Carbon Steel Edition, my favorite of the watch-style activity trackers on the market, automatically figures out when you fell asleep and woke up without you having to do anything at all. 

In my testing, the heart rate monitor/oximeter just wasn't reliable. To use that function, you have to pop the device out of its band or clip, press the top button until you get to a screen showing a heart and moon, tap the heart, and then hold your finger to the back of the device. In other words, you can't be moving around. You can see a red light beaming into your finger, and you have to hold still for about 15 seconds. But it only worked about half the time.

Sometimes a reading would fail entirely, at which point the Pulse O2 displays Xs instead of numbers for both readings. Sometimes one of the two readings would appear, but I'd get an X for the other. A few times, everything seemed to be going smoothly, but then the device would flash on screen, "hello!" as if it had just rebooted. And back-to-back readings were occasionally off, like the time my pulse appeared to jump from 59bpm to 75bpm a minute later. All the while, I was sitting perfectly still.  

Perhaps more to the point, the Withings Pulse O2's heart rate monitor isn't a continuous heart rate monitor, so it does you no good whatsoever while working out. Compared with the Basis Carbon Steel Edition, which reads your heart rate all day and all night, the Pulse O2's occasional readings seem almost worthless. Anyone can take their resting heart rate by pressing two fingers to their throat. The real data that's important is your heart rate while running, bicycling, hiking, or even playing actively with your pets or children.