I have two watches on my left wrist, another on my right arm, a ring on my finger and a sensor embedded in my bra. No one should ever wear this many fitness trackers simultaneously. But in this moment, I am letting the latest heart-rate-sensing, sleep-capturing, workout-recording wearables from Apple, Fitbit, Garmin, Whoop and Oura capture all my data, to see which ones do the best job.

Whether you are training for a race or trying to lose the Quarantine 15, a fitness tracker can provide a helpful motivational nudge. But choosing...

I have two watches on my left wrist, another on my right arm, a ring on my finger and a sensor embedded in my bra. No one should ever wear this many fitness trackers simultaneously. But in this moment, I am letting the latest heart-rate-sensing, sleep-capturing, workout-recording wearables from Apple, Fitbit, Garmin, Whoop and Oura capture all my data, to see which ones do the best job.

Whether you are training for a race or trying to lose the Quarantine 15, a fitness tracker can provide a helpful motivational nudge. But choosing the right one depends on your preferred activities and health goals—and requires an understanding of what data is most useful to you.

New trackers are much more than pedometers and heart-rate monitors. They also measure blood-oxygen levels, a sign of overall health and altitude acclimation. And they capture heart-rate variability, the difference in time between each heartbeat, which can provide feedback about your body’s ability to recover from exercise. But how accurate are the metrics?

The Right Metrics

Studies have shown that wrist-based wearables’ optical sensors—which beam light onto the skin to detect pulse—are generally accurate during rest, but less so during workouts with unpredictable movement, such as strength training.

The data can still be valuable. Zakkoyya Lewis-Trammell, an assistant professor of kinesiology at California State Polytechnic University, Pomona, said that while wrist-based trackers aren’t clinical devices, she considers them a reliable tool for comparing day-to-day changes.

One helpful approach: Pick metrics that are as close to the sensor’s data as possible—such as resting heart rate over time, said Seth Martin, a cardiologist at Johns Hopkins Medicine. Avoid obsessing over things like calories burned, which is just a calculated guess, he added.

A new metric popping up in wearables looks at whether your body is primed to work out. Fitbit and Oura call it “Readiness.” On Garmin, it’s “Body Battery.” Whoop assigns you a “Strain” score. Several Apple Watch apps, including Training Today, offer similar measurements. A high score means you’re ready for intense exercise; a low score indicates your body needs rest.

A new metric uses heart-rate variability to assign a score: High means ready for exercise, low means get some rest; from left, Garmin, Oura, Whoop and Fitbit.

Photo: Nicole Nguyen/The Wall Street Journal

It isn’t as valid for people who have diabetes, heart disease or who are pregnant, Dr. Lewis-Trammell said. In those cases, she said, the data should be brought to a physician for evaluation.

For people who do outdoor workouts, GPS accuracy is important for logging distance. Dr. Lewis-Trammell found Garmin devices have a better GPS than others. I confirmed this in my own testing.

On one ride testing the wearables’ GPS-tracking capabilities, the Apple Watch and Garmin tracks were very close to my actual route. The Fitbit didn’t lock a GPS signal until about half a mile into my workout, so it displayed less total mileage. It also cut corners (literally) and showed me riding through neighbors’ homes! The route had trees and buildings, which might block GPS signals. Fitbit did better during a beach workout under an open sky.

GPS routes tracked by the Garmin Forerunner 945, blue, and by Apple Watch Series 7, red, matched my ride accurately. The Fitbit Charge 5, green, often cut corners.

Photo: Nicole Nguyen/The Wall Street Journal

A spokeswoman for Fitbit’s owner, Google, shared best practices: Bring a phone to speed up the initial GPS lock-in, don’t start the exercise until the GPS is connected, and make sure the band is secure, but not constricting, for the best signal.

SHARE YOUR THOUGHTS

How do you track your workouts? Join the conversation below.

A word about data privacy: While all of these devices capture potentially sensitive information about your body, none of them sells it to third parties. Google says Fitbit doesn’t use any data, even internally, for the purpose of advertising. If you are concerned, however, you should read the terms of service of the product—and the data-privacy statements of their makers—before buying.

When picking the best tracker for your life and fitness regimen, ask yourself three questions:

What’s my preferred workout type? If you like treadmill running, spinning or pool swimming, you can spend less and get a basic tracker such as the Fitbit or Oura. Outdoor activities require the advanced GPS capabilities of the Garmin and Apple Watch. And if you often go on long hikes or run marathons, you’ll definitely want a Garmin, which has multiday battery life.

What am I willing to wear daily? Most are intended to be worn during sleep, which is when the tracker can most accurately record resting heart rate. Wrist-based wearables range from Oreo-size watches (Garmin) to slender bracelets (Fitbit). If you want stealth, Oura’s ring or Whoop’s clothing-compatible sensor are better—you just can’t get any information from them until you check the app.

What are my goals? Are you looking to improve your health generally, or are you focused on training and performance? The Fitbit, Apple Watch and Oura are sufficient for the former, while self-described athletes should lean toward the Garmin watch or Whoop’s subscription-based coaching service. For the most accurate heart-rate reading during a workout, opt for a chest strap.

Keeping your answers to these questions in mind, review the pros and cons of the models I tested:

Fitbit Charge 5.

Photo: FITBIT

Fitbit Charge 5

Price: $180 ($80 a year for optional Fitbit Premium); fitbit.com

Target user: People just getting into exercise

Battery life: 7 days; up to 5 hours with continuous GPS

The Charge 5, released in September, has a bright touch screen and a sleek design. The friendly companion app, which thoughtfully explains why each metric matters, is great for workout newbies. A recent update turned on electrocardiogram capabilities and, for premium subscribers, a readiness score. On the down side, though the Charge 5 has GPS, it can’t record open-water swims—just pool workouts. Strangely, the device can’t be powered down. (Fitbit says users won’t need to save power because of the device’s long battery life.) And, after a six-month trial included with purchase, the app persistently tries to upsell its Premium membership, which includes mindfulness content and deeper sleep insights.

Apple Watch Series 7.

Photo: Apple

Apple Watch Series 7 and SE

Price: Starts at $279; apple.com

Target user: iPhone owners who want smart features

Battery life: 18 hours; up to 7 hours with continuous GPS

The Apple Watch is incredibly versatile, and its interface is the easiest to use. Activity options range from Tai chi to snow sports, and you can listen to podcasts or music sans phone during workouts. The cheaper SE model ($279 and up) offers most of the Apple Watch experience, minus the blood-oxygen and ECG sensors. The Series 7 ($399 and up) has a slightly larger display. This watch is best used with third-party apps, such as Strava, as the accompanying Fitness and Health apps are fairly basic. And yes, the device still requires daily charging.

Oura Ring, third generation.

Photo: Oura

Oura Ring

Price: $299 ($72 annual membership); ouraring.com

Target user: People who don’t like stuff on their wrist

Battery life: Up to 7 days

The third-generation ring is lightweight and screenless, an ideal tracker for someone who doesn’t want distractions. It can record heart rate continuously, analyze sleep and use its thermometer to track menstrual cycles. There’s no built-in GPS, though it autologs activities such as runs or even yard work. Without the membership, you can see only readiness, sleep and activity scores.

Garmin Forerunner 945 LTE.

Photo: Garmin

Garmin Forerunner 945

Price: Starts at $600; garmin.com

Target user: People who are very active outdoors

Battery life: Up to 2 weeks; 36 hours with continuous GPS

Of Garmin’s many models, its Forerunner watch offers the most capability in the slimmest package. It is the ultimate wearable for athletes focused on training, and even stores your plans onboard. It can broadcast heart rate to any Bluetooth-enabled or ANT+ device such as a Peloton. The model I tested is pricey. The slightly more affordable Forerunner 745 ($500) offers the same core functionality, minus its big sibling’s longer battery life and increased storage.

Whoop 4.0.

Photo: WHOOP

Whoop 4.0

Price: Starts at $18 a month; whoop.com

Target user: Athletes who want to improve performance

Battery life: Up to 5 days

Whoop bills itself as a “personal and health fitness coach.” You don’t pay for the tracker up front—you get it when you subscribe to the service. Like Oura, the Whoop 4.0 is a screenless sensor with automatic workout detection. The device can be worn on your wrist, or embedded in a variety of compatible apparel. Using the app’s journaling feature, you can mark a custom set of behaviors, such as alcohol or caffeine consumption, and Whoop will create an assessment of how those influence your workout performance and sleep. Whoop was one of the first devices to use heart-rate variability to suggest optimal activity intensity to help prevent injury and overtraining. It is a pricey service—and the others are quickly copying the tech for less, but they don’t yet offer the same level of detail.

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Write to Nicole Nguyen at nicole.nguyen@wsj.com