I Tried a Smart Bracelet That Measures Body Temperature During Workouts, but I Haven’t Decided on Its Use Yet.
The smartband market is becoming quite crowded. The Fitbit Air is hard to beat for price/performance, the Whoop for performance analytics, and anything in between struggles to stand out. But the Speediance caught my eye with its unusual feature: a sensor that claims to measure body temperature during workouts.
Does the Speediance strap actually measure body temperature?
The only other device I’ve seen with similar claims is the Core 2 temperature sensor, which I used during summer workouts in the heat and found valuable and useful (albeit with some caveats). A Speediance representative told me their device works similarly to the Core 2: it has a skin temperature sensor and another one for heat flow, which can estimate how hot you are during a workout.
Core 2 requires pairing with a chest heart rate monitor, as its algorithm can only correlate sensor data with your body temperature if it knows your heart rate. It also requires wearing the sensor on the chest heart rate monitor so it’s as close to your actual body temperature as possible. Speediance doesn’t have these requirements; you can get approximate temperature estimates without removing the device from your wrist or requiring any additional equipment.
To test Speediance’s claims, I used both devices simultaneously during several workouts and compared the heat exposure readings I experienced, as reported by each device. While the Core 2 focuses on heat adaptation, Speediance uses temperature data primarily to tell you when you’ve completed your warm-up and when your performance may be declining due to heat.
How I tested the Speediance bracelet’s primary temperature measurement features
The Speediance app displays your “thermal load” during workouts right on the same screen where your heart rate, time, and distance are displayed. At the start of a workout, you’ll see a message indicating you’re not warmed up enough. Once your body temperature rises by approximately 0.5–1 degree Celsius (approximately 1–2 degrees Fahrenheit), you’re considered in the “optimal performance” zone. The load index number increases as your temperature rises; a value above 5.0 indicates “decreased performance,” while a value above 7.0 indicates “thermal overload.”
By comparison, the Core 2 sensor uses a slightly different scale, or at least has different thresholds: in the range of 1.0 to 2.9, you increase your training load due to overheating, while in the range of 3.0 to 6.9, you increase it more effectively. Above 7, training is not recommended, as the risk of overheating is too high.
When I tested the Speediance strap, it was fall and the weather was beautiful, so I wasn’t able to test the feature in extreme temperatures. I noticed that during a strength training session in the garage, my warm-up was supposedly over before I could do anything. And by the end of several relatively cool runs, I’d reached a “performance decline” level due to heat stress.
To get a better idea of how well the Speediance measures temperature, I took it along with the Core 2 on an hour-long trail run on an unusually warm day for the time of year (temperatures were around 25 degrees Celsius) and compared the heat load readings at various points during the run.
The numbers didn’t exactly match; the Speediance bracelet always showed a few points higher than the Core 2 sensor and seemed to show more fluctuations. But both readings increased more or less simultaneously. I wouldn’t say the Speediance bracelet is as accurate—and in any case, I don’t have a reference for comparison—but it seems useful for tracking how your performance changes with temperature in hot weather.
How the Speediance bracelet handles activity and sleep-related tasks.
If you’re considering the Speediance for its temperature-monitoring features, you’ll likely also use it to track your overall activity and sleep. I have a lot of good news on that front. It’s comfortable to wear, with a soft strap that’s a bit nicer than the Whoop. I often completely forget I’m wearing it.
The app has a lot of nice little features. You get a 360-degree visual representation of your day, showing when you slept and when you exercised. My only complaint is that the background is a generic image that creates the impression you’re looking at a day/night chart, but it doesn’t align with the actual sunrise and sunset times. Combining these times with the visual representation would provide a much better indication of how your habits align (or misalign) with your circadian rhythms.
The bracelet and app measure all the usual metrics and display the data well: heart rate variability, skin temperature, and resting heart rate are all displayed as vital signs, and I can easily see how they’ve changed over the past week, month, and year. Among my daily metrics is a “TRIMP” number, which I know means the same thing other apps call cardio load . The name is a bit confusing, but it’s a good metric to track. The app suggests a bedtime, and its sleep tracking seems relatively accurate (aside from the time it mistook a dentist appointment for a nap—though, to be fair, I did lie still for half an hour).
But I was disappointed by a few things. One drawback is that, since Speediance is primarily a home gym company, you can’t log a strength training session without creating an exercise program. (I ended up logging my strength training as “mixed cardio.”) If you want to share your workout data as graphics, you’re presented with default images of Speediance equipment as backgrounds.
And I still couldn’t sync my activities with Strava correctly. After attempting to connect, I received a message in Chinese, which, according to the app’s translation, meant “linking successful!” The app’s English settings indicated that Strava was connected. However, nothing appeared on my Strava page. This is extremely frustrating, both for my everyday use of Strava and for me personally, as I could use Strava to upload more detailed data about my activities than simply viewing them in the Speediance app.
Is the Speediance strap worth the money?
The $159 price tag doesn’t convince me. The temperature measurement features on the Speediance strap could be useful for those who want to track the impact of heat on their workouts, especially in the summer. It doesn’t track heat adaptation, so it can’t replace the Core 2 sensor, but I appreciate that the Speediance offers more than just another heart rate sensor.
But the shortcomings (especially the lack of Strava connectivity) are still a dealbreaker, and I haven’t been able to get much information about how well it measures heart rate. If I could overlook them, I don’t think this bracelet is significantly worse than the Polar Loop or Garmin Cirqa, which are slightly more expensive. But I definitely can’t say it’s better than the Fitbit Air or Whoop. Perhaps this device will improve over time, but for now, I’m more inclined to observe improvements than recommend it in its current form.