Fitness bands like the Apple Watch and the Fitbit aim to track your vitals, like heart rate. But early models weren't all that accurate.

苹果手表和Fitbit等运动手环的设计初衷是追踪你的生命体征，比如心率。但是运动手环的早期型号却并不那么精确。

"We thought of them a little bit like random¹ number generators². They really didn't seem to be providing anything that bore any relationship to heart rate," said Euan Ashley, a cardiologist who studies wearables at Stanford University.

斯坦福大学研究可穿戴设备的心脏病专家尤安·阿什利表示：“我们认为这些手环有点像随机数生成器。它们似乎并没有提供任何与心率有关的数据。”

He and his colleagues have now tested seven newer fitness bands -- from brands like Apple, Fitbit and others -- and he says those heart rate stats have gotten way better. "Yeah we were pleasantly surprised actually by how good the accuracy of the heart rate monitoring was."

目前，他和他的同事已经测试过苹果、Fitbit和其他品牌的7种新型运动手环，他表示，所监测到的心率数据准确多了。“是的，我们对运动手环的心率监测准确性竟然如此之高感到惊喜。”

For most of the devices, the error rate was less than five percent -- good enough for your doctor. But where all the devices failed to measure up was estimating calories burned.

大多数手环的错误率小于5%，对你的医生来说这足够好了。但是，在估算卡路里消耗值方面，所有运动手环都不合格。

Even the most accurate devices were off by 27 percent, compared to lab measurements of energy expenditure³. One device was off by more than 90 percent.

与能量消耗的实验室测量值相比，就连最准确的手环也有27%的误差。其中一个手环的差错率甚至超过90%。

If you think about going to the gym and working out for an hour and maybe that's around 400 calories, in reality that could be anything from 200 to 800.

如果你去健身房锻炼一小时，运动手环可能显示你消耗了大约400卡路里，但实际上你的消耗值从200到800卡路里都有可能。

And that's a big difference if you're thinking about somebody who's incorporating those estimates into their lifestyle and thinking about what to eat that evening based on the workout they did that afternoon.

如果有人将这一数据融入其生活方式当中，依据下午的锻炼情况来决定晚上吃什么，那就有很大差异了。

The reason for the discrepancy⁴, Ashley says, could be that we all burn energy at different rates—and that's hard to reckon from simple input⁵ stats like weight and height.

阿什利表示，产生这种差异的原因可能是我们燃烧能量的速率不同，而且这很难通过身高体重这类简单的输入数据估算出来。

"Some people are incredibly efficient and look incredibly elegant when they run. And others really clearly look like they're burning a lot more calories to cover the same amount of ground," he said.

他说道：“有些人跑步时很优雅，而且还能高效地燃烧能量。而其他人跑步时则很费力，虽然是跑同样的里数，但看起来却消耗了更多的卡路里。”

So if you own a wearable, it's probably safe to trust the heart data. What it can't tell you is whether your time on the treadmill⁶ really justifies⁷ that chocolate shake.

因此，如果你有可穿戴设备，它可能会提供安全可信的心率数据。但它无法告诉你的是，你在跑步机上的锻炼是否能消耗掉那杯巧克力奶昔的热量。