This hidden Apple Watch stat tells you whether to exercise or rest

Imagine if your Apple Watch could tell you which days were best for you to do a workout, and what kind of workout you should do. Well it can, sort of, thanks to a hidden feature that few people have yet discovered or know how to use. Heart rate variability, or HRV, is a new metric that reveals your stress level and whether you have recovered from your last workout. It has been added to lots of high-end sports watches in recent years, including Apple Watch since watchOS 4 & iOS 11. Here’s how you can use it to optimize your training, reduce your risk of injury, and know when to take a well-earned rest day.

How do you know when you should train, and how hard? If you’re feeling aches and pains, should you take a break or train through it? A lot of people say you should listen to your body, and never work out if you don’t feel great. The problem with that advice is that if you took it literally, you might end up never working out at all. Running, cycling, swimming and most other sports come with an element of pain. That’s all a part of the deal. “No gain without pain,” as the saying goes. If it was easy, then everyone would do it. If you want exceptional results, you must push yourself harder than most people are willing to do. But how hard?

It’s a perennial problem for every athlete, whether you are a serious baller or a weekend warrior. Push yourself too hard and you risk getting injured, which could set you back months. Don’t push yourself hard enough and you’ll never make any gains. Heart rate variability is a way of detecting the level of stress your body is under. Generally speaking (and a little counterintuitively), the higher your HRV, the lower your stress levels.

Lots of different things put your body under stress. Some of the most common causes are sickness and worrying, which are obviously not good things. But stress is not always a bad thing. Exercise, for example, is good for you, but it also increases your stress levels, sometimes for many hours after you finish a workout.

So HRV can be a complicated stat to interpret. However, if you follow the trends in your HRV over time, you can start to recognize patterns in how it changes, and how your body is responding to exercise, anxiety and sickness. When your HRV is unusually low, that may be a sign that you are under the weather, in which case you should probably skip a workout. Or at least go for a low-intensity workout, or do something relaxing instead, like some meditation or yoga…

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