Researchers from the University of Southern Denmark investigated how eight weeks of high-intensity interval training affect the structure of mitochondria—the parts of muscle cells that produce energy.

Unless you've been living under a rock – or haven't checked your Insta feed – for the last few years, it's likely you've heard of high-intensity interval training, or HIIT. It's the type of training ...

Here's who it might help and harm, and how to tell if you're overdoing it.