Go hard. Rest briefly. Go hard again. That's the essence of high-intensity interval training, from the 400-meter track repeats elite marathoners do to boost their aerobic capacity to the gut-busting CrossFit-style weight circuits used by NFL players to build strength and power. Now, after decades of research, exercise physiologists are finally explaining the outsize health and fitness impact of these ferocious little workouts – vastly greater, for every minute invested, than longer, slower efforts – and how the rest of us can apply them to everyday life. "Balls-to-the-wall efforts remodel the cardiovascular and skeletal muscle metabolic systems," says Martin Gibala of McMaster University in Ontario and the author of more than a dozen research papers on the subject – meaning they make your heart, lungs, and muscles stronger in short order. "You can get a lot of fitness benefit with these very short-burst workouts," says Steven T. Devor, associate professor of kinesiology at the Ohio State University. Same for the health benefits: "It's becoming quite clear that you get lower body fat, lower risk for disease, lower LDL cholesterol and higher HDL, and lower blood pressure," he says.
You don't even have to go "balls-to-the-wall." Researchers have since tested a variety of less extreme but slightly longer routines – two to four minutes of output divided by equal or shorter rest intervals. "The reality is they all pretty much work, as long as you can achieve high intensity and the recovery is long enough to catch your breath but not long enough to sit down and fully rest," says Chris Jordan, a former fitness trainer for the U.S. Air Force and now the director of exercise physiology for the Johnson & Johnson Human Performance Institute.
As for which exercises and routines work best, think in terms of three broad categories used by elite athletes: bodyweight-only circuits for core strengthening, free-weight and gymnastic combinations for muscle mass, and cardio intervals to burn more fat and boost lung capacity. There's one caveat: Stick to "compound multijointed movements," says Devor, "meaning you're not going to stand there and do old-school bicep curls." Aside from the fact that muscles can't function in solitude, single-muscle movements wouldn't push the heart and lungs enough. In all, you need to keep the intensity up with well-placed rests, and mix the muscle groups. Here are three workouts to get started.