For the middle-aged or older, high-intensity exercise has its perils.

"Somewhere around age 45 for men and 55 for women, you worry about the dark side of exercise," he said.

"Studies are clear that when people have catastrophes, such as heart attacks, they are almost always related to inappropriately high-intensity exercise."

Sometimes high-intensity routines are just too uncomfortable to be habit-forming, he said.

"Yes it can be done. You can do the work in 10 minutes but then it takes you 40 minutes to recover."

Foster predicts the focus on high-intensity interval training will wane.

"It will be like seasoning in food: you want to feel like an athlete, so let's do some of it in the middle of the workout," he said.

Before you leap into high-intensity training, he urges, be sure to have at least six months of normal training behind you. Then gradually add five or 10 seconds that are a little harder.

The biggest risk is that people who've been sedentary will suddenly decide to get in shape with high-intensity training.

"For middle-aged and older people, high-intensity training can be a trap that leads to health problems," Foster said.

"The people who are moderately active on a routine basis are the people who don't get heart attacks."

(Editing by Paul Casciato)