As Kenny points out, high intensity intervals are a good training technique, but there is some preparation to be ready for them.
For instance, on the Biggest Loser program, before admitting participants, they put them through a battery of tests first to make sure they are healthy enough to tolerate the weekly workouts seen on the show.
I have read of track teams that will not accept a new member who can not first run 5 miles. Does not matter the event, they want them healthy enough to handle any workouts.
The fitter you are, the more exercise you can tolerate without strain. I'm 57, with massive heart disease but excellent fitness.
Another one of the tricks to outstanding fitness is to limit very hard workouts to twice a week. One gains the most from adequate rest around good workouts.
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