According to Peter Jaret in a New York Times article, the high-intensity interval phase is typically 1-4 minutes of exercise at 80-85% of your maximum heart rate and should be long and strenuous enough that you are out of breath. Recovery periods should last long enough for your pulse to come down, but not long enough for your pulse to return to its resting rate. This is consistent with how I was taught to do interval training by a personal trainer. You want to remember to adequately warm up before the first interval, and you can use interval training workouts any way you want - running, cycling, swimming, on elliptical trainers, even walking if you alternate a speed walk and slow walk. Joggers can alternate walking and sprints. Swimmers can complete a couple of fast laps, then four more slowly. Many coaches recommend varying the duration of activity and rest. Be sure to include a cool down period after the last high-intensity interval, typically 3-5 minutes of a low-intensity jog or walk.
Coaches advise that, ideally, people should not do interval work on consecutive days. More than 24 hours between such taxing sessions will allow the body to recover and help them avoid burnout. Anyone in good health may consider doing interval training once or twice a week.
For anyone with heart disease or high blood pressure — or who has joint problems such as arthritis or is older than 60 — experts say to consult a doctor before starting interval training.
I do interval training several times each week, first thing in the morning. I warm up by walking to a park that’s about a mile from my house. When I get to the park, I run intervals with my dogs, so it’s a fun workout for all of us. By the time I start working I’m energized and can think clearly!