Is it better to break my exercise sessions up, or exercise for a longer period?

(from Ken Raich raichk@ix.netcom.com)

There are TWO distinct thoughts on this issue: Both present fairly reasonable arguments.

First, It takes 15-20 minutes to get your metabolism into the fat burning zone many people desire for an aerobic workout. Once you have reached this level, your body tends to obtain more of its energy from fat than carbohydrates. Using this argument, a single 90-minute workout will allow you to exercise in this "fat burning" zone for at least 70 minutes while two 45-minute sessions would allow you to be in this zone for at least 50 minutes (2 x 25). This logic supports a single, longer workout.
Second, For 6 or so hours after a workout your body remains in "afterburn" mode, burning more calories at rest than it would have if you had not worked out. Using this logic, two sessions would produce two afterburn periods and result in more fat being burned than would be for a single workout session.
The real bottom line is that if you exercise for 90 minutes a day, you're interested in more than just fitness(don't get me wrong, this is perfectly OK). To stay in reasonable cardiovascular shape, you need to perform aerobically for 20-30 minutes at least 3 times a week. If you wish to be in better than "the minimum acceptable" shape, remember that the returns are not geometric (you won't be in twice as good shape if you workout for twice as long). Therefore, if you're going to workout for 90 minutes a day, splitting the time between one or two sessions probably doesn't make a significant fitness-level difference.

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