So I've discovered the secret of all those ultra-short workouts that one segment of the fitness industry keeps trying to sell us.

Ready?

They take longer than the 10 minutes they say they do. On top of the actual workout, there's the warm-up and cool-down. They never mention that, but if you don't want to blow a joint or worse, you need to tack that on to the beginning and the end.

And did I mention that the 10 minutes (I don't know why 10 is so popular, unless some marketing consultant has determined that it seems like a short time, without seeming inconsequential) usually involves only one area of the body?

For example, Amy Bento's recent addition to the Ten Minute Solution DVD series is really five 10-minute workouts — one for shoulders and arms, one for abs, one for thighs, one for aerobic fitness, one for flexibility.

Put them together and what do you get? A fine workout. It's just not 10 minutes long. And, in case you were wondering, 10 minutes a day won't get you a body like Madonna's.

The "Total Workout in Ten!" DVD released by Women's Health?

That'd be four 10-minute segments — upper body, lower body, core strength and a "cardio blast."

Now, don't misinterpret. Ten minutes is better than nothing, and if that is all people can spare for themselves, so be it.

But I find the marketing disingenuous, a pitch that may get people off the couch but seems likely to discourage them in the long run when they realize not much is happening.

There are some sound principles underlying short workouts. "Burst training," or "high-intensity interval training," is a staple of athletic coaching and can help reasonably fit people who want to vary their routine or better their performance. It involves alternating short bouts of intense effort with a longer recovery period: Think of a 30-second, all-out sprint followed by a slow, 90-second jog, followed by another sprint, and another jog and so on.

Intervals can be worked into running, cycling, swimming and a variety of other activities for which endurance and speed matter; and by sort of supercharging the metabolism, they help with weight loss and maintenance as well.

These shorter, more intense sessions are typically only part of a weekly program. You wouldn't want to exercise this way every day. Muscles need recovery time, and the risk of overtraining or injury would increase.

The most recent American College of Sports Medicine recommendations recognize that shorter, more vigorous workouts have a role in overall fitness but that bouts of less than 10 minutes aren't so effective.

Keep in mind: Their basic recommendation remains 30 minutes of exercise a day.

To get a sense of how a rigorous but time-efficient workout could be structured, I visited Rio Sport & Health in Gaithersburg, Md., recently and sat in on trainer Kenn Kihiu's "80/20" class.

The name was drawn from Italian economist Vilfredo Pareto's observation that about 80 percent of an outcome often comes from about 20 percent of the resource being used.

He uses a device called "MyGym" for the half-hour class — an aerobic stepper with resistance bands crisscrossed beneath it and different handles and straps attached to the ends.

There is not a second's rest, and by combining exercises and compound movements — a lunge coupled with a biceps curl, a reverse crunch coupled with a triceps raise — he keeps the heart rate elevated. In my case, it averaged in the mid-140s, peaking at about 160, and I used close to 500 calories.

More telling, however, was Kenn's attitude about the workout. If students come twice a week as he recommends, he tells them they should still do at least one other day of training with free weights, as well as other cardio sessions.