No Time to Work Out? Get Fit in a Flash

"If you don't give your body a chance to recover between exercises, it must get in better condition in order to repair itself for the next bout of activity -- so you're automatically getting more out of each workout," says Spencer.

By decreasing rest periods, you can also do more work in the same amount of time, he says, and that means better (and faster) results.

Even in a 30-minute workout, Novak says, reducing rest periods will also increase your challenge level - which, in turn, will increase your body's ability to recover. So you end up in better shape without increasing your workout time.

"The idea is not to increase intensity, but to challenge your body by forcing it to recover more quickly," says Novak.

Franklin agrees: "A body at rest tends to remain at rest; a body in motion tends to remain in motion. So the more you move in any given time period, the easier it becomes to keep moving."

Workouts That Work Harder

According to the American Heart Association Choose to Move program, certain activities definitely yield more results than others. The general rule of thumb: The more vigorous the activity, the less time you need to do it to get optimum results. And the more leisurely your activity, the longer your exercise session should be.

According to Choose To Move, spending 15 minutes climbing stairs, jumping rope, or sprinting a mile will give you results equal to that of playing volleyball or touch football for 45 minutes, walking 1 3/4 mile in 35 minutes, or dancing fast for 30 minutes. And you'll get the same result from bicycling 4 miles in 15 minutes as from mowing the lawn for 45 minutes.

You don't even have to do the short bouts of exercise all at one time, Franklin says.

"You don't have to put the dollar bill in the piggy bank all at one time -- you can put in four quarters, and get the same benefit -- and exercise is the same way," he says. In fact, Franklin tells WebMD, there is some evidence that several shorter bouts of exercise may be better for reducing body weight and fat than one long workout.

When it comes to working out in the gym, Spencer says you'll get the biggest result from your efforts if you trade in treadmill walking for cycling or spinning.

"If you walk on the treadmill for the same amount of time you cycle, you may build cardiac endurance, but you're not building muscles the way you are when you're spinning," he says.

And, he says, any exercise that conditions the heart while building muscle causes your body to work harder -- even when it's at rest.