Scientists define "vigorous" activity as anything with a metabolic equivalent (MET) intensity level greater than 6.0. Your MET can range from 0.9 (sleeping) to 18 (running at 10.9 mph-a 5.5-minute mile). Try working these vigorous activities into your routine.

RUNNING

Add some stairs and boost the burn by about 90 percent.

BIKING

Cruising around doesn't count. You've gotta be doing at least 12 mph.

STEP AEROBICS

One set of risers (6 to 8 inches) is plenty to get your heart rate pumping.

JUMPING ROPE

It doesn't matter how fast - just keep jumping. If you're not feeling coordinated, ditch the rope and circle your arms.

BOXING

Want to get more out of it? Imagine that the punching bag is your ex-significant-other.

TENNIS

Lose the partner. A game of singles burns about 33 percent more calories than doubles.

SOCCER

Playing goalie? Jog in place during the game (that is, when the ball's not coming your way).

SWIMMING LAPS

Try alternating strokes between butterfly, backstroke, breaststroke and freestyle to maximize your calorie burn.

BACKPACKING

Carry more than 10 pounds and you'll blast about 7 percent more calories.

CANOEING

You don't have to be an expert, but try to row at 4 mph or faster.

MOUNTAIN CLIMBING

Not the adventurous type? Walk hard up a big set of stairs and get about the same heart-pumping (though not as thrilling) effect.

ROLLER-SKATING

Pick up a pair of inline skates instead - you'll up the intensity by about 71 percent.