WHEN GUYS SET out to start losing weight, their mission usually begins in one familiar place: the treadmill.

In fact, this old gym touchstone—that cardio = weight loss—is so well-worn that it’s practically a law of the universe. We hear guys say it over and over: “I need to keep my weight down, so I’ll do more cardio.” Then they trundle over to their gym’s Treadmill Row, flick on ESPN, and settle in for 45 minutes of mostly unpleasant sweating.

But here’s the thing: That’s not necessarily true. Cardio doesn’t necessarily translate to losing weight. In fact, one could argue that they’re two totally different modes of training.

“I think a lot of people look at cardio as the easy way out toward weight loss,” says Matthew Ibrahim, C.S.C.S., a strength coach at Boston Underground Strength Training. “Going for a run outdoors or on the treadmill without any direction or specific goal is easy to do, and we often see people who do this for years without any real body composition change.”

Sound familiar? Truth is, while cardio is one of the quickest ways to trim down, most guys don’t go about it correctly. Here’s why you shouldn’t necessarily equate cardio with losing weight—and how you can make cardio work translate to a smaller waistline.