I’m just going to come out and say it: The aerobic machines at the gym are seeing way too much action. And while both men and women overuse the elliptical and treadmill, often it’s the ladies populating the cardio corral.

Whether you know someone who relies too heavily on steady state cardio to keep them in shape—or if even you’re guilty of this on occasion—I need to set the record straight: despite all the sweat and burn, going cardio crazy will not create strong, shapely muscles. That’s because there are two laws of physiology by which your muscles operate:

1. The all-or-nothing law. This means a muscle either activates, or it doesn’t (blow-drying your hair with your arm over your head means some muscles are working to do that task, but others aren’t). This is why compound movements like squats and pull-ups are more effective at building strength and size; they involve many more muscles.

2. The size law. This dictates that the smallest, slow-twitch muscle fibers (Type I) will do the work first, and only when the load has surpassed their ability to lift it, will the bigger, fast-twitch fibers (Type II) start to help out. Therefore, you must load your muscles with enough weight to require the bigger fibers—the ones with far greater potential for developing strength and size—to get to work.

But what about genetics? Are some of us predestined for more muscle and a better response to training? Sort of. Genes determine this much about muscles: how many fibers you get as a whole, the percentage of fast-twitch versus slow-twitch, and the shape of your muscle when fully developed. So maybe your bum is small and flat thanks to Grandma Jane, or you’ve never been a fast runner, but the baseline principles still apply to you: your muscle can grow, even if you are genetically challenged, you just need to give it the right environment. The fast-twitch fibers in your glutes, for instance, will never live up to their potential if you don’t give them the right stimulus. Your best body is dependent on how well you can apply the principles that govern your muscular potential.

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It’s important to understand that merely exhausting a muscle, or “feeling the burn,” to affect muscle size and shape is only part of the picture. Going back to where cardio is concerned, doing a ton of low intensity exercise for long periods of time means you aren’t engaging your larger fibers very much. Since most of us hover in the 50/50 range for slow and fast twitch fibers, training for shape and size means you must lift heavy enough to really engage your muscles.

“A shapely physique isn’t made on the elliptical, so cross the gym floor and lift something heavy.”

Here’s how to hit your muscle-building potential:

1. Prioritize exercises that use the most muscle mass. Those are your squats, deadlifts, presses and pulls. Let isolation work take second place.

2. Lift progressively. Take the time to learn technique and good form with lighter weight, but then progress!

3. Do enough work. A blend of power, strength, and hypertrophy training will guarantee no muscle fiber is left out. It’s important not only to work to achieve your goals, but to do the right kind of work.