Common knowledge is overrated. Hence why common accomplishments are just so-so. Don't let stale fitness rules hold back your fitness gains. Break these four to get more from your gym time.

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1. Cardio is the King of Weight Loss

Those tortuous treadmill sessions might not be worth it, after all. In a recent Obesity study, researchers at the Harvard School of Public Health followed 10,500 healthy men over the course of 12 years and found that the guys who spent 20 minutes a day lifting weights put on less belly fat than those who performed cardio for 20 minutes per day.

Why? In a word, muscle, says Jacquelyn Brennan, C.S.C.S., a personal trainer to collegiate and professional athletes and cofounder of Mindfuel Wellness. While, during your workout, cardio often burns more calories than lifting weights, strength-training comes with an awesome "afterburn." After each strength-training workout, your body burns extra calories for up to three days as it works to repair and grow your muscles, she says. Meanwhile, cardio builds far less muscle and, in some cases, can actually contribute to muscle losses, not gains.

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And, over the long term, the more muscle you have, the greater your metabolic rate."Strength-training an average of three times a week on a regular basis at a moderate intensity for 20 to 30 minutes increases your resting metabolic rate by 5 to 7 percent," she says. That equals an extra 110 to 120 calories burned each and every day—just sitting on your keister.

2. Your Knees Should Never Move Past Your Toes

If you have healthy knees, there's no reason not to let your knees eek past your toes, Brennan says. After all, when you walk up a flight of stairs, do your knees always stay behind your toes? We're thinking not.

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Meanwhile, there's plenty of reason to let your knees creep forward: It allows you to squat and lunge more deeply, which translates into greater strength gains in your legs, butt, back, and even ankles, she says. Plus, research out of the University of Memphis shows that unnaturally forcing your knees back behind your toes increases the stress placed on your hips and back.

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3. Never Squat Below Parallel

If your personal trainer tells you that squatting below parallel is bad for the knees, fire him right then and there. Your knees' protective ligaments are designed to help you squat low. And, get this, those ligaments don't do much of anything to protect your knees when your thighs are parallel to the ground, says strength coach Tony Gentilcore, C.S.C.S., cofounder of Cressey Sports Performance. Problem is, this most vulnerable spot is where most guys stop their squats. Far safer for your knees—granted you don't have any previous injuries, mobility deficits, or postural issues—is actually dropping below parallel before pausing and pressing back up, he says. What's more, dropping low is the key to a better backside, Brennan says. By taking you through a fuller range of motion (which, by the way, is much more functional than stopping short), deep squats burn more calories and recruit more muscle fibers, especially in your glutes.

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4. You Should Always Stretch Before a Run

It feels good before a run, but static stretching (think: bend and hold) can actually prevent you from hitting new PRs, Brennan says. That's because static stretching is essentially like stretching out a rubber band—and then trying to run on it. In fact, a recent Scandinavian Journal of Medicine and Science in Sports review of more than 100 studies on static stretching concluded that static stretching before a workout reduces strength, power, and explosive performance.

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"Dynamic stretching is preferred because it increases blood flow, increases range of motion, and can improve your athletic performance," she says. Before your next run, warm up with some hip circles, leg swings, monster walks, skips, high knees, and butt kicks.

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