The myth: It's the safest way to work your quadriceps, or thigh muscles.
The truth: Physiologists at the Mayo Clinic determined that leg extensions place significantly more stress on your knees than squats. Why? Because the resistance is placed near your ankles, which leads to high amounts of torque being applied to your knee joint every time you lower the weight. What's more, Auburn University scientists found that people who squat long-term have tighter, stronger knee ligaments than those who don't squat at all.
The alternatives: Free weight squats, split squats, and lunges-performed with perfect form-are all better choices for working your quads and protecting your knees.