7. Seated leg extension
Much like the back extension, people mistakenly believe the leg extension prevents injuries. Instead, it places dangerous torque on your knee joints because it pulls your shins back as you lower the weight.
Also, knee extensions develop an imbalance between your quads and hamstrings, which causes knee problems. As you move, your quads always work with other muscles, never in isolation—even when you kick a soccer ball, you flex your hips, twist your trunk, and stabilize through your other leg.
The best way to strengthen your quads is to involve your entire lower body as well.
Use these instead:
Front Squat (shown in the picture above)