You get greater overall muscle and strength gains from the squat than from any other exercise. Because the exercise involves the largest muscle group in your body (glutes, hamstrings, quads) and requires a high amount of energy, the movement triggers what is known as a neuroendocrine response. In short, this response causes the release of extra testosterone and growth hormone into your bloodstream, which in turn will help you to build muscle not only in your legs, but your upper body as well. In addition, squats require a great deal of core strength.
There are some out there who will tell you that squats will ruin your knees. This is wrong. There have been numerous studies conducted which prove that on the contrary, when done properly, squats not only strengthen and tighten your knee ligaments, but are also very effective in building core and upper body strength. It has actually been determined that squats place less stress on your knees than leg extensions! And perhaps my favorite part, squatting is a functional movement. I can't think of a single example where a leg extension would translate over to something you would have to do in a real life situation. On the other hand, you squat every time you get out of or sit down in your chair at work.
My recommendation, do squats 2-3 days a week in conjunction with a full or upper body workout for stronger knees, a tighter core, and improved lower and upper body strength.
Yours In Health,