Probably no weight training movement has been more maligned than the back squat. Over the years it has been blamed for ruining knees and backs, causing disk problems and building a big butt. There is absolutely no ground to any of these claims when we are talking about squatting properly. For one, squats do not destroy the knees or spine.
Squats that are bounced out of at the bottom are bad for the knees, and loading up on tremendous weight and doing partial squats is a good way to overload the lower lumbar area of the spine. But squats done with a weight that you can do in good form and going past parallel is the best way to ensure that you avoid any issues with knee and back problems.
Shallow squatting is one of the worst things you can do, because you tend to keep adding weight and then - bang - one day, there will be an attempt that will injure you because you did not train the movement in a full range of motion and develop the stabilizer muscles involved properly. Partial squats can also cause an imbalance where the quadriceps get stronger out of proportion to the strength of the hamstrings, which in turn contributes to pelvic imbalances and low back issues.
So, stay away from the nonsense you see in some gyms where the guys is doing heavy "partials" with his "partner" bear-hugging him from behind - this is useless for athletic conditioning and muscle building.There are many who feel the deep wide stance squat done by powerlifters is the only way to squat. It is true that one can typically move bigger weights squatting like this, but we are interested in the muscular development of the thighs. I personally feel that the "Olympic" style squat with the bar held high on the traps puts a far greater load on the thighs and less on the low back and hips, and will avoid thickening this area as well.
The most important consideration is that the trainer keep the bar so that it is constantly at a 90 degree angle straight above the mid-thigh and also the arches of the feet (see image below to see the line of gravity drawn through the mid thigh). Find the bar height placement that best suits your individual body mechanics and keeps the bar in line over the femur and mid-foot. One advantage too with the Oly style squat is that one can get far greater depth and range of motion from this method. keeping the bar as low as some powerlifters do, on the shoulder blades practically, doesn't allow for the best balancing of the load between the hips, legs and back.
This is why so many of them have to do a fair volume of good mornings to compensate. To see proof of what kind of squats build better thighs, all one has to do is look at the thighs of the average Olympic lifter vs powerlifter. In many cases the former has well balanced thigh development and the latter has thighs that look big at the top and small below mid-thigh - why? Because all of the powerlifters strength in the squat comes from the posterior chain - the hips and not the thighs.
With low-bar squats one can lift more weight because the leverage is shortened. Add to that, Olympic lifters focus mainly on the front squat, which I would argue is the best overall quad builder. Our focus isn't on lifting weights but on building bigger, proportionately developed muscles while gaining in athletic ability and overall strength.
How To Squat Properly
Before removing the bar from the rack, grasp the bar very tightly with both hands and use as narrow a grip as you can, which will make the traps bunch up and avoid any stress on the neck. Also with the hands in tight, you will be able to push the chest out and thus cut down on the tendency to lean forward.
The feet and hips must be directly under the bar before the bar is lifted off the racks and stay like that throughout the movement. Place the feet about shoulder width apart and toes pointed slightly outwards. The head is held in a neutral position, not looking down or up. A big breath should be taken and the chest pushed outward and shoulders back. The final step involves straightening and locking the muscles of the back before pushing upward on the bar.
After lifting the bar off the rack and moving backward in as few steps as possible, the feet, hips and torso should still maintain their position and the whole body must be kept as tight as possible. You should also concentrate on squeezing your glutes as much as possible and keep them tight as well. (this helps in all full body movements: bench, military press, cleans, deadlifts, etc!) The elbows should be behind the bar.
The descent is initiated by "breaking" the hips and moving them backwards and downwards as if you were sitting down on something. Never should the movement be initiated by knee movement. After the hips break downwards and backwards the knees will bend automatically. You should feel the weight balanced over the middle of the foot - not the balls of the feet or heels. The downward movement should be controlled and you should never allow your body to drop. Stay tight.
The knees should travel directly out over the feet. This notion that the knees should always stay behind the toes is rubbish. As long as the hips break backwards first, there will be no exorbitant strain on the patellar tendons when the knees slightly extend over the toes. This is the norm in the Olympic squat.
Once you have gone deep enough - just below parallel - its time to reverse the downward motion by driving up by, again, initiating movement from the hips. As in the descent, the ascent of the squat is started with hip movement. The heavy weight causes the hips to move up and back and in turn causes both the knees to move toward one another and the torso to bench forward.
To counteract these natural movements, and in order to keep the bar over the proper base of support - directly over the arches - the hips must be moved forward under the bar. This is best done slowly by pushing the knees outward once the upward acceleration has been established bu the hip drive. At no point should the torso be relaxed. You should slightly slow the motion just before lockout to act as a brake on the explosive movement.