The Back Squat
The back squat requires the structures of the lower body and core to work synergistically. Optimal performance requires an adequate range of motion at the ankles, hips, and knees; superior lower-body strength; and a tremendous amount of core stability. - CrossFit.com
Hip drive and the posterior chain
The back squat is literally the only exercise in the entire repertoire of weighted human movement that allows the direct training of the complex movement pattern known as hip drive.
“Posterior chain” is a term that refers to the muscles that produce hip extension—i.e., straightening out of the hip joint from its flexed (or bent) position in the bottom of the squat. The muscles that accomplish hip extension are the hamstrings, the glutes, and the adductors or groin muscles, and together these are referred to as the posterior chain.
The initial movement up out of the bottom of a full squat is hip drive, and is best thought of as a shoving-up of the sacral area of the lower back, the area right above your butt. This is the hardest thing to teach in my method of squatting, and by far the most important.
This is because the squat is the only exercise in the weight room that trains the recruitment of the entire posterior chain in a way that is progressively improvable, and that is one of the things that makes the squat the best exercise you can do with barbells and, by extension, the best strength exercise there is.
Build to 1 RM Back Squat