All hail the barbell squat, the king of all exercises, according to many athletes. The high level of admiration for the squat is well warranted; not many exercises can match the high level of muscle stimulation of the entire lower body which is experienced with the lift.
While many see the barbell squat as primarily a quadriceps exercise, the muscles responsible for the extension of the hip are also heavily involved during the lift, namely the hamstrings, gluteus and spinal erectors. The core is also very much targeted, with the abdominals and obliques contracting to maintain a stable trunk.
The squat should ideally be performed so that the thighs reach parallel to the ground. By doing so, the glutes and hamstrings especially, but also the quadriceps, receive maximum stimulation. Some athletes place a box or bench behind them when squatting, which is set at a height which ensures they reach full depth. As they descend, they allow their backside to touch the box (to indicate they have reached the required depth, the athlete should never sit on the box) before ascending back to the starting position. This is known as a box squat.