A very underused (or wrongly used) exercise by many in the health and fitness community, the squat should be one of the foundational movements for any strength, fitness, or bodybuilding routine. The squat keeps the knee joint healthy and works almost every muscle in the body as either a primary mover or in stabilization.
Prepare for the squat by commandeering a squat rack, cage, or stand and setting the bar hooks/uprights and safety catches to the correct height. The hooks or uprights (which are what holds the bar before and after the lift) should positioned so the bar is slightly under shoulder level. This allows the lifter to lift the bar off the hooks by just standing up. The safety catches should be low enough to allow full range of motion without the bar hitting them, but high enough that the lifter can relax in the bottom position and set the bar on them if he or she fails the lift.
Position the body under the midpoint of the bar. While many people have the bar across the back of the trapezious, this author prefers the put the bar on top of the traps, on the natural shelf that appears there when the shoulders are pulled back, as they are to keep the hands on the bar. Either placement is acceptable; the lifter should use whichever feels more secure.
Stand up under the bar, lifting it up off the hooks/uprights, and take a step back. Position the feet slightly wider than shoulder width, toes pointing slightly out.
Begin the descent by bending at the knees. Keep the back arched, chest up, and weight on the heels while continuing to descend.
The bottom of the squat is at least when the line drawn between the hip crease and knee is parallel to the ground, or when the hamstrings come in contact with the calves. For most trainees it is difficult to reach the bottom of the squat while maintaining good form, especially when first attempting it. However, barring certain medical conditions, through continual practice, and improving flexibility through stretching, it is possible for pretty much anyone to work up to performing a full squat.
From the bottom position, simply stand up, pushing through the heels of the feet. Make sure to extend both the knees and hips at the same time, so that the squat is one smooth movement.
Once standing upright, return the bar to the catches, or do another rep.
Some people, either due to ego, misinformation, or both, will do squats with a very small range of motion. They end up doing only quarter or half squats. Those who partial squat for ego like to have more weight on the bar than they would have if they did full squats. In the end, however, they're really just short-changing themselves, missing out on strength and muscle development through the full range of motion. Other people mistakenly believe that full squats are bad for the knees and that partial squats are safer. In fact, studies have shown that the opposite is true. There are forces that can damage the knee when squats are improperly done. In a partial squat, these sheering forces are entirely on the knee when changing from down to up. However, in a full squat, once the upper leg drops to parallel or below, the forces are distributed to other tissues around the knee, which reduces the chance of injuring the knee and strengthens the whole area.