Whether squatting "ass to ground" (ATG or full squat) is more beneficial, or hazardous, than squatting with upper or lower thighs parallel to the ground (parallel or half squat) is a perennial question in weight training and weightlifting circles. Here is my view of it.
In ATG you go right down so that your butt is closest to the ground. This requires almost maximum flexion then extension of the knee joint under load as you lower then push "out of the the hole" to stand upright. With parallel and half squats you only go low enough so that your thighs are parallel to the ground or even higher with knee joints at about 90 degrees or a bit more. Even less flexion is sometimes called a quarter squat.
The first thing to note is that if you do Olympic lifting -- snatches, and clean and jerks -- the ATG or full squat is a part of the formal lift protocol and technique that you need to learn.
Beyond that, there is a common belief that ATG squats are superior to parallel or half squats because the full range of motion promotes balanced and superior muscle and strength development. The implication is that parallel squats don't involve the hamstrings and gluteus (butt) muscles like full squats and therefore you get a muscle strength imbalance between the quadricep muscles at the front of the thigh and the posterior chain, which includes the hamstrings and the glutes. This belief seems to be widespread because it's repeated regularly.
I could find no justification for this position. In studies of muscle activation comparisons between half and full squats, the main hamstring muscle, the biceps femoris, is involved almost equally in full or half squats. The main butt muscle, the gluteus maximus, is involved slightly more in the full squat but full squats are likely to utilize less heavy weights so that any general advantage in muscle or strength development may be minimal for full squats. And somewhat contrary to widespread opinion, the rectus femoris muscle of the front of the thigh -- in one study at least -- got hammered twice as hard in the ATG squat as the parallel squat. Muscle imbalance development with parallel squats is unlikely to be a problem. In this context one could almost argue that full squats are more likely to cause muscle imbalance by emphasizing the rectus femoris compared to the posterior chain.
Finally, some sports medicine authorities claim that full squats can damage the knees. Experienced Olympic lifters tend to dispute this claim -- they have the experience to know -- and there is little medical evidence to support the idea that full squats are inherently dangerous. Even so, there are additional compression forces involved in full squats, so for novices starting out, or for people who have less than ideal biomechanical knee joint structure or pre-existing injury, caution is warranted. But that goes for any exercise, including parallel squats. If it hurts, don't do it. Proper form and technique is paramount for injury prevention.
Summing up, there are few compelling reasons to exclude either full or half squats from your program. Naturally if you train for Olympic lifting you will need to do full squats. I mix it up by doing both.
Photo by Ryan McVay / Getty Images
J Strength Cond Res. 2002 Aug;16(3):428-32. The effect of back squat depth on the EMG activity of 4 superficial hip and thigh muscles. Caterisano A, Moss RF, Pellinger TK, et al.
12th Annual Congress of the ECSS, 11-14 July 2007, Jyväskylä, Finland. An electromyographic analysis of four methods in squat training. Sogabe Akitoshi (Konan University, Japan)