Should you do half or full squats?
This topic bothers me a bit because I often see people dispensing and accepting bad advises, without checking to see if they are sound or well-researched. This is especially so in the internet age where people can easily find any exercise routine online. People will follow as long as the program is from someone who looks good and/or is a famous personality, never mind their fitness credentials if they have any in the first place. These followers would then forward the same thing to the rest. Blind leading the blind.
Before I go into the discussion, I need to clarify what exactly are the half squats and full squats.
A correctly done full squat is one that at the lowest point of the descend, the hamstrings are touching the calves and the butt is close to the floor like this:
A half squat is one where the lowest part during the descend, the thighs are running parallel to the ground, like this:
Note the depth difference.
Back to the question: is half-squat better than full squat because the former puts 'less' stress on the knees and safer as claimed by many 'experts'? Such 'experts' include many fitness trainers, fitness enthusiasts, and medical doctors who often teach people to avoid full squats, in order to prevent injuries to the knees. The prevalence of such advice is so common that it can be heard everywhere.
Just google 'squat exercise' and you can see almost all exercisers and drawings are demonstrating 'half squats'. Rarely can I find a photo depicting a properly done full squat ( I did eventually after scanning through like 100 pics).
Look! Everyone is doing half or partial squat. Surely 99% of the fit-looking models and their writers can't be wrong. (Oh btw, speaking of fitness models, you would be surprised many of them don't even exercise, they just happened to look good. How do I know? I have written for many health magazines, worked with such models on many occasions and they self-admitted).
Now, if one has a case of bad knee due to ACL/PCL, meniscus, flexibility issues etc, such advice is warranted. Otherwise, doing half squats consistently may do you more harm than good in the long run, ending with the knee issues just mentioned. Such irony.
Hear me out.
Full squats, done properly, are one of safest and the healthiest thing you can do for your leg muscles and your body, for that matter. Even if you can only do half-squat or even only quarter-squat, one should progressively work towards increasing the depth of the squat.
Here are the reasons why going deeper is better:
Full squats build more shape in your butt.
Full squats activate more glute muscles, hands down . So, if you want a fuller-looking, sexy set of bums, squat low and deep.
Full squats build stronger legs all round.
Full squats activate the hamstrings, adductors, and glutes, so exercisers will develop a balanced set of leg muscles. In contrast, partial squatting contributes to an imbalance in the quadriceps to hamstring strength ratio. This imbalance increases the risk of hamstring tears.
Full squats are better for knee health.
This is an important part of the argument.
This imbalance caused by doing partial squat also increases the risk of an ACL tear. An increased quadriceps to hamstring ratio is a major factor in increasing the amount of anterior tibial sheer. Research suggests that “the deep squat presents an effective training exercise for protection against injuries and strengthening lower extremities” .
In addition to knee pain, the squatting depth can affect the amount force put on the knee. The amount of force put on the knee can cause an individual to not squat as low as they desire due to discomfort. Individuals have various squatting depths to produce more or less force on the knee.
There two major forces acting on the knees.
Shear force. It is force acting from the sides. In the case of the knee, shear force would be loads that go crosswise to the shinbone
Compression force. It is force acting downwards. In the case of the knee, compression force would be loading along the length of the bone.
Important: Joints are better able to withstand compressive forces than shear forces. Imagine this: if you apply a large compression force on a spring (which mimics the meniscus as it is the only concern for compression force), would it cause much damage to the integrity of the spring?
The answer is an obvious no.
On the other hand, if you apply large shear pressure to knee, the damage exerted would be great as it could hurt the ligaments (anterior cruciate ligament, the ACL, and posterior cruciate ligament, the PCL) and the meniscus. High shear force is the reason why leg extension exercise causes knee pain. Ouch!
The diagram suggests that the greatest shear forces on the knee will occur with a squat at different knee angles.
As you can see, the forces acting on the ACL peak at between 15-30 degrees and then reduce considerably past the point of 60 degrees. Similarly, the forces acting on the PCL rise gradually to a knee angle of 90 degrees, and then drop significantly thereafter. Furthermore, when holding a squat position at 90 degrees, as often occurs in transition between the eccentric and concentric phase of the exercise, the time the knee is exposed to the maximum levels of shear force is also increased. Thus for many, deep knee squatting may indeed be better for knee health than squatting to 90.
Full squats are safer for the spine.
"Concerns about degenerative changes of the tendofemoral complex and the apparent higher risk for chondromalacia, osteoarthritis, and osteochondritis in deep squats are unfounded. With the same load configuration as in the deep squat, half and quarter squat training with comparatively supra-maximal loads will favour degenerative changes in the knee joints and spinal joints in the long term" 
We were meant to squat fully
Every single human being knows how to squat perfectly right from the young. Here, my daughter and her cousins show you how it's done (circa 2008).
That is until we have chairs, toilet bowls and stools. If you look at certain cultures that have not had exposure to seated in chairs from an early age, you will see that they rest and eat in a full squat position, even as they grow old. This where 'Asian squats' come from. If going by the logic of full squats can damage knees, explain this: