Squat is the crowned king of lower body exercises. The benefits of this powerful exercise are associated to hypertrophy - overall increase in lean muscle mass, calorie expenditure and hormonal release. Squats invariably can be performed with different variations, these variations include Back Squat, Front Squat, Ball Squat and Unilateral squats. The movement involved in squatting is a co-ordinated natural movement that is normally practiced while performing activities of daily living. In an Asian population, an average individual do certain movements to a full flexion of the knee (111degree – 165 degree). A perfect example would be sitting cross legged; one requires 90 degree – 100 degree of hip flexion and a full knee flexion . However, there has been a healthy debate amongst coaches, strength athletes and fitness professionals that squatting is bad for your knees. Specifically, the knee joint goes through a lot of stress that will invariably compromise the stability of the knee joint, if it passes the toe line.
Squatting is a compound and a multi-joint movement exercise that requires the involvement of three joints – ankle, knee and the hip where flexion and extension is involved. Moreover, the movement is initiated on all three joints. Any pre-dominance of any joints, especially, during the initiation of the movement would result in the over loading of that specific joint.
Restricting the knee joint (not ahead the toe-line) while squatting invariably with a dominance on hip flexion. The body has to then compensate that leads to increase spinal loading, especially, lumbar loading. Researchers have suggested that the knee torque was slightly less in squat when the forward displacement of the knee was restricted . However, hip torque was substantially increased. They suggested that restricting the knee resulted in the forces being inappropriately transferred to the hips and the lumbar region. Therefore, appropriate joint loading while squatting may require freedom of movement on the knees i.e. to move slightly passed the toes.
Squatting deep is bad for the knees
In our activities of daily living (ADL), we normally squat deep and this happens on a regular basis. Researchers have suggested that the force experienced on the knee in 70, 90 and 110 degree flexion of the knee joint. 70 degree can be explained as a quarter of a squat, 90 degree can be explained as a parallel squat where the femur is parallel to the floor and 110 degree can be explained as a full squat. In 110 degree, knee flexion the knee passes the toes. This study suggested that the amount of stress or force experienced on the joint was not that significant in all the three range of movement. In simpler terms, the amount of stress experienced by the knee joint did not warrant an immediate contraindication on this particular movement restriction.
Now as a trainer we need to be aware that the tibiofemoral and patellfemoral compressive force are greatest in the closed kinetic chain exercise at full flexion and squat is a good example of a closed kinetic chain exercise. Interestingly, the greatest tension on the anterior cruciate ligament (ACL) was actually found in the open kinetic chain exercises, at or near the full extension position 
Now the question that keeps arising is should we allow a clients’ knees to go past the toe line? The answer to this question is subjective. An individual with specific knee based issues have to go through rehab to develop the muscles and ligaments surrounding the knees. The job of fitness trainers are multidimensional and the trainer has to be aware of the clients’ condition to assess and analyse the functionality of the exercise. This can be only done through proper counselling and health screening of the clients. It is very important to teach a member to squat using their body weight, as this is an important preconditioning before loading the members with weight.
It is common knowledge that the body is not designed to work in just one plane but it is designed to work in quite a few planes together. Hence, exercises like the hack squat, leg press and smith machine are fixed axis machine and makes the body adjust to one line of axis. This unnatural movement makes the body to compensate and in time may lead to injury.
 S.J. Mulholland and U.P. Wyss (2001). Activities of daily living in non-Western cultures: range of motion requirements for hip and knee joint implants. International Journal of Rehabilitation Research, 24, 191 – 198.
 Fry AC, Smith JC, Shilling BK. (2003). Effect of knee position on hip and knee torques the barbell squat. Journal of Strength and Conditioning Research,17(4): 629-633.
 Escamilla R, Fleisig G, Zheng N, Barrentine S, Wilk K, Andrews J. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sports Exerc. 30(4); Pp 556-569. 1998.