There are some long standing myths in bodybuilding which simply refuse to die.
This is one of them.
It is believed that allowing the knees to travel past the toes while squatting puts a lot of stress on the knee joint and causes injury.
Let’s break this down.
In 2003, Fry et al. conducted a study in which subjects were made to perform 3 unrestricted squats and 3 restricted squats where a wooden board was placed in front of both feet so that the knees were prevented from moving forward past the toes.
They found that unrestricted squats produced greater forces at the knee joint compared to restricted squats. However, restricted squats resulted in greater forces at the hip joint since the subjects had to lean more forward and round their back to execute the lift. The latter can be problematic as the lower back is more susceptible to injury as compared to other joints.
To quote directly from the study: “While it is critical to protect the knees from unnecessary forces, it is also important to avoid unnecessary forces acting at the hips. These hip forces will ultimately be transferred through the lower back and therefore must be carefully applied. The net result is that proper lifting technique must create the most optimal kinetic environment for all the joints involved. Exercise technique guidelines should not be based primarily on force characteristics for only one involved joint (e.g., knees) while ignoring other anatomical areas (e.g., hips and low back).”
Yes, as your knees go into a greater degree of flexion, the forces at the joint will increase but there is no “magic point” beyond which this will suddenly become injurious! Your knees go past your toes all the time when you run, jump, walk, sit down, and stand up.
Squatting technique also depends on your anatomical proportions and the type of squat you’re performing. If you have long femurs, chances are your knees will go further past the toes as compared to someone who has shorter femurs. If you’re performing a front squat, your knees will again travel past the toes, and if they aren’t, then you’re most likely performing them incorrectly.
How you squat depends on a lot of factors, and varies from person to person.
There are no rules set in stone.
So sit back, and perform the exercise in a manner which feels comfortable, and allows you to move some heavy ass weight!