One of the most widely held exercise beliefs is that you should never let your knees go past your toes when squatting. You’ll hear this “rule” echoed like a mantra over and over by the majority of personal trainers: “Keep the knees behind the toes!”
Fact is, though, there’s little evidence to back up such a claim. It is true that as the knees move anteriorly (i.e. forward) during the squat, the forces acting on the knee joint increase. However, there is no “magic point” where these forces suddenly become dangerous. The plane of the toes has been misguidedly used as a line of demarcation despite a complete lack supporting research. What’s more, intentionally preventing the knees from going past the toes can create additional problems at other joints that are potentially more injuries.
An eloquent study by Andy Fry and colleagues (2003) looked at this very topic. Seven recreationally-trained males performed 3 unrestricted squat lifts and 3 restricted lifts where a wooden board was placed immediately in front of both feet so that the knees were prevented from moving forward past the toes. As expected, knee torque was greater when the knees went past the toes compared to restricted squatting (~150 vs. 117 newton-meters). Sounds like intentionally keeping the knees behind the toes is a good thing, right? Not so fast…
Restricted squats resulted in significantly greater torque at the hip joint compared to unrestricted squatting, with the differences here much greater than those seen at the knee joint (302 vs. 28 newton-meters). Perhaps even more problematic is that results were attributed to a greater forward lean when performing restricted squats. Why is this an issue? Well, in order to squat while keeping knees behind toes, lifters tend to compensate by increasing their forward lean. Studies have shown that an increased forward lean is associated with greater lumbar shear forces. And since the lower back is more susceptible to injury than other joints, this would seem to be a poor tradeoff.
So what’s the take home message? I’ll quote directly from the Fry et al. study as they sum things up very nicely: “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).”
I would note that the same rules do not apply for lunges. Since the lunge involves stepping forward, there is no issue with maintaining an erect posture during performance. The biggest mistake I see is that people tend to push forward on their front leg, which significantly increases shear at the knee joint. Instead, your focus should be centered on dropping the rear leg. In doing so, your front leg will stay perpendicular to the ground, minimizing stresses to the knee joint without negatively impacting the hip or the spine.
Fry AC, Smith JC, Schilling BK. (2003). Effect of knee position on hip and knee torques during the barbell squat. J Strength Cond Res. 17(4):629-33.