If your knee hurts during or after squatting, you've probably heard the same old advice: "never let your knee go past your toes." That rule has been repeated in gyms for decades, but it solves very little — and sometimes creates a new problem, since it forces you to lean your torso forward and out of the position it was supposed to protect. Knee pain during the squat almost never comes from how far the joint travels forward; it comes from how it's aligned and how much your ankle and hip can actually move. Understanding that difference changes how you adjust your training, keeps you from dropping the squat out of fear, and lets you fix the real cause instead of just treating the symptom.
The myth that your knee can't pass your toes
The rule against letting the knee travel past the toes came from a real concern: the further the knee moves forward, the more force goes through the joint. The problem is that, in practice, blocking that forward travel doesn't remove the load — it just shifts it to the hips and torso, forcing you to lean your body forward to keep your balance. That tends to make the squat shallower and reduces activation of the hamstrings at the bottom of the movement, which is the opposite of what most people are actually training for.
Stronger by Science's technique breakdown is blunt about this: letting the knee travel forward naturally usually allows for a deeper, more efficient squat, and locking that travel down to "sit back" isn't a universal safety rule. The more useful reference point isn't an imaginary line above your toes — it's the direction your knee points relative to your foot, which is what the next section gets into.
Source: Stronger by Science
What actually matters: knee direction, not knee travel
The movement pattern linked to knee pain during squats and single-leg exercises is dynamic valgus: the knee caving inward relative to the foot instead of staying lined up with the toes. Studies on people with patellofemoral pain show a larger valgus angle during squatting and single-leg landing than in pain-free people, which points to that inward collapse as a factor associated with pain at the front of the knee.
In practice, that means the right question isn't "did my knee pass my toes?" — it's "is my knee pointing the same direction as my foot, from the top of the movement to the bottom?" Filming your own squat from the front, with your phone propped on the floor, is the simplest way to answer that without relying purely on how it feels — the eye alone usually catches valgus too late, after the pain has already shown up.