The knee bends when walking, running and landing. This is seen as the knee traveling towards/over the toes. The knee moving past the toes is, contrary to popular belief, perfectly safe. All movements are safe, with the proviso that it is in a controlled manner. If the knee cannot move towards the toes as much as is needed by the movement, it is forced into a different plane of movement. If this happens, the knee will travel towards the other leg, usually this is caused by high impact forces greater than walking alone. This forced motion causing excess rotation of the knee, at speeds that place great stress upon the ACL (anterior cruciate ligament). Over time the ACL may become damaged and weak due to the consistent high stress it undergoes.
To test your ankle dorsiflexion (the amount your knee can move towards your toes), try the knee to wall test explained on our Youtube channel. Aim for 10+cm and you should be safe from usual conditions.