The Knee Axis

The axis that your knee spins around moves about while you go through flexion and extension. You actually can't perfectly ever align the axis of the knee with something like a leg extension or a leg curl perfectly, because even if you had x-ray an amazing vision, so you'd only be set up for one point of it. The reason it spins around is the size of the contact surfaces between the femoral condyle and that tibial plateau that it's sitting on changes. When you get into deeper amounts of knee flexion, there's a smaller contact surface area than when your leg is straight.