 If you train on machines, the extension machine, PEC-DEC, you'll see the instruction, line up the joint with the axis of the machine. So single joint machines imply that all of your joints act like hinges. If you do kettlebell type classes or other kind of free weight TRX, that type of stuff, and they have you loading your joints in a lot of different directions, that implies that your joints are all swivels, like they're all ball and socket. In biomechanics, the joints are blends of these six basic types. They're not pure hinges and they're not pure ball and socket. So for instance, with the knee, you really can't line up the axis of the knee with the machine axis. Because if the knee was a hinge, if the knee was a pure hinge, it would rotate this way. And that would line up with the machine axis. If the machine was a rolling joint, when you moved your shin, it would roll off. And if it was a sliding joint, it would slide this way. What happens with the knee is it's a rolling and sliding joint, so it rolls and slides in place. The axis in that far right corner, the axis changes depending on how far bent your knee is. So technically, you can't line up the axis of your knee with the machine because that axis is always moving. So now if we're looking at full range of motion with a knee extension, for instance, and you're really working at locking your knee out as far as it can go, the issue with that is the screw-hole mechanism, terminal rotation. So for the middle of the knee range of motion, it does function like a hinge. At the end, the bones of the shin and the quad rotate on each other to lock. So the shin will rotate externally or the femur will rotate internally and locks together. The point of that is so when you're standing up straight or you're walking, you're not using any muscular effort. It is a passive lock at the knee. So if you're on the leg extension machine and you're really diligently trying to lock your knee out against resistance, what's happening is you're taking this passive bony lock, you're loading an external weight on it, and you're applying great internal force to jam this bony lock into position that's designed to be a passive lock with your body weight standing upright. So your quads will burn as you're trying to lock the knee out, but you're really just bringing on a lot of unnecessary joint stress for the sake of feeling the exercise. Practically what you do is you watch someone on a knee extension and at the end of the motion you'll see what happens if your knees are free to move. When you lock your knee out, the shin will rotate externally or your femur will rotate internally. When you're strapped in on a leg extension machine and you're trying to lock out, that movement still has to happen so you'll see some shifting going on in the seat. The thing to do would be to watch and as soon as the shifting starts, that's the end of the repetition. Because past that point, you're just trying to force that bony lock, again that's designed to be a passive lock. At the back end, that was for locking the knee out. As you're bending the knee, as you pull your knee more and more into flexion, again the way the bones are shaped in the femur and the shin, your foot is going to move towards the center line.