Why are leg extensions hard on the knee joint?
To understand why this is, it is helpful to understand the concept of shear. Shear in this case just refers to a horizontal force on the joint. Imagine two cans stacked on top of one another, and imagine that a piece of masking tape joins them. Then, imagine what happens if you hold the top can still while you push the bottom can sideways. Eventually that tape will snap. This is a simplistic description of what happens to the knee joint in a leg extension. In a squat your feet are on the ground (hopefully), and the force of the load is transmitted downwards, along the length of the bones. In a leg extension machine, there is a pad against the front of your shin or ankle, and you press against it to move the weight. The foot swings upward in an arc. Thus, the pressure is coming across the shin bones, not along their length. This creates the problem in the knee joint as the shin is pressed backwards. Leg extensions do have their place, usually in rehab. If the leg extension machine is us
