are leg extensions potentially dangerous because of the shearing force they put on the knees?
“In fact, contrary to popular belief, squatting actually places less stress on the knee joint than leg extensions. This has to do with the way that force is applied during exercise performance. Stay with me here as I need to cover a little exercise physiology. In the leg extension, loading is applied perpendicular to the long axis of the tibia (one of the bones in the lower leg) – a fact that creates tremendous shear force in the patellar region. Alternatively, loading during the squat is mainly compressive, with forces applied parallel to the long axis of the tibia. Since a joint is better able to withstand forces from compression as opposed to shear, it therefore follows that squats are more joint-friendly than leg extensions. What’s more, leg extensions tend to overstress the anterior cruciate ligament (ACL). During performance, the quadriceps reacts by pulling the tibia forward (a phenomenon called tibial translation). The ACL in turn opposes the quadriceps by trying to prevent tra
