University of South Wales. Years ago when I was in school to become an engineer, I wrote a paper on this very subject, and how over time heavy shear force will open up the knee joint up. Try to think of a normal free weight movement that would involve knee extension with resistance. High Repetition Loading The leg extension prevents force from the back, destabilizing the knee. The most shearing occured in the final third of a full range of motion during the concentric phase, at the very top of the leg extension. ... Biggest thing for shear force on the knee is the knee doing past the toes, teaches it bad neuromuscular firing patterns and increases likely hood of an ACL tear. When you squat, the thickest part of the cartilage is loaded during the squat's maximum force. Thus, by starting from about a 90 range of motion, decreases the shear force on the knees. Unlike squats and lunges, which load the knee primarily with compressive (downward) force, quad extensions apply shear (front-to-back) force. jwcalla Rookie Posts: 35 Joined: Wed Jul 19, 2006 11:01 pm. Shear force during leg extensions Thank you standapart for that link! 14th Jun, 2018. Now look at Walker. Leg extensions are infamous for being bad for your knees. Furthermore, because the angle of the leg was measured throughout the movement, it allowed the researchers to study the shearing forces of the limbs. This can lead to cartilage damage. Never before has this happened. Leg extensions place a lot of shear force on the knees. The leg extension places maximum force on the back of the patella ("knee cap"), the thinnest portion of the cartilage. Post by jwcalla » Fri Mar 09, 2007 5:10 pm Thanks guys for all the info. It is not natural to load the knee while the foot travels through space with open-chain movement. George Zafiropoulos. I recently started being very consious not to let my knee go past my toes after watching scott herman on youtube profess this point. The aggressive extension of his lead leg builds force under his foot, causing it to pop up and move away from the ball (the opposite direction that he was pushing). Top. 9. The leg extension is a popular way to train the quadriceps at the front of your knee. this is how I feel as well, the problem is the knee joint is built for compressive force, and the leg extension places shear force on the knee. If leg extensions are bad because of shear forces at the knee joint, what about skullcrushers and cable pushdowns? That exercise, because if it's very nature (Open-kinetic-chain), puts excessive shear force on the knees. And fear not, I wouldn't dare replace squats with leg extensions. Cite. And just as recently on my 45 degree leg press ive been developing pain in my patella. No matter the state of your knees, safely and effectively performing leg extensions relies on good form. The leg extension is a popular way to train the quadriceps at the front of your knee. But heavy weights and … This is because the movement places a lot of shear force on to the back of the patella (your knee cap), which forces the tibia (the bone in your lower leg) towards the femur (your thighbone). But heavy weights and improper form increase your risk of knee injury. Unstable knees are more vulnerable to injury from shear stresses as opposed to compressive forces. [/quote] Shear Force On The Knees The degree of shear force on the knee occurs primarily in the bottom part of the Leg Extension. Knee extension acts as an unloaded function of the quads. It's appreciated. Leg extension also maximize shearing force on knee and patella. Close. Kenny Croxdale. Of a normal free weight movement that would involve knee extension acts as an function! 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