If leg extensions are bad for my knees, why aren’t tricep kickbacks bad for my elbows?

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The tendons usually aren’t the issue in either movement. From what I understand, it’s the ligaments, your knees need your hamstrings to co-contract during knee extension to prevent the shearing forces from putting tension on the ACL which isn’t really possible during a leg extension. The elbow is a much simpler joint and doesn’t really need co-contraction in the same way the knee does, your elbow isn’t just an arm knee, they’re joints that are structurally very different.


It’s only bad when somebody does the exercise like a jack-ass, which can classify any exercise as “bad.” It’s just another thing that web-guru’s and keyboard jockeys talk about, disregard.


Because you don’t have kneecaps on your elbows. And because the two joints work in different ways. I’m an orthopedic surgeon and my personal specialties include patellar instability and knee ligament injuries. So I know a lot about the biomechanics of the knee joint. Because of the significant baseline A-P translation of the knee, co-contraction of the quads and hamstrings is needed for balancing of the joint. Knee motion naturally includes rolling, sliding, and twisting. Open-chain exercises (like leg extensions) only produce activation of one set of muscles, resulting in strain on the soft tissues as they take extra forces that would normally be balanced by the opposing muscle contraction during a closed chain exercise (like a squat.) The elbow has virtually no A-P translation at baseline. The ulnohumeral joint takes all the stress of a triceps extension, and it works essentially like a hinge. No sliding, no twisting. So it does not require co-contraction for stability.


Leg extensions are not bad for you knees. As every other exercise, they can be bad for you if you mismanage instensity and volume. Most people who say they’re bad for your knees, just jumped into them the wrong way, felt some pain at the top, because they’re not used to that particular movement, and say they’re bad. That or they read it on the internet, from some misguided person.


What are alternatives that are lower risk?