The problem with this machine is that it pushes your knee past the point of comfort, then forces you to contract your quad to push the weight back up. The muscles in your quad are already in a stretched position, and when you try to tense the quad while it is in a fully stretched position, you’re asking for trouble!
It gets worse, because this exercise is often prescribed as a remedy for knee pain or a means to prevent knee injury, when it actually places dangerous torque on your knee joints. That’s because it pulls your shins back as you lower the weight, forcing your knee to bear the brunt of the load.
It also applies constant tension to the ACL (a major ligament in your knee), so it’s especially hazardous if you have a ligament injury.
Why Are Full-Arc Knee Extensions So Bad?
Imagine your body is a bridge with a broken section in the middle that needs repair. Would your solution be to isolate this area and put more strain on it? Absolutely not! Unfortunately, that’s exactly what the full arc knee extension forces your knees to do. It’s unhelpful, illogical, and it really, really hurts.