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I want to change my workout routine a little and do Leg Extensions and Curls. Since I'm already lifting heavy on other exercises, I want to do the two with relative light weights but thererfor high reps.

I have heard about so many people complaining about knee pain or even injuries after training with these two machines. And often these people are the ones going heavy on Leg Extensions and Curls.

So I'm wondering, when performing with light weight, which I can easily perform let's say 12-15 reps with, are these two exercises as harmful or even harmful at all?

My main goal is to get hypertrophy going and build some muscle. I'm not debating on weither these exercises are necessary or helpful towards my goal, because I've found that opinions in such cases differ very much. I just want to know about the injury/pain risk to feel safe when performing these, so to say.

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The reason why the leg extension is particularly harmful to your knee is the way your leg is partially loaded on the seat, and the rest isn't, causing a "break point" that can be compared to how a twig would break under the same circumstance.

Seated leg curls I would argue are even worse, though in this case due to the risk of back injury rather than knee injury. This is because we tend to shift in the seat over time, causing a posterior pelvic tilt that pulls on the back.

Personally, I think it's a no-brainer. Ditch these machines, and do better, safer alternatives.

For reference, here's some extra viewing materials that give a more visual explanation of the risk factors, and offer some good alternatives.

Regarding leg curls: https://www.youtube.com/watch?v=_Anl0osYkg8

Regarding leg extensions: https://www.youtube.com/watch?v=VvLpDniGiDM

As for whether the exercises are safer with lighter loads and higher reps: not really. While you're doing away with the instant heavy load on the knees, you're replacing this with repeated loads, which decreases the risk of acute injuries, but increases the risk of long-term injuries like tendinitis.

And the case can be made that if 12-15 reps is such an easy weight, there's no point in doing it. Progress is made when you challenge yourself.

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  • Thanks for the detailled answer. The videos really helped me too. What would be some alternatives? I found Lunges and free Leg Curl variants (non-machine ones). Would that be safer options?
    – Suimon
    Sep 25, 2018 at 11:58
  • Yes. Personally I'm a fan of the leg curls on the physio ball. Resistance can be added by holding weight on your pelvis.
    – Alec
    Sep 25, 2018 at 17:27
  • Well I guess Lunges are easy to implement into my workout routine. With the Leg Curls I might have some trouble due to the rather spartan equipment of my gym. They don't got physio balls or one of these fancy glute-ham-raise machines. What would be a good alternative for the Leg Curl?
    – Suimon
    Sep 26, 2018 at 6:10

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