20# for leg extensions is doable. When I used to use a machine to do them, I could usually do 45# (both legs) or more.
However, I don't think leg extensions are the safest exercise for the knee, and I've read several authors who agree (or whom I agree with? LOL!).
If you are talking about straight-leg lifts done while lying on the floor, it's possible to work up to that weight, I suppose, but they are challenging with lighter weight.
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Kathryn
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2010 season speed-walking 5K PR: 35:47
2011 season race-walking 5K PR: 34:42
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