If it was too much for your knee, it likely wouldn't hurt while you're doing the exercise, but you might have swelling and/or pain 12-24 hours later. If that is happening, I'd back off the leg press for now. If you choose to continue to use it be VERY sure that each time you push on the plate, you lift your toes slightly so that the push goes through your heels. This small adjustment keeps the brunt of the force from your knee joint.
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