Specific Exercise Needed

I have been using the elliptical and have been getting weakness behind and above the knees. I used to get this same weakness when I was running and it forced me to stop until the weakness went away.

I am wondering if there might be an exercise that would strengthen other muscles in the area that help with the load while exercising. Perhaps there is another machine that would work better for this problem? (I want to exercise to get my heart rate up for half an hour.)

I really would like to continue working on the elliptical but if I don't do something then this will just continue to injure me until I have to stop.

Cheers -

george

Specific Exercise Needed