I have a knee problem rear its head occasionally and this does stop all exercise. Cycling is not too bad but it has to be gentle. So while laid up- I try to do some leg exercises without putting any pressure on the knees. Even walking is out due to pain so I have to be carefull. I go back to the quad exercise that I have done for years. Lie on my back and raise the leg without bending the knee. Once I can raise the leg 10 times- I put a weight on the foot. just a pair of boots initially but gradually raise it to a 2 lb weight (The weight of a bag of sugar in old money over here)

So while I am laid up- I can still put some exercise into the legs- and build up the quads at the same time.