Knee problems from an old Injury and Cycling does not affect it- But I do make certain that I keep my quad muscles strong. Couple of times a week I am doing 1 hour sessions to make certain they are strong.

And on the Mountain biking- The only time that hurts is if I ever walk up a hill. (That sideways pressure on the knee joint) Thats one of the reasons I learnt to climb hills on the Bike.

Since getting the road bike- I rarely even get a twinge- but That is why I am doing the Quad muscle exercises. Mountain biking kept them in better shape.