I'm an experienced downhill mountain biker and have been one for over 20 years. At 44 I'm still fairly fit, slim and although I don't go quite as fast as I used to, and take it easy on the big jumps I'm still fairly active, riding 5 miles almost every day and doing a fairly tough downhill about once a week. In just the last few months I've started experiencing a stiffening-up of my knees during my 5 mile runs. They don't give me problems after I've had a hot shower and a rest but they can get quite painful during the run. Have any other old codgers experienced this during rides, and if so, could they offer my any advice on how to avoid the problem ?