That is your sign to change what you are doing, and change it NOW. Cycling, skipping rope, and squats are all excellent alternatives to running in your case. My knees are so damaged that I simply cannot run, but can tolerate all the above exercises. Cycling in particular is good to lubricate the knee before moving on to other exercises. Kicking, if you are able, is an alternative to the warm-up provided by cycling.
Originally Posted by dragon1952
Arthritic knees can lose mobility in both the ability to flex/bend and the ability to extend/straighten the knee. The use of a stationary bike has been demonstrated to be effective in improving knee mobility. The easy repetitive motion of the knee that occurs with stationary biking stimulates production of synovial fluid, the natural lubricant of the knee. Riding a stationary bike 10 to 15 minutes on a regular basis can keep the joint lubricated and improve the ability of the knee to bend.