Water exercises may be the best exercise for your arthritis. Swimming laps, water aerobics, or even just walking in the shallow end of a pool all make the muscles around your spine and legs stronger.
Ask your doctor if you can use a stationary bike. Be aware that if you have arthritis of the hip or knee cap, biking can worsen your symptoms.
If you are not able to do water exercises or use a stationary bike, try walking, as long as it does not cause too much pain. Walk on smooth, even surfaces, such as the sidewalks near your home or inside a shopping mall.
Ask your physical therapist or doctor to show you gentle exercises that will increase your range of motion and strengthen the muscles around your knees.