Although all muscle groups must be strengthened with light weights, focus on the extensor muscles that open up the joints. Work with a physical therapist or an exercise trainer to design an individualized exercise program.
Use small hand weights (2 to 2.5 lbs) which can be created from soup or juice cans, shampoo bottles, etc. Choose a weight that can be easily lifted for one set of 10 repetitions and work up to two to three sets of 10, three sessions per week.
Focus on one muscle group at a time, and use slow, fluid movements.
Do not strength-train the same muscle groups on consecutive days.