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Stretch your quadriceps muscles. Begin basic strength training. Use a leg extension machine for more strength training. Do eccentric squats on raised heels.
If your doctor recommends that you exercise as part of a rehabilitation program, you'll probably need to stretch the quadriceps muscles in front of your thighs. Try one of these stretches:  Do the hip flexor stretch by putting one knee on the ground and the other foot in front so the knee is bent. Push your hips forward while you keep your back straight. Hold your ankle and pull the leg behind you toward your back. You should feel a gentle stretching in your quadriceps muscles. Hold the stretch for 10 seconds and release it. Once your knee no longer feels painful, start simple exercises to contract and relax your quadriceps. Stand or lay down and pull your knee towards your chest. Hold your knee for 5 seconds before you slowly release it. Do 3 sets of 8 repetitions for each knee. You can eventually work up to holding the knee for 10 seconds. Do 4 sets of 12 repetitions for each knee once you feel comfortable. If you're not comfortable with putting weight on the knee while you exercise, sit down and use the leg extension machine. Extend a single leg with light resistance and try to do 3 sets with 10 repetitions when you start out. Work up to 3 sets of 10 to 12 repetitions once your knee feels stronger. Stand against a slanted foam roller or board so your heels are raised up at the back. Slowly squat down and then stand up a little quicker. Put your weight on the good knee instead of the knee that's healing. Do 3 squats 10 times a day when you start out. Work your way up to 3 squats 15 times a day.