Start exercising your injured leg when some of the swellings have gone down and you are able to put about half of your weight on that leg.
Quad sets: sit on the floor with your injured leg straight and your other leg bent. Press the back of the knee of your injured leg against the floor by tightening the muscles on the top of your thigh. Hold this position 10 seconds. Relax. Do 2 sets of 15.
Seated quad sets: sit in a straight-back chair with your injured knee bent at a 90-degree angle. Try to tighten the top of your thigh muscles without moving your leg. Hold for 10 seconds. Do 2 sets of 15.
Knee stabilization: wrap a piece of elastic tubing around the ankle of your uninjured leg. Tie a knot in the other end of the tubing and close it in a door at about ankle height.
Straight leg raises: Lie on your back with your legs straight out in front of you. Bend the knee on your uninjured side and place the foot flat on the floor. Tighten the thigh muscle on your injured side and lift your leg about 8 inches off the floor. Keep your leg straight and your thigh muscle tight. Slowly lower your leg back down to the floor. Do 2 sets of 15.
Wall squat with a ball: stand with your back, shoulders, and head against a wall. Look straight ahead. Keep your shoulders relaxed and your feet 3 feet (90 centimeters) from the wall and shoulder's width apart. Place soccer or basketball-sized ball behind your back. Keeping your back against the wall, slowly squat down to a 45-degree angle. Your thighs will not yet be parallel to the floor. Hold this position for 10 seconds and then slowly slide back up the wall. Repeat 10 times. Build up to 2 sets of 15.
Step-up: stand with the foot of your injured leg on a support 3 to 5 inches (8 to 13 centimeters) high --like a small step or block of wood. Keep your other foot flat on the floor. Shift your weight onto the injured leg on the support. Straighten your injured leg as the other leg comes off the floor. Return to the starting position by bending your injured leg and slowly lowering your uninjured leg back to the floor. Do 2 sets of 15.
If you have access to a wobble board, do the following exercises:
Wobble board exercises