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Place the fitness ball on a wall behind you. Stand with your legs about shoulder-width apart, and lean back against the fitness ball for support. Slowly bend your knees and lower your tailbone until you are almost in a sitting position. Hold the pose for 10 seconds and slowly return to a standing position.  Perform several 4-5 sets of 8 to 10 repetitions. When doing these wall squats with the fitness ball, position the ball low enough against your body that it will still offer you full support once you are in a squatting position. Start with your feet far enough from the wall so when you squat, your knees are above your ankles and not your toes. Stand with your feet planted out past each side of your body, leaning with your back against the fitness ball. Your feet should be about 4 feet (1.2 m) apart and your torso should stay upright. Lower your body slowly, supporting your weight in your quadriceps and glutes, and hold the pose for 10-12 seconds. Stand up slowly and repeat. As with traditional squats, try doing 4-5 sets of 8-10 repetitions each.  To prevent injury, do not extend your knees past the tip of your toes when bending into position for squats. Make sure your knees are above your ankles, not your toes, as you complete the squat. As with wide-stance squats, place the fitness ball on the wall behind you. Stand with your legs close together and with your back against the fitness ball on the wall. Your legs should only be slightly narrower than your hip width. Bend your knees and lower your body to nearly a 90-degree angle. Hold for 10-12 seconds and slowly return to standing position. Keep your torso upright and your back straight. Do 2-4 sets of 7 to 9 repetitions. As your quads strengthen, you'll be able to increase the number of sets and reps that you can do.

Summary:
Perform traditional squat moves for the quadriceps. Practice wide-stance squats to strengthen your inner quads. Do narrow-stance squats to strengthen your outer quads.