The quadriceps muscle group is the best shock absorber your knee has. When you are walking, especially down an incline, you use your quads to minimize the shock to your knee. Remember, your knee has to absorb 4 times your body weight when you walk on flat ground, and up to 7 times your body weight when going down stairs. Weak quads equate to accelerated wear and tear in your knee joint. This particular exercise is an example of isometric exercise — one that allows you to work your muscles without movement of your knee.
Sit to Stand
This exercise combines quad strengthening (see Quad Set) with balance work (see One Leg Balance)
This is another simple exercise that improves strength in your quads (see Quad Set) and improves balance (see One Leg Balance)
Another way to strengthen your quads without having to bear weight.
This exercise strengthens your hip adductors. This muscle strengthening can put you in better alignment if you have a valgus deformity (the tendency to be knock-kneed).
This exercise strengthens your hip abductors. This is a good exercise if you have a varus deformity (the tendency to be bow-legged).
This exercise is a great one to strengthen your quads, but it also helps strengthen your low back muscles and your abs. Core strengthening is a focus for spine health, so this is a great one if you have arthritis in your back as well.
Specific exercises are needed to strengthen your vastus medialis oblique (VMO if you have a knee cap (patella) tracking issue. This muscle is the weakest of your quadriceps group, the first to atrophy and the last to recover. The VMO is used in the last 15 degrees of extension and is responsible for keeping your knee cap in proper alignment within your femoral groove. These exercises are sometimes complicated and usually require a physical therapist to train you how to perform them appropriately.