What are some exercises for knee strength?

Here are some knee-strengthening exercises:

Straight-leg raise: To protect your knees, do this exercise to strengthen your quadriceps, the large muscle in the front of your thigh. Lie on your back and tighten the thigh muscles of one leg with your knee fully straightened. Lift your leg several inches and hold for 10 seconds. Lower slowly. Repeat until your thigh feels fatigued, then switch to the other leg.

Wall sit: This exercise also strengthens the quadriceps. While standing with your back against the wall, bend your knees, lowering yourself into a sitting position. Do not lock your knees. Hold this position for 20 seconds. Repeat.