The quadriceps are located above the front of your knee while the hamstrings are located on the back side of your leg above the knee. Both muscles are responsible for stabilizing your knee. Regular strengthening exercises performed every other day should have you noticing results within weeks.
Knee extensions require a weight machine. Position the foot pad on the top of your ankles and ensure the seat of the machine allows you to sit with your back straight so you maintain proper posture during the workout. Choose a weight that causes muscle fatigue by your eighth repetition and perform three sets of eight. Use your ankles to push the padding upward toward the ceiling in a slow and controlled motion. Hold this position for one to two seconds and then slowly lower your ankles to a starting position. The weights should barely hit each other as you resume the starting position. If the weights make a loud bang, the quadriceps are not controlling the weight.
Hamstring curls require a weight machine. Lie on your stomach and place your achilles tendon under the foot pad. Reach under the machine and grasp the handle bars under the bench to ensure your upper body does not help with the hamstring curls. In a slow and controlled motion, lift your legs up so the foot padding moves toward your buttocks. Hold this position for one to two seconds and then slowly lower your ankles to the starting position. Again, the weights should barely touch as you resume the starting position. Choose a weight that causes muscle fatigue by your eighth repetition and perform three sets of eight.
Hold dumbbells in each hand with your palms facing inward. Stand with your feet shoulder-width apart and carry your weight evenly on both feet. Keep your neck aligned with your spine by looking at the wall in front of you during the entire squat. Tighten your abdominal muscles to help support your spine during the squat. Simultaneously bend your knees and hips while moving your buttocks backward as if trying to sit in an invisible chair. Do not round your spine. Once your thighs are almost parallel with the floor, exhale and push your heels into the floor as you return to the starting position. Complete 15 repetitions of squats.
Perform these exercises every other day so your muscles have at least 24 hours to recover. These exercises can result in injury if performed incorrectly, so it's best to consult with a personal trainer before attempting exercises unsupervised. Consult with your doctor before starting a new exercise program, especially if you've had an injury in the past or have an underlying health condition.