The patella, femur and tibia come together to form the knee joint. For the knee to be stable it relies heavily on its ligaments, as well as the health of the meniscus--a cushioning layer between bones. A large percentage of people have muscular imbalance on the inner quadriceps muscle near the medial (inside) of the knee, which leads to knee instability. Knee extensions are a great way to strengthen this area.
To strengthen quadriceps, do knee extensors: Place weight on ankle. Sit on a chair or bench. Place a towel roll under the knee of your exercising leg. Keep trunk stable. Contract your quadriceps to extend your leg up. Lock your knee. Hold for 5 seconds at the end of the movement. Slowly return to start position. Repeat for several sets of 8–10 reps. Gradually increase weight over time, as you become more proficient.
Strengthen the knee by working not only the quadriceps but the hip adductors and abductors as well.