The quadriceps are one of the most important group of muscles for having good healthy knees. The quad muscle group, as evidenced by the name, are made up of four muscles that attach at the knee, and they provide support and stability to the knee joint and the kneecap, or patella. With any knee injury, pain, or surgery increasing and working on that quad strength is imperative. We obviously also want to work on the other muscles that support the knee, such as the hamstrings and hip/glute muscles, but strengthening the quads provides many benefits to the knees and with many functional and day-to-day activities.
The quads are involved in almost any activity we do while on our feet. They are active while walking, running, standing up from a chair, going up and down stairs, squatting, jumping, and absorbing shock when landing from a jump. Without adequate strength of the quads, the knees would buckle more easily and be less stable, increasing the risk of knee injury. Being able to absorb the shock through the quads (and glutes as well) helps decrease the pressure and impact put on the knees when landing from a jump. It also helps to prevent knee buckling when walking or going down stairs. Multiple studies have shown how increasing quad strength with people with knee arthritis has helped to decrease pain and improve functional ability, even helping people feel better to the point that they can put off having a knee replacement. People who have worked on their quad strength are able to stand up from lower seats and squat with less difficulty and increased stability of the knee. The quads act as a natural brace for the knee to keep it in proper alignment during different dynamic movements.
There are many different ways to strengthen the quads. It depends on how advanced in your recovery from injury or surgery you are. You can isolate the muscles and motion itself with just working on straightening out your knee from either a slight bend while lying down or from a 90 degree bent position while seated, and adding weights as you get stronger. The leg press machines at the gym are good as well. However, for someone who is at the right stage of rehab or is healthy, squats and lunges are always the most effective. These are dynamic motions that work the whole lower body from hips down to the feet, and are great for strengthening the quads. They also are very functional movements that translate over to daily life. They can also be progressed and translated over to jumping and sprinting, as the landing motion in a jump is similar to doing a squat. These exercises are all great ways to strengthen one of the most important muscle groups, and to having and maintaining good knee health.