The movements and the stabilization of the knee joint are supported by the quadriceps and the hamstrings. The quadriceps is actually composed of four individual muscles located on the anterior upper leg. These muscles are the vastus lateralis, vastus medialis, vastus intermedius and rectus femoris. These muscles fuse, forming the quadriceps tendon. The quadriceps straightens the knee by pulling the patella up on contraction.
The hamstrings are the muscles that attach to the tibia, specifically at the back of the knee. It consists of three individual muscles: biceps femoris, semitendinosus and semimembranosus. The hamstrings functions by flexing or bending the knee joint. This muscle group also provides stability on both sides of the knee.
The stability of the knee largely depends on the four major knee ligaments: the medial collateral ligament, lateral collateral ligament, anterior cruciate ligament and posterior cruciate ligament. Ligaments are the tough but slightly elastic bands of connective tissues that hold two or more bones together. Excessive movements, such as hyperextension or hyperflexion, at the knee joint are restrained by these ligaments, stabilizing the knee joint and keeping the bones in their correct alignment during movements.
Medial and lateral collateral ligament
The medial collateral ligament, or MCL, resists excessive forces coming from the knee’s outer surface, or valgus forces. The lateral collateral ligament, or LCL, resists the forces coming from the inner surface of the knee, or varus forces. These ligaments are located on the outside of the knee joint and are able to heal on their own.
Anterior cruciate ligament and posterior cruciate ligament
The cruciate ligaments are located within the center of the knee, where they are constantly bathed by the synovial fluid. These ligaments form a cross right in the center of the knee joint. The anterior cruciate ligament and the posterior cruciate ligament, are closely interconnected and are commonly termed the central pivot (Johnson, 2004). If one of these ligaments is injured, the damage can completely disrupt the function of the entire knee joint. Without timely and appropriate treatment, a cruciate ligament injury may result in degenerative arthritis.
The anterior cruciate ligament is considered one of the most important structures of the knee. Functioning as the knee’s major stabilizer, the ACL runs from the thigh bone to the shin bone through the center of the knee, and provides stability and reduces stress across the knee. It prevents hyperextension of the knee by restraining the excessive forward movement of the tibia in relation to the femur. Working with PCL, the ACL limits the rotational movements of the knee. If one of these ligaments is damaged, instability of the knee occurs whenever the foot of the affected leg is planted on the ground or during pivoting. A major injury involving the ACL usually requires extensive surgery and prolonged therapy
The three bands of ACL supporting the knee through full range of motion are mostly composed of type-1 collagen fibers. Collagen is a naturally occurring protein that is produced by the fibroblasts. These fibers provide the tensile strength and elasticity of the ACL.
In contrast to ACL, the posterior cruciate ligament is not frequently injured. The PCL is almost two times stronger than the ACL and is more resistant to damages of excessive forces. Not much is known about this ligament; however, it is believed that the PCL functions as a central axis during knee rotation.
That is the end of Part 3. I will have part 4 up very soon.
If you are interested in knee pain, knee injuries or ACL injuries, these other posts may interest you:
If you are looking for stuff with more details on knee pain and ACL injuries, you can attend the live course called Exercise Rehabilitation of the Knee or instantly download my video presentation on Exercises for Prevention, Rehabilitation and Overcoming Knee Injuries.
Rick Kaselj, MS