Many people wonder if there are muscles located directly inside the knee joint. Muscles that enable knee movement are not found within the joint capsule but rather surround the knee, extending from the thigh and lower leg. These powerful muscles exert force that allows the knee to bend and straighten, working in concert with the joint’s internal components.
What’s Inside the Knee Joint
The knee joint is a complex structure where the thigh bone, or femur, meets the shin bone, known as the tibia. A smaller bone, the patella or kneecap, sits at the front of the joint, protecting it and improving the leverage of the thigh muscles. These bones are covered by articular cartilage, a smooth, slippery tissue that reduces friction and allows for fluid movement between the bone ends.
Within the joint, two C-shaped pieces of cartilage, called menisci, act as shock absorbers and help distribute weight evenly across the joint surface. The medial meniscus is on the inner side of the knee, while the lateral meniscus is on the outer side. The entire joint is enclosed within a capsule, which contains synovial fluid. This fluid lubricates the joint, nourishing the cartilage and ensuring smooth, pain-free motion.
The Muscles That Power Your Knee
The quadriceps femoris, a group of four muscles located on the front of the thigh, are primary movers for knee extension, which is the straightening of the leg. These include the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The rectus femoris also crosses the hip joint, contributing to hip flexion, while the vasti muscles specifically act on the knee.
On the back of the thigh, the hamstring muscles are responsible for knee flexion, or bending the leg. This group comprises the biceps femoris, semitendinosus, and semimembranosus. The biceps femoris has two heads, a long head that also extends the hip and a short head that only flexes the knee. The semitendinosus and semimembranosus also contribute to hip extension.
A calf muscle, the gastrocnemius, also crosses the knee joint. It plays a minor role in knee flexion, particularly when the foot is flexed. All these muscles attach to the bones surrounding the knee via tendons.
The Essential Role of Connective Tissues
Connective tissues are important for the knee’s stability and function, linking muscles to bones, or bones to other bones. Tendons are cord-like structures that connect muscles to bones, transmitting force generated by muscle contraction. For instance, the quadriceps tendon connects the quadriceps muscles to the patella, and the patellar tendon then continues from the patella to the tibia, enabling the knee to straighten.
Ligaments, distinct from tendons, are strong, fibrous bands that connect bones to other bones. They play a primary role in stabilizing the knee joint by limiting excessive movements. Key ligaments within the knee include the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which prevent the tibia from sliding too far forward or backward relative to the femur. The medial collateral ligament (MCL) and lateral collateral ligament (LCL) also provide stability against side-to-side forces, preventing the knee from bending inward or outward.