What are the muscles that flex the knee?

Among all the joints in the human body, the knee joint is particularly prone to injury. When people exercise or work, this joint will bear a great load, so various injuries will occur in this part. For athletes, if they get injured, the impact will be very large, and they may miss the game for a long time. So what are the main muscles of the human knee joint?

What are the muscles that flex the knee?

The main movements of the knee joint are flexion, extension, and small internal and external rotation after flexion.

The main muscles involved in the movement are:

Flexion: biceps femoris, semitendinosus, semimembranosus, sartorius, gracilis, popliteus, and gastrocnemius.

Extension: The knee extensors are primarily the quadriceps femoris.

Internal rotation: The muscles involved in internal rotation of the calf are the semitendinosus, semimembranosus, sartorius, gracilis, and popliteus.

External rotation: The muscle involved in external rotation of the calf is the biceps femoris.

The joint capsule of the knee joint is thin and loose, attached to the periphery of each joint surface, and reinforced with ligaments around it to increase the stability of the joint. The main ligaments are:

1. Patellar ligament

It is the central fibrous cord of the quadriceps tendon, extending from the patella downward to the tibial tuberosity. The patellar ligament is flat and strong, and its superficial fibers pass over the patella to connect to the quadriceps tendon.

2. Peroneal collateral ligament

It is a tough, cord-like fibrous cord that starts from the lateral epicondyle of the femur and extends downward to the head of the fibula. Most of the ligament surface is covered by the biceps femoris tendon and is not directly connected to the lateral meniscus.

3. Tibial collateral ligament

It is wide and flat and is located on the inner and posterior part of the knee joint. It originates from the medial epicondyle of the femur, attaches downward to the medial condyle of the tibia and adjacent bones, and is tightly integrated with the joint capsule and medial meniscus. The tibial collateral ligament and the fibular collateral ligament are tight when the knee is extended, loose when the knee is flexed, and are most loose when the knee is half-flexed. Therefore, a small amount of internal and external rotation of the knee joint is allowed in the semi-flexed knee position.

4. Oblique ligament

It extends from the semimembranosus tendon, originates from the medial condyle of the tibia, slants upward and outward, and ends at the lateral epicondyle of the femur. Some fibers fuse with the joint capsule to prevent hyperextension of the knee joint.

5. Cruciate ligament of knee

Located slightly behind the center of the knee joint, it is very strong, lined with synovium, and can be divided into two front and back strips:

The anterior cruciate ligament originates from the inner side of the tibial intercondylar elevation, connects with the anterior horn of the lateral meniscus, and runs obliquely to the posterior, superior and lateral sides, with the fibers attached to the inner side of the lateral femoral condyle in a fan shape.

The posterior cruciate ligament is shorter, stronger, and more vertical than the anterior cruciate ligament. It originates from the posterior side of the intercondylar eminence of the tibia, moves obliquely forward and upward to the medial side, and attaches to the lateral side of the medial femoral condyle.

The cruciate ligament of the knee firmly connects the femur and tibia, preventing the tibia from shifting forward or backward along the femur. The anterior cruciate ligament is tightest when the knee is extended and prevents the tibia from sliding forward. The posterior cruciate ligament is tightest when the knee is flexed and prevents the tibia from sliding back.

The synovial layer of the knee joint is the widest and most complex of all the joints in the body. It is attached to the periphery of the articular surfaces of the bones in the joint and covers all structures in the joint except the articular cartilage and meniscus. The synovium protrudes upward above the upper edge of the patella to form a suprapatellar bursa about 5 cm deep between the quadriceps tendon and the lower part of the femoral body. On both sides of the midline below the patella, part of the synovial layer protrudes into the joint cavity to form a pair of wing-shaped folds, which contain adipose tissue and fill the space in the joint cavity. There are also synovial bursae that are not connected to the joint cavity, such as the deep infrapatellar bursa located between the patellar ligament and the upper end of the tibia.