Corpus: Quadriceps femoris muscle

The quadriceps femoris muscle is located on the ventral side of the thigh, above the articularis genus muscle. It consists of four parts: the rectus femoris, vastus intermedius, vastus medialis, and vastus lateralis muscles.

2.1. Rectus femoris muscle

from Latin: rectus - right, upright

The rectus femoris muscle has two heads: the straight head which originates from the anterior inferior iliac spine, and the reflected head, a highly variable structure that originates from the upper edge of the acetabulum. The muscle’s tendon begins approximately ten centimeters above the patella. As a bi-articular muscle, the rectus femoris contributes to both knee extension and hip flexion.

2.2. Vastus intermedius muscle

from Latin: vastus - wide

The vastus intermedius originates from the front and lateral surfaces of the upper two-thirds of the femur. About halfway down the femur, the muscle fibers converge into a tendon. This muscle is generally located beneath the rectus femoris.

2.3. Vastus lateralis muscle

The vastus lateralis muscle, the largest of the quadriceps heads, originates mainly from a broad aponeurosis attached to the upper part of the intertrochanteric line, the greater trochanter, and the linea aspera of the femur. Some fibers also originate from the lateral intermuscular septum.

2.4. Vastus medialis muscle

The vastus medialis muscle originates on the medial side of the femur, from the intertrochanteric line and extending along the medial lip of the linea aspera down to the supracondylar line.

The common tendon of the quadriceps muscles attaches to the upper part of the patella. Some fibers extend over the patella, enclosing it as a sesamoid bone. Force is transmitted to the lower leg through the patellar ligament, which inserts at the tibial tuberosity.

The quadriceps femoris muscle is innervated by the femoral nerve (L2-L4 segments) from the lumbar plexus.

The quadriceps femoris muscle is the primary extensor of the knee joint, essential for straightening the body (e.g., from a squatting position). It works against gravity and is significantly stronger than the hamstrings. The rectus femoris also flexes the hip joint, though its effect is relatively weak. Full knee extension by the quadriceps is only achieved when the hip is extended, as the rectus femoris becomes insufficiently effective when the hip is flexed.

The quadriceps also stabilize the patella within its groove. Uneven muscle development in this group can lead to patellar dislocation.