Quadriceps Femoris Muscle
The quadriceps femoris muscle is a four-membered muscle of the thigh which almost completely covers the anterior femur. It ranks among the strongest muscles in the human body (physiological cross-sectional area > 150 cm2). It significantly forms the lateral contours and the ventral side of the thigh. Its innervation is carried by the femoral nerve (L2-4).
Rectus femoris muscle
The Rectus femoris muscle has two origins at the anterior inferior iliac spine of the pelvis and the upper margin of the acetabulum. Distally its fibers end in the common insertion tendon (quadriceps tendon).
Vastus medialis muscle
The Vastus medialis muscle runs spirally around the shaft from the linea aspera and intertrochanteric line of the femur and merges with the quadriceps tendon for the most part. A second part - referred to as medial patellar retinaculum - bypasses the patella medially and inserts at the medial condyle of the tibia.
Vastus lateralis muscle
The vastus lateralis muscle originates at the linea aspera and greater trochanter of the femur, loops around the shaft and mainly runs into the quadriceps tendon. Mirror-inverted to the vastus medialis muscle a small part goes around the patella laterally and inserts at the lateral condyle of the tibia (lateral patellar retinaculum).
Vastus intermedius muscle
The vastus intermedius muscle begins at the front side of the femur and ends in the common insertion tendon. In the height of the patellar base a small part splits off and inserts at the suprapatellar recess of the knee joint capsule (articularis genus muscle). Even though it does not count as an independent muscle it is sometimes considered as the “fifth head” of the quadriceps.
The quadriceps tendon runs above the ventral side and through the periosteum of the patella and finally inserts at the tuberosity of the tibia. The part below the patellar apex is referred to as the patellar ligament.
The quadriceps is the only extensor of the knee joint. Therefore it plays a key role in every movement involving the stretching of the knee (e.g. walking, climbing stairs, rising from the sitting position). In addition it keeps the knee from buckling when standing. Furthermore the rectus femoris muscle forces a flexion of the hip joint. To a small extent the vastus medialis muscle is involved in the inward rotation and the vastus lateralis muscle in the outward rotation of the knee joint. The articularis genus muscle is directly linked to the knee joint capsule and the suprapatellar bursa. During the knee extension it pulls both structures proximally and by this means prevents their entrapment between patella and femur.
The quadriceps tendon utilizes the patella as a sesamoid bone. This has two advantages:
- The lever arm is lengthened which effectively increases the torque (torsional moment). Thus the muscle needs less power in order to move the bone.
- The patella protects the knee joint from damage through the quadriceps tendon.
Clinically the quadriceps is the reference muscle for the nerve roots L3 and L4. An absent patellar reflex may reveal a spinal disc herniation at the height of L3 or L4. In contrast a paralysis of the quadriceps by a peripheral lesion is typically demonstrated by the inability to walk adequately and to climb the stairs. Interestingly the stand can be initially compensated by the help of the ligaments and capsule of the knee joint, and by leaning over the upper body.
From all muscles in the human body the rectus femoris muscle is mostly prone to shorten and is therefore particularly vulnerable to injury. Normally one should be able to touch his buttocks with his heel. In order to prevent a shortening of the quadriceps it is strongly recommended to do stretching exercises after each workout.