Knee Muscles Anatomy

Knee Muscles Anatomy that act on the knee joint complex are the quadriceps, the hamstrings complex, the gastrocnemius, the popliteus, and the hip adductors.

See Also: Knee Ligaments Anatomy

Knee Muscles Anatomy

Quadriceps Muscle Group

The four muscles that make up the quadriceps are the rectus femoris, the vastus intermedius, the vastus lateralis, and the vastus medialis. The quadriceps tendon represents the convergence of all four muscles tendon units, and it inserts into the anterior aspect of the superior pole of the patella.

The quadriceps muscle group is innervated by the femoral nerve.

The quadriceps muscles can act to extend the knee when the foot is off the ground, although more commonly, they work as decelerators, preventing the knee from buckling when the foot strikes the ground.

Rectus Femoris

The rectus femoris, which originates at the anterior inferior iliac spine (AIIS) is the only quadriceps muscle that crosses the hip joint. The other quadriceps muscles originate on the femoral shaft. This gives the hip joint substantial significance with respect to the knee extensor mechanism in the examination and intervention.

The line of pull of the rectus femoris, with respect to the patella, is at an angle of about 5 degrees with the femoral shaft.

Vastus Intermedius

The vastus intermedius has its origin on the proximal part of the femur, and its line of action is directly in line with the femur.

Vastus Lateralis

The Vastus Lateralis is composed of two functional parts: the VL and the vastus lateralis oblique (VLO).

The VL has a line of pull of about 12–15 degrees to the long axis of the femur in the frontal plane, whereas the VLO has a pull of 38–48 degrees.

Vastus Medialis

The vastus medialis is composed of two parts that are anatomically distinct the vastus medialis obliquus (VMO) and the vastus medialis proper, or longus (VML).

The VML appears to have little biomechanical significance unlike its counterpart, the VMO.

Vastus Medialis Obliquus:

The VMO arises from the adductor magnus tendon. The insertion site of the normal VMO is the medial border of the patella, approximately one-third to one-half of the way down from the proximal pole.

If the VMO remains proximal to the proximal pole of the patella and does not reach the patella, there is an increased potential for malalignment.

The vector of the VMO is medially directed, and it forms an angle of 50–55 degrees with the mechanical axis of the leg. The VMO is least active in the fully extended position and plays little role in extending the knee, acting instead to realign the patella medially during the extension maneuver. It is active in this function throughout the whole range of extension.

The VMO is frequently innervated independently from the rest of the quadriceps by a separate branch from the femoral nerve.

The vastus medialis is the weakest of the quadriceps group and appears to be the first muscle of the quadriceps group to atrophy and the last to rehabilitate.

Vastus Medialis Longus:

The VML originates from the medial aspect of the upper femur and inserts anteriorly into the quadriceps tendon, giving it a line of action of approximately 15–17 degrees off the long axis of the femur in the frontal plane.

Since the quadriceps group is aligned anatomically with the shaft of the femur and not with the mechanical axis of the lower extremity, any quadriceps muscle contraction (regardless of knee flexion angle) results in compressive forces acting on the patellofemoral joint.

Hamstrings Muscles Group

The Hamstrings Muscles Group primarily function to extend the hip and to flex the knee. The hamstring muscles are innervated by branches of the sciatic nerve.

Semimembranosus Muscle:

The semimembranosus muscle arises from the lateral facet of the ischial tuberosity and the ischial ramus. This muscle inserts into the posteromedial aspect of the medial tibial condyle and has an key expansion that reinforces the posteromedial corner of the knee capsule. The semimembranosus pulls the meniscus posteriorly, and internally rotates the tibia on the femur, during knee flexion, although its primary function is to extend the hip and flex the knee.

Semitendinosus Muscle:

The semitendinosus muscle arises from the upper portion of the ischial tuberosity via a shared tendon with the long head of the biceps femoris. From there, it travels distally, becoming cord-like about two-thirds of the way down the posteromedial thigh. Passing over the MCL, it inserts into the medial surface of the tibia and deep fascia of the lower leg, distal to the gracilis attachment, and posterior to the sartorius attachment. These three structures are collectively called the pes anserinus at this point. Like the semimembranosus, the semitendinosus functions to extend the hip, flex the knee, and internally rotate the tibia.

Biceps Femoris:

The biceps femoris muscle is a two-headed muscle. The longer of the two heads arises from the inferomedial facet of the ischial tuberosity, whereas the shorter head originates from the lateral lip of the linea aspera of the femur. The muscle inserts on the lateral tibial condyle and the fibular head. The biceps femoris functions to extend the hip, flex the knee, and externally rotate the tibia. The superficial layer of the common tendon has been identified as the main force creating external tibial rotation and controlling internal rotation of the femur. The pull of the biceps on the tibia retracts the joint capsule and pulls the iliotibial tract posteriorly, keeping it tight throughout flexion.

Gastrocnemius Muscle

The gastrocnemius originates from above the knee by two heads, each head connected to a femoral condyle and to the joint capsule. Approximately halfway down the leg, the gastrocnemius muscles merge to form an aponeurosis. As the aponeurosis gradually contracts, it accepts the tendon of the soleus, a flat broad muscle deep to the gastrocnemius.

The aponeurosis and the soleus tendon end in a flat tendon, called the Achilles tendon, which attaches to the posterior aspect of the calcaneus. The two heads of the gastrocnemius and the soleus are collectively known as the triceps surae.

Although the primary function of the gastrocnemius–soleus complex is to plantar flex the ankle and to supinate the subtalar joint, the gastrocnemius also functions to flex or extend the knee, depending on whether the lower extremity is weightbearing or not.

It has been proposed that weakness of the gastrocnemius may cause knee hyperextension. In addition, it has been theorized that the gastrocnemius acts as an antagonist to the ACL, exerting an anteriorly directed pull on the tibia throughout the range of knee flexion–extension motion, particularly when the knee is near extension.

Popliteus Muscle

The popliteus originates from the lateral femoral condyle near the LCL. The muscle has several attachments, including the lateral aspect of the lateral femoral condyle, the posteriormedial aspect of the head of the fibula and the posterior horn of the lateral meniscus. The larger base of this triangular muscle inserts obliquely into the posterosuperior part of the tibia above the soleal line.

The muscle has several important functions, including the reinforcement of the posterior third of the lateral capsular ligament and the unlocking of the knee during flexion from terminal knee extension. It performs this latter task by internally rotating the tibia on the femur (a good example of an arcuvial muscle), preventing impingement of the posterior horn of the lateral meniscus by drawing it posteriorly, and, with the PCL, preventing a posterior glide of the tibia.

Since knee joint injury frequently involves some component of transverse-plane rotation and the popliteus muscle has been described as an important, primary, dynamic, transverse-plane, rotatory knee joint stabilizer, an understanding of its function in relation to other posterolateral knee joint structures is important.

Attached to the popliteus tendon is the popliteofibular ligament, which forms a strong attachment between the popliteal tendon and the fibula. This ligament adds to posterolateral stability. medial portion of the popliteus penetrates the joint, becoming intracapsular with the lateral meniscus. This part of the popliteus tendon is pain sensitive, and an injury here can often mimic a meniscal injury on the lateral aspect of the joint line. Differentiation between these two lesions can be elucidated with the reproduction of pain with resisted flexion in an extended and externally rotated position of the tibia if the popliteus is involved.

Hip Adductors

The hip adductors, which play an indirect role in the medial stability of the knee. The exception to this is the two-joint gracilis muscle, the third member of the pes anserinus group, which in addition to adducting and flexing the hip, assists in flexion of the knee and internal rotation of the lower leg.

Tensor Fascia Latae

The tensor fascia latae (TFL) arises from the outer lip of the iliac crest and the lateral surface of the anterior superior iliac spine (ASIS). Over the flattened lateral surface of the thigh, the fascia latae thickens to form a strong band, the iliotibial tract.

When the hip is flexed, the TFL is anterior to the greater trochanter and helps maintain the hip in flexion. As the hip extends, the TFL moves posteriorly over the greater trochanter to assist with hip extension. The TFL is also a weak extensor of the knee, but only when the knee is already extended.

The muscle is innervated by the superior gluteal nerve, L4–L5.

See Also: Hip Muscles Anatomy

Iliotibial Band (Tract)

The ITB or tract begins as a wide covering of the superior and lateral aspects of the pelvis and thigh in continuity with the fascia latae.

It inserts distal and lateral to the patella at the tubercle of Gerdy on the lateral condyle of the tibia. Anteriorly, it attaches to the lateral border of the patellar. Posteriorly, it is attached to the tendon of the biceps femoris. Laterally, it blends with an aponeurotic expansion from the VL.

Like the patella tendon, the ITB can be viewed as a ligament or a tendon. Its location adjacent to the center of rotation of the knee allows it to function as an anterolateral stabilizer of the knee in the frontal plane and to both flex and extend the knee.

During stationary standing, the primary function of the ITB is to maintain knee and hip extension, providing the thigh muscles an opportunity to rest. While walking or running, the ITB helps maintain flexion of the hip and is a major support of the knee in squatting from full extension until 30 degrees of flexion. In knee flexion greater than 30 degrees, the iliotibial tract becomes a weak knee flexor, as well as an external rotator of the tibia.

Action	Primary Muscles	Peripheral Nerve Supply	Nerve Root Derivation
Knee Flexors	Biceps femoris Semimembranosus Semitendinosus Gracilis Sartorius Popliteus Gastrocnemius Tensor fascia latae	Sciatic Sciatic Sciatic Obturator Femoral Tibial Tibial Superior gluteal	L5, S1–2 L5, S2–2 L5, S1–2 L2–3 L2–3 L4–5, S1 S1–2 L4–5
Knee Extensors	Rectus femoris Vastus medialis Vastus intermedius Vastus lateralis Tensor fascia latae	Femoral Femoral Femoral Femoral Superior gluteal	L2–4 L2–4 L2–4 L2–4 L4–5
Internal rotation of flexed leg (non–weight-bearing)	Popliteus Semimembranosus Semitendinosus Sartorius Gracilis	Tibial Sciatic Sciatic Femoral Obturator	L4–5 L5, S1–2 L5, S1–2 L2–3 L2–3
External rotation of flexed leg (non–weight-bearing)	Biceps femoris	Sciatic	L5, S1–2

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