Popliteal Fossa

The popliteal fossa (fossa poplitea, in Latin) is an anatomical region located on the posterior surface of the knee-joint region. It is rhomboid in shape. Superiorly and medially, it is bounded by the semimembranosus and semitendinosus muscle. Superiorly and laterally, it is limited by the biceps femoris muscle. Inferiorly, it is confined by both heads of the gastronemius muscle and the plantaris muscle. The floor of the fossa is constituted by the popliteal surface of the distal end of femur and the posterior surface of the knee-joint capsule.

The region of the popliteal fossa is crisscrossed by a network of arteries that supply the different muscles and bones of the distal portion of thigh and the leg. These are the branches of the popliteal artery, which in turn is the continuation of the femoral artery. It is innervated by the popliteal nerve as well as by the tibial and the common peroneal nerve, all of them being branches of the sciatic nerve.

Below, diagram showing the popliteal fossa region, with the different muscles, arteries and nerves.

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Muscles of Thigh

The muscles of thigh are the longest and most powerful muscles of the human body. They consume a large amount of calories (glucose/fatty acid) when we run. Not only are the muscles of thigh long but some of them are also broad and spindle-shaped. The sartorius is the longest and narrowest, while the adductor magnum is the broadest, with the quadriceps femoris being the largest. They are innervated by branches of the femoral, the sciatic, the tibial, and the peroneal nerve and they are supplied by the femoral artery and its branches. Some of them arise from the different anatomical parts of pelvis, but others from the upper third portion of femur, inserting into the tibia.

Anatomically, they are classified into three groups: 1) anterior group; 2) medial group; 3) posterior group.

Anterior Group

The muscles of this group are located on the anterior aspect of thigh. It is formed by the sartorius, which is the longest muscle of the body and the most anterior of the thigh, arising from the anterior superior iliac crest and inserting into the tubercle of tibia (tibial tuberosity); and the quadriceps femoris, which is composed in turn of four muscular heads: the rectus femoris, which lies anteriorly; the vastus medialis, which is located towards the medial side of thigh; the vastus lateralis, lying towards the lateral aspect of upper limb; and the vastus intermedius, which also lies anteriorly but under the rectus femoris.

Medial Group

There are five muscles in this group as their function is the adduction of the thigh (pulling it inwardly as when we ride a horse); they all lie on the medial (inner) aspect of thigh. It is composed of the gracilis, which is long and flattened; adductor longus, which is also long but rather triangular in shape; the adductor brevis; adductor magnus, which is the broadest of them all; adductor minimus; and pectinius muscle, which is small, flat and rather square. These muscles originate from the pubic border of pelvis and insert into the middle line of femur, and into the tibia. They are innervated by branches of the femoral and obturator nerve and they are supplied by the femoral artery. 

Posterior Group 

Three long muscles make up this group. The biceps femoris, which is situated along the postero-lateral side of thigh; the semimembranosus, which lies along the medial border and posterior side of thigh; the semitendinosus, which is narrower than the other two muscles of the group, being situated along and over the semimembranosus. All three of them arise from the tuberosity of ischium (of pelvis) and are inserted into the superior portion of tibia. Action: they flex the leg and extend the thigh. They are innervated by the sciatic, tibial, and peroneal nerve. They are supplied by the medial circumflex, perforating, and the popliteal artery, which are branches of the femoral artery.

Below, anterior group of muscles of thigh. The sartorius and the quadriceps are visible in this picture of right thigh. The vastus intermedius cannot be seen as it is located under the rectus femoris. However, three muscles of the medial group can be observed (adductor longus, adductor magnus, and gracilis).

Below, medial group of muscles. View of medial aspect of right human thigh. The adductor brevis is not visible as it lies beneath the gracilis and the adductor longus.

Muscles of thigh. Posterior group: semitendinosus, semimembranosus, and biceps femoris. The adductor magnus and gracilis belong to the medial group.

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Quadriceps Femoris

The quadriceps femoris is the strong extensor muscle which lies on the anterolateral aspect of thigh. It is made up of four smaller muscles; they are the rectus femoris, the vastus lateralis, the vastus medialis, and the vastus intermedius, forming a large dense mass covering the front and sides of the femur.

Each of the heads of these muscles arises independently, but as they reach the knee-joint the form one tendon. This means that the individual tendons of these four muscles that constitute the quadriceps fuse into a common tendon at the distal part of the thigh and inserts into the patella and the tibial tuberosity.

The rectus femoris occupies the anterior surface of thigh, arising from the anterior inferior iliac spine by a fine tendon. The vastus lateralis lies on the anterolateral aspect of thigh, originating from the greater trochanter and the lateral lip of linea aspera of femur. The vastus medialis is located on the anteromedial side of thigh, taking origin also from the lateral lip of linea aspera of femur. The vastus intermedius lies on the anterior side of thigh, beneath the rectus femoris muscle, arising from the anterior surface of femur.

Action

Contraction of all the heads of the quadriceps femoris muscle extends the leg at the knee-joint, while the rectus femoris alone flexes the hip-joint. All of them contract when we rise up from a sitting position, and when we run and do sit-ups in the gym.

Innervation

This group of four muscles is innervated by the femoral nerve (from L2, L3 and L4).

Blood Supply

The quadriceps femoris is supplied by the lateral circumflex femoral and the profunda femoris artery.

Below, a diagrammatic picture of right thigh, showing three of the four muscles that make up the quadriceps group. The vastus medialis cannot be seen because it lies underneath the rectus femoris muscle.

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Posterior Group of Thigh Muscles

The posterior group of thigh muscles is sometimes referred to as the hamstring muscles. It is composed by three long powerful muscles, all of them arising from the ischial tuberosity of pelvis, stretching along the full length of femur. Their upper (proximal) portions are covered by the gluteus maximus. They are:

1- The semimembranosus muscle, which lies on the medial border of the posterior side of thigh. Origin: ischial tuberosity. Insertion: medial condyle of tibia and oblique posterior ligament of knee.

2- The semitendinosus muscle, which lies closer to the medial border, extending over the semimembranosus. Origin: ischial tuberosity. Insertion: tubercle of tibia.

3- The biceps femoris muscle, which extends vertically on the lateral border of posterior side of thigh. Long and flattish, it arises from the ischial tuberosity of pelvis ischium and get inserted into the proximal end of tibia.

Action/function

They extend the thigh at the hip-joint, flexing the leg at the knee, rotating medially and laterally.

Innervation

The posterior group of thigh muscles are innervated by the tibial (L4-L5-S1) and sciatic nerves (S1-S2).

Blood Supply

They receive oxygenated blood from branches of the medial circumflex femoral, perforating, and popliteal artery.

Below, the schematic picture of posterior aspect of right thigh, showing all three muscles of posterior group.

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Biceps Femoris Muscle

The biceps femoris is a long muscle lying on the lateral border of posterior aspect of human thigh. Thus, it belongs to the posterior group of thigh muscles, together with the semitendinosus and semimembranosus muscle. It has two heads, a long and a short head, which fuse together to form a single common belly.

The long head of the biceps femoris muscle arises from the ischial tuberosity by a narrow, flat tendon. The short head originates from the lateral lip of linea aspera of the distal half of femur. Then the two heads get together to become a strong muscle belly, which stretches down along the posterior lateral border of thigh, parallel and next to the semitendinosus muscle. The distal portion of the biceps femoris becomes a long narrow tendon, which curves around the lateral epicondyle of femur to be inserted into the head of fibula.

However, some bands of its distal tendon runs almost horizontally to be inserted into the edge of superior articular surface of the tibia. The lower bursa of the biceps femoris muscle is lodged between its tendon and the lateral ligament of the knee.

Action/Function

It extends the thigh at the hip-joint. It also flexes the leg at the knee-joint, rotating it laterally.

Innervation

The long head of the biceps femoris in innervated by branches of the tibial and sciatic nerve (S1, S2). The short head receives branches from the common peroneal nerve (L4, L5, S1).

Blood Supply

It receives oxygenated blood from the medial circumflex femoral, perforating, and popliteal artery.

Above, a schematic picture of posterior side of right thigh, showing the biceps femoris muscle. You can observe that its proximal (upper) portion is covered by the gluteus maximus.

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Semitendinosus Muscle

The semitendinosus muscle belongs to the posterior group of muscles of human thigh. It is long and narrow and stretches along the full length of femur, over the semimembranosus muscle. It lies closer to the medial (inner) border of the posterior aspect of thigh, next and parallel to the biceps femoris mucle. Its proximal end is covered by the gluteus maximus muscle.

The semitendinosus muscle arises from the tuberosity of ischium, traveling downwards along the length of thigh, partially covering the semimembranosus muscle. At its distal extremity, it ends up in a long tendon that curves around the medial femoral epicondyle as it runs down slightly beyond the knee-joint. Then the tendon stretches on to the anteromedial surface of tibia to be inserted into its tuberosity.

Action/Function

The semitendinosus muscle flexes the leg at the knee-joint and it also extends the thigh at the hip joint.

Blood supply

It is supplied by the perforating arteries, which are branches arising from the deep artery of thigh, also known as the deep femoral artery.

Innervation

It is innervated by branches springing from the tibial nerve (L4, L6, S1, S2)

Below, diagrammatic picture that shows the thigh posterior side muscles, which include the semitendinosus, semimembranosus, and biceps femoris muscle.

 The superficial aspect of posterior side of side. The semitendinosus muscle can be perceived as it stands out distintively under the skin in athletic humans, such as soccer and football players.

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Semimembranosus Muscle

The semimembranosus muscle lies on the medial border of posterior side of the human thigh. Its lateral half is covered by the semitendinosus muscle, leaving a mark in the form of a wide longitudinal groove. With the medial margin of the muscle being free, it stretches along the full length of the femur. It is part of the posterior group of muscles of the thigh.

The semimembranosus muscle originates from the ischial tuberosity of pelvis by a flat, strong tendon. Then it travels downwards vertically along the length of thigh, partially covered by the semitendinosus. It ends up in a distal flat tendon which narrows gradually. This tendon winds around the medial epicondyle as it runs to the anteromedial surface of tibia where it becomes wider and divides into three bands; the first one is inserted into the medial condyle of tibia; the second band fuses with the fascia covering the popliteus muscle; and the third one is inserted into the oblique posterior ligament of knee. 

Function/action 

It extends thigh straight at the hip-joint. It also flexes and rotates leg medially. 

Nerve supply 

The semimembranosus  muscle is innervated by the tibial nerve, which is a major branch of the sciatic nerve. 

Blood supply 

It receives oxygen-rich blood from a branch from the deep femoral artery, which arises from the femoral, and the gluteal arteries, which spring from the internal iliac artery.

Below, posterior aspect of muscles of right thigh, showing the semimembranosus, semitendinosus, and biceps femoris muscle.

The semimembranosus muscle, with the semitendinosus cut away to expose the groove along its lateral half.

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