Posterior Thigh Muscles
The upper part of the lower limb, or region between the hip joint and knee joint, is referred to as the thigh. The thigh is divided into three muscular compartments: the anterior, medial compartment and posterior compartments.
The posterior compartment is separated from the anterior compartment by a lateral intermuscular septum. However, there is no septum separating the posterior compartment from the medial compartment. The posterior compartment comprises three muscles collectively known as the hamstrings. These include the:
- Biceps femoris
Origins: (Superolateral impression of) Ischial tuberosity
Innervation: Tibial division of sciatic nerve (L5-S2)
Function: Hip joint: Thigh extension, Thigh internal rotation; Knee joint: Leg flexion, Leg internal rotation; Pelvis stabilization
Origins: (Posteromedial impression of) Ischial tuberosity
Innervation: Tibial division of sciatic nerve (L5-S2)
Origins: Long head: (Inferomedial impression of) Ischial tuberosity; Short head: Linea aspera of femur (lateral lip), Lateral supracondylar line of femur
Innervation: Long head: Tibial division of sciatic nerve (L5-S2); Short head: Common fibular division of sciatic nerve (L5-S2)
This article will talk about the action of these muscles and their anatomical locations as well as their vascular supply and nerve innervation. Finally, it will go over the pathology of the hamstrings in a clinical setting.
The muscles of the posterior compartment cross both the hip and knee joints and thus have actions at both of these joints. The action of the hamstrings is to extend the thigh at the hip joint and flex the leg at the knee joint. When the knee is in the flexed position, the biceps femoris can externally rotate the leg whereas the semimembranosus and semitendinosus can act as internal rotators at the knee joint.
The semimembranosus is located medially in the posterior compartment, varies in size and has a flat, membranous shape. It originates from the superolateral impression of the ischial tuberosity, by a long, flat tendon. The tendon descends deep to the long head of biceps femoris and to the semitendinosus muscle and forms an aponeurosis. At about midthigh, muscle fibres arise from this aponeurosis. The muscle then converges with another aponeurosis and then forms a rounded tendon. This tendon descends to attach to the horizontal concavity on the posterior aspect of the medial tibial condyle.
The semitendinosus is also situated posteriomedially in the thigh, but runs superficially to the semimembranosus. It arises from the posteromedial impression of the ischial tuberosity by a shared tendon with the long head of the biceps femoris. A muscular belly arises from this tendon, which is fusiform in shape. A rounded tendon then arises just below midthigh. The great length of this tendon gives the muscle its name. It then crosses the medial collateral ligament before attaching to the proximal part of the tibia medially, behind the tendon attachment of the gracilis, via the pes anserinus.
The biceps femoris is once again situated posteriomedially, however, it consists of two heads or proximal attachments. The long head of the biceps femoris arises from the inferomedial impression of the ischial tuberosity through a shared tendon with the semitendinosus. The short head arises from the femur at the lateral lip of the linea aspera and the lateral supracondylar ridge. The long head crosses the sciatic nerve before forming an aponeurosis. The short head joins with this aponeurosis on its deep surface. A tendon is formed from this aponeurosis distally and inserts into lateral aspect of the head of the fibula.
Semimembranosus - perforating arteries (predominantly the first), which branch from the profunda femoris artery. The muscle is also supplied by the inferior gluteal artery.
Semitendinosus - first perforating artery of the profunda femoris artery and the inferior gluteal artery.
Biceps Femoris - the long head is supplied by the first and second perforating arteries of the profunda femoris. The short head is supplied by the second or third perforating arteries of the profunda femoris. Both heads are also supplied by the inferior gluteal artery as well as muscular branches of the popliteal artery.
The nerve innervation of the hamstrings is by the tibial division of the sciatic nerve (L5-S2) except for the short head of the biceps femoris, which is supplied by the common fibular division of the sciatic nerve (L5-S2).
The power of the hamstrings is clinically tested by active flexion of the knee against resistance. The power of the individual hamstring tendons can be assessed more accurately whilst the subject is prone. A strain or tear of the hamstrings, sometimes known as a pulled or torn hamstring is a common sports injury in individuals who are required to start and stop often or run very hard e.g. in those who play basketball and football etc.
An avulsion or a tear may occur when an individual violently exerts a muscle. This commonly causes the proximal tendons to tear away from the ischial tuberosity, resulting in bruising (or a contusion), muscle fibre tears and blood vessel rupture which produces a haematoma. Tearing of the hamstring muscle fibres usually results in a sharp, intense pain. Stretching and adequate warming up help prevent this injury.