Part four. Posterior compartment of the thigh
The posterior or hamstring compartment of the thigh extends from the buttock to the back of the knee. It is separated from the anterior compartment by the lateral intermuscular septum, but there is no septum dividing it from the medial or adductor compartment, for the adductor magnus is a muscle consisting of fused adductor and hamstring components.
The cutaneous nerve supply is by the posterior femoral cutaneous nerve, which runs vertically downwards deep to the fascia lata, pierces it behind the knee, and ends halfway down the calf. It sends branches through the fascia lata to supply the overlying skin.
Hamstring muscles
The hamstring muscles all arise from the ischial tuberosity and are inserted into the tibia or fibula; they thus span both the hip and knee joints. They are the semimembranosus, semitendinosus and biceps femoris (long head). The short head of biceps arises from the back of the femur. The hamstring part of adductor magnus is described on page 122.
Semimembranosus
This extends from the ischial tuberosity to the medial condyle of the tibia. It arises from the lateral part of the ischial tuberosity (Fig. 3.16), above the part that bears weight in sitting. This origin is a long flat tendon, from which muscle fibres commence about mid-thigh. The distal attachment of the muscle is by a rounded tendon into the horizontal concavity on the back of the medial condyle of the tibia (Figs 3.17 and 3.19). From this insertion three expansions diverge. One passes forwards along the medial surface of the condyle deep to the tibial collateral ligament of the knee, separated from it by a bursa. A second expansion passes obliquely upwards to the lateral femoral condyle as the oblique popliteal ligament (see p. 137), while the third forms a strong fascia overlying popliteus and reaches the soleal line of the tibia.
Figure 3.16 Left ischial tuberosity, from behind.
Figure 3.17 Right hamstring muscles. Semitendinosus and long head of biceps are retracted laterally to show the sciatic nerve lying on adductor magnus lateral to the tendinous upper end of semimembranosus.
Figure 3.19 Right knee joint from behind, showing the reinforcement of the capsule by the oblique popliteal ligament, and popliteus emerging from the capsule. The fabella is a sesamoid bone sometimes present in the lateral head of gastrocnemius.
Semitendinosus
This arises, in common with the long head of biceps, from the medial part of the ischial tuberosity
(Fig. 3.16). The fleshy belly diminishes in size from above downwards to a cord-like tendon that lies on the posterior surface of the muscular belly of semimembranosus (Fig. 3.17). The tendon of semitendinosus passes behind the medial condyle of the femur and then curves forwards to be inserted behind gracilis into the upper part of the medial surface of the shaft of the tibia.
Biceps femoris
This muscle has two heads of origin. The long head arises, in common with semitendinosus, from the ischial tuberosity (Fig. 3.16). It passes downwards and laterally, crossing the sciatic nerve on its posterior aspect, and joins the short head which has a long origin, from the whole length of the linea aspera and the upper part of the lateral supracondylar line of the femur (Fig. 3.50B). Distally the single tendon of the muscle is folded or split around the fibular collateral ligament of the knee joint and is inserted into the head of the fibula (Fig. 3.26).
Figure 3.26 Ligaments of the left knee joint, with the knee partially flexed: A from the front; B from behind.
Blood supply of the hamstrings. The hamstring compartment receives its blood supply mainly from the profunda femoris artery and its perforating branches. These pierce the adductor magnus and give off large branches to the hamstrings and the overlying fat and skin. The blood supply of the upper part of the hamstrings is derived from the inferior gluteal artery and that of the lower part from the popliteal artery. These and the perforating branches of the profunda femoris form a series of anastomoses along the back of the thigh. The highest is the cruciate anastomosis.
Nerve supplies. The three muscles are supplied, with the ischial part of adductor mangus, by the tibial component of the sciatic nerve (L5, S1), with the exception of the short head of biceps, which is supplied by the common peroneal (fibular) part of the sciatic nerve.
Actions. Acting from above the hamstrings flex the knee joint, and acting from below they extend the hip joint. With the knee semiflexed, biceps femoris is a lateral rotator, while semimembranosus and semitendinosus are medial rotators of the leg.
Test. While lying face downwards with the limb straight, the knee is flexed against resistance, and the tendons of biceps and semitendinosus palpated above the knee on the lateral and medial sides respectively.
Sciatic nerve
The sciatic nerve runs vertically through the hamstring compartment, lying deep to the long head of biceps, between it and the underlying adductor magnus (Fig. 3.17). At the apex of the popliteal fossa, a hand's breadth or more above the knee joint, it divides into its tibial and common peroneal (fibular) components; but the division may occur at a higher level (see p. 127).
The surface marking of the nerve is from the midpoint between the ischial tuberosity and greater trochanter to the apex of the popliteal fossa.