Pathology
The hamstring muscle group is composed of three posterior thigh muscles: the semitendinosus and semimembranosus on the medial side and the biceps femoris on the lateral side. The biceps femoris has two heads, so in some ways it may be considered two separate muscles. The long head of the biceps femoris originates on the ischial tuberosity and the short head originates along the linea aspera on the posterior side of the femur. Both heads of the biceps femoris share a common insertion on the lateral aspect of the fibular head. The semitendinosus and semimembranosus both originate on the ischial tuberosity and insert on the medial portion of the proximal tibia.

When contracting concentrically, the hamstrings produce knee flexion. The semitendinosus, semimembranosus and long head of the biceps femoris also contribute to hip extension. The short head of the biceps femoris is not involved in hip extension, as it doesn’t cross the hip joint. The hamstrings also act on knee rotation when the knee is in a flexed position. The semimembranosus and semitendinosus medially rotate the knee while the biceps femoris laterally rotates it.

Strains may occur in any region of the muscle, but are most common at the musculotendinous junction. Tensile forces are particularly high at the proximal musculotendinous junction because it is the common attachment for all three major heads of the hamstrings. Consequently, strains are common here. However, strains may occur in the middle of the muscle belly as well.

A strain involves muscle-fiber tearing from excessive tensile stress. However, excess stress alone does not create the injury. Instead, muscle strains occur most often when the muscle is exposed to tensile (pulling) stress while it is contracting. Tensile stress during contraction is most common during eccentric contractions. Forces on the muscle are greater in an eccentric contraction than in an isometric or concentric contraction, which is why so many strains occur from eccentric overloading.

There are three grades of muscle strain: first degree (mild), second degree (moderate) and third degree (severe).

In a first-degree strain, few muscle fibers are torn. While there may be some post-injury soreness, the individual usually returns to normal levels of activity quickly.

With second-degree strains more fibers are involved in the injury. There is a greater level of pain and a clear region of maximum tenderness in the muscle tissue.

A complete rupture of the muscle tendon unit occurs with a third-degree strain, although some strains are classified as third degree when they still have some fibers intact. In either case, there is likely to be significant pain at the time of the injury, along with swelling, bruising, and a clearly palpable defect in the muscle.

Some cases of third-degree strain also involve an injury called an avulsion fracture. An avulsion fracture occurs when the tendon is torn away from its attachment and pulls a small chunk of bone with it. One of the more common locations for avulsion fractures is the proximal hamstring attachment at the ischial tuberosity.

Hamstring strain is a common lower-extremity injury, and there are several reasons why. One reason involves strength imbalances between the hamstrings and quadriceps. Because the quadriceps muscles are usually much stronger, the hamstring muscle group may be less resistant to fatigue, which is a cause of strain injury. Another reason the hamstrings are prone to strain relates to their anatomy and biomechanical function.

Because all three hamstrings cross both the hip and knee joints, with the exception of the short head of the biceps femoris, they are considered multi-articulate muscles. Multi-articulate muscles are more susceptible to strain because they act across more than one joint at the same time. For example, the hamstrings are forced into eccentric contraction at the very end of the swing-through phase of gait, when they are in a lengthened position across both joints they cross (Figure 2). Consequently, attempting to contract the hamstrings while they are lengthened this way frequently leads to strain.

In addition to strength imbalances and biomechanical demands, other factors play a role in the onset of hamstring strain. These factors include lack of flexibility or proper conditioning, muscle fatigue and insufficient warm-up. Hamstring strain is usually acute, but low levels of stress may accumulate and cause a chronic strain as well.

An overlooked contributing factor to hamstring strain is excess tension on the sciatic nerve. The increase in neural tension causes sensory irritation and produces an elevated level of tonus in the muscle. When the muscle tonus is increased, the muscle is tighter and more susceptible to strain injury.

The process of neural tension and hamstring strain, may become a vicious cycle, because adverse neural tension may result from hamstring strain as well as cause it. The strain may increase neural tension because fibrous adhesions from scar tissue restrict mobility of the sciatic nerve. When the sciatic nerve’s mobility is restricted, neural tension results. From the neural tension, the hamstring muscles are more prone to strain injury.