Hamstring strains are among the most common muscle injuries, particularly in athletes. Hamstring strains increase in incidence with age and are most common in football, soccer, and sports that require sprinting. The hamstrings consist of three muscles: the semimembranosus and semitendinosus muscles medially and the long and short heads of the biceps femoris muscle laterally.

Hamstring strains constitute a range of injuries from delayed-onset muscle soreness to partial tears to complete rupture of the muscle-tendon unit [1]. Injuries can occur from direct or indirect forces. Direct forces refer to lacerations and contusions. Complete avulsion of the proximal hamstring origin from the ischial tuberosity has been described, most commonly in water-skiers [2,3]. These injuries occur when forced hip flexion is sustained while the knee remains in complete extension.

Most hamstring injuries, however, occur from indirect forces with exertional use of the muscles, such as running, sprinting, and hurdling. Most hamstring injuries occur at the myotendinous junction during eccentric actions when the muscle lengthens while developing force, most commonly in the lateral hamstrings [4]. The biceps femoris has two heads with different origins and dual innervation and is therefore considered a “hybrid” muscle [5]. Dyssynergic contraction of the muscles is one of many proposed etiologic factors predisposing the hamstrings to strain. Other proposed etiologic factors include the hamstring’s being a two-joint muscle, insufficient hamstring flexibility (Fig. 68.1), insufficient warm-up and stretching before activity, strength imbalance between the hamstrings and quadriceps, strength imbalances between the right and left hamstrings, previous injury to the hamstring, higher running speeds, and poor strength or endurance of the hamstrings. Hamstring strains can occur in a variety of patients from young to old and in any level of athletics from the “weekend warrior” to the elite athlete.

f68-01-9781455775774 — FIGURE 68.1 Tight hamstring muscles may lead to an imbalance between the quadriceps and hamstring muscles, placing an athlete at increased risk for injury.

The hamstrings function over two joints. Like other biarticular muscle groups, such as the quadriceps femoris, the gastrocnemius, and the biceps brachii, the hamstrings are more susceptible to injury. The hamstrings cross the hip and knee joint (with the exception of the short head of the biceps femoris). During the latter part of the swing phase of gait, the hamstrings act eccentrically to decelerate knee extension; and at heel strike, the hamstrings act concentrically to extend the hip. During running, this rapid change in function puts the muscle at risk for injury; the higher the running speed and angular velocity, the greater the forces at heel strike [6,7]. Any large strength imbalance between the larger and stronger quadriceps and the hamstrings will put the hamstrings at a disadvantage. If the synergy of antagonists is altered, a vigorous contraction of the weaker muscle may result in injury. Any factor that adversely affects the neuromuscular coordination during running, such as lack of proper warm-up, poor training, or muscle fatigue, may result in a strain injury.

Hamstring strains can be divided into three grades according to their severity:

1. Grade I, or first-degree, strain: mild strain with minimal muscle damage (less than 5% of muscle fiber disruption). There is associated pain but little or no loss of muscle strength.

2. Grade II, or second-degree, strain: moderate strain with more severe partial tearing of the muscle but no complete disruption of the myotendinous unit. Pain is present with loss of knee flexion strength.

3. Grade III, or third-degree, strain: severe strain involving complete tearing of the myotendinous unit. This injury presents with severe pain and marked loss of knee flexion strength [1,8].

Avulsion of the hamstrings tendon from its origin on the ischium or distally from the tibia or fibula is not graded like the classic myotendinous strains. These injuries are usually complete or partial avulsion injuries and described as such.

Symptoms

At the time of injury, patients typically report a sudden, sharp pain in the back of the thigh. Some describe a “popping” or tearing sensation. There is generalized pain and point tenderness at the site of injury. The patient may complain of tightness, weakness, and impaired range of motion. Depending on the severity of the injury, the individual may or may not be able to continue the activity and occasionally is unable to bear weight on the affected limb. Swelling and ecchymosis are variable and may be delayed for several days. The ecchymosis may descend to the thigh and be manifested at the distal thigh or back of the knee, calf, or ankle. The injury may occur in the early or late stages of activity, and patients may give a history of inadequate warm-up or fatigue.

Rarely the patient may complain of symptoms of numbness, tingling, and distal extremity weakness. If these are present, further investigation into a sciatic nerve irritation is warranted. Complete tears and proximal hamstring avulsion injuries can cause a large hematoma or scar tissue to form around the sciatic nerve [9,10].

Alternatively, any change in training patterns and increased eccentric exercise in a previously untrained subject can lead to hamstring injury with delayed-onset muscle soreness. This is thought to be the result of microscopic damage followed by a local inflammatory response [11].