Practice Essentials
The hamstrings are a group of muscles (ie, semimembranosus, semitendinosus, biceps femoris) located on the back of the upper leg. [1, 2, 3] This article focuses on injuries to the hamstring muscles. The word "hamstrings" was derived from the fact that it is these muscles by which a butcher would hang a slaughtered pig.
The hamstrings are a common source of injury and chronic pain in athletes. Injuries to the hamstring muscles primarily occur proximally and laterally, and they usually involve the biceps femoris. The severity of injury to the hamstring muscles is classified according to the following grades:
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Grade 1 is a mild strain, with few muscle fibers being torn.
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Grade 2 is a moderate strain, with a definite loss in strength.
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Grade 3 is a complete tear of the hamstrings.
Hamstring injuries almost always occur at the proximal myotendinous junction. In the biceps femoris, this junction extends over most of its entire length. Injury usually does not occur within the tendon itself unless there is preexisting pathology.
Bony avulsion at the ischial origin may occur as well, but this is usually associated with sudden, large-force, hip-flexion injuries. [4] Avulsions are commonly seen in individuals who have been involved in waterskiing accidents in which the knee is extended and the hip is suddenly flexed as the skier falls forward. [5]
One study involving 47 football players with hamstring injuries reported an average of 14 days of convalescence before return to play.
Signs and symptoms
The onset of pain and/or weakness is usually sudden and may occur during an explosive movement, such as sprinting. Patients may report hearing an audible pop at the time of injury.
Physical findings are absent in many hamstring injuries.
See Presentation for more detail.
Diagnosis
Most often, the diagnosis can be made on the basis of the history and physical examination alone. Laboratory and imaging studies are usually not needed.
See Workup for more detail.
Management
Treatment of hamstring injuries varies according to the severity of the injury. Patients with minor strains may progress quickly to strengthening exercises, whereas those with full-thickness ruptures require surgery.
See Treatment and Medication for more detail.
Patient education
For patient education resources, see the First Aid & Emergencies Center, as well as Pulled Hamstring (Hamstring Injury) and Muscle Strain.
Related Medscape topics
Resource Center Adolescent Medicine
Resource Center Exercise and Sports Medicine
Resource Center Joint Disorders
Resource Center Trauma
Specialty Site Orthopaedics
Etiology
The major predisposing factors are lack of warm-up, poor flexibility, fatigue, and a hamstring-to-quadriceps ratio less than 50%.
A previous hamstring injury is the most recognized risk factor for injury. A study of elite track and field athletes with acute, first-time, unilateral hamstring muscle strains found that low-grade hamstring muscle injuries may lead to a higher risk of reinjury than high-grade injuries. [6, 7, 8]
Poor running style, especially overstriding, predisposes some runners to hamstring injuries. Overstriding stretches the hamstring and places it in a position of active insufficiency.
Dyssynergia secondary to dual innervation of the hamstring muscles may also be a factor that contributes to hamstring injuries.
Rapid growth seen during adolescence sometimes leads to tight hip flexors with a resultant anterior hip tilt. This can cause a natural predisposition to hamstring injuries for this age group.
Epidemiology
United States statistics
As a percentage of lower-extremity injuries, hamstring injuries peak at 33% in persons aged 16-25 years, and they most often occur in sports in which the hamstrings can be stretched eccentrically at high speed. [9, 10, 11, 12, 13, 14] Prime examples of such sporting activities include sprinting, track and field, and other running contact sports, such as football and soccer. Recreational sports such as waterskiing, in which the knee is fully extended during injury, are also common causes of hamstring injuries. [5]
One study analyzed data from the National Football League’s Injury Surveillance System and found that a high percentage of hamstring injuries occur in the preseason. Over the 10-year study period, 1716 hamstring strains were noted, with slightly more than 50% occurring during the preseason. Players on the special teams units as well as wide receivers and defensive secondary were found to have an elevated risk for injury. [15]
International statistics
An Australian study involving 1614 individuals with hamstring injuries revealed that such injuries compose 54% of the injuries in rugby, 10% of the injuries in soccer, 14% of the injuries in track, and less than 2% of the injuries in tennis, squash, ballet, and gymnastics. Another Australian study showed that hamstring injury accounts for 10% of all injuries in field-based team sports (soccer, rugby union, field hockey, Gaelic football, hurling, and Australian football). [16]
A 13-year longitudinal analysis by Ekstrand et al that included 36 European Football clubs and 1614 hamstring injuries reported that training-related hamstring injury rates increased by 4.0% per year since 2001, while match-related injury rates remained stable. [17] More recent results from the same investigators show that the proportion of hamstring injuries rose from 12% in 2001 to 24% in 2022. From 2014 to 2022, the incidence of hamstring injuries increased during both training and match play. [18]
Functional Anatomy
The hamstrings are composed of 3 muscles, as follows:
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Biceps femoris muscle (long head and short head)
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Semimembranosus muscle
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Semitendinosus muscle
Origins and insertions
All of the muscles of the hamstrings originate on the ischial tuberosity. The second head of the biceps femoris (ie, short head) originates medial to the linea aspera on the distal posterior femur.
The short head of the biceps femoris crosses only one joint to insert with the long head of the biceps femoris onto the fibular head and lateral tibial condyle.
The other hamstring muscles cross 2 joints to reach their insertions. The semitendinosus muscle forms the pes anserinus with the sartorius and gracilis tendons to insert on the medial tibial metaphysis. The semimembranosus muscle interweaves with the fibers of the semitendinosus to eventually insert onto the posteromedial tibial condyle.
Innervations
The short head of the biceps femoris muscle is also unique in that it is innervated by the peroneal portion of the sciatic nerve, whereas the long head of the biceps femoris, semimembranosus, and semitendinosus are innervated by the tibial portion of the sciatic nerve.
Sport-Specific Biomechanics
In track and field events in which the hamstring is eccentrically contracted, the risk of a hamstring injury can be high. Contact sports such as football can result in contusions of the hamstring muscle. [19, 20] The contusion is superficial when the muscle is contracted on impact, and it is deep when the muscle is relaxed on impact. Waterskiing accidents have an association with proximal, bony avulsions because the individual's knee is extended when the hip undergoes a violent, forceful flexion as he/she falls forward.
Prognosis
With minor activity limitations and proper rehabilitation, the prognosis is good for hamstring strains and even partial tears. Complete tears also heal but require a significantly longer and more intensive rehabilitation program.
Complications
Returning to play too early is a common factor leading to chronic hamstring pain and injury. Reinjury rates as high as 77% may be related to areas of calcification and inflammation in the hamstring after injury. Scar formation may impinge the sciatic nerve, resulting in hamstring syndrome. [21] Surgery has rarely been used to break up painful scar tissue.
