Hamstring Strain

What Is a Hamstring Strain?

A hamstring strain is a partial or complete rupture of the muscle fibers of the posterior thigh group — formed by the biceps femoris, semitendinosus, and semimembranosus. It is the most frequent muscle injury in sport, accounting for approximately 12-15% of all injuries in athletes of disciplines that involve high-speed running.

The most common mechanism of injury is eccentric overload during the late swing phase of running, when the hamstrings contract to decelerate knee and hip extension before foot contact with the ground. This intense eccentric demand is particularly pronounced during sprints, accelerations, and abrupt changes of direction.

The most consistent risk factor in the literature is history of prior strain — athletes with previous injury have a two- to six-fold higher risk of recurrence, reflecting both incomplete healing of muscle tissue and residual neuromuscular deficits that frequently persist after return to sport.

Pathophysiology

The hamstrings are biarticular muscles that cross the hip and knee. During high-speed running, in the late swing phase, they contract eccentrically to brake knee extension and hip flexion — generating internal forces that may exceed the muscle fibers' resistance capacity. The injury results from this imbalance between load and resistance.

The biceps femoris (long head) is the most frequently injured muscle, accounting for about 70% of cases. This is due in part to its dual innervation (tibial nerve to the long head and fibular nerve to the short head), which can produce asynchronous muscle activation, and to its greater contribution to knee deceleration during sprinting.

Muscle healing after a strain involves three overlapping phases: inflammatory (0-5 days), proliferative (5-21 days), and remodeling (21 days to months). Fibrotic scar tissue forming at the myotendinous junction is the main factor predisposing to recurrence, since this tissue is less elastic and less resistant to eccentric loading than the original muscle.

Signs and Symptoms

Clinical presentation varies with injury grade. In grade I strains, pain may appear only during or after sports activity. In grade II and III injuries, onset is typically acute and incapacitating, often with a "pop" or "snap" sensation in the posterior thigh during sprinting.

Diagnosis

Hamstring strain diagnosis is predominantly clinical, based on a typical injury-mechanism history and physical examination findings. Imaging studies are indicated to grade the injury, guide prognosis, and rule out alternative diagnoses.

Differential Diagnosis

Although the acute presentation of hamstring strain is generally characteristic, other conditions may mimic or coexist with the muscle injury. Differentiation is particularly important in subacute and chronic presentations.

Treatments

Treatment of hamstring strain follows a progressive rehabilitation protocol, the duration of which depends on the grade of injury. The contemporary approach emphasizes early initiation of exercises within pain limits, avoiding both prolonged immobilization and premature return to sport.

Medical Acupuncture

Acupuncture can support recovery from hamstring strain, acting as an adjunct to rehabilitation. Proposed mechanisms include modulation of local pain through segmental inhibition, release of endogenous opioids, improvement of tissue microcirculation, and reduction of reflex muscle spasm that frequently accompanies the injury.

Electroacupuncture at low frequencies (2-4 Hz) may be particularly useful in the pain control phase, facilitating progression of the exercise program. Stimulation of local points in the injured region promotes increased blood flow and may favor the healing process, although specific data for muscle strain remain limited.

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