Hamstring Strain from Kicking or Running

A hamstring strain is one of the most common sports injuries. The hamstrings, on the back of the thigh, are made up of three muscles: the biceps femoris (long and short heads), the semitendinosus, and the semimembranosus.

The mechanism of injury usually occurs in the late swing phase of the gait cycle. At that point the hamstrings contract eccentrically to decelerate knee extension, then contract concentrically to act as hip extensors. The eccentric contraction generates greater tension, and in the late swing phase the biceps femoris reaches a peak, about 12 percent higher than in the standing position.

There are several reasons a hamstring strain can happen, including:

• The muscle crosses two joints and can be pulled in different directions.
• The short head of the biceps femoris tendon has a longer tendon.
• The long and short heads are innervated by different branches of the sciatic nerve, which can lead to uncoordinated contraction.

By the degree of injury, hamstring strains can be graded clinically into three levels:

• Grade 1: only a small number of muscle fibers are damaged; range of motion is normal, but stretching causes mild swelling and pain with a slight loss of strength.
• Grade 2: a partial tear of muscle fibers causing functional limitation, with difficulty walking.
• Grade 3: complete rupture of the muscle fibers with loss of muscle function.

Muscle-injury grading is based primarily on clinical diagnosis. With advances in medical imaging in recent years, grading systems based on ultrasound and MRI have gradually developed: grade 1 shows increased T2 signal on MRI with no fiber disruption, grade 2 shows fiber disruption under 50 percent, and grade 3 shows fiber disruption over 50 percent.

By location of injury, hamstring strains fall mainly into two types:

• The muscle-tendon junction of the proximal long head of the biceps femoris. This occurs during the eccentric contraction of the late swing phase of high-speed running, as the swinging lower limb is decelerated.
• The tendon of the proximal semimembranosus. This occurs with excessive lengthening of the hamstring during high kicking, dancing, or a sliding tackle in football. A proximal semimembranosus tendon strain has a longer recovery period.

Epidemiology

Among the three muscles, the long head of the biceps femoris is the most commonly injured, accounting for about 80 percent, the semimembranosus about 10 percent, and the semitendinosus about 5 percent, with the muscle-tendon junction most often affected. The short head of the biceps femoris tendon has the lowest injury rate but is often underestimated and usually injured together with the long head.

Overall, the prevalence of hamstring strain may reach 20 to 30 percent of people in related sports, and the re-injury rate is also 12 to 31 percent. These are study figures and do not predict any individual’s outcome.

Risk factors

Related factors include strength imbalance, poor flexibility, inadequate warm-up, weak core strength, uneven strength (a difference of more than 10 to 15 percent in hamstring strength between the injured and opposite legs, or a hamstring-to-quadriceps ratio under 0.6), leg-length discrepancy, and muscle fatigue. The most important and best-established factor is a history of prior injury, which was associated with a two- to six-fold higher risk of injury in the studies reviewed.

Diagnosis

History

• Acute symptoms are usually a sudden pain in the hamstrings, bruising, and a feeling of a “pop.”
• More serious red-flag symptoms include pain when sitting, severe loss of function, and difficulty walking; extensive bruising appearing after a few days may indicate a complete tendon rupture.
• Some chronic symptoms present mainly as swelling, tightness, and a pulling sensation.

Physical examination

• Begin by observing gait pattern and function, then examine active and passive range of motion of the hip and knee, usually compared with the opposite side.

• Palpate the whole muscle, noting the location of pain and any muscle defect.

• In addition to resistance testing, common physical examinations include:

• Puranen-Orava Test: with the heel placed on a raised surface, the patient reaches for the toes; sensitivity 0.76, specificity 0.82.

• Bent-Knee stretch test: in supine, with maximal hip and knee flexion, the knee is slowly passively extended; sensitivity 0.84, specificity 0.87.

• For active range of motion of the lower-limb joints, note the maximal tolerance during a straight-leg raise. In one study, if the difference in straight-leg-raise angle between the two sides was under 20 degrees the prognosis tended to be better, with most recovering within two weeks; if it exceeded 30 degrees, return to sport might take more than six weeks.

Diagnostic tools

• X-ray: can rule out an avulsion fracture of the ischial tuberosity.
• Ultrasound: relatively inexpensive and easier for diagnosing acute injuries.
• MRI: may not show abnormalities in the first 2 to 3 days, and MRI findings currently cannot accurately predict the timing of return to sport.

Imaging can show a hematoma and an ischial tuberosity avulsion.

On ultrasound, a hematoma first appears as a hypoechoic lesion, then becomes a hyperechoic acute lesion, and gradually turns hypoechoic again. On MRI, a muscle strain appears differently depending on the time since injury; when the muscle is completely torn, the hematoma and muscle edema can sometimes be hard to distinguish. A hematoma is absorbed over roughly 6 to 8 weeks. An intermuscular hematoma tends not to collect like a mass and disperses more quickly, whereas an intramuscular hematoma clears more slowly and may need intervention. Because of the irritation from the hematoma, there may be a tendency toward cramping, inhibited normal contraction, and muscle atrophy, and the clot may also compress a nearby nerve.

An ischial tuberosity avulsion usually occurs proximally and is harder to diagnose with ultrasound, though not impossible; it requires a full physical examination and palpation and tracing the whole muscle to its attachment. Many athletes have well-developed gluteal muscles, and once the ultrasound passes through the thick gluteal muscle layer the diagnostic capability weakens, so MRI is another tool to consider, and attention should also be paid to whether any nearby nerve is compressed.

Treatment

Basic management and principles

First, control pain, swelling, bleeding, and adhesion of muscle fibers; basic principles include immobilization, rest, and compression, and it may be best to avoid overusing anti-inflammatory painkillers, since current research on hamstring strains suggests these medications offer little benefit and may interfere with the repair process. In addition, PRP may help with pain and functional recovery in hamstring injuries. Athletes with a hamstring tear need six weeks or more to return to their sport. A basic reference framework is as follows:

• Phase I: RICE (reduce pain and swelling, limit scar-tissue formation, and improve neuromuscular control), with moderate range-of-motion stretching while avoiding over-stretching.
• Phase II: increase range of motion, add eccentric resistance training, and progressive neuromuscular training.
• Phase III: enter this phase when full strength has returned and you can jog at 50 percent of maximum speed; add sport-specific training.

Advanced treatment

A PRP injection may be considered within 24 to 48 hours after an acute injury, and ultrasound-guided injection is recommended. Research findings on PRP are heterogeneous with mixed opinions, but some studies support a reduction in pain and a small benefit for early return to sport. Alongside this, strength is built gradually through core training and lower-limb stability work. The traditional training idea of progressing from isometric to concentric to eccentric contraction does not necessarily apply to the hamstrings; based on patient tolerance, adding eccentric contraction earlier may be more helpful.

Surgery

Surgery is usually not needed unless there is a complete proximal rupture. If all three muscles have an avulsion fracture, surgery within two weeks of injury is recommended, because beyond that time scar tissue increases around the sciatic nerve and makes surgery more difficult.

Without an obvious avulsion fracture, surgery is not the main treatment, and conservative treatment is more appropriate. In addition, surgery may be considered if displacement exceeds 2 centimeters or if two tendons have retracted by 2 centimeters or more.

Postoperative rehabilitation

In the first two weeks after surgery, treatment focuses mainly on pain and swelling control, avoiding massage. After four weeks, core-stability exercises can begin, using neural-facilitation techniques while avoiding excessive tension; use crutches when needed and maintain only partial weight-bearing.

In weeks 3 to 6, gradually increase weight-bearing with the goal of returning to a normal gait, aiming for knee flexion greater than 90 degrees and full active hip and knee motion. In weeks 7 to 12, begin strengthening hamstring strength and stability without resistance.

After three months, resistance weight training can be done; jogging should be possible after four months, and after six months activity can resume and reach 80 percent of the opposite side’s strength, with activities including short-distance sprints.

According to a systematic review, for patients who meet the indications and need surgery, early surgery (under 4 weeks) leads to better recovery of strength and endurance than delayed surgery (beyond 4 weeks). About 80 percent of patients can return to sport 5 to 6 months after surgery. Surgery still carries a few risks, including sciatic nerve injury, muscle re-rupture, muscle atrophy, and persistent residual pain, along with a few cases of weakness, cramping, and difficulty walking.

Complications

• Sciatic nerve injury: about 8 percent of surgical cases.
• Hamstring re-strain: the most common, at about 12 to 31 percent.
• Poor healing of an ischial tuberosity avulsion fracture: more likely if the avulsion is displaced by more than 2 centimeters.
• Peroneal nerve injury: most recover.

Follow-up and prognosis

About 84 percent of patients recovered their pre-injury strength and 89 percent recovered their pre-injury endurance.

Mild symptoms may gradually ease to pain-free within 10 days of injury, but full recovery usually takes weeks to months. In-season athletes generally return to sport within six weeks as symptoms and pain improve, and should be careful about the possibility of re-injury. The re-injury risk remains high in the first two months, and the first-week examination after returning is valuable for judging whether re-injury has occurred, including checking the feel of the posterior thigh, local tenderness, degree of knee swelling, and hip and knee range of motion.

Risk factors for re-injury include age and injury history; in addition, a history of ACL injury also increases the risk of a hamstring strain.

Certain factors, including a deeper muscle strain, a strain area over 50 percent, or a bleeding volume greater than 21 cc, generally indicate a poorer prognosis.

Following up with MRI to assess the timing of return to sport is not recommended; a set of tests, including tenderness, stretch testing, the H test, the single-leg bridge test, and sport-specific tests, can more effectively assess return-to-sport readiness.

Both the eccentric-training and running portions have three-stage programs to reference, starting at 25 percent of maximum running speed and gradually increasing to 50 percent for the first stage, then to 80 percent for the second stage, and to 100 percent for the third stage.

References

• Orthobullets: Hamstring Injuries
• Clinical Sports Medicine, 5th edition
• The 5-Minute Sports Medicine Consult
• Rehabilitation and return to sport after hamstring strain injury. J Sport Health Sci. 2017 Sep;6(3):262-270.
• Hamstring Strain Injury Rehabilitation. J Athl Train. 2022 Feb 1;57(2):125-135. doi: 10.4085/1062-6050-0707.20.
• Platelet-rich plasma injections for the treatment of hamstring injuries: a randomized controlled trial. Am J Sports Med. 2014 Oct;42(10):2410-8.
• Platelet-Rich Plasma Injection for the Treatment of Hamstring Injuries: A Systematic Review and Meta-analysis With Best-Worst Case Analysis. Am J Sports Med. 2021 Feb;49(2):529-537.