- are among the most common lower extremity injuries in athletes, accounting for up to 29% of all injuries in various sports;
- are most common in sports requiring rapid acceleration, such as running, hurdling, jumping, and kicking sports, with hamstring strains accounting for 50% of muscle injuries in sprinters and the most common injury among hurdlers;
- in the National Football League are sustained most often during non-contact activities, with sprinting as the primary activity. "Speed positions" (receivers, defensive backs, and running backs) have significantly higher rates of hamstring muscle strains compared to "strength positions" (offensive and defensive linemen);
- also occur in sports with ballistic movements such as skiing, dance, skating, and weight-lifting;
- range from mild muscle damage to complete muscle tearing; and
- may result in prolonged impairment and a high risk of re-injury.
- inadequate warm-up;
- strength imbalance (a difference in hamstring strength between legs greater than 10 to 15%, or a ratio of hamstring-to-quadriceps strength in the same leg less than 0.6);
- lower extremety flexibility (although whether this is a risk factor remains controversial, with studies reporting inconsistent results);
- core stability;
- muscle weakness;
- fatigue (more hamstring injuries occur toward the end of an athletic competition);
- time of season (pre-season in the NFL has been identified as the most vulnerable period);
- dehydration; and
- previous history of hamstring injury (the most significant risk factor, increasing risk of reinjury by 2 to 6 times; previous injury may lead to formation of weakened scar tissue at the injury site, thereby lowering the threshold to a recurrent injury).
Grades of injury
A detailed history, a thorough physical examination, and a complete review of imaging (MRI) studies will confirm the presence of a hamstring injury.
There are three ways to classify hamstring injuries. Traditionally, hamstring injuries have been classified by grade:
- Grade 1 (mild): overstretching but minimal loss of structural integrity of the muscle-tendon unit;
- Grade 2 (moderate): partial or incomplete tearing; and
- Grade 3 (severe): complete rupture (rare, tend to occur with pre-existing tendinopathy).
Hamstring injuries are also classified using an MRI grading system. Alternatively, hamstring injuries can be classified based on the anatomic site, pattern, and severity of injury in the acute state, as assessed by MRI or ultrasound.
Important to the management of hamstring injuries is the early determination of time required to return to full competition. Neither the traditional clinical nor general MRI classification have been precisely correlated with time to return to play after a hamstring injury, although a new MRI scoring system tested on 38 NFL players with acute hamstring strains which took into account factors including age, number of muscles involved, location of injury appears to be highly predictive of time missed from competition but requires further study.
- activity modification
- early physical therapy
- non-steroidal anti-inflammatory drugs (NSAIDs)
- sport-specific activity progression carried out with attention to athlete's symptoms
- electrical stimulation
- shockwave therapy (shown in recent study to be safe, effective, and superior to traditional treatments in professional athletes)
- intramuscular corticosteroid injection (advocated as early treatment to prevent prolonged disability, and shown in NFL players with severe injuries to decrease time to return to full play without a risk of further injuries or complications)
"Traditional management has been disappointing, with unpredictable time lost, high reinjury rates, and poor restoration of preinjury status," say the authors of a 2013 review in the American Journal of Sports Medicine.
New, cutting-edge biological therapies are therefore being investigated, including:
- Platelet-Rich Plasma: A concentrated source of platelets and growth factors they naturally contain, PRP "has the potential to improve the healing process, and thus, its use has been proposed for the management of many musculoskeletal injuries, including hamstring strains. The results of studies conducted so far have shown promising results, with a more rapid return to play in NFL players with acute hamstring injuries and, most encouragingly, a 0% recurrence rate, but there are no so-called 'Level 1' studies (high quality, randomized trial or prospective study) to prove PRP's superiority. Because of concerns that the injections may lead to the growth of scar tissue, the authors of the 2013 literature review (1) state that, "at present, there is no evidence to recommend or discourage the adoption of PRP in clinical practice."
- Stem cell therapy: Stem cells directly participate in tissue regeneration and thus influence muscle healing. Animal models of muscle injury have shown that muscle-derived stem cells improve both muscular structure and muscle regeneration. Two potential techniques for skeletal muscle engineering (in vitro and in vivo) are being explored, but the clinical use of stem cells at this time is limited, and "further research is necessary to identify the mechanisms involved in muscle regeneration to exactly understand the therapeutic potential of stem cells."
Essential to the treatment of hamstring injuries is the early recognition of the injury and timely referral to the proper specialists. When surgical repair is necessary, experts recommend that it occur within 4 weeks of injury, as later surgery makes repair more difficult, leading to increased surgical complications and possibly inferior outcomes.
Postoperative rehabilitation is an essential part of surgery.
- Immediately after surgery: The leg is placed in a custom-fitted orthotic for the hip that restricts hip flexion.
- First 4 weeks: Patients are prescribed aspirin to minimize the risk of deep vein thrombosis.
- First 10 to 14 days: Toe-touch weight bearing with crutches
- Weeks 2 to 5: continued use of hip orthotic and 25% weight-bearing
- Week 2: Passive exercises with physical therapist to increase range of motion of hip begins
- Week 4: begin active range of motion exercises
- Week 6:
- discontinue brace to allow full weight bearing;
- begin gait training along with isotonic exercises;
- introduce aqua therapy along with isotonic exercises, core strengthening and closed chain exercises; range of motion is increased with caution for extreme ranges.
- Week 8: dynamic training and isometric strengthening begins
- Week 10: Isometric strength evaluation performed with knee at 60 degree of flexion.
- Week 16: Full isokinetic evaluation with knee at 60/120/180 degrees, with results compared to non-operative leg on isokinetic testing.
- Between 6 and 10 months: return to sport
Recurrence and prevention
Prevention of hamstring injuries is essential by:
- emphasizing eccentric exercise over concentric hamstring movements
- improving hamstring strength and endurance during sprinting in order to increase the ability of the hamstrings to absorb repeated eccentric loads before and during the heel strike.
- identifying and correcting muscle imbalance and optimizing neural timing alogn with progressive sprint training for high-risk athletes
- incorporating prevention programs as part of a preseason training program, although the effectiveness of hamstring injury prevention programs remains controversial and deserves more attention.
Source: Ahmad C, et al. Evaluation and Management of Hamstring Injuries. Am J Sports Med 2013;20(10). doi:10.1177/0363546513487063 (published online ahead of print May 23, 2013).