Evidence-Based Treatment of Hamstring Tears

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1 COMPETITIVE SPORTS AND PAIN MANAGEMENT Evidence-Based Treatment of Hamstring Tears Spencer T. Copland, John S. Tipton, and Karl B. Fields Moses H. Cone Sports Medicine Fellowship and Family Medicine Residency, Moses Cone Health System, Greensboro, NC COPLAND, S.T., J.S. TIPTON, and K.B. FIELDS. Evidence-based treatment of hamstring tears. Curr. Sports Med. Rep., Vol. 8, No. 6, pp. 308Y314, Hamstring tears are exceedingly common in a variety of athletic populations and contribute to a significant amount of morbidity and time lost from sport. Many modifiable and nonmodifiable risk factors have been identified with hamstring injury. There is strong evidence that Nordic hamstring exercises can decrease the risk of hamstring injury, limited evidence that sports specific anaerobic interval training and isokinetic strengthening can reduce injury rates, and limited evidence that daily static stretching after injury can increase recovery rate. The majority of medical, surgical, and rehabilitative intervention studies have limitations based on the total number of hamstring injuries included in a given study, reliance on retrospective cohort studies, and conclusions based on case series that limit the utility of the information. Most do not provide a level of evidence greater than expert opinion. INTRODUCTION Hamstring muscle injuries are common in both recreational and elite athletes. Depending on the severity of the injury, the athlete may require considerable time off from sport (9,11,14,29,31,33,35,41). Hamstring tears traditionally have been classified as mild (grade I), moderate (grade II), and severe (grade III). Grade I injuries signify a small tear of the muscle or tendon, minor swelling, and pain with no or minimal strength loss. Grade II strains are more complete partial tears, with definite loss of strength and pain. Grade III tears are complete ruptures of the musculotendinous unit with a complete loss of muscle function, and they typically develop a large hematoma. EPIDEMIOLOGY Rates of hamstring muscle injury vary by differing injury definitions and sporting populations. Prevalence rates range from 8% to 25%, with return to sport occurring from 2 wk to never, with large variability based on severity. One study Address for correspondence: Karl B. Fields, M.D., Director, Moses Cone Sports Medicine Fellowship, Moses Cone Health System, Family Medicine Residency and Sports Medicine Fellowship, Professor and Associate Chairman, Department of Family Medicine, UNC-Chapel Hill, 1125 N. Church Street, Greensboro, NC ( bert.fields@mosescone.com) X/0806/308Y314 Current Sports Medicine Reports Copyright * 2009 by the American College of Sports Medicine reports a single-season prevalence rate greater than 50% in elite soccer players (3). Recurrent hamstring injury rates generally are higher than initial injuries. Most studies found rates of reinjury for the ensuing sporting season higher than 30% and some up to 60%Y70% (29,31,35,41). RISK FACTORS Several risk factors have been attributed to hamstring injury and can be divided into modifiable and nonmodifiable factors and their interplay. Modifiable risk factors include fatigue, low hamstring strength, lack of warm-up, greater training volume, poor muscle flexibility, cross-pelvic posture, poor lumbo-pelvic strength, and biomechanical problems. Nonmodifiable factors include age, previous hamstring and lower extremity muscle injuries, and being of black or Aboriginal ethnic origin (9,11,30,31). The key identifiable risk factor for hamstring strain is a previous injury. Most studies involved Australian Rules football and noted that athletes with a previous history of hamstring strain were two to six times more likely to suffer subsequent strains during their lifetime (2,8,18,31,45). Most subsequent strains occurred within the first 2 months after return to sport, although the risk continued over time (2,8,15). In some reports, athletes were three times more likely to suffer another hamstring injury even after a year (18). Researchers theorize that regeneration and remodeling of an injured muscle may continue for up to 9 months after injury. A debate still exists regarding whether recurrence of hamstring injury arises from inadequate rehabilitation and premature return to sport, or whether an intrinsic risk is 308

2 created by the initial injury (2,33,43,45,46). Recurrence remains high even with thorough rehabilitation and functional improvement. Some researchers believe that skeletal muscles, including hamstrings, are at risk for reinjury because of scar tissue formation and muscular architectural reorganization (11,45), although others dispute this contention (46). Another issue involves size (measured by imaging) and severity (defined by number of days missed from competition) of the initial hamstring injury and subsequent association with recurrent injury within the same season. While some found no relationship (23), one study noted a high risk of recurrence within two seasons in athletes with severe strains (Q18 d missed) (48). Other injuries may predispose the athlete to hamstring strain, which reinforces the notion of hamstrings belonging to a larger kinetic chain. History of previous calf or quadriceps muscle injury, knee injury, or osteitis pubis increased the risk of subsequent hamstring injury (31,45). A potential explanation is that the biomechanics of running after any lower extremity injury is altered and predisposes athletes to hamstring injury (31). A recent meta-analysis suggests that hamstring flexibility has no significant association with injury (35). Confounders affecting this type of research include questions about how best to measure hamstring flexibility, since motion occurs at the hamstring and at the lumbo-pelvic junction. In addition, because athletes already were relatively flexible because of a regular stretching routine in training, hamstring inflexibility was less likely to be an issue in those with hamstring injuries (7). The flexibility of other muscles groups in the thigh, particularly the quadriceps, may be of more significance than that of the hamstring group. In one study, an inverse relationship was found between increased quadriceps flexibility and incidence of hamstring strain. The athletes who were able to achieve greater than 51- knee flexion in a modified Thomas test were less inclined to suffer a hamstring strain. In the same study, tight hip flexors also posed a significant risk for hamstring injury. However, older-aged athletes in this subgroup were a potential confounder (14). A possible biomechanical reason explaining why tight hip flexors may predispose athletes to hamstring injury is that tight muscles create higher potential energy during hip extension and knee flexion in the pre-swing phase of gait. This generates increased forward propulsion of the leg during swing due to passive recoil of these muscles, which then increases the eccentric load on the hamstrings to decelerate the limb (17). Isolated decreased hamstring strength appeared to be a risk factor in a prospective study on track and field athletes. In these runners, the injured extremity had significantly less hamstring strength compared with the uninjured side (49). Similar findings were demonstrated in the largest published hamstring risk factor study with 672 total hamstring injuries in Australian Rules footballers (32). Not all studies support this finding. Another study of Australian Rules football noted that a 10% between-leg strength discrepancy was not a significant predictor of future hamstring strain in their sample of 12 injuries (8). Strength imbalances between quadriceps and hamstrings may play a larger role than isolated hamstring strength. A significantly reduced ratio of hamstring strength to quadriceps strength (H:Q) in the injured side was found in several studies (11,32,49). The hamstrings eccentrically slow the lower limb during the swing phase of running before extending the hip to achieve forward motion. This braking function also is critical in kicking motion (17,33,48). The ability to exert lower limb swing force possibly is greater in individuals with increased quadriceps strength relative to hamstring strength. This potentially places a greater requirement on the hamstring to decelerate the lower limb. Some researchers speculate that professional players, particularly in kicking sports, may be developing too much quadriceps strengthening, which may predispose them to hamstring injury (35). One study with low injury rates did not support the role of strength imbalance as a risk factor for hamstring injuries (8). Increasing age appears to be the most prominent intrinsic risk factor for hamstring injury, with several studies of Australian footballers reporting a significant relationship (14,15,17,18,31,45). Specifically, athletes older than 23 yr were 1.3 to 3.9 times (17,31) and athletes older than 25 yr were 2.8 to 4.4 times (14,15) more likely to suffer a hamstring injury than younger players. Data suggest that risk of injury increases by 30% annually (45). Different theories have been proposed linking hamstring injury and age. One theory suggests that age promotes a reduction of crosssectional area of the hamstrings such that the muscles can no longer produce sufficient tension to resist load before failure (35). Confounders in this observation include that the pivotal studies were performed on athletes who were relatively young. A second novel theory is that hamstring strain may be caused by age-related lumbar degeneration leading to L5 and S1 nerve impingement and subsequent hamstring muscle fiber degeneration (31). Studies also point to race and ethnicity as intrinsic factors, with athletes of black descent being significantly more likely to suffer hamstring strain (45,48). In Australian Rules football, professional Aboriginal footballers were 11.2 times more likely to suffer hamstring strains than non-aboriginal footballers (45). In another study involving the English professional football leagues, hamstring injury was not specific to any one nationality or ethnic group. Rather, injury was related to all players of black racial background (48). Body mass index (BMI) inconsistently was associated with risk of either initial or recurrent hamstring strains, with two positive and two negative studies (2,15,31,45). Weight also was linked variably with risk of hamstring injury where some prospective cohort studies did not find a significant association with incidence of hamstring strain (2,15,32,45), whereas others studies did (14,31). SPORT-SPECIFIC RISKS A higher level of competition was a risk factor of hamstring injury. One study involving soccer in the English Premier League (EPL) showed a significantly higher prevalence of hamstring strain in the Premier division versus Division 2 (48). Similarly, a study performed on Australian Rules football demonstrated a significantly higher prevalence (920%) in the Australian Football League (AFL) versus the lower division South Australian National Football League Volume 8 c Number 6 c November/December 2009 Treatment of Hamstring Tears 309

3 (SANFL) (45). This may reflect the increased physical burden in the higher leagues where the tempo of the games may be faster and training more demanding. Playing positions that required more running also correlated with hamstring injury rates. Outfield players exhibited a higher incidence (22% to 37%) of hamstring strain compared with goalkeepers in English soccer and Australian Rules football (32,48). In soccer, Australian Rules football, and in the rugby union, the players who ran more, kicked more, and covered more of the field had the highest risk of hamstring injury (9,48). Another injury mechanism for hamstring strains is slowspeed stretching exercises carried out to an extreme joint position, as in ballet dancers. Hamstring strains in different sports, with similar injury conditions to dancers, show a resemblance in symptoms, injury location, and recovery time to dancers. These particular hamstring injuries occurred during movements reaching a position with combined extensive hip flexion and knee extension. The incidence and prevalence rates during these types of injuries were not calculated. While not reaching statistical significance, there was a trend suggesting that the time to return to preinjury status was longer in athletes with strains of the stretching type than in athletes with strains during high-speed running (mean 31 wk vs 16 wk) (4,5). PHYSICAL EXAMINATION Hamstring injury evaluation follows a pattern similar to evaluation of other musculoskeletal injury: inspection, palpation, range of motion, strength testing, and special maneuvers. Any swelling, ecchymosis, atrophy, and scars should be noted during the inspection. Particular attention also should be placed on gait analysis, including motion at the hips, knees, and feet. The length of the hamstring should be palpated for tenderness or any muscle defects from the popliteal fossa to the ischial tuberosity, where the semitendinosus, semimembranosis, and the long head of the biceps femoris originate. The semitendinosus can be followed to its insertion at the pes anserinus, where it is felt as the most posterior and inferior tendon. The biceps femoris tendon is found easily on the lateral knee where it should be palpated to its insertion on the fibular head. The semimembranosis inserts deep to the pes anserinus on the posterior tibia and can be felt through the semitendinosus and gracilis tendons (25,38,39). Knee flexion and extension range of motion can be estimated or easily quantified with a goniometer. To evaluate hamstring strength, have the patient in the supine position with his or her knee flexed to 90-. The patient should flex concentrically his or her knee towards the buttock region against resistance, while stabilizing the thigh above the knee. To place more emphasis on the biceps femoris, the knee should be rotated externally, while internal rotation will activate the semitendinosus and semimembranosis. Additionally, eccentric hamstring strength should be assessed with the patient supine and with knee flexion from 15- to 30-. Overall strength of hamstrings also can be tested concentrically at 90- knee flexion and eccentrically at 15- of knee flexion with the patient prone, which allows the examiner to observe for defects or fasciculations while testing. Evaluation of range of motion, strength, and for muscle defects allows the clinician to grade the severity of injury (grades I, II, or III), which directly relates to rehabilitation and return to play. The Lasègue test (straight leg raise) should be performed if radiculopathy is present. Dynamometer strength testing frequently was used in published trials, but it is less relevant for day to day use by clinicians (28,38,39,50). INJURY LOCATION AND IMAGING In the majority of studies, the biceps femoris was the most commonly injured hamstring. Injury to the semimembranosis was less common, followed by the semitendinosus (4,13,23,45). However, these numbers differ in other studies and are complicated by the fact that more than one muscle often is injured (5,13). Proximal injuries including tendon avulsion (based on their relationship to the origin of the short head of the biceps femoris) were more common than distal injuries, regardless of which muscle was involved. Most occurred in the musculotendinous junction, which is really a 10- to 12-cm transition zone in which myofibrils contribute to form the tendon. Bony avulsion of the ischium was rare in adults and usually occurred in the skeletally immature (4,5,13,23,47). In the setting of hamstring injuries, ultrasonography and magnetic resonance imaging (MRI) are the modalities of choice. Both provide detailed information about the injury with respect to localization and characterization. Ultrasound is attractive because it is less expensive, portable, and can be implemented in the office setting. Moreover, dynamic assessment of the hamstring tendon with ultrasonography provides additional information about its integrity in varying degrees of resisted contraction. Ultrasound is highly user-dependent but has excellent sensitivity in the acute phase of injury when inflammatory fluid is in the soft tissue. As the fluid resolves (usually within 2 wk), ultrasonography becomes less accurate in showing myofibrillar abnormalities, while MRI remains sensitive. MRI is more reliable in depicting hamstring tendon and osseous avulsion injuries and injuries that are in the deeper musculotendinous junction. MRI also allows accurate assessment of the degree of tendon retraction and of tendon edge morphology (23). This gives the surgeon important information since tendon avulsion may require surgical repair (24Y26,38,39,47). When experienced musculoskeletal ultrasonographers are available, this should be the initial form of imaging because the cost is much lower and the sensitivity is good. However, in much of the United States, MRI remains the imaging modality of choice. TREATMENT EVIDENCE There is no consensus regarding treatment for hamstring tears. Many interventions commonly are done, but limited randomized controlled trials and quality prospective studies guide the medical, surgical, and rehabilitation treatments prescribed by clinicians. Moreover, many of the published risk factor studies are retrospective or assess only a small 310 Current Sports Medicine Reports

4 number of injured athletes. Even when randomized controlled trials have been conducted, most have low total numbers of injured athletes, which potentially explains the variability among study results. A comprehensive analytical review by Bahr from the Oslo Sports Trauma Research Center provides invaluable insight into the methodological deficits of the majority of published hamstring studies (6). This methodological and statistical approach to evaluating sports injury evidence and its specific relationship to hamstring injury seems sound and points out the limitations of our ability to analyze hamstring injury risks and treatment interventions. Published cohort risk factor studies all have low numbers of total injuries, with one exception (32). They are necessarily powered to detect only a strong relationship, and a negative result potentially could be untrue from Type 2 error V failing to observe a difference when in truth there is one. Reproducibility and precision of measurement at baseline of a modifiable risk factor affect its ability to determine associations. If a specific measurement is less precise, then greater numbers of test subjects and injuries are needed to detect an association. Bahr used statistical modeling to determine the number of injuries needed in studies to have adequate power to determine the association of given risk factors. Risk factors with a small to moderate association to hamstring injury would only be detected in studies with 200 or more injured patients. Even risk factors with a strong association to hamstring injury would require a study with 20 to 50 injured athletes (6). The following section discusses evidence-based treatment and also includes the largest human retrospective studies and case series when no other evidence is available. REHABILITATION Many rehabilitation programs exist for hamstring tears, but only a few are based on randomized controlled trials. A 2007 Cochrane Review included three randomized controlled or comparative trials dealing with hamstring rehabilitation (29). A randomized controlled trial by Malliaropoulos et al. found that static stretching started 48 h from injury in grade II hamstring tears four times daily compared with once daily decreased athlete_s time to normal range of motion (5.6 vs 7.3 d; P G 0.001) and unrestricted activity (13.3 vs 15 d; P G 0.001) (28). Sherry and Best performed a prospective randomized comparison study of 24 athletes with acute hamstring strain into two groups: static stretching, isolated progressive hamstring resistance exercise, and icing (STST group) or progressive agility and trunk stabilization exercises and icing (PATS group). Reinjury was less likely in the PATS group at 2 wk after return to sport (RTS) (0% vs 54.5%; P = 0.003) and at 1 yr (8% vs 70%; P = 0.006), but there was no statistical difference in injury time to RTS (41). A randomized controlled trial comparing manipulation of the sacroiliac joint to control in athletes with hamstring strains and sacroiliac dysfunction showed increased hamstring muscle peak torque, but did not evaluate clinical outcomes (10). Early mobilization including stretching and strengthening following a brief period of immobility of 2Y6 d depending on severity of injury has some evidence for decreasing scar formation and reruptures. Delaying the onset of rehabilitation and stretching programs for at least 48 h, including those mentioned previously, is based on this earlier work (21,22,28,29,41). Physical therapy treatment modalities such as cryotherapy, heat, compression, elevation, ultrasound, electrical stimulation, and massage frequently are used to treat muscle injuries. There is a lack of randomized controlled trials in their use for the treatment of hamstring tears (29,33,35). Cryotherapy appears safe and can be used for pain relief, and it is the only modality other than mobilization (manipulation) that has evidence to support its use. However, there is some evidence suggesting that early heat application to the injury may prolong rehabilitation (21). Multiple meta-analyses have found therapeutic ultrasound to be no better than placebo, and there is conflicting evidence regarding the use of electrical stimulation and laser therapy in muscle injury (21,29). A PubMed search and review did not find any adequately powered human trials to recommend for or against anti-fibrotic manual therapy, such as ASTYM or Graston techniques. MEDICAL THERAPY The physiological impact of COX-1 and COX-2 inhibition and impaired prostaglandin function by nonsteroidal antiinflammatories (NSAID) and subsequent decrease in inflammatory cells and pain perception is well-established. There is only one published study examining the role of NSAID in hamstring strain healing. In a double-blind, randomized controlled trial (N = 75), patients were given either one of two NSAID or placebo, and all completed the same physical therapy course including RICE (rest, ice, compression, elevation), ultrasound therapy, deep transverse friction massage, stretching, and strengthening. In this study, there was no difference in pain, swelling, strength, or endurance among the three groups. In their severe injury subgroups, the placebo group had significantly better pain scores compared with the NSAID groups at day 7 (37). The use of corticosteroid for muscle or tendon injury is controversial, and many physicians have a clinical concern that corticosteroid may increase the likelihood of complete rupture, which has been reported in hamstring and other muscle-tendon-bone unit injuries. Theoretically, corticosteroids could suppress pain and inhibit fibroblast proliferation and collagen synthesis V scar formation V by inhibiting the inflammatory cascade and cytokine production (40,42). One retrospective case series of 58 National Football League players with palpable grade II and grade III hamstring tears had 100% of included athletes return to play following intramuscular corticosteroid injection with no known ruptures or other complications (27). Injection of an antifibrotic agent has been beneficial in animal laboratory studies, but no human data exist. SURGERY Due to the relative rarity of hamstring rupture, published evidence on hamstring surgery outcomes are all based on case Volume 8 c Number 6 c November/December 2009 Treatment of Hamstring Tears 311

5 series and expert opinion, and to our knowledge, there are no randomized controlled trials or prospective surgical outcome studies. The usual mechanism for proximal hamstring avulsion is forceful eccentric contraction with knee extension and hip flexion, and it is seen most commonly seen in waterskiing. Proximal avulsions can be divided into complete, partial, and bony apophyseal injury, typically seen in the skeletally immature, where classically, surgery is reserved for retraction greater than 2 cm (39,47). Many partial avulsions do well with nonoperative treatment, but in limited case series, complete ruptures treated nonoperatively did poorly with the great majority ultimately seeking surgery. When conservative treatment fails in partial proximal avulsions, surgical treatment has allowed 87%Y100% of patients return to sport (24,47). Surgical outcomes from repair of complete proximal hamstring avulsions have yielded good to excellent results in 71%Y100% of cases in the largest case series, and postoperative strength returned to 84%Y98% of the contralateral side. Delayed surgical repair in complete proximal ruptures has yielded worse postoperative strength and potential sciatic nerve involvement with surrounding scar formation (24,38,39,47). Lempainen et al. report the largest case series (N = 18) on surgical treatment of distal partial and complete hamstring tears, including all cases at Mehiläinen Sports Trauma Research Center from 1992 to In their series, 14 of 18 patients returned to sport at preinjury levels, and 13 of 18 reported excellent results. However, all (4/4) partial semimembranosus tears treated with surgery did poorly (25). There has been recent attention in the medical literature to persistent radicular pain at the ischial tuberosity and the gluteal and proximal hamstring region. It is associated with prior hamstring injury and variably has been termed the hamstring syndrome (36), proximal hamstring syndrome (50), and proximal hamstring tendinopathy (26). Different authors have postulated that this entity is caused by taut tendinous band or scarring around the sciatic nerve. But a recent article by Lempainen et al. is the first to evaluate histopathological findings. All surgical samples from these tendons showed signs of tendinosis. In their surgical series, they achieved 89% good to excellent results and return to sport with semimembranosis tenodesis and reattachment to the biceps tendon without neurolysis (26). Other authors achieved similar results (77%Y88% RTS) surgically with either tendon debridement (50) or partial tenodesis (36). But in contrast to Lempainen, all other authors contended that complete dissection of the sciatic nerve from adherent tissue was a critical component. PREVENTION While there is limited evidence-based research on the prevention of hamstring strains, several studies have recently been published. Included in the literature are eleven prospective studies that specifically examine an intervention that influences injury incidence in a human population. No studies have evaluated preventative measures that decrease incidence of complete hamstring avulsion. Four studies evaluated eccentric strength training as a tool to prevent hamstring injuries (1,3,9,16). Askling et al. evaluated 30 elite soccer players, where 15 players underwent a 10-wk preseason hamstring training program using a device designed for eccentric hamstring overload for 10 wk. The incidence of injury in the training group was significantly lower (3/15) compared with the control group (10/15) during the 10-month study (3). A randomized controlled trial of community-level Australian Football players (N = 220) compared Nordic hamstring eccentric training (Fig.) to basic stretching (control) and showed no statistically significant difference in injury rates. In this study, five training sessions were completed in a 12-wk period, and there was a 47% dropout rate from session 1 to session 2 (16). A study of British professional rugby players showed that the group using stretching, concentric strengthening, and Nordic hamstring exercises decreased hamstring injury incidence to 0.39 per 1000 player hours (95% CI 0.25Y0.54) compared with strengthening without Nordic hamstring exercises 1.1 (0.74Y1.1) and strengthening plus stretching 0.59 (0.34Y0.84) (9). Arnason et al. recently published a much larger scale prevention study that compared Nordic hamstring eccentric training (Fig.) or a hamstring flexibility program to teams that opted not to participate (52%). The study included players from the Norwegian and Icelandic top soccer leagues Figure. Nordic hamstring lowers. An eccentric strength training exercise where one person stabilizes an athlete_s lower extremities, and starting at 90- the athlete lowers himself or herself during the eccentric stage of muscle contraction. The forward movement should be resisted by flexing the hamstrings for as long as possible, and subsequently a push-up motion is used to elevate the body with minimal concentric hamstring contraction to the initial position for another repetition. 312 Current Sports Medicine Reports

6 TABLE. Strength of recommendation taxonomy (SORT). SORT: Key Recommendations for Practice A preventative program of Nordic hamstring exercises can decrease injury risk. Isokinetic strength training to correct for hamstring-quadriceps imbalances could reduce injury rates. Sports specific anaerobic interval training could reduce injury rates. Daily static stretching started 48 h after injury could increase recovery rate. Warm-up exercise and clothing may decrease injury rates. Cryotherapy appears safe and may have a role in pain control. Progressive agility and trunk stabilization exercises should be considered during rehabilitation and may make reinjury less likely. When conservative treatment fails in partial proximal hamstring avulsions, surgery has given most patients good to excellent results. For complete proximal hamstring avulsions, delayed surgery has resulted in worse postoperative strength and possible sciatic nerve involvement. Evidence Rating References A 1 B 12,20 B 44 B 28 C 43 C 21 C 41 C 24,47 C 24,38,39,47 A = consistent and good-quality patient-oriented evidence; B = inconsistent or limited quality patient-oriented evidence; C = based on consensus, usual practice, opinion, disease-oriented medicine, or case series for studies of diagnosis, treatment, prevention, or screening. for the 1999Y2002 seasons, representing over 330,000 exposure hours and 183 injuries. The eccentric strengthening group underwent warm-up stretching and a gradually increased eccentric hamstring protocol described by Mjølnes et al., then three times weekly during the preseason and two times weekly during the competitive season. This intervention yielded a significantly lower injury rate compared with those teams that did not do eccentric training (RR = 0.43, P = 0.01) and compared with baseline data from previous years (RR = 0.42, P = 0.009). Flexibility training alone did not reduce injury risk (1). One prospective study with military basic training compared three times daily hamstring stretching, and the injury rate for all lower extremity injuries decreased from 29% in the control to 17% in the intervention group (19). A larger (N = 1538) randomized controlled trial from the Australian military showed that a daily stretching program was no better than normal military training without stretching (34). Multiple studies have evaluated the role of hamstring flexibility in hamstring injury prevention, but the data are conflicting (1,19,33Y35,44). One study of Australian Football players evaluated the impact of sports-specific training, anaerobic interval training, and stretching fatigued muscle, and found a significant difference in hamstring injury rates compared with preintervention rates (44). Two studies implemented an isokinetic preseason evaluation assessment and isokinetic training regimen to correct for quadriceps-hamstring (H/Q) imbalances (12,20). Crosier et al. performed a multi-year, multinational intervention study of professional soccer players that found that athletes with H/Q ratio imbalances had a much higher injury risk (RR = 4.66, CI = 2.01Y10.8). Normalizing the H/Q ratio with an isokinetic strength training program made the risk no different from those athletes with a normal H/Q ratio preseason (Absolute Risk Reduction = 1.6%, Number Needed to Treat = 63) (12). Warm-up exercise and clothing designed to keep hamstring muscles warm frequently have been cited as potentially modifiable factors for injury prevention, but few human intervention studies exist. One study in club rugby players with prior hamstring injury examined 1.5-mm neoprene shorts as a temperature intervention, and found that players who intermittently wore them were less likely to injure their hamstrings when wearing neoprene (3 vs 57, out of 1000 playing hours). Only five players wore the shorts all season (43). CONCLUSION The majority of the medical, surgical, and rehabilitative interventions commonly used to treat hamstring injuries lack strong evidence. The most compelling evidence for hamstring tear treatment lies in recent prevention studies. Arnason_s prospective intervention study exhibited 183 injuries in 330,000 exposure hours; thus the results carry statistical power. There is significant evidence that an eccentric hamstring program of Nordic hamstring exercises can decrease injury risk (1). There is limited evidence that daily static stretching can increase recovery rate after injury (28). Two interventions, sports-specific anaerobic interval training and isokinetic hamstring strength training, have limited evidence that they could reduce injury rates (12,44). For other hamstring research, the limitations based on total hamstring injuries included in a given study, reliance on retrospective cohort studies, and conclusions based on case series limit the utility of any information that can be concluded from the papers and do not provide a level of evidence greater than expert opinion (Table). Hamstring injuries are a major problem in sports, and their occurrence inherently is influenced by multiple factors. If we are to establish an evidence-based approach to their prevention and treatment, future studies should be prospective cohort studies and randomized controlled trials that include adequate numbers of participants and injuries to reach statistical significance even after multivariate analysis is applied to enhance validity. References 1. Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand. J. Med. Sci. Sports. 2008; 18:40Y8. 2. 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