Hamstring injuries (HSI) are commonly encountered among athletes who participate in sports involving high speed sprinting such as track and field, American football and soccer [1,2,3]. It occurs frequently in both elite and recreational athletes and account for 75% of all lower extremity strains in running sports . Moreover, nearly one third of these injuries recur within the first year following a return to sport, with subsequent injuries often being more severe than the original .
The purpose of this review is to discuss the epidemiology, risk factors, diagnosis as well as management of hamstring injuries. We will also discuss the rehabilitation protocols and indication for surgical referral. These injuries are notoriously challenging and associated with significant missed playing time.
The strongest risk factor for hamstring injury is a history of previous hamstring injury. Literature reviews have identified risk factors that contribute to recurrent hamstring injuries, including a history of prior hamstring injury, persistent muscle weakness, muscle strength imbalances between the quadriceps and hamstrings, poor flexibility, hamstring muscle fatigue, and inadequate warm up [8,9,10].
Hamstring muscle group functions primarily in knee flexion and hip extension. Specifically the semimembranosus and semitendinosus allows extension of the thigh at the hip, and internal rotation of the knee when its flexed. The biceps femoris long head flexes the knee, extends the hip, laterally rotates the lower leg when the knee is slightly flexed, and assists in the lateral rotation of the thigh when the hip is extended. The short head flexes the knee, laterally rotates the lower leg when the knee is slightly flexed . In summary, the short head is not involved in hip extension but assists with knee flexion .
The origin of all three muscles is the tuberosity of the ischium, except the short head of the the biceps femoris which has its origin at the linea aspera of the femur and lateral supracondylar ridge of the femur. The distal attachment of the semitendinosus is the medial surface of the tibia, the semimembranosus attaches to the medial tibial condyle, and the two heads of the biceps femoris attaches to the lateral side of the head of the fibula.
The semitendinosus and semimembranosus receive its blood supply via branches of the internal iliac, popliteal and profunda femoral arteries. While the biceps femoris is supplied by perforating branches of the profunda femoris, inferior gluteal and medial circumflex arteries . The Hamstrings are innervated by the tibia branch of the sciatic nerve except the biceps short head which is innervated by the common peroneal.
The two (2) most common mechanisms of injury include the inefficiency of hamstring eccentric contraction during the swing phase of running, and secondly, inadequate lengthening of the hamstrings during active hip flexion with knee extension [4, 5]. Although some studies suggest that contact activities are the cause of hamstring injuries, most studies show that more than 90% of injuries occur without contact, with the classic injury being sustained by a water skier who gets pulled up by the boat [21, 22].
Studies of running biomechanics have found the hamstrings are active for the entire gait cycle with peaks in activation during the terminal swing and early stance phases . During the terminal swing phase the hamstrings are required to contract forcefully whilst lengthening to decelerate the extending knee and flexing hip [47, 48]. It is also in terminal swing that the hamstrings reach their maximum length [47-52]. Of the three biarticular hamstring muscles, BF long head (BFL) undergoes the greatest stretch, reaching almost 110% of the length in upright standing during terminal swing, whilst semimembranosus and semitendinosus reach 107.5% and 108.2%, respectively .
In summary, the presence of a high force eccentric contraction during the stance and swing phases likely contributes to the high rates of HSIs during maximal speed running [47-52]. The terminal swing phase is considered the most hazardous as the hamstring muscle-tendon units are at their longest length of the gait cycle and are most heavily activated [47-52].
A detailed history of an athlete with a suspected hamstring injury is critical for timely diagnosis and management. Athletes who present with acute hamstring injuries will often describe pain in the posterior thigh, which results in a decrease or cessation of physical activity in their sport of choice. They may also describe hearing a “pop”, experiencing a sensation of “pulling”, perceive swelling, bruising or muscle spasms. Typically, they will describe pain with weight bearing exercises and difficulty with hip and knee flexion. Occasionally, patients who present with either acute or chronic tears may report a pins and needles sensation along the sciatic nerve distribution [23,26,27]. Sprinters often say they know exactly when a HSI happened, for example “i was sprinting around the track and I felt a pop and had to pull up”.
Palpation: Tenderness upon palpation of the posterior thigh may localize the point of injury. However, this may not always be practical, especially if the injury is not superficial. In grade II or III injuries, a palpable defect may appreciated.
Range of Motion: Range of motion of should be assessed. Patient can be placed in a prone position to test knee flexion. Patients are typically restricted in hip passive hip flexion with the knee extended. Muscle strength may be tested in this position as well and most often have pain with hip or knee extension True strength deficits are uncommon.
Special Tests: Special tests used to identify hamstring strains or tendinopathy include the Puranen-Orava test, the Bent-Knee Stretch test and the Modified Bent-Knee Stretch test. In all three tests, an increase in pain in the posterior thigh with extension of the knee is indicative of a positive test:
1. The Puranen-Orava test: Active stretch of the hamstring with the patient standing. The hip is flexed at 90 degrees. The knee is then fully extended and the heel is held on a support.
2. The Bent-Knee Stretch test: The patient is supine with the hip and knee flexed. The hip and knee are then passively extended.
3. The Modified Bent-Knee Stretch test: The patient is supine and the affected leg is fully extended. The examiner will flex the hip and knee, then rapidly extend the knee
All three test have been shown to have validity and reliability in identifying hamstring sprains and tendinopathy .
Patients may report pain with pulling their shoe off with the other foot. This maneuver has been described as the ‘taking off the shoe test (TOST)’ by Zeren et al. The conducted a non blinded observational study of the TOST with one hundred and forty professional male soccer players with a history and clinical findings of a pulled hamstring muscle had an ultrasound-proven grade I or II biceps femoris muscle injury. The study showed the TOST had a sensitivity and specificity of 100% for hamstring injury .
Plain radiographs of the pelvis and a lateral radiograph of the affected hip should be obtained to rule out any apophyseal avulsions, particularly to the ischial tuberosity in adolescent athletes and to evaluate for other pathology . The two best imaging modalities for diagnosing suspected hamstring injuries are musculoskeletal ultrasonography (US) and magnetic resonance imaging (MRI). Both imaging modalities provide information about the location and extent of the injuries , and can aid in management of HSI.
In the United States, the gold standard for diagnosing hamstring injuries is magnetic resonance imaging (MRI). It allows visualization of partial tears and degree of tendon involvement as well concomitant hematoma. Retraction of the two tendons more than 2 cm typically renders the intact tendon functionally impaired. This level of retraction is clinically comparable to a complete rupture and requires surgical intervention .
US boasts several unique advantages including low cost, ease of accessibility and portability. The information gained using US assist clinicians in understanding real-time status of underlying tissues . It also allows for follow up imaging post rehabilitation. Limitations of ultrasound include its efficacy depends on operator experience, patient body habitus, and can be time consuming for the busy physician.
A study by Connel et al compared sonography and MRI for assessing hamstring injuries in professional football players (Australian football) 3 days, 2 weeks, and 6 weeks after an injury. The study showed that at baseline, MRI identified abnormalities in 42 (70.0%) of 60 patients, whereas sonography found abnormalities in 45 (75%) of 60. At 2 weeks, 29 (59.2%) of 49 scans showed abnormalities on MRI and 25 (51.0%) of 49 showed abnormalities on sonograms. Of those players who were injured at baseline, 15 (35.7%) of 42 and 10 (22.2%) of 45 still showed abnormal results on scans at 6 weeks on MRI and sonography, respectively . The author’s conclusion was that Sonography is as useful as MRI in depicting acute hamstring injuries and because of lower costs may be the preferred imaging technique. However, MRI is more sensitive for follow-up imaging of healing injuries . Limitations include study used two radiologists and had a small sample size.
However there is a growing body of evidence that suggest MRI is more reliable than US, especially when used for injury follow-ups and observing injuries near the musculotendinous junction [14,15]. MRI has been shown to be more sensitive in the follow up management of acute hamstring injuries when compared to ultrasound. Specifically, MRI better demonstrates the full extent of the muscle strain in which longitudinal length has a strong correlation with time required for rehabilitation and recovery . Ultimately the astute physician will be able use both modalities to drive optimal management for the injured athlete.
For mild to moderate hamstring injuries (strain, partial tear), treatment is directed toward the “PRICE” protocol, i.e. protection, rest ice, compression and elevation. In addition, early physical therapy and non-steroidal anti-inflammatory drugs (NSAIDs) are valuable at this time [16, 17]. Early treatment minimizes scar tissue formation and aid in myofiber regeneration. The first 48 hours after injury should be focused on rest, ice and compression with elevation. Early pain free range of motion exercises and controlled monitored exercises(or muscle contraction) regimens promotes angiogenesis and increases the likelihood delivering muscle derived stem cells to the injured area .
Subsequent days to weeks will involve stretching, neural mobilization, strengthening exercises focused on the gluteal muscles as well core muscle strength. Neuromuscular control exercises such as squats, single leg balance drills may performed once the patient can perform free focused hamstring exercises such as Nordic Hamstring exercises, and using a yo-yo machine. Once the athlete experiences pain free walking and has completed hamstring strengthening exercises with no pain they can begin active rehab with a running program on a day on/day off basis as well as sports specific agility drills.
Rehabilitation of a HSI is broken down into three phases with goals and progression criteria for advancement and return to sport . Phase one focuses on minimizing pain and edema while protecting the healing area, especially directly following insult . Low intensity, pain‐free activity encompassing the entire leg and core region are initiated through the pain free range in order to reduce atrophy and increase neuromuscular control of lumbopelvic stabilizers . Range of motion and intensity of the interventions as well as eccentric strengthening are progressed according to the patient’s tolerance and transition into phase two of rehabilitation . Phase three integrates return to sport activity with more sport specific treatment interventions through the full, pain free ROM . At this point the patient should be ready to begin assimilating back into their sport without restriction.
Three randomized clinical control trials have evaluated the efficacy of a particular intervention for acute hamstring strains [28, 29, 30]. In patients with acute hamstring strain, Best and Sherry demonstrated a rehabilitation program consisting of progressive agility and trunk stabilization exercises is more effective than a program emphasizing isolated hamstring stretching and strengthening in promoting return to sports and preventing injury recurrence . Several studies have shown that performing eccentric exercises such as Nordic hamstring exercises promote recovery and serve to decrease injury recurrence. Petersen and colleagues  assigned a 10-week Nordic hamstring programme  to 461 of 942 Danish soccer players. These athletes were monitored for injury across a single season. Interestingly, players in the intervention group experienced 71% fewer first-time and 85% fewer recurrent hamstring injuries than players in the control group; however, it should be noted that athletes in this study  had no known history of strength training. Furthermore, van der Horst and colleagues  allocated 292 of 597 sub-elite Dutch soccer players to a similar 13-week Nordic hamstring strengthening programme and reported that players who completed the training experienced 69% fewer hamstring strains that those who did not (odds ratio 0.3, 95% CI 0.1–0.7).
Lumbopelvic stability is also an important consideration during the rehabilitation process . Sherry, et al completed a randomized controlled trial investigating a lumbopelvic and agility program compared to a conventional program consisting of ice, stretching and strengthening . The athletes’ who participated in the core stability program significantly reduced both the time to return to sport (22.2 vs 37.4 days) and re‐injury rates two weeks and 12 months following injury compared to athletes’ in the conventional program .
These studies support the use of, Trunk stabilization, Nordic Hamstring exercises to aid in recovery and decrease re-injury rate. However, Progression through phases of rehab must be tailored to the patient and not be time dependent. The athlete must be able to perform all exercises without pain prior to progression and ultimately return to sport. Following an optimal physical therapy regiment, return to activity timeframes range from; 1‐3 weeks (Grade I); 4‐8 weeks (Grade II); and 3‐6 months (Grade III.) .
Recurrence is dependent on the modifiable and non modifiable risk factors aforementioned. If the clinician has a high index of clinical suspicion for recurrence, identifying the modifiable risk factors such as eccentric hamstring strength, training volume, running biomechanics may assist with management however to date there is no accepted standardized regimen used to decrease recurrence rate and shorten the injury duration.
As previously mentioned typical management of HSI would involve, PRICE therapy, comparative US or MRI and then a graded rehabilitation program. However it is important to note that MRI findings have not been found to be a useful prognostic tool for determining reinjury risk  however it can be used to quantify the extent of the injury and determining progression of the injury.
Rehabilitation programs focused on eccentric loading in a lengthened position and lumbopelvic stability have been demonstrated to be effective in reducing recurrence after injury . Furthermore, dynamic and functional testing techniques may be useful in predicting return to injury time and identifying impairment when clinical exam appears normal .
The astute physician will ensure that other diagnoses are not masquerading as a recurrent HSI, specifically lumbar radiculopathy, femoral neck stress fracture, posterior thigh compartment syndrome, adductor muscle strain, piriformis muscle syndrome, and sacroiliac joint dysfunction .
Glucocorticoid Injections. Intramuscular corticosteroid injection have been used as a treatment to prevent prolonged disability in hamstring injuries. A study by Levine and Bergfeld demonstrated that the use of intramuscular steroid injections in the National Football league (NFL) afforded players a shorter time to return to play . However this study lacked a control group and thus it is not frequently used to aid in recovery. It is not recommended to use intramuscular corticosteroid because of the catabolic effects of glucocorticoids upon skeletal muscle and the absence of strong evidence supporting the benefits of this therapy. In addition, glucocorticoids may cause tendon degradation and increase the risk of tendon rupture .
Shock Wave Therapy. Cacchio et al provided a randomized controlled trial regarding the use of shockwave therapy in the treatment of proximal hamstring tendinopathy . Forty professional athletes with chronic proximal hamstring tendinopathy were observed for a mean of 10.7 months (range 1-12 months). Patients were randomly assigned to receive either shockwave therapy, or traditional conservative treatment consisting of nonsteroidal anti-inflammatory drugs, physiotherapy, and an exercise program for hamstring muscles. The visual analog scale (VAS) score for pain and Nirschl phase rating scale (NPRS) were used as primary outcome measures. Results demonstrated that the shockwave therapy group experienced equal or greater reduction in pain when compared to the conservative treatment group (P < .001).This study suggests that Shockwave therapy is a safe and effective treatment for patients with chronic proximal hamstring tendinopathy (PHT). However this was a small sample size and the authors admit they would like to a longer study time to determine effects of this modality on quality of life .
Platelet Rich Plasma (PRP). Currently, there is not enough research exhibiting the direct effects of platelet-rich plasma on hamstring injuries. Studies presently have conflicting data for the benefits of PRP in hamstring injury. Wetzel et al demonstrated some benefits of PRP injections, however, this prospective study consisted of a small sample size. On the other hand, levy and Lucas et al investigated the efficacy of an ultrasound-guided PRP injection in the treatment of patients with Proximal Hamstring tendinopathy, The study sample consisted of 22 females and 7 males with a mean age of 45.2 years (95% CI 40.8-49.5) . When comparing pre-injection VISA-H scores (mean: 43.90; 95% CI 37.77-50.03) with 8-week post-injection VISA-H scores (mean: 51.14; 95% CI 43.39-58.88) in the total sample of patients, no statistically significant difference was found (p=0.14). When performing separate analyses for patients with mild (n=9), moderate (n=16) or marked (n=4) PHT, no statistically significant difference was found in pre-and post-injection VISA-H scores for any of the groups (p=0.86, p=0.13, p=0.28 respectively). 69% of patients reported no change in their ability to undertake sport or other physical activity at 8-weeks follow-up . Ultimately the study concluded that patients with proximal hamstring tears receiving PRP injections, did not improve in clinical outcomes at an 8-week follow-up [19,20,21].
Hamstring strains are a common cause the for posterior thigh pain. Among athletes, these injuries are associated with a significant amount of missed playing time. The average number of days until return sport is 8 to 25 days and up to one third of hamstring injuries will recur; with the greatest risk being during the initial two weeks following return to sport .
There are multiple studies evaluating various rehabilitation protocols for hamstring injuries. . The objective of these protocols is to safely recover from an acute injury, to limit hamstring injury recurrence and allow for the athlete’s safe return to play. Rehabilitation programs identify the role of progressive agility and trunk stabilization exercises, versus isolated hamstring stretching and strengthening .
The clinician must be able to quickly diagnose this injury via history, physical exam, and confirm with use of MRI and/or Ultrasound. A progressive rehabilitation program is an integral part of the treatment. Use of ICE, NSAIDs, maybe helpful. Injection of corticosteroids into the area of concern provides good relief however is controversial. Newer interventions such as shockwave therapy and PRP may be promising but further research needs to be conducted. Pain free walking indicates patient is ready to be progressed in rehab. Ultimately successful completion of rehab exercises and normal training activities with no limitation are indications that the athlete can return to play.
Stephen Henry DO MS CAQSM
Naima Stennett MD MS PGY 2 Resident Physician
Thomas M Best, MD, PhD, FACSM
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