A common injury among running, kicking, sprinting, and cutting and turning sports, a hamstring strain often occurs at the long head of the biceps femoris during the end phase of the leg swing during a sprint. It usually happens toward the hip joint rather than toward the knee, but the painful symptoms may be felt at the back of the knee.
Hamstring strains can put athletes out of commission for many weeks or months, depending on the severity of the injury and recovery time. Field sports, like American football, soccer, and rugby, have higher prevalence of acute hamstring strains than court sports, like tennis and basketball. Likewise, these strains are more common during pre-season than in-season or post-season, and they are common during competition than during practice.
For example, a three-year-study of injuries from track and field athletes at the Penn Relays found that hamstring strains make up about 24% of all 489 treated injuries. Among lower-body strain injuries, hamstring strains accounted more than 75%. In this population, older athletes (ages 40-plus) and men are more likely to sustain a hamstring strain than younger athletes and women, respectively. Also, those who participated in longer relays (400 meters) have a higher risk than those in shorter relays (200 and 100 meters).
In American football, 10 years of data from one study published in the American Journal of Sports Medicine found that hamstring strains ranked number two after knee sprains among players in the National Football League camp.
Meanwhile, about 37% of soccer injuries are hamstring strains and other hamstring injuries. This is based on nearly 3,000 injuries reported between from 2001 to 2009 among 51 teams in the Union of European Football Associations.
Dancers (classical, modern) also have been shown in a Swedish study to sustain similar acute hamstring strains as sprinters and field athletes because of the flexibility strain they do during practice or a performance. Interestingly, the sprinters got a strain while they were sprinters while the dancer got a strain from slow stretching exercises.
While hamstring strains are biological in nature, it can influence the patients’ behavior, which can affect how long and how well they recover and resume daily activities. A thorough understanding of hamstring strain from a biopsychosocial perspective should help maximize recovery and save time (and money) on treatments.
Hamstring strain causes
Hamstring strains are caused primarily by various biomechanical and physiological factors. These include fatigue, eccentric contraction of hamstrings, and previous hamstring and posterior knee injuries, such as a PCL tear or hamstring tendinitis.
While many research suggests that eccentric contraction phase of the hamstrings — more specifically, during the final phase of the leg swing during a running gait — an international team of researchers suggested that this is not enough to explain how this mechanism can cause a hamstring strain.
Led by physiotherapist Shaun Huygaerts from UCAM of Murcia in Spain, the team highlighted that even though the number of research about hamstring strains has increased, “the potential injury mechanisms are not well defined, and injury incidence seems to have either remained about the same or even increased.”
They argued that these studies were done from computer simulations and prediction models, not live humans, which do not account “all aspects of muscle tissue that influence function” during a run or sprint, such as anatomical variability and tissue adaptation to mechanical stress.
There is still much speculation on whether the fascicles in the hamstring muscles lengthening during the end phase of the leg swing, as well as potential errors in measuring the stretch of the tendon tissues. These errors, Huygaerts et al. suggested, would include a lack of account for muscle fatigue.
They wrote that if one of the hamstring muscles (biceps femoris, semitendinosus, semimembranosus) is working harder than the others, “its metabolic demand would be higher and fatigue would develop sooner.” Fatigue would also change the coordination of muscle contraction, which may cause a chain reaction of altered running mechanics, such as reduced activity in the rectus femoris muscle, increased activity in the semitendinosus and biceps femoris, decreased hip extension, and increased knee extension — among many more changes.
While there is little evidence that such changes in running mechanics may significantly increase the risk of getting a hamstring strain, Huygaerts et al. suggested that a reduced running efficiency (such as adopting an anterior pelvic tilt during a fatigue) and greater force upon the joints may likely increase such risk.
Eccentric contraction is where a muscle lengthens under stress. If you were to do a prone hamstring curl exercise, an eccentric contraction is where you extend your knees as you lower the weight in a controlled manner. When you run, your hamstrings decelerate during the end phase of the leg swing. They also eccentrically contract where the hamstring tendons reach their highest amount of strain. It is this phase of movement that many researchers on hamstring strain investigate.
While most of the current literature emphasize on the final phase of a leg swing during a run, earlier research on hamstring strain hypothesized that the early phase of a leg swing has the greatest risk of getting a strain. Dr. John W. Orchard, who is a sports physician at the University of Sydney, is one of the proponents who supported the early leg swing phase. He argued in an editorial in 2012 that although the hamstrings do work the hardest during the end phase, the inertia that created the tension is not the force that causes the strain.
“It is a strong force (moment, torque) in the opposite direction (causing hip flexion and knee extension) that strains a hamstring, which is most likely to be due to [ground reaction force],” he wrote.
He pointed out that there is no direct evidence to support the eccentric contraction idea, such as a video that shows an actual tearing of the hamstring muscle or tendon — as he had demonstrated with a calf muscle strain on video in 2002.
However, perhaps there is no clear cut answer to this problem. Researchers from China stated that both phases of the leg swing are “potentially hazardous times for injury during high-speed running.”
Rather than looking at both phases as separate moments, Liu et al. suggested in 2017 that these two phases should be considered as a “swing-stance transitional period.” This means that this movement is continuous, not just a snapshot of a moment in time. The hamstrings flex the knee and extend the hip throughout the running gait.
In a 2019 narrative review by a team of Canadian and Australian researchers, they cited several “gaps” of knowledge regarding this part of hamstring strain research: a lack of studies among elite athletes, a lack of direct measurement of hamstring muscle stimulation during high-speed running, the need to look at the how hamstring muscles and tendons each contribute to lengthening and velocity, and a lack of prospective studies that examine the relationship between running mechanics and hamstring injury.
Currently, the most recent consensus still weighs highly on eccentric hamstring contraction during the late phase of the leg swing cycle, as a 2020 systematic review of 26 studies indicated. The researchers, however, warned that the included studies’ conclusions were based on “estimations of the hamstring injury mechanism,” not the injury “per se.” Some of these studies had subjects running on a treadmill at a slower pace than an actual sprint on a grassy field where most of the hamstring strains happen. So, a better way to edge closer to the truth would be to improve study designs and get larger sample sizes, the researchers suggest. This would likely further reduce the research bias.
The lack of good flexibility of the hamstring may seem like an obvious cause of a hamstring strain, but — like most scientific research — there is no strong certainty that this is the case.
One 2012 literature review found mixed results regarding whether a lack of flexibility and a reduction of muscle length could cause a pulled hamstring. In one study the researchers cited, athletes from the Australian Football League (AFL) with a history of hamstring strains “may have shorter optimum hamstring muscle lengths” than those with no such history. However, a prospective study of 44 sprinters in Hong Kong found no “significant difference in the knee flexion angle for the peak knee flexion torque” between the injured and non-injured during the pre-season.
For general hamstring and knee flexibility, there is “inconsistency” among various studies. The researchers speculated that it is because of different methods and factors used, such as different control groups, control of risk factors, and how injury risks were measured.
A larger and more recent study of more than 500 young Gaelic football players and hurdlers found that poor hamstring flexibility and previous injury “were unable to predict those at risk of sustaining a hamstring strain…”
Looking at the bigger picture, a 2020 systematic review of 78 athletic studies by Green et al. found that the degree of flexibility, mobility, and range of motion in the hips, knees, and ankles showed no clear relationship with the risk of getting a hamstring strain.
Previous hamstring injury
As with most types of muscle and joint injuries, having a history of hamstring strains would increase your chances of sustaining another pulled hamstring. Green et al. also found that in addition to older age, a history of having hamstring strains, ACL tear, and calf strain “were significant risk factors” for hamstring strains.
This is similar to a previous systematic review in 2014 that “revealed a relationship between previous injury and re‐injury in each lower extremity injury that was studied.” These injuries include hamstring strains, ACL injury, Achilles tendon injury, and ankle sprains.
Fulton et al. summarized that elite Australian football players were more than four times more likely to pull another hamstring than their peers who never had such injury. Also, about 27% of all hamstring strains in the AFL were caused by previous hamstring strains. However, the researchers do not know if the reinjuries were due to “accumulated microscopic muscle damage or the presence of a single injurious event.”
Hamstring strain symptoms
The sharp pain that you may feel when you get a pulled hamstring is a common symptom of a hamstring strain. You may feel the pain in the middle of the hamstrings, toward the hip attachment, or near the back of your knee.
Research has shown that most people with a hamstring strain have pain at their biceps femoris, which is the tear-shaped muscle that you often see among bodybuilders. Hamstring strains are more common toward the hip rather than toward the knee. However, the pain may radiate down or may be felt at the back of the knee.
Hamstring strains are classified into I (mild), II (moderate), or III (severe), which is based on the degree of pain, loss of range of motion, and loss of strength. They can be a mild tear in the muscle or tendon or a complete tear.
Interestingly, hamstring muscle tears or injuries do not necessarily cause pain or dysfunction. In a 2016 study of 506 hamstrings from 253 patients with no hamstring or knee pain, 15% of them had a partial tear on both hamstrings and 2% had complete tears on both sides. These tears seem to be more common among older adults than younger adults, as reflected in the Penn Relays and the Green et al. studies.
Hamstring strain treatments
Because there are no clear-cut answers for what the primary causes are for everyone, it is difficult for clinicians to determine the best course of treatment. Everyone likely wants to learn how to heal a hamstring strain fast so they can return to playing and functioning without pain, but the treatments would need to be individualized based on the existing evidence, which is one of the keys to applying science with clinical practice.
Most types of hamstrings strains can be treated and managed conservatively. Although some of the treatments for minor to moderate acute strains, such as the RICE method, lack strong evidence of effectiveness, they are still commonly used for home remedies and a doctor’s office.
Erickson and Sherry proposed a three-step general guideline for hamstring strain rehabilitation:
- Minimize pain and edema, restore some neuromuscular control and function with exercises at slow speeds, and prevent excessive scar tissue formation while “protecting the healing fibers from excessive lengthening;
- Increase exercise intensity and speed gradually to the patients’ ability and tolerance, start eccentric strength training;
- Integrate high-speed exercise training and sports-specific training.
Surgery may be needed if there are symptomatic tendon avulsion, where a hamstring tendon tears of a piece of the bone at one of its attachments. In a review by Ernlund and Vieira, such an injury prevents athletes from returning to compete, and the results of surgery and conservative treatments are mixed.
For example, in one 2014 study by Hoffman et al., about 30% of the 19 older patients with a hamstring tendon avulsion were unable to play at their pre-injury level, and about half of them “regretted not having undergone surgical treatment.”
Ernlund and Vieira suggested that hamstring avulsion — especially at the long head of the biceps femoris — should be surgically repaired “as early as possible,” ideally within two weeks after the injury. This can minimize the risks of further complications of the hamstring tendons and muscles and the sciatic nerve.
In fact, sciatica and weakness of the hip extensors may develop due to nerve problems associated with delayed surgery, including the nerves in the back of the thigh (posterior femoral cutaneous nerves, lower gluteal nerve).
As with most treatments, surgical or otherwise, consult with your physician or another qualified medical professional for your best course of action for your hamstring strain and similar leg injuries. Do not substitute this article for medical advice.
Exercise has been shown to minimize the effects of scar tissue formation and injury recurrence, such as strengthening and stretching programs. Both concentric exercises (shortening of muscle fibers under resistance) and eccentric exercises (lengthening of muscle fibers under resistance) can improve recovery, as well as open-chain exercises (where your body is in a fixed position and your limbs are free to move) like a hamstring curl on a machine. While some research favors eccentric strength training over concentric training to improve hamstring strength and function as well as minimizing re-injuries, the evidence is still quite mixed.
Several trials have found that eccentric training is superior to concentric training for muscle power and general muscle strength. One 2008 systematic review of seven studies found that there is limited evidence that eccentric training should be the primary and secondary preventive treatment for hamstring strains because of several study limitations. These include lack of control groups, eccentric training not studied in isolation, and small sample sizes.
Another systematic review in 2020 from Brussels, Belgium, that examined the biceps femoris found similar results, given that there is “limited to moderate evidence” that eccentric strength training can increase muscle fascicle length. There was also conflicting evidence that eccentric training is better than concentric training for general strength development.
One exercise researcher from the University of Calgary in Canada argued that hamstring injuries may “have nothing to do with eccentric loading and mechanics of the muscles in sprint running.” Dr. Walter Herzog proposed other factors that might influence a hamstring strain, such as a “loss of motor control” or a “lack of focus when sprinting.” These factors are also psychosocial in nature, so they should be included when considering treatment for hamstring strains.
“Maybe hamstring injuries are not mechanically mediated, not caused by eccentric contraction, but caused by a small change in hamstring activation when attempting to come from behind in a sprint race, or when tightening up in a losing position,” Herzog concluded.
Although most of the research on hamstring strains are based on competitive athletes, the average gym-goers and other non-athletes may benefit from some of these findings. Finding the right treatment for you and finding out the mechanisms and risk factors of getting a hamstring strain are still challenging for scientists and clinicians, even though this topic has been researched for decades. Perhaps a better approach — if not the best — is to have a combined and adaptable intervention that is borrowed from the biopsychosocial model of pain and health.
Since exercise has been shown to be the most effective and fulfills the biopsychosocial domains, this should be one of the primary interventions for hamstring strain recovery and prevention with the help of a physical therapist, an athletic trainer, or a qualified personal trainer. While there is no shortcut to healing a hamstring strain quickly, having a good balance of rest, activity, nutrition, and social and work life would be an ideal start.