Pelvic Prime's Anatomy Academy Series
Hamstring Strain Injury in Athletes
Hamstring strain injuries are common in sports and activities that involve a lot of running, jumping, kicking, or quick changes in direction. This includes sports like track and field, soccer, football, and rugby. In non-contact sports, about 8.7% of hamstring strain injuries occur, while in contact sports, the rate is 9.6%. For professional male European soccer players, during competitions, 30-40% of hamstring injuries happen. Most hamstring strain injuries in men’s football, men’s soccer, and women’s soccer occur during practice (68.2%). A professional soccer team with 25 players can expect around seven hamstring strain injuries each season.
The risk of getting a hamstring strain injury increases with age. As athletes grow older, their muscles and tendons become more prone to strains and tears, making them more susceptible to hamstring injuries. This is particularly concerning for professional teams where player availability is crucial. The physical demands of sports that require explosive movements, combined with the natural aging process, create a perfect storm for hamstring problems.
Hamstring strain injuries often cause players to miss 3 to 28 days or more of competition, depending on the severity of the injury. The chance of reinjury is high, ranging from 13.9% to 63.3% for Australian rules football and track and field athletes. Players who have had a hamstring strain injury before are 3.6 times more likely to get another one, often because they returned to play too soon.
Muscle Fiber Composition and Injury Risk in Hamstring Strains
Skeletal muscle is made up of slow (type I) and fast (type II) muscle fibers. It is believed that the hamstring muscle group has more type II fibers than other thigh muscles, making it more prone to injury. The long head of the biceps femoris muscle is the most commonly injured hamstring muscle, involved in 79% to 84% of hamstring strain injuries.
An increased forward tilt of the pelvis can stretch the hamstring muscles more, possibly increasing the chance of injury. Using ultrasound imaging during different levels of muscle contractions, injured hamstring muscles had shorter muscle fibers and a larger pennation angle (the angle at which muscle fibers attach to the tendon) compared to uninjured muscles.
Signs of Hamstring Strain Injury
Sudden pain,
injury during running or acceleration,
tenderness in the back of the thigh,
pain when contracting the hamstring muscle.
The "taking-off-the-shoe" test has been found to be 100% accurate for diagnosing grade I and II biceps femoris injuries compared to ultrasound diagnosis.
Risk Factors for Hamstring Strain Injuries
Risk factors for hamstring strain injuries can be divided into two categories: nonmodifiable and modifiable. Nonmodifiable factors are characteristics that cannot be changed, like a person’s age or their history of previous injuries. Modifiable factors are those that can be altered, such as muscle characteristics and performance.
Nonmodifiable Risk Factors
Previous Injury
Studies show that having had a previous hamstring strain injury greatly increases the risk of getting another one. For example, male sprinters with a past hamstring strain injury are almost three times more likely to get injured again compared to those who never had one. Additionally, a recent injury (within the last eight weeks) increases the risk even more than an older injury. Athletes with a history of injuries like anterior cruciate ligament injuries, calf strains, or other knee and ankle ligament injuries are also at higher risk. However, a history of quadriceps strain or chronic groin problems does not seem to increase the risk.
Physical Characteristics
Getting older is a significant risk factor for hamstring strain injuries. Athletes older than 23 are at greater risk compared to those 23 or younger. In Australian rules football, players older than 25 have a much higher risk of injury. While height and preferred kicking leg are not risk factors, ethnicity can be. For example, African-American athletes and Aboriginal Australian rules football players have a higher risk.
Modifiable Risk Factors
Weight and Body Mass Index
Weight and body mass index do not seem to be risk factors for hamstring strain injuries.
Muscle Characteristics
Flexibility does not appear to affect the risk of hamstring strain injuries. However, shorter muscle fibers and stiffer hamstring muscles are related to a higher risk. There is conflicting evidence about whether tight hip flexors or limited ankle movement contribute to the risk.
Muscle Performance
There is limited evidence suggesting that weak hamstrings are a risk factor for injury, and this may depend on how and when the strength is measured. Increased quadriceps strength can be a risk factor, and there are mixed findings about the importance of hamstring-to-quadriceps strength balance. Changes in how trunk and gluteus muscles work and control movements might also be risk factors.
Performance Characteristics
High-speed running demands are a known risk factor, particularly in sports like soccer, American football, and rugby. Rapid increases in high-speed running can be particularly risky. Certain sprinting styles, like increased forward tilt of the pelvis and sidebending of the thoracic spine, are associated with hamstring strain injuries. Performance measures like the single-leg hop for distance and differences in jumping ability have lower levels of evidence as predictors.
Clinical Course of a Hamstring Strain Injury
A hamstring strain injury can happen anywhere along the muscle, but it most often occurs in the upper part of the biceps femoris muscle where it connects to the tendon. When the injury happens, a person feels a sudden, sharp pain in the back of the thigh. Sometimes, they might hear or feel a popping sound or sensation. This usually happens during activities that overload or overstretch the hamstring muscle. The person will likely stop what they are doing because of the pain and difficulty moving.
The chance of reinjury ranges from 13.9% to 63.3% over the same and following seasons. Injuries that cause more damage to the muscle and tendon are more likely to happen again and delay the return to playing sports or other activities.
The healing process depends on how severe the injury is. In mild cases, only the small muscle fibers are damaged. In more severe cases, the force of the injury tears the muscle fascia, basal lamina, and blood vessels. This leads to the release of muscle enzymes and proteins, causing inflammation and bleeding.
Grades of Hamstring Strain Injuries
Hamstring strain injuries are classified into three grades based on severity:
Grade I (Mild Strain)
Minor tearing of a few muscle fibers
Localized pain in a small area
Tightness and possible cramping in the back of the thigh
Slight pain with muscle stretching or activation
Stiffness that may go away during activity but return after
Minimal strength loss
Less than a 15-degree deficit in the active knee extension (AKE) test
Grade II (Moderate Strain)
Moderate tearing of muscle fibers, but the muscle remains intact
Pain covering a larger area than in a grade I strain
Greater pain with muscle stretching or activation
Stiffness, weakness, and possible bruising
Difficulty walking, especially in the first 24 to 48 hours
A 16 to 25-degree deficit in the AKE test
Grade III (Severe Strain)
Complete tear of the muscle
Swelling and bleeding
Possible lump of muscle tissue at the tear site
Extreme difficulty or inability to walk
A 26 to 35-degree deficit in the AKE test
The healing process has three phases: inflammation, proliferation, and remodeling.
Inflammation Phase
This phase starts right after the injury and lasts about 3 to 5 days. Blood vessels widen and become more permeable, causing fluid to build up and leading to further muscle damage and swelling.
A few days after the injury, cells called phagocytes enter the area to clean up the damaged tissue and activate stem cells to start rebuilding.
During this phase, the person usually feels pain, swelling, and has a limited range of motion.
Proliferation Phase
This phase can overlap with the inflammation phase and last for several weeks.
During this time, satellite cells help repair damaged muscle fibers, and the collagen and blood vessels are rebuilt.
The person might experience muscle weakness, stiffness, swelling, and limited function. If these symptoms last too long, recovery may be affected.
Remodeling Phase
Depending on the injury's extent, this phase can last up to 2 years. It involves the final formation of collagen to support the injury site.
Proper alignment of the extracellular matrix is needed for the muscle fibers to heal correctly. Early movement and soft tissue mobilization after the injury can help create organized scar tissue with fewer adhesions.
As this phase progresses, the person will feel less pain and can handle more stress on the muscle.
Return to Play and Reinjury Risk
Hamstring strain injuries often happen again, causing athletes to miss training and competition. For example, athletes who had a hamstring strain injury are about three times more likely to get another one. This can be very costly for professional sports teams. It's important to assess the risk of reinjury and make good decisions about when an athlete can safely return to play. This is especially crucial after a severe injury that needs a longer recovery.
Reducing Reinjury Risk
Doing agility and stabilization exercises after a hamstring injury can lower the risk of getting hurt again. This is more effective than just doing stretching and strengthening exercises. A study found that a special treatment program for soccer players reduced the risk of reinjury compared to a standard exercise program.
Effective Rehabilitation
Adding eccentric exercises to a rehabilitation program can significantly reduce the time to return to play. For example, hamstring strain injuries with greater range of motion deficits need longer rehabilitation times. Early and proper rehabilitation can help athletes return to play sooner and reduce the risk of reinjury.
Imaging
Imaging is usually not needed for grade I or II hamstring strain injuries. MRI is recommended for suspected grade III injuries. MRI and ultrasound imaging can be helpful for individuals with unusual symptoms or those not improving with treatment. X-rays are generally not needed unless there is a suspicion of a fracture.
Measurement Recommendations
Strength Measurement: Clinicians should measure knee flexor strength using a handheld dynamometer or isokinetic dynamometer.
Length Measurement: Clinicians should use an inclinometer to measure hamstring length by assessing knee extension deficit with the hip flexed to 90 degrees.
Tenderness: Clinicians may use the length of muscle tenderness and its proximity to the ischial tuberosity to help predict the time needed to return to play.
Posture and Control: Clinicians may assess for abnormal trunk and pelvic posture and control during functional movements.
Activity Limitation and Participation Restriction
Guidelines for assessing activity limitations include checking for pain-free walking, jogging, running at 70% speed, changing direction, and running at full speed. Repeated sprint tests can reliably measure performance in athletes and may indicate decreased speed in those with a previous hamstring strain injury.
Interventions After Injury
Eccentric Strengthening Exercises:
Begin hamstring-strengthening exercises, including eccentrics, early in the rehabilitation process, guided by patient pain tolerance.
Exercises should be done 2 to 3 times per week, with both load and ROM increased as tolerated.
Starting and progressing exercises and running should be done carefully to avoid aggravating symptoms.
Clinicians should use eccentric training to patient tolerance, added to stretching, strengthening, stabilization, and progressive running programs.
Agility and Trunk Stabilization:
Adding progressive agility and trunk stabilization exercises to a rehabilitation program that included stretching and strengthening may decrease reinjury rates.
Use progressive agility and trunk stabilization exercises and a running program that includes acceleration and deceleration phases with increased speed and distance throughout the rehabilitation process.
Stretching:
There is insufficient evidence to support stretching alone as an effective treatment for hamstring strain injuries.
Comprehensive Treatment Programs:
An individualized treatment program that includes a range of exercises reduced the risk of reinjury compared to a standard Nordic hamstring exercise (NHE) program.
Neural Tissue Mobilization:
It is recommended to include neural tissue mobilization to reduce adhesions and therapeutic modalities to control pain and swelling early in the healing process.
Martin, R. L., Cibulka, M. T., Bolgla, L. A., Koc, T. A., Jr., Loudon, J. K., Manske, R. C., Weiss, L., Christoforetti, J. J., & Heiderscheit, B. C. (2022). Hamstring Strain Injury in Athletes: Clinical Practice Guidelines Linked to the International Classification of Functioning, Disability and Health From the Academy of Orthopaedic Physical Therapy and the American Academy of Sports Physical Therapy of the American Physical Therapy Association. *Journal of Orthopaedic & Sports Physical Therapy, 52*(3), CPG1-CPG44. https://doi.org/10.2519/jospt.2022.0301