Hamstring Injuries: Causes, Classification, and Prevention

Hamstring injuries are among the most common injuries in sports, particularly in activities involving high-speed running and rapid changes in direction. Understanding the causes, classification, and strategies for prevention is crucial for athletes and active individuals alike. Below, we break down the key points in simple terms.

Causes of Hamstring Injuries

1. High-Speed Running

One of the most common causes of hamstring injuries is high-speed running. During sprinting, the hamstrings play a vital role in the “eccentric phase.” This is when the hamstrings act as brakes to control the forward motion of the leg as it swings forward. This braking action places a significant load on the muscles, especially when they are stretched.

• Different Muscles, Different Positions: The hamstring group consists of three muscles: the biceps femoris, semitendinosus, and semimembranosus. The type of injury often depends on the position of the body:
  • Upright running: Injuries are more likely to occur in the biceps femoris, the muscle on the outer side of the hamstring group.
  • Acceleration or bent-over positions: These are more likely to strain the semimembranosus, located on the inner side.
• High-Speed Running Injury Types: The high-speed running type of injury typically affects the long head of the biceps femoris (BFlh) and has a shorter recovery time compared to the stretching type of injury, which commonly affects the semimembranosus.

2. Overload

Hamstring injuries can also result from overloading the muscles, often due to insufficient recovery between workouts. Without adequate rest, the hamstrings may become fatigued, increasing the risk of strain.

Classification of Hamstring Injuries

Hamstring injuries are classified based on the extent and location of the damage. An MRI scan is considered the gold standard for diagnosing and grading these injuries. Here are some key factors:

• Size of the Injury: Larger tears often take longer to heal.
• Muscle vs. Tendon Involvement: Injuries involving the tendon (the tissue that connects muscle to bone) can be more complex than those confined to the muscle.

What are Muscles and Tendons? Muscles are soft tissues that contract to produce movement, while tendons are tough, fibrous tissues that connect these muscles to bones. Together, they form a seamless unit that powers and stabilizes our movements.

One of the common classification systems used utilising MRI scans is the British Athletics muscle injury classification (BAMIC/BAC) System.

Classification

• grade 0
  • grade 0a: focal neuromuscular injury with normal MRI
  • grade 0b: generalised muscle soreness with normal MRI or MRI findings typical of delayed onset muscle soreness (DOMS)
• grade 1 (mild): high STIR signal that is <10% cross-section or longitudinal length <5 cm with <1 cm fibre disruption
• grade 2 (moderate): high STIR signal that is 10-50% cross-section; longitudinal length 5-15 cm with <5 cm fibre disruption
• grade 3 (extensive): high STIR signal that is >50% cross-section or longitudinal length >15 cm with >5 cm fibre disruption
• grade 4: complete tear

Grades 1-3 are subclassified depending on site:

• a: myofascial (peripheral)
• b: myotendinous junction / muscular
• c: tendinous

For grade 4 (complete tears), the distinction is only made between tears involving muscle versus those solely involving the tendon, as follows:

• grade 4: myofascial, muscular, or myotendinous
• grade 4c: tendinous

Although expensive (MRI scan), it can help provide athletes with a more comprehensive prognosis. Some minor strains may be fine to return to sport in a few weeks, others may require months.

Prevention of Hamstring Injuries

1. Improving Biomechanics

Poor biomechanics can increase the strain on your hamstrings. Key factors include:

• Pelvic Tilt: An anterior pelvic tilt (when the pelvis tilts forward) can place extra tension on the proximal hamstring (where it attaches to the pelvis). [Insert diagram of hamstring proximal attachment here.]
• Overstriding: Taking excessively long strides during running can overload the distal hamstring (closer to the knee), increasing injury risk.

At Physiotherapy Performance Institute (PPI), we offer biomechanical assessments and running sessions to help optimize your running form and reduce injury risk.

2. Strengthening Through Range

Developing strength in the hamstrings across their full range of motion is essential. This includes both concentric (shortening) and eccentric (lengthening) exercises tailored to your needs.

3. Improving Fascicle Length

Hamstring fascicle length refers to the structure of the muscle fibers. Research shows that training methods like Nordic hamstring curls are particularly effective for improving fascicle length, enhancing the muscle’s resilience to strain.

4. High-Speed Running Exposure

Gradually exposing your hamstrings to high-speed running is critical for building their capacity to handle the demands of sport. This involves carefully planned doses of sprinting, progressing intensity and volume over time to avoid overload.

• Hamstring Loading at Different Speeds (meters per second):
  • Slow to Moderate Running: Approximately 3–5 m/s.
  • High-Speed Running: Speeds ranging from 5–8 m/s, often seen in sports-related sprints.
  • Maximal Sprinting: Speeds exceeding 8 m/s, which heavily tax the hamstrings during the terminal swing phase.
• Application in Field Sessions: Athletes can set up cone-to-cone running drills over 40–50m distances to practice upright running. By timing these efforts, you can estimate running speeds in meters per second and use this as a structured hamstring loading exercise. Gradually increasing speed and volume helps build capacity while minimizing injury risk.
• Weekly Planning: Proper dosage and weekly planning are essential. This includes balancing sprint sessions with appropriate loading and strength training to ensure recovery and prevent fatigue-related injuries.

A well-rounded strength program should target various parts of the hamstrings (for example):

• Nordic hamstring curls fascicle length development and eccentric loading.
• Single-leg loaded deadlifts targets more the hip extensor action of the hamstring unit.
• Prone/laying hamstring curls to target the distal hamstring.

Depending on where someone is at in terms of their competitive season; stage of rehab; sports demands etc, the dosage of these exercises and frequency per week will vary.

An example of an out of season week may be:

• Strength 2/week: Mon / Wed (reps 4-8 range; 3-5 sets/ex)
• Field session: Tue: normal training and technical / biomechanical work
• Friday: sprint/high speed session
• Saturday: skill/lengthier sports specific session

Takeaway: Hamstring injuries are often preventable with a combination of proper biomechanics, strength training, and structured exposure to high-speed running. At PPI, we specialize in personalized programs to keep you moving at your best!