Hamstring strains occur when one or more of the three hamstring muscles (biceps femoris, semitendinosus, and semimembranosus) are overstretched or torn. They are categorized into three grades:

• Grade 1: Mild strain with minimal fiber damage and no significant loss of function. Athletes may feel tightness or discomfort but can typically walk without pain. Sprinting or explosive movements, however, may feel weak or restricted.
• Grade 2: Partial tear with moderate pain, swelling, and some loss of strength. Bruising may appear within a few days, and flexing the knee against resistance often triggers pain. Walking is usually possible but with noticeable discomfort. Athletes with a Grade 2 strain often struggle with explosive acceleration, and simple movements like rising from a seated position may cause discomfort.
• Grade 3: Complete tear, often requiring extensive rehab and sometimes surgery. This injury results in severe pain, immediate bruising, and an inability to bear weight. A visible deformity in the muscle may be noticeable in some cases. Athletes may hear or feel a “pop” at the time of injury, and any attempt to contract the hamstring will result in sharp pain and weakness.

Athletes can perform at-home tests to gauge the severity of their injury. A simple single-leg bridge test can help determine functional deficits—if lifting the hips while keeping the injured leg elevated causes sharp pain or weakness, a more severe strain may be present. Additionally, a passive straight leg raise can assess flexibility limitations compared to the uninjured leg. The heel slide test, where an athlete tries to slide their heel toward their glute while lying down, can also help identify the degree of strain—pain or weakness during this motion suggests a more significant injury.