Basic Muscle Physiology in Relation to Hamstring Injury and Repair

A hamstring strain injury has long been considered as a skeletal muscle injury, and the connective tissue associated with the muscle, i.e., the tendon/aponeurosis, has not been addressed sufficiently. A hamstring strain is a traumatic injury which very often occurs at the myotendinous junction (MTJ), which is the interface between the muscle and the tendon. The MTJ is formed during development and animal research clearly shows the interdependence and interaction between the muscle-derived and the connective tissue-derived cells during developmental processes. Additionally, several key molecules are indispensable for the MTJ formation. Although the MTJ is the most affected site after a strain injury, the research in adaptations to loading, unloading, and regeneration of the human MTJ is scarce. Skeletal muscle has a pronounced healing capacity, but the connective tissue in contrast is associated with a long repair period and incomplete repair. The differences in tissue healing and regeneration may complicate MTJ repair after hamstring strain injuries and might be the underlying factor why these sports injuries have a high recurrence rate. Re-injuries might occur as the repaired tissues have inferior mechanical properties as it is often described when scar tissue forms following tissue damage. Prolonged inflammation has been tightly associated with scar formation in several tissues, and recent data on human strain injuries support the idea that inflammation is present for an extended time after strain injuries.