Early and persisting increase in fat content after human muscle strain injuries in adults.

Acute muscle strain injuries belong to the most frequent sports injuries with high recurrence rates. Biopsies obtained from previously strain-injured muscles showed fat intra- and intermuscular, but quantification of fat content in the entire muscle volume following a strain injury has not been performed previously. The aim of the study was to evaluate fat fraction in the injured and the contralateral, uninjured muscle acutely, and 3- and 12-months after a strain injury. DIXON fat fraction was analyzed acutely after the injury, 3- and 12-months post injury in the injured and the contralateral, healthy muscle in 29 individuals with an ultrasound verified strain injury in either the calf or the hamstrings. There was a significant increase in fat content 3 and 12 months post injury in the injured, but not in the contralateral, uninjured muscles. There was a significant negative correlation between the increase in fat fraction and loss of muscle mass, and a significant positive correlation between the enlargement of the aponeurosis and the amount of fat in the injured muscles. Strain injuries lead to a rise in fat fraction in muscles from healthy, sports active individuals. The increased fat fraction appears permanent as fat accumulation in the muscle after the injury was not reversed after 12 months.