An update on the grading of muscle injuries: a narrative review from clinical to comprehensive systems

Muscle injuries are recognized to be among the most frequent injuries occurring in the sporting and athletic population, and they account for more than 30% of all injuries in professional soccer players. Despite their considerable frequency and impact, there is still a lack of uniformity in the categorization, description and grading of muscle injuries. Dozens of systems based on clinical signs, ultrasound imaging (US) appearance or magnetic resonance imaging (MRI) findings have been proposed over the years. Most of them are three-grade systems that take into account pain, ROM limitation, swelling and hematoma, hypoechoic or hyperintense areas on US or MRI, and muscle gap or tendon involvement; however, they still lack evidence-based prognostic value. Recently, new comprehensive classification systems have been proposed, with the aim of developing uniform muscle injury terminology and giving each severity grade prognostic value. The systems that combine detailed MRI and US features with the clinical presentation, such as the Munich Muscle Injury Classification, the ISMuLT classification, and the British Athletic Classification, if used extensively, could improve the diagnosis, prognosis and management of muscle injuries.