Background: Muscle injuries are the most common injury in soccer and account for almost 30 percent of all time-loss injuries. The aim of this study was to investigate the muscle length as a risk factor to sustain a muscle injury and to grade and monitore muscle injuries during the rehabilitation. Methods: 110 young male football players of 5 soccer teams (U15, U16, U17, U18 and U21) with (age 16,8 (14,3–21) years, height 174,5 (146,1–190,0) cm, weight 65,7 (35,5–84,6) kg) were monitored for 12 months in a prospective cohort study. Initially, muscle length in all players were measured. Every muscle injury was documented and monitored in a standardized protocol and graded clinically based on the Muscle-­Injury-Score.