Background Muscle injuries constitute a large percentage of all injuries in football.

Objective The aim of this study was to investigate the incidence and nature of muscle injuries in male professional footballers.

Design Prospective cohort study design.

Setting Teams from the first division of a professional football league.

Patients (or Participants) Seven football teams, comprising 152 players, were followed prospectively.

Main Outcome Measurements Team medical staff recorded individual player exposure and time-loss injuries. A muscle injury was defined as ‘‘a traumatic distraction or overuse injury to the muscle leading to a player being unable to participate fully in training or match play.’'

Results In total, 114 muscle injuries were registered. On average, a player sustained 0.75 muscle injuries per season. A squad of 25 players can thus expect about 19 muscle injuries per season. Muscle injuries constituted 38.5% of all injuries. Sixty-eight percent of all muscle injuries were muscle strain injuries. The four most commonly strained muscle groups were adductors/groin (40%), hamstring (35%), calf muscles (16%) and quadriceps (9%). Sixteen percent of the muscle injuries were reinjuries. The incidence of muscle injury is increased with age.

Conclusions Muscle injuries are a substantial problem for players and their clubs. They constitute more than one-third of all time-loss injuries in men's professional football and affect the four big muscle groups in the lower limbs.