Background Risk for injury in professional soccer is one of the largest in sports. Next to negative aspects for players' health, in professional soccer the absence of players due to injury leads to high costs. Therefore it is important to gain knowledge on injury risk factors within this specific athletic population to draw up measures that may prevent injuries.

Objective To systematically review intrinsic and extrinsic risk factors which are related to injuries in male professional soccer players.

Design A computerised literature search was carried out in Medline, Embase, SportDiscus and PsycInfo to search for relevant original studies. A study was included if: (1) it contained data of professional, adult, male soccer players; (2) it concerned a prospective or retrospective cohort study covering at least half a season; (3) it addressed data of at least one team; (4) it assessed risk factors for injuries. All relevant studies were assessed for quality.

Results In total 313 articles were found, of which 14 were identified as being relevant and of sufficient methodological quality. In total 14 potential intrinsic and nine potential extrinsic risk factors were investigated in the included studies. From these, previous injury, preseason strength imbalances, decreased muscle flexibility and a preseason ultrasonic abnormality in the Achilles tendon were reported in the literature as intrinsic risk factors for injury. With regard to extrinsic risk factors artificial turf, match result, and time of the season were associated with injury occurrence.

Conclusion To date, aetiological studies in professional soccer have mainly charted player characteristics, for example, age, playing position, strength, and ankle stability. According to the literature 23 potential risk factors have been investigated, and only seven have been associated with the occurrence of injury. Future research should further explore modifiable extrinsic risk factors, and translate ascertained factors into practical preventive efforts.