Table 1 Description of the 7 criteria designed to assess the risk of bias of external validity quality in the studies

1. Definition of soccer-related injury

Studies that aimed to investigate soccer-related injuries should present a definition of an injury informing what was considered as an injury in the study.

Studies that present a definition of time-loss injury received a star for this criterion.

2. Representativeness of the exposed cohort

(a) Truly representative of the average soccer players in the community^a; (b) somewhat representative of the average soccer players in the community^a; (c) selected group of users; (d) no description of the derivation of the cohort.

3. Ascertainment of exposure

(a) Secure record^a; (b) structured interview^a; (c) written self-report; (d) no description.

4. Demonstration that outcome of interest was not present at the start of the study

(a) Yes^a; (b) no

Studies that described that all soccer players included were injury-free at baseline received a star for this criterion.

5. Assessment of outcome

(a) Independent blind assessment^a; (b) record linkage^a; (c) self-report; (d) no description.

6. Was follow-up long enough for outcomes to occur?

(a) Yes^a; (b) no

Studies that carried out a follow-up period of at least 12 weeks received a star for this criterion.

7. Adequacy of follow-up of cohorts

(a) Complete follow-up of all subjects accounted for^a; (b) subjects lost to follow-up unlikely to introduce bias (up to 20% loss) or description provided of those lost^a; (c) follow-up rate < 80% and no description of those lost; (d) no statement.