Table 1

Operational definitions

Training sessionTeam training that involved physical activity under the supervision of the coaching staff
MatchCompetitive or friendly match against another team
InjuryAny physical complaint sustained by a player that resulted from a football match or football training and led to the player being unable to take full part in future football training or match play.
Hamstring injuryA traumatic distraction or gradual onset injury to the hamstring muscle group
Functional muscle injuryAcute indirect muscle disorder without macroscopic evidence of muscle tear
Structural muscle injuryAcute indirect muscle injury with macroscopic evidence of muscle tear
Injury severityThe number of lay-off days caused by the injury between the injury date and the date when the medical team declared the player as ready for full participation in team training and availability for match play
Recurrent injuryInjury of the same type and at the same site as an index injury occurring previously during the same season
Early recurrenceRecurrent injury that occurs within 2 months after return to full participation from the index injury
Injury incidenceNumber of injuries per 1000 player hours ((Σ injuries/Σ exposure hours)×1000)
Injury burdenNumber of lay-off days per 1000 player hours ((Σ lay-off days/Σ exposure hours)×1000)