Table 1

Operational definitions used in the study

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 a full part in future football training or match play
Slight/minimal injuryInjury causing 0–3 days lay off
Mild injuryInjury causing 4–7 days lay off
Moderate injuryInjury causing 8–28 days lay off
Severe injuryInjury causing >28 days lay off
Traumatic injuryInjury with sudden onset and known cause
Overuse injuryInjury with insidious onset and no known trauma
Hamstring injuryA traumatic distraction or overuse injury to the hamstring muscle group (the musculotendinous complex of biceps femoris, semitendinosus and semimembranosus)
ReinjuryInjury of the same type and at the same site as an index injury occurring within 2 months after return to full participation from the index injury (ie, early recurrent injury)
Injury prevalenceThe proportion of players affected by at least one injury during a season
Injury rateNumber 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)