Operational definitions
| Training session | Team training that involved physical activity under the supervision of the coaching staff |
| Match | Competitive or friendly match against another team |
| Injury | Any 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 injury | A traumatic distraction or gradual onset injury to the hamstring muscle group |
| Functional muscle injury | Acute indirect muscle disorder without macroscopic evidence of muscle tear |
| Structural muscle injury | Acute indirect muscle injury with macroscopic evidence of muscle tear |
| Injury severity | The 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 injury | Injury of the same type and at the same site as an index injury occurring previously during the same season |
| Early recurrence | Recurrent injury that occurs within 2 months after return to full participation from the index injury |
| Injury incidence | Number of injuries per 1000 player hours ((Σ injuries/Σ exposure hours)×1000) |
| Injury burden | Number of lay-off days per 1000 player hours ((Σ lay-off days/Σ exposure hours)×1000) |