The most commonly used criteria for defining a sport injury is: The amount of game time lost.
During a sports event someone will probaly get hurt, the way people know that theres and injury is by the amount of game time lost.
So lets say someone was playing football and, they fell and broke there knee. The coaches, and others look to see the at the time to see if any time was lost. If there where people would be sent to make sure that the player is okay.
In many cases, it may be a short amount of game time lost and there alread sending someone to assisnt the fallen player.
So the answerwould be: Time lost from play as the indicator of injury.
