One non-peer-reviewed study has estimated the risk of death from bare-knuckle boxing at 14,000 deaths per million participants. This is 184 times more deaths per million participants than for modern professional boxing, which has 76 deaths per million participants (according to the same study). Data for the number of fights and deaths from the bare-knuckle era is incomplete, and also that there were many differences in rules and medical care. Bare-knuckle boxing matches were usually fought until one fighter could not continue, with bouts sometimes lasting hours, and a few fighters dying after they were carried to their mark to restart the fight when they would otherwise have been unable to continue. (The London Prize Ring Rules later specifically stated that a fighter must "walk to his own side of the scratch unaided" (emphasis added) or lose the fight. ) Bare-knuckle rules also allowed grappling and throws, and some deaths were caused by a fighter hitting his head on a stone or rail.