Heading a ball may damage footballers' brains, research shows.
A study has found evidence of mental impairment caused by the action. Scientists say the effects suggest "mild traumatic brain injury of the frontal lobes".
Dr Anne Sereno, from the University of Texas, and colleagues wrote in the online journal Public Library of Science ONE: "To our knowledge, these results provide the first evidence that even sub-concussive blows in football could lead to measurable, even if possibly transient, cognitive changes in young football players."
Increased time spent playing the sport each week had a significant effect on response time, while the number of years in the sport only had a marginal effect.