Concussion damage 'lasts months'
The damage caused by concussion can be detected months after the injury and long after patients feel like they have recovered, brain scans show.
Concussion has become highly controversial in sport, with concerns raised that players are putting their brain at risk.
Researchers at the University of New Mexico said athletes may be being returned to action too quickly.
While UK doctors said the attitude to head injury was "too relaxed" in sport.
Debate over concussion and head injury has lead to resignations over new rules in rugby, controversy in football after a player was kept on the field after being knocked out, and has been a long-standing issue in American football.
The US study, published in the journal Neurology, compared the brains of 50 people who had mild concussion with 50 healthy people.
Initially there were problems memory, headaches and dizziness - but these cleared up after a few weeks.
But four months later, doctors were still able to see differences in the way fluid moved through the brain suggesting the brain still had not healed.
Lead researcher Dr Andrew Mayer compared the impacts of concussion to a burn in which the symptoms, such as pain, can disappear long before the tissue has healed.
He told the BBC: "The big take-home message is that even though people report feeling better, the brain might not be completely healed."
"In the US, after concussion, athletes sit out for a week or 10 days because that's when people seem to be normal, but that might be premature as it doesn't mean the underlying tissue is healthy."
He said there was evidence that multiple small injuries in a short time were much worse than a single big injury. "It's a vulnerable system," he said.
Commenting on the research, Prof John Hardy, from University College London and one of the UK's leading researchers on brain disease, said he was not surprised by the results and warned against rushing people back into action.
He said: "If there's knee damage they'll have three months off, but after a head injury they're back on the field once they can count backwards from 10."
"I think there's much too relaxed an attitude to head injury, we need to minimise the occurrence and when it happens it needs to be taken seriously and have the proper time off no matter how long it takes."
Prof Hardy has previously called for boxing to be banned, after the repeated blows to the head were linked with dementia later in life.
The link with dementia has since been found in other sports involving regular head injuries.