There are renewed concerns about the dangers of head knocks and the effects they have on the brain after a series of experiments looked at the link between Alzheimer's and brain injuries.

Boston University's Dr Lee Goldstein says we shouldn't be letting children under 12-years old play contact sports.

He told Mike Hosking it's far too early for serious knocks on a developing brain.

"We know that the risk increases with repetitive injuries and the more accumulative injuries one has the greater the risk of developing the disease later in life."

"These repetitive injuries add up and ultimately can lead to really devastating problems."

Goldstein said even if a child doesn't suffer a concussion, they can still injure their brain, putting themselves at risk.

"This is not about concussions, this is about repetitive head injuries."