I am writing a report on brain injuries in sport and submitted it to my tutor, who has said that it is too account-based. She also said it 'lacked criticality and theoretical underpinnings' What does this mean and how can I avoid writing in an account-based style?
I'm guessing the same as Mark - that your tutor thought the report put a lot of emphasis on the individuals' own accounts of what happened, or the accounts of others present - both of which could be considered anecdotal.
Criticality could be achieved by comparing and contrasting accounts, or by relating them to physical evidence (eg. hospital reports).
Theoretical underpinning would include points like "an injury to this area of the brain has been shown to cause this difference in mood or behaviour", which is where citations of professional/academic papers will be your best friend.