My dad used to always tell me the story of someone in our extended family who got in an accident while riding his bike (he had his helmet on) and rode his bike himself to the hospital only to die the same day.

Not a medical professional, but that could easily be caused by something like a subdural hematoma.

I'm reminded of a talk at a science fiction convention years ago mostly targeted at writers (and done by a physician) - "Maim 'em Right." Basic message was that when writing injuries, make them realistic. If someone takes a blow to the head that knocks them unconscious for a significant amount of time, they're not resting for a day then getting up and going on to lots of exertion and winning the day. They're suffering from concussion, maybe subdural hematoma/brain bleed, etc.

Also discussion of TBI, including things like shaken baby syndrome - in the skull, the brain has a consistency somewhat more durable than thick jello. Get one of those brain jello molds available around Halloween, make a batch, then instead of unmolding it keep it sealed and give it a good shaking. THEN unmold it and see how much damage there's been.