It would appear that players returning from concussions do not hit as well:
The study identified 66 position players who had concussions between 2007 and 2013, including some who never went on the disabled list. The study then compared their performance in the weeks before and after the injury.
The gap was noticeable. In the two weeks before their injuries, the players hit .249 with a .315 on base percentage and a .393 slugging average. For the two weeks after the injury, their line was .227/.287/.347.
Baseball instituted a seven day DL for concussions, but:
“I would say that what they’re doing now is a good start,” the senior author of the study, Dr. Jeffrey Bazarian, a professor of emergency medicine at the University of Rochester, said of baseball’s protocol. “But if they integrated some kind of analysis about how the player is swinging the bat, then you could see if they’re really back to where they were.”
I see nothing in the article, however, about comparing this to other injuries. Any player who takes time off is going to be rusty, so how do these numbers compare to hitters returning from a hand injury? It would be interesting to see the comparison to wrist fractures and hamstring tears.
You don’t hit with your head (physically, mechanically). A hand injury may linger and affect the swing. Same with a leg injury or oblique injury. Controlling for rust alone is hard to do, although maybe it can be estimated. Anything beyond rust is related to the brain injury. Concussions as you know cause brain damage. Might be temporary, or linger
Its a well known fact that concussions affect reaction times, which is important for hitting.
http://www.ncbi.nlm.nih.gov/pubmed/21402367
In fact, the study suggests the poor performance may be due to players coming back too quick. Reaction times should probably be measured to ensure a player is ready to return. This could be done by getting a baseline for all players
As a retired athletic trainer, I see any concussion study through a grateful lens. For decades concussions were dismissed and belittled. Serious efforts are finally underway to gather and share data and these are coupled with prudent management protocols. We are finally in a place where athletes well-being is prioritized.