Testing the Lasting Effect of Concussions

Pitchers are expected to lose command after Tommy John surgeries. Prolific base stealers coming back from hamstring injuries are expected to take it slow for a week or two before regularly getting the green light on the basepaths. A broken finger for a slugger is blamed for the loss of power; a blister for a pitcher might mean a loss of feel on their breaking ball. What is not well publicized, however, is how a player recovers from and reacts to returning from a concussion. For an injury that has been talked about in the media so often in the last few years, we know very little about the actual long-term, statistical impacts that concussions have on players that experience them.

Players often talk about being “in a fog” for some time after suffering a concussion – often even after they return to play. The act of hitting is a mechanism that involves identifying, reacting, and deciding on a course of action within half a second. With that in mind, I wondered: do concussions change the quality of a batter’s eye and discipline at the plate? Do brain injuries add milliseconds to those individual steps? Even though each injury is different, do varying lengths of disabled list stints due to concussions change a player’s performance on the field after they return? The most direct route to answering those questions might be studying the impact of concussions on strikeout and walk rates.

To start with, I’ve pulled data for all of the position players that went on the DL due to a concussion from the start of 2011 to the end of 2013. There were 37 of them, starting with Corey Patterson at the beginning of 2011 all the way up to Joe Mauer at the end of 2013. Mauer is actually the player that started my interest in this question, so it’s fitting that he should bookend the player sample. I then compared each player’s strikeout and walk rate statistics from a calendar year before they suffered the concussion to the calendar year after they were activated off the DL following the concussion. There are many factors that could go into a player’s growth or decline outside of having a brain injury, but for simplicity’s sake, I’ve compared these two timeframes. The idea is to find out whether concussions have long-term effects on these rate statistics.

Out of the 37 players, I was able to get useable data on playing time out of 26 of them. This is due to some being sent to the minors for a long spell of time after their return from concussion, having other serious injuries in the year after the brain injury, and finally, a simple lack of playing time. Even though it is a small sample size of players, walk rate and strikeout rate stabilize at a much lower plate appearance mark than other statistics, and I’ve taken out all of those players that don’t meet the requirement.

To get an idea of whether longer stays on the disabled list due to concussion resulted in different performance changes, I’ve separated out the length of stays into three divisions of players: seven days (the shortest possible stay), eight to 14 days, and over 14 days. I’ve also supplied an overall table combining all of the data. Let’s get to it.

First, the seven-day DL, in which many cases were termed “minor” concussions on the injury description:

Baseball is really neat sometimes, and I mean that literally. This data set was a combined 4,732 plate appearances, and there was basically zero change in the rates before and after the concussions. It was if these players had a week to rest and then went on their merry way, striking out and walking slightly less than your average player.

Now, the eight to 14-day DL stint:

Pretty much the same – a slight downturn in walk rate, and the strikeout rate rises just a touch. A few of these players were activated after eight days on the DL, so these could have been situations with an off day at the end of a week. The point is evident: there isn’t a big, eye-catching difference.

Finally, the players that were on the DL for over 14 days for a concussion:

Here we have some noticeable separation. These are the players with long-term, complicated recoveries: Brian Roberts (just under three calendar years to fully recover); Justin Morneau (concussed in July of 2010 and admits he still isn’t the same); Denard Span (over 100 days before activation); Jason Bay, David Ross. These players represent the often-debilitating head injuries we still don’t know much about.

I’ve also created a table showing the change for each length of DL stint:

I also find it important to note that the return dates for all of these players, regardless of how long they were on the disabled list, were often murky – I came across phrases like “lost year” and “long, twisting road back” very often when going through the research. That fact also points to a disturbing trend when individuals returned from a head injury: players were often activated from the DL after a concussion only to be placed back onto it shortly afterward, frequently with a different but related reason, such as “migraine symptoms”. The leeway afforded to teams with respect to naming the cause for a DL visit seems to manifest itself nowhere greater than in cases of head trauma. I’ve done my best to identify concrete dates, but the best point to take from this research is that concussions are still not well understood, nor is it easy to put an end point on them.

Do concussions have a long-term impact on strikeout and walk rates? It’s hard to say with complete certainty. Though we see a downturn in walk rate and rise in strikeout rate following seven days on the disabled list, especially with those players that have concussions that keep them out of the game for over 14 days, it’s difficult to separate that from a potential natural downturn when someone doesn’t play baseball for an extended period of time. What catches my eye is that many of the players who suffered serious head trauma had a track record of success prior to concussion and dropped off precipitously following the injury. Though he is just one example, Morneau’s return to near peak form in 2014 (with a career-best strikeout rate) tells me that there could be something to the long-term offensive impact of these types of injuries. Further research will broaden the sample size, look at plate discipline statistics, and try to isolate exact approach changes as they relate to concussions. Here’s hoping that can finally lift the fog, and allow us to better understand the nature of head injuries.