Do Hitters Wear Down Over a Season?

While watching and listening to games this past postseason, I heard quite a few comments on players wearing down as the season dragged on. The narratives were all over the place. Are the older players able to pace themselves over the season because of experience? Do the younger players have youth on their side and continue producing? Is there a difference? When I went searching for an answer, the results surprised me.

To see how hitters perform as the season wears on, I compared their production in the first half (1H) to the second half (2H). One issue with doing these comparisons is the decline in the offense environment that started after the 2005 ban on performance-enhancing drugs. To help adjust for this drop in offense, I used wRC+, which is adjusted for the league and season run environment. This is not so important with the 1H to 2H comps, but I also compared one year’s second half to the next season’s first half. There, it is more valuable.

First, hitter change in performance from first half to second half.  (There’s a note on my methodology at the end of this article.)

The data are not perfectly smooth, but some trends can be noted. Hitters can expect to see some improvement from the season’s first half to second half in their age 21 to 25 seasons. From age 26 to 34, some decline occurs, with even more from age 35 on. I weighted each of the changes grouped into three age brackets and here are the results:

Age Range: Change in wRC+

• 21 to 25: +2.1
• 26 to 34: -2.1
• 35 to 40: -5.1

We need to understand that each of these values should probably be more positive depending on survivor bias. Hitters who are over-performing in the first half are more likely to get more plate appearances in the second half than an identically talented player who struggled to start the season. The over-performing player will on average regress to his true talent level in the season’s second half, which will give the appearance that he is wearing down.

The above numbers do pass the smell test. Over the same time frame, all hitters have a 102 wRC+ in the season’s first half and a 97 wRC+ in the second half. Overall league offense sees a decline from the season’s first half to the second. The first thought to explain the change is that the average temperature is warmer in the season’s first half. Higher temperatures  should mean more offense because for every 10 degree rise in temperature, a baseball will travel an additional four feet. But that’s not the case. Using Retrosheet data from 2005 to 2013 (2014 data are not available yet), the average temperature in the first half is 70.0 degrees; it jumps to 76.6 in the second half. On average, hitters should be performing better at higher temperatures, but they aren’t.

Other than those in the 25-and-under grouping, most hitters see a small drop in ability over a season, but how do they produce in the first half of the next season? I ran the same process for the last half of Season One to the first half of Season Two. Here are the results:

Age Range: Change in wRC+

• 21 to 25: -4.8
• 26 to 34: -4.7
• 35 to 40: -6.4

Talk about some negative numbers! The higher values from 26 to 40 are sort of easy to understand. If a player is slowing down, losing eyesight and/or loses muscle mass, he would expect to see more decline over a nine-month span than in the three months from the first to the second half of the same year.

The young player decline puzzles me. The decline amount is just a bit more than the amount for the 26-to-34 year-old group. These players improved as the season went on, but declined from the end of one season to the start of the next half season. The colder weather could be some of the effect, but not all. I don’t know the answer for sure, but here are some theories.

• Teams have more time over the offseason to find out what works against certain new young hitters and can make changes going into the next season.
• Younger hitters don’t take the offseason as seriously as older hitters. They don’t come into the season ready to play and struggle early on. Veterans, who struggled in the past, are going to put in more effort.
• Survivor bias is even more of an issue because teams thought the player they saw in the second half would be the same next year. They don’t have any options available when the player regresses to a lower talent level to start off the next season.

The sweet spot for lack of decline from youthful innocence on what it takes to be a big leaguer and the body letting a player down occurs around age 25. This drop can be seen in the overall wRC+ aging curve from the same time frame.

A couple of notes. First, aging curves used to have more of an up and then down, with a later peak, but just recently I noticed a change post PEDs, which major league teams, like the Pirates, have also noticed.

Second, the aging curve was created by the delta method by weighting plate appearances and innings pitched using their harmonic means, explained here. With this method, there’s a small survivor bias, which was summarized by Mitchel Lichtman in these internet pages back in 2009:

“… survivor bias, an inherent defect in the delta method, which is that the pool of players who see the light of day at the end of a season (and live to play another day the following year) tend to have gotten lucky in Year 1 and will see a “false” drop in Year 2 even if their true talent were to remain the same. This survivor bias will tend to push down the overall peak age and magnify the decrease in performance (or mitigate the increase) at all age intervals.”

Age Range: Average decline in wRC+

• 21 to 25: +0.8
• 26 to 34: -4.2
• 35 to 40: -9.7

While the year-to-year numbers don’t add up because of survivor bias, the trend is generally the same. We have an early improve/constant talent period to age 25. Hitters then begin to slowly decline from age 26 to age 34. After age 34, the decline rate doubles.

Now, getting back to the original question: How do hitters perform as the season progresses? Generally, hitters 25 and younger improve during the season, while older hitters get worse. The reasons could be injuries or just wearing down. The big surprise in aging happens between seasons, when players — who shouldn’t be experiencing any wear and tear on their bodies — get worse. Batters of all ages see their performance decline significantly from the end of one season and the start of the next. The reason is not obvious, but it could be a combination of weather, conditioning, regression or teams having more time to prepare. All players eventually wear down; it is surprising to see this aging happen during the offseason instead of during the season.