Catcher fatigue: Did the Royals break Salvador Perez?

[Editor's note: This is the site's first piece by new contributor Matt Jackson! Welcome him aboard.]

Last season, Salvador Perez struggled offensively down the stretch and through the playoffs, leading some to speculate that his downturn in production was the result of his league leading 1248.2 innings behind the plate.

Before examining whether Perez wore down during the regular season, look how his workload compared to other catchers. The following figure shows how the three most-used backstops accumulated their innings. Notice that Perez split from Lucroy and Montero toward the end of August as the Royals pushed to take the AL Central.

Although Perez logged 5% more innings behind the plate than Jonathan Lucroy, the Brewers catcher appeared defensively in more games during the regular season (155), playing 129.1 innings at first base. In the 2015 Hardball Times Annual, Shane Tourtellotte introduced opponent’s plate appearance (OPA), a proxy for catcher workload. Each batter faced while the catcher is behind the plate counts as a unit. He accounts for exertion from time spent at other defensive positions by assigning two OPA for each of these innings. Using this method, Lucroy (5,191 OPA) comes close to matching Perez (5,217) for top workload, while Montero continues rounds out the top three (4,883).

So the margin by which Perez had the heaviest regular season catching workload may be smaller than previously thought, but did it affect his performance? Other studies in catcher fatigue have used OPS+ as an indicator, which was appropriate given the sample sizes they had, but they were not looking at individual player splits.

In 2013, a Vanderbilt University research group showed that MLB hitters chase pitches outside the strike zone (Fangraphs O-Swing%) with greater frequency in the last month of the season than the first and suggested this decrease in strike-zone judgement may be due to fatigue. Since pitches accumulate more quickly than PAs (albeit less so for Perez than nearly any other major league regular), I’ll use O-Swing% as an indicator of fatigue.

First, consider all catchers as the baseline. In September, they swung at 2.8% more pitches outside the strike zone than they did during the first month of the season. The change in Salvador’s strike zone judgement was certainly more dramatic. A free swinger to begin with, his O-Swing% shot up from 38.2% in March/April to a staggering 50.6% in September.

Although Perez was thrown more than 150 pitches outside the zone each month, the 95% confidence intervals are still rather large. Instead, I'll compare the difference between his first and second half splits to all MLB catchers as well as Lucroy and Montero.

Both Perez and Miguel Montero showed worse strike zone judgement in the second half of the season, though Perez lapped the field in that department. They also made less contact on pitches outside the zone (O-Contact%) which contributed to an increase in swinging strikes (SwStr%). Finally, the pair lost distance on their fly balls (hit further than 150 ft.). Perez also increased his already sizeable infield fly ball rate (IFFB%) by 5.5%. Lucroy, on the other hand, shirked the trend, swinging at 4% fewer pitches outside the zone in the second half.

Player (BMI)	Δ O-Swing%	Δ O-Contact%	Δ SwStr%	Δ IFFB%	Δ Avg. FB Distance (ft.)
Perez (30.0)	11.1%	-6.7%	2.0%	5.5%	-21.2
Lucroy (26.4)	-4.4%	0.8%	-0.5%	3.1%	8.0
Montero (30.0)	4.7%	-3.3%	2.0%	1.1%	-18.4
All Catchers	0.7%	-0.1%	-0.1%	1.8%

I include BMI in the table not to suggest that either Perez or Montero is moderately obese (as the scale would indicate if one chooses to ignore its multitude of limitations), but rather to state the obvious: larger catchers may not sustain high workload seasons as well as their smaller brethren. That Perez and Montero seemed to wilt in the second half while Lucroy improved may be influenced by their body types.

So what will happen to Perez next season? Is his 2015 campaign doomed even before it begins? We can’t know, of course, but Royals fans may look to Russell Martin for a small measure of reassurance. After a strong rookie campaign in 2006, the Dodgers leaned heavily on Martin the next season, pushing him to an OPA of 5,302 in 2007. He posted a third straight above average offensive season in 2008 while logging as staggering 5,680 OPA. It took two grueling seasons before Martin faltered, posting a career low 86 OPS+ in 2009 despite remaining healthy enough to play the full season.

Aside from their height, speed, and pitch framing abilities, Perez and Martin are similar in their track record of excellence on both offense and defense. For this reason, their managers are tempted to push them to their limits. However, based on comments from Royals brass that Perez would spend less time behind the plate in 2015, it appears we won’t get the chance to see whether Perez can endure a second 5,000+ OPA season. Based on what appeared to be fatigue at the end of 2014, that’s probably not a bad idea.

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All statistics courtesy of FanGraphs, Baseball-Reference, ESPN, and Baseball Heat Maps.

Matt Jackson is a contributor to Beyond the Box Score. You can follow him on Twitter at @jacksontaigu.