Catching, for lack of a better word, is hard. You strap on 18 interlocking pieces of gear, some of them plated in solid gold, and then squat for hours at a time working with maybe 25 percent visibility, getting hit by bounced splitters and backswings, feeling the cartilage twist and fray inside your knees, worrying about what each batter wants to hit and what pitch the pitcher wants to throw and how well you can fake that awful slider into almost being a strike. And then you head back into the dugout, strip off all the armor, head to the plate and get cheered half-heartedly by the fans who don’t understand why you don’t hit as many home runs as the right fielder. You develop late, fade early, play through constant pain and tear, and maybe get Sundays off.

For much of the sport’s life, catchers were the most underappreciated men on a baseball field. The numbers on their baseball cards rarely matched up with their peers, thanks to all those days off, routine or otherwise. The value of pitch framing, long a trade secret, has finally been exposed, but so many of the other facets of catching, like the game theory of sequencing and the psychology of working with pitchers, remain sadly unquantifiable. Bill Bergen, gainfully employed for a dozen years behind the plate, is left with a legacy as the game’s worst ballplayer, mostly because the secret of what made him worth the paycheck, sadly, died with him.

Catchers see better treatment than their forefathers, at least. Among the top 100 catcher seasons, by playing time, in the postwar era, only three names can be found from the past decade: Salvador Perez, Russell Martin, and one last hurrah from the unbreakable Jason Kendall. Teams have shown more willingness or inventiveness in giving their backstops regular rest, or at least rotating them around to DH or an easier position.

Even with the more progressive handling, it remains a long season, and the average fan would assume that, more than any other position player, catchers would tend to wear down as the days go by. Somewhere along this line is a breakeven point, where the diminished performance of the starter is exceeded by the rested, inferior substitute. Has the league settled into this balance?

At first blush, it would seem that catchers do tail off rather heavily compared to their unarmored comrades. A very simple chart:

Common wisdom dictates that the pitchers get a head start on the league, while the cold air saps the urgency out of foul balls. The effect is perhaps not quite as dramatic as advertised, but for catchers at least, the September swoon is noticeable, even compared to the league as a whole. All those bumps and bruises accumulate, all those hours spent squatting in a position that no human being does for more than five minutes voluntarily. It makes sense. But is it actually true?

September baseball isn’t real baseball, the careful, measured sport we depend upon. For the decent teams, when seasons hang in the balance, all measure of decorum and patience get tossed aside; no days off for the future when there may be no future. For the struggling teams, and for the very best, the effect is the opposite: it’s instead a time to employ that strangest of baseball rules, the 40-man roster. This is where we discover a problematic aspect to our tidy narrative above: rookie catchers aren’t good hitters. The league, as rookies, puts in a wRC+ in the high 80s; catchers come in a full 10 points south of that figure. And September call-ups fare even worse: over the past five years, rookie catchers who only played in the final month totalled a collective 63 wRC+. They’re not only defensive-oriented backups, they’re ones who aren’t ready.

The real question is not whether catchers are worse in September, but whether the guys who have ground out the whole year are worse in September. By comparing each player’s offensive output in September to their season total, it’s not difficult to measure:

Over the past decade, catchers who cross the threshhold into what many people would consider “overuse” are the ones most likely to maintain offensive consistency. (Only Yadier Molina and his 15 wRC+ in the final month of 2015 prevented the group from actually improving down the stretch.) From there it trails down to a level 95 percent productivity. There are lots of variables involved here, not least of which is regression: players who overperformed the majority of the season would be more likely to be given the chance to fall back to earth, while underperformers are more likely to see their playing time lost to the team backup or Double-A fill-in. Still, there seems to be very little evidence on a league-wide level that the grind of being a catcher has much effect on their bats as the seasons turn to fall.

The sample size of month-long splits, even over the course of 10 years, is great enough to make any inquiry into individual performance problematic. Brian McCann, for example, has struggled in the final month as a rule, putting up only 80 percent of his usual offensive performance, and only twice out of 10 times improving on it. But this seems true regardless of whether he starts 90 games behind the plate of 120, and either says more about Brian McCann than it does about catching, or more likely, says nothing at all. It should merely be noted that Salvador Perez, the modern champion of overtime, has performed 40 percent better in September than he has as a whole, making him a champion of the stretch run as well.

This research is rough, and incomplete. It fails to take into account a catcher’s defense over time. Given that catcher framing appears to decline with age, it would also seem to decline with fatigue, but this is an assumption to be tackled another day. Nor does it tackle the long-term effects of overwork. The heyday of hardworking catchers, the 1970s, left a graveyard of replacement-level backstops in the 1980s and early '90s; a glance at the career path of one Randy Hundley, who started 448 games behind the plate from 1967-1969, is a sobering anecdote to counteract the good feelings of Perez fans last paragraph.

The caution many modern teams display toward their starting catchers, whether out of genuine concern or a better understanding of defensive value, is still gratifying. But if you’re a San Francisco Giants fan finding yourself nervous about Buster Posey’s workload as the postseason approaches, don’t worry. Buster Posey in September will probably still be great, because he’s Buster Posey, tired or no.

Thank you for reading

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I tried to investigate the same subject in one of my first BP pieces. Not sure every effect I found was statistically significant, but here it is.