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Astros Sabermetrics: GBs & Pitcher Aging

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Discussing A Recent Sabermetrics Article On Groundball Pitcher Aging

Jonathan Dyer-USA TODAY Sports

"How are you going to accumulate a team W, if the pitcher doesn't get his W? You gotta have a W."--Hawk Harrelson debating sabermetrics with MLB Network's Brian Kenny

This article doesn't directly deal with the quotation, but it does address pitching.  A recent fangraphs article analyzed pitcher aging among extreme groundball pitchers.  Let's evaluate the article and consider implications for the Astros.

We here at TCB encourage our readers to become more educated about sabermetrics.  Therefore, if you are interested in more reading material, here is a fangraphs overview of the groundball pitching statistic.  And here is a Hardball Times article which used neural network analysis to suggest that pitch velocity and pitch movement are associated with pitcher injuries. Contrasting somewhat, another Hardball Times article finds no statistically significant relationship between pitch f/x variables, such as pitch type, and pitcher injuries.


Fangraphs recently posted an article by Eno Sarris entitled "Maybe Groundball Pitchers Are A Bad Bet."  The article is a continuing saga reacting to Bill James' assertions that groundball pitchers are overrated.  Two years ago, James gave his admittedly subjective opinion that extreme groundball pitchers are more susceptible to injury.  As noted by Sarris, Bill Petti and Russell Carleton showed that the data doesn't support the notion that groundball pitchers are subject to more or worse injuries.   Sarris' article addresses the second prong of James' contention--that extreme groundball pitchers are more susceptible to premature aging syndrome.  As a result, James contends that the aging profiles of groundball pitchers should be truncated. The hypothesis is that grounder pitches place more internal rotation pressure on the shoulder, leading to deteriorating performance over time which may not show up in injury data, but will affect aging data.

Sarris attempts to test this hypothesis, and surprisingly the data is consistent with the aging effect claimed by James.  Sarris examines a large sample of extreme groundball pitchers (>50% GB rate) vs. the overall population of pitchers.  His conclusion:

It certainly looks like James was right: right around 30 years old, ground-ball guys start to age worse. They add four runs to their peak RA/9 two years earlier than the average pitcher. The attrition is much worse, too: between 32 and 34 years old, 34% of all innings pitched leave the game, while 74% of all innings pitched by plus ground-ball pitchers disappear.

He goes on to point out that the number of qualified extreme GB pitchers over age 30 in 2015 was fairly sparse (three pitchers, two of whom won't pitch this season).  Interestingly, a number of big name GB pitchers (including Felix Hernandez, Jake Arrieta, and Jon Lester) have recently turned 30 or will exceed that age this season, again testing James' contention.

Why is this relevant to the Astros?  Well, let's start with Dallas Keuchel, baseball's premier extreme GB pitcher, who will turn 30 in two years.  If the notion of premature decline is correct, the implication could change our opinion about  the merits of locking up the Astros' ace to a multi-year contract.  In addition, the Astros have several extreme groundball pitchers in the minors and frequently take off-season risks on signing older groundball pitchers.  If you had dreams of Doug Fister reverting to his "No. 2 rotation form" from earlier in his career, this aging hypothesis may shoot holes in that hope.

Although this study is useful and perhaps suggestive of conclusions, I don't think it proves a linkage between premature aging and the mechanical process of throwing groundball-inducing pitches.  Although my evaluation is hindered because I do not have access to the study's data/samples, alternative interpretations of the data may explain the results.  In order to understand the contexts for comparing groundball pitchers, I used fangraphs' leaderboard to rank pitcher groundball results for ages 26-29 in 2008-2011 and for over 30 age in 2012-2015.

Consider the following alternative explanations.

One of the comments regarding the fangraphs article raised a good point: is it the ground balls/sinkers behind the early aging, or is it because extreme groundballers typically post lower strikeout rates than flyballers, giving them less margin for error if either their strikeout rate declines further or their ground ball rate deteriorates?

The "over 30" group of groundballers for 2012-2015, doesn't totally support this explanation, but there may be some kernals of truth.  This group of 50%+ groundball pitchers contains two strike out pitchers (Burnett and Liriano) and three pitchers with low strike outs (Hudson, Roberto Hernandez, and Maholm).   The latter three low strike out pitchers historically have been been low strike out pitchers, but pitched with some effectiveness well into their 30's.  Maholm and Hernandez have pitched with roughly the same effectiveness (mediocre) throughout their career.  Hudson, Burnett, and Liriano were good pitchers both in their 30's and earlier in their careers. This small sample is a mixed bag in terms of explaining the effect of strike outs on pitchers' ability to pitch past age 30.

However, there may be some truth to the contention that higher strike out rates provide a greater margin of error as the pitchers age.  Burnett and Liriano were both 94+ mph pitchers earlier in their career.  Like most pitchers, their velocity declined as they aged; however, their velocity began at a high level, which enabled them to continue to pitch in the 92 mph range after the decline.  As a result, their strike out rate after age 30 continued to be above average for starting pitchers. Jon Lester is another illustration of a high strike out groundball pitcher who adjusted as he aged.  At age 26-29, Lester produced a 52% groundball rate; however, for the period 2012-2015 when he turned 30, his groundball rate declined to 45.5%.  Pitch f/x for Lester shows that he dialed back his usage of the sinker in recent years, relying more on his 4 seam fastball.  This adjustment presumably was made possible by Lester's consistent 92 mph velocity and above average strike out rates throughout his career. Perhaps Lester felt that the effectiveness of his sinker was declining, or maybe he was adjusting to the defense behind him.  Lester's career groundball rate is below 50%, and it is unclear if he is included in the extreme groundball group in the fangraphs study.

Hernandez and (to a lesser extent) Maholm demonstrate that low strike out groundball pitchers can adjust to their declining velocity by improving their walk rates.   Both pitchers suffered significant declines in their velocity past age 30, but offset the declining K rate with better control.

Another possible explanation for the study results is that elite pitchers may be over-represented in the non-groundball pitcher sample.  Here is my hypothesis:

Elite pitchers (for example, pitchers whom regularly rank among the top 20 starting pitchers in the majors) are much more likely to be "survivors," pitching into their 30's.  They have the mix of plus pitches and run prevention skills that permit them to adjust to age-related deterioration.  Elite pitchers are not as heavily represented among extreme groundball pitchers, because their versatile repertoire does not produce extreme groundball rates (which the study defines as greater than 50%).

Since I haven't examined the samples of pitchers used in the study, I am uncertain about the accuracy of the first statement, above.  But my review of  groundball pitcher rankings makes me think that its correct.

Dallas Keuchel is a very unusual pitcher. (His 62% groundball rate, combined with a 8.38 K/9 rate, last year is almost unprecedented.)  Keuchel is an elite pitcher.  But few, if any, elite pitchers produce 60%+ groundball rates.  Indeed, as far as I an tell, few elite pitchers consistently produce groundball rates above 50%. (Brandon Webb is such an elite pitcher, and I have written about the comparison between Webb and Keuchel.) They might have a season or two at that level, but few elite pitchers have career groundball rates over 50%. (This is more difficult to verify for pitchers prior to 2002, since GB rate data is unavailable for earlier periods.)  In some ways, Andy Pettitte is a similar pitcher to Keuchel, but his career groundball rate is "only" 48.5% despite four seasons with a rate above 50%.

However, many elite pitchers are strong groundball pitchers; for example, among over-age 30 pitchers during the period 2012-2015, several elite pitchers produced strong groundball rates which fell below the 50% threshold: Wainwright (48.7%); Greinke (48.3%); and James Shield (46%).  Presumably, the study would classify these pitchers in the "All Pitchers" group which aged better than extreme groundball pitchers.   However, they are so strongly associated with groundball pitching that it seems odd to use their comparative performance to prove that groundball pitches lead to premature aging.  The 50% dividing line may be somewhat arbitrary for purposes of the study.

The findings of the study are interesting.  But I'm not convinced that groundball pitching is the cause of premature  decline among starting pitchers.  Factors other than the groundball pitches, themselves, may be the underlying reason for the study's results.

On the other hand, I think it would be foolish to completely ignore the implications for Dallas Keuchel's aging profile. The aging pattern of extreme groundball pitchers could make the Astros less comfortable in offering a lengthy contract extension. But, it's notable that Keuchel is unique in his combination of extreme groundball rates and strike out ability.  Arguably, there may not be comparable pitchers for evaluating his aging profile.

As for Doug Fister, so far his spring training results have been encouraging.  But, then again, it is spring training.  And it is early. Some of the focus this spring has been on Fister's fastball velocity.  As discussed above, the groundball pitcher's ability to maintain his fastball velocity may improve the capacity to make adjustments in his post age 30 career. My guess is that Fister, who recently turned 32, can settle into a Scott Feldman-like level of performance.  And that can be very useful, if not exciting.