Pitcher Aging Curves: Maintaining Velocity

Bill Petti published the first two parts of a series on pitcher aging. Bill’s first article focused on pitchers, in general, and the second was on the difference between starters and relievers. For the third installment, I’ll look at aging patterns for pitchers who maintain a relatively constant velocity from year-to-year.

From the previous articles, the average pitcher loses about 4 mph from their fastball from ages 21 to 38. In essence, most pitchers’ stats degrade as their fastball speed drops. Using the same methodology, I wanted to know how pitchers age when they don’t lose velocity on their pitches.

I examined pitchers who had their fastball speed within +/- 0.5 mph, compared with the previous season. I ran into a problem in that 60% of the samples saw a decline in pitch speed — while 40% stayed the same or increased. The initial curves mimicked the ones of the general population. To get a sample that had an equal number of samples above and below zero, I limited the number of samples below zero to a random sample equal to the number above zero. Those results were better.

Even with this change, though, I have two biases in the data. First, there were more declining values for older pitchers than for younger ones. This can be seen in the up and down of the velocity curve. The second problem was that there are larger velocity drops than velocity increases. For example, there are more -0.5 mph declines than +0.5 mph increases. When I tried to correct for each of these problems, the data sample size became too simple.

With that in mind, here are the fastball aging curves for pitchers who maintained a constant velocity:


— From ages 21 to 38, a pitcher’s FIP will basically remain constant with the pitcher’s peak years being from 25 to 30 years of age.
— Pitchers improve their walk rates until age 24 and then the number stays relatively constant.
— BABIP goes up steadily during a pitcher’s career, possibly because hitters become more familiar with pitchers.

Most of these curves are somewhat similar to the curves for all pitchers. The main difference is in the amount of change in values. Here are some select curves when the velocity is kept constant, compared with the curves for all pitchers.

(“= Velo” is for data with constant velocity)


— Even though the problems stated earlier show a decline in velocity, it’s a total of only -0.3 mph from ages 21 to 38. The entire population loses 3.7 mph.
— The constant velocity FIP initially drops and eventually goes back up to the age-21 value. For the general population, their FIP increases by 1.7.

Roy Halladay has been able to maintain a relatively consistent fastball speed over several seasons. Since 2002, his fastball speed has ranged between 90.3 mph and 92.7 mph with an average value of 91.9 mph. Over that same time, his FIP has a range between 2.20 and 3.79 and averaged out at 3.11. Here’s how is how Halladay’s K/9, BB/9 and velocity has aged when compared the average of players with constant velocities:

– While Halladay has been able to main a relatively constant fastball velocity over the years, it has bounced around a little from year-to-year.
– Whenever Halladay’s velocity moves up, his K/9 goes up and his BB/9 goes down. Likewise, when his velocity drops, his K/9 and BB/9 increase. Crazy how that works, right?

As long as a pitcher is able to maintain a certain velocity, like Halladay has, the player can generally pitch with the same results year after year. It’s only when the pitcher begins to lose velocity that he sees his stats degrade at a higher rate. Sure, vvelocity isn’t everything with a pitcher — but it’s important. Pitchers who maintain their velocity don’t really “age” — they stay the same.

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Jeff writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won three FSWA Awards including on for his MASH series. In his first season in Tout Wars, he won the H2H league. Follow him on Twitter @jeffwzimmerman.

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