Friday night was the rockiest game of the season for Astros right-hander Brad Peacock. From the jump, it was clear that he didn’t have good command, and he limped through four innings of what turned into an ugly loss for the Astros. His transition from short relief to a starting role has been slow and uneven. He’s still yet to crack 90 pitches in any start, and has two good and two bad outings as a starter.

On the whole, though, the season has been a huge success for Peacock, the former Nationals prospect who (just a year or so ago) seemed likely to never find his way to consistent success in the majors. In parts of six previous seasons, Peacock had been worth -1.1 WARP, but this year he’s posted a 91 DRA- and has been worth 0.7 WARP in under 35 innings. A sample that small might tempt one to dismiss the performance as fluky, especially since Peacock’s cFIP so far is an uninspiring 103. On the other side of the scales, though, sit a career-best 45 percent ground-ball rate and an eye-popping strikeout rate (48 of 143 batters faced, even after fanning just one of 18 batters Friday night), plus the fact that he’s allowed just one home run.

The evidence that something real is happening here runs deeper than that. Specifically, two new things he’s doing are driving those positive results. One is that he’s dramatically altered and improved his slider. The other, and perhaps the more interesting, is that his average spin rate on four-seam fastballs has risen by 87 revolutions per minute over last year’s total. Of the 197 pitchers who have thrown at least 150 four-seamers in each of the last two years, Peacock’s increase in spin rate ranks 13th-largest. He’s gone from 2,260 RPM (about 0.2 standard deviations below the average) to 2,347 (about half a standard deviation above it), and in so doing he’s more than doubled the rate at which he’s induced whiffs on that pitch.

Brad Peacock, Swinging Strike Percentage on Four-Seam Fastballs, 2015-2017


Swinging Strike Percentage







The increase in spin rate is interesting, because it just isn’t something pitchers can often, intentionally, or reliably do. Kyle Boddy, the founder of Driveline Baseball, did an interview last month during which he was asked if the company has found any drill or conditioning technique that specifically improves spin rate. He said they haven’t, and that jibes with the general consensus around spin rate so far—that it’s fairly intrinsic. If you want a hitter to increase his launch angle, or even his exit velocity, no problem. That can be trained. Velocity can be increased. Spin axis and resulting movement can be changed. Spin rate itself, though, changes only unpredictably, and usually as a result of other changes.

If Peacock found a way to alter his spin rate, and especially if he did so consciously, it would be big news. Peacock spoke to BP’s David Brown about the change last Thursday in Kansas City. “Not on purpose,” Peacock said, when asked whether he’d intentionally bolstered his spin. “I didn’t know [about the change in the spin data].”

Peacock did make a change, though, under the tutelage of Astros pitching coach Brent Strom. It might have unlocked his best delivery, and by extension, the best versions of his two top pitches, even if only by accident. “Strommy’s always preached to me, ‘Throw your backspin four-seamer.’ … [He] was working with me a lot in the bullpen, starting in spring training, trying to perfect what I could with the fastball,” Peacock said.

But what Peacock discovered, after a career of using an over-the-top delivery, was that he was more comfortable and more effective when he lowered his arm slot:

Strom actually liked me better coming over the top—because it creates backspin—but it just feels so much better coming out of my arm at three-quarters. You can’t even tell on video that I did it but, to me, I feel something. I’ve noticed that [the four-seamer] has got a little more tail on it—not like true four-seamers that are straight. It’s got a little tail to it and I like that a lot. It’s kind of got a rise effect to it. It’s not actually rising, but it has that appearance. It stays on the same plane, it doesn’t drop as much and has more of a side-to-side movement. That’s a big key.

What Peacock is describing is exactly what’s happening. His four-seamer has a bit less rise to it than it had in the past, but considering that it’s coming from a lower arm slot, hitters are almost certainly perceiving more rise than they had in the past. At the same time, the pitch is moving horizontally more than ever, freed by the change in spin axis that occurred naturally when Peacock lowered his angle.

That makes the pitch much more of a weapon (and explains why he’s getting much weaker contact on it than he has in the past). It’s also notable that Peacock’s perception of what’s going on so nicely fits what’s really going on. It’s a reminder that part of his success, even beyond the concrete physical changes, is Peacock’s improving self-knowledge. He’s also correct about the changes his slider has undergone this year—changes that helped his opponents' whiff rate on that offering jump from 10.8 percent to 19.7 percent.

“The slider has been more side to side instead of up and down. I changed my slider last year, in the middle of the year at Triple-A, and this year. The slider was really getting hit, so I had to make another adjustment there. My buddy, Jordan Jankowski, showed me his slider, and I brought it into my next start and it worked out,” Peacock explained, adding that Jankowski inspired him to change his grip, and that naturally begot his change in arm slot.

That’s where things really begin to come together. It would seem, based on all the available evidence, that Peacock almost backed into the discovery of an arm slot that might have worked better for him all along. The changes to his two most important pitches that opened up for him after that mechanical alteration are significant. Last year, even after switching the grip on the slider, he had a hard time throwing the slider and the heat from the same slot.

As he worked throughout the spring and smoothed out that problem, he found the perfect balance—a slider and a fastball that stay on the same vertical plane for a long time, but with more velocity separation than the two pitches used to have, and with enough movement on each pitch to keep them off opponents’ barrels, when they make contact at all. He’s also giving batters less time to distinguish the pitches by getting better release extension: he went from having almost perfectly average extension on fastballs last season (6.14 feet) to roughly one-third of one standard deviation better than average this year (6.29 feet). On top of the sheer impact of that small improvement, that kind of change suggests that Peacock’s delivery has become more efficient since he changed that arm slot.

This is, for my money, a fascinating look at what can happen for pitchers when talent, good instruction, new-age information, and the humble maturity bestowed by years of adversity converge. Speaking of that adversity, Peacock had one other thought as to how he’s made such big progress.

I’m healthy, man. I had back surgery in ’15. Last year, I was coming back from back surgery and still had kind of a rough start to the season, but started feeling better towards the middle. And felt right toward the end of the season. It was nice to have a normal offseason instead of rehabbing. I was able to lift my normal amount, and that was big. Being healthy.

It’s impossible to say whether a merely healthier, but not fundamentally changed, Peacock would be having this kind of (admittedly minor, so far) breakout, but it’s worth mentioning. There are a number of interesting cases like Peacock’s this year, and others a bit less like his, but sharing that trait of increased fastball spin. High fastball spin rates are awfully nice things to have, so we’re going to dig more into what permits pitchers to achieve them. For Peacock, the answer seems to be some of everything—an arm slot change, a grip change on a secondary pitch (who knows? It might have retrained his muscles ever so slightly), a mentality centered on applying backspin to the ball, and good health.

Thanks, or thanks again, to David Brown for speaking to Peacock.

Thank you for reading

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Really interesting, thanks, I loved this.
Thank Dan Straily as well....last season the Astros had to essentially give Straily away when he ran out of options,only to see him blossom elsewhere. Peacock faced the same situation this season, but Houston remembered what happened with Straily and were sure to keep Peacock as a long reliever.