If the Ball Isn’t Juiced, Then What Explains the Homer Surge?

Ringer staff writer and FanGraphs podcaster Ben Lindbergh exhibited some excellent reporting earlier this week, obtaining MLB’s study on the properties of its baseballs. It was a valiant attempt to learn whether the ball itself is responsible for the game’s curious home-run surge. Last year, Lindbergh and Rob Arthur went as far as dissecting some balls to study, wondering if juiced balls were the “new steroids.”

This year, the home-run rate on fly balls is 12.8%. Last year, it was the same. Both marks are the highest on record and certainly grab our attention.

Lindbergh has been on the case of the juiced ball for a while. In light of that fact, it’s somewhat unsatisfying that the report he obtained doesn’t support the juiced-ball theory. While this conclusion naturally depends upon the assumption that MLB’s study was conducted in good faith, Dr. Alan Nathan — friend of FanGraphs and professor emeritus of physics at the University of Illinois — was asked by MLB to review the research as an independent source. Lindbergh spoked with Nathan.

“Quite frankly, I was disappointed at that result, because I was hoping I’d find something,” Nathan, who was compensated by MLB for the time he spent studying the BRC report, tells me by phone. However, he says, “I saw nothing in the data that was presented that suggests that the ball has been altered at all.”

Wrote Lindbergh in conclusion:

If the spread of dingers has less to do with COR or seam height than with a wave of Yonder Alonso–like breakouts by hitters who’ve tailored their swings to lift low pitches, then pitchers could exploit those uppercuts by raising their own sights … The historic performance we’ve seen since mid-2015 still supports at least a little skepticism about the true roots of baseball’s home run revolution; without witnessing the tests, we can’t consider these findings definitive. But the “juiced ball” hypothesis does seem much less likely than I thought it did two days ago. “It has every look of being suspicious,” Nathan says about the timing of baseball’s big-fly bailout. “But as I said, there’s nothing I could find that suggests anything amiss.”

Everyone interested in the home-run surge has a theory about its causes.

Eno Sarris recounted a story to me via Slack about an experience he had while in the Detroit Tigers’ clubhouse earlier this season. (We’re protecting the name of the players involved since it wasn’t a formal interview.)

[I] made an offhand comment about the ball being juiced. The pitcher I was talking to leapt from his chair, made a beeline for a crew of hitters sitting at the other end of the clubhouse and then yelled to me: “Come over here and tell them what you just told me!” I sheepishly recounted the joke, and then was implored to recite the evidence that the ball was juiced. While I did so, particularly when I pointed out that the explosion of home runs per ball in play came in the middle of a month in August of 2015, the pitcher was demonstrative. “See!” he said. The hitters were not quite convinced. “In a court of law, you’d have to open up the ball,” one said.

Lindbergh has opened up baseballs. So has MLB. It appears, however, that the answer isn’t found within.

But perhaps there isn’t one big unifying theory that explains the surge. Perhaps there are many small answers, many that have been discussed and written about, which add up to an explanation for this home-run surge. It’s a power spike that has allowed the game to escape from the suppressed run-environment of earlier this decade, despite the continuing rise of strikeouts.

If you’re familiar at all with what I’ve written to date this year, then it should come as no surprise I believe the fly-ball revolution explains part of the home-run surge. And we’ve continued to see a number of hitters increase their fly-ball rates early this season. But at a global, league-wide level, it’s complicated. As Marc recently wrote at USS Mariner, we’re not seeing an increase in fly-ball and line-drive rates despite a number of public comments by hitters to the effect that they’d like to add loft to their swings — and some of those hitters demonstrating real change. What’s going on?

My idea is that we have two large-scale trends that may be counteracting each other. The past several years have seen more and more low pitches thrown, as the strike zone expanded downward. This year, 54.9% of pitches are in the bottom third of the zone or below. Last year, it was 51.3%,* and in 2015, 51.2%. Back in 2012? Just 48%. Pitch height and ground balls are correlated; throwing a pitch lower makes it more likely to be hit on the ground. This study by Gerald Schifman goes into a lot of detail, but for now, the point is that with more and more low pitches (and more and more non-fastballs), we should be seeing a spike in ground balls. As we just saw, we’re not.

Marc is on to something. While we haven’t witnessed notable league-wide changes to fly-ball rate — even as players endeavor to lift the ball — we are perhaps seeing something of a cancellation effect of balls down in the zone, perhaps because of changes to swing plane. Back in March, for example, Jeff Sullivan found that more home runs were being hit down in the zone last season and suggested a cure for pitchers.

If we take for granted that the optimum bat plane for most hitters is a slight uppercut swing — which allows the bat to meet a pitch traveling on a slightly downhill trajectory from the mound — then such a swing should be effective against pitchers lower in the zone. And the average launch angle on pitches in the lower third of the zone and below has increased from 4.6 degrees in 2015, to 5.2 degrees last season and 5.3 degrees this season, according to Statcast data via BaseballSavant.

When evaluating the overall quality of contact, consider expected wOBA by year via Baseball Savant:

2015: .309
2016: .316
2017: .316

A juiced ball could influence many things, but not launch angle. Batters, it seems, are making better quality contact more often — even if that conclusion isn’t immediately revealed in the league-wide fly-ball rates. Maybe it explains a large part of what’s going, maybe just a small part. But it explains some of it.

Maybe another part is simply that these guys — these young guys — are really good. We have a historic class of under-25 shortstops adding power production at a position that usually doesn’t include much of it. Since 2015, Kris Bryant has debuted, Freddie Freeman and Bryce Harper and Manny Machado have broken out. Francisco Lindor and Rougned Odor have found new power levels.

Players 25 and under have produced a 13.6% HR/FB rate since 2015. All other players have produced a 12.0% mark. Overall the game has a 12.2% HR/FB rate since that time.

It has typically been thought that young players grow into their power. Perhaps now they’re growing into it earlier and earlier — or, alternatively, arriving to the majors with the tool. While players under 25 represent just a small part of the playing population, this class of player is perhaps another contributing factor. And it could be the first of a wave of a new type of player who arrives more advanced, more powerful, to the majors.

Another idea discussed here and elsewhere is that the strike zone is shrinking, perhaps compelling more drive-able pitches to be thrown. According to Jon Roegele’s research, it shrunk for the first time on record in the PITCHf/x era last season, and as Jeff noted in late March looked to be shrinking again in spring training. Perhaps pitchers, consciously or subconsciously, are throwing more pitches to hit? Lindbergh found the average height of a fastball is up, though pitchers are actually throwing fewer pitches in the zone. (More breaking pitches are being thrown below the zone,) Perhaps pitchers are being compelled to throw more hittable fastballs? Major-league hitters slugged .443 against two- and four-seam fastballs in 2015, and the homer surge began in August. MLB hitters slugged .457 against two- and four-seam fastballs last year, and .454 to date this year.

What else might be going on?

With the data tools at our disposal today, I’d like to think — and hope — journalists, bloggers, and journalist-bloggers, could better detect when something is amiss in the game, whether it’s with the game ball, or elsehwere. The media world did not do so well a generation earlier in regard to PEDs.

Given that MLB has one of the stricter drug-testing and penalty systems in place among professional sports leagues, I have not heard many suspect PEDs explain the game’s home-run surge. (Well, plenty seem skeptical of Eric Thames, although they probably shouldn’t be.) It would be naive to think some players will not seek an edge. Starling Marte, for example, just received an 80-game PED suspension. But there’s no evidence that PEDs have returned in a significant way. It would be naive to think drug creators cannot eventually get ahead of the drug testers. But hopefully that’s a story for another day, another era. However, never is a long time.

Even without chemical assistance, modern strength training and conditioning is helping batters become stronger, just as it’s probably helping pitchers to throw harder.

Pitching injuries are on the rise this decade, and the Tommy John outbreak has been categorized as an epidemic. But velocity has continued to rise, the average fastball is sitting at 93.0 mph this season for the first time in the PITCHf/x era, and K-BB% rates are also increasing. So overall arm talent doesn’t seem to be in decline.

The change doesn’t seem to be tied temperature spikes, which Lindbergh and Arthur explored last year, or new ballparks. SunTrust Park is the first new ballpark to open since Marlins Park in 2012.

There could be a lot of things going on, but the idea of the ball being juiced appears to have been ruled out, at least for now, along with some environmental and ballpark factors. Lindbergh should be applauded for looking for an answer. The media should always be looking for answers. Perhaps the answer is a series of small causes, not one big one. Or perhaps there is a significant issue that we are missing that will eventually be discovered. The truth is out there.