There are a few trends that have become something like the natural laws of modern baseball: Unencumbered, strikeouts go up, game times get longer, and fastball usage goes down. MLB has stepped in on the game time front and has attempted to work on the strikeouts indirectly, but the fastball graph is still looking like a smooth, if slow, downward slope.

Despite ever-increasing velocity (a borderline natural law itself that has recently begun to hit a plateau), fewer and fewer of the pitches thrown in major-league games are pure fastballs. In 2013, 56.9% of all pitches were either four-seam fastballs or sinkers. In 2022, that rate dipped below 50% for the first time since the advent of pitch tracking in 2008, and in 2023, it has dropped further to 47.3%.

Especially since Statcast began providing more granular data on pitch qualities in the mid-2010s, teams and players have leaned into a cycle of spin. In particular, sliders have become a much more frequently used weapon. The prototypical slider is designed to look like a fastball and sit in a velocity band just below the fastball but swerve down and to the pitcher’s glove side just before reaching the plate. Sliders made up 14.3% of MLB pitches in 2015, the year Statcast data became public.

In 2023, they have proliferated so thoroughly that there are now two categories where there used to be one: sliders and the newly designated sweepers. Put together, they make up 22% of MLB pitches, and that’s before considering the also-related cutters that have similar but less extreme movement profiles at fastball-esque speeds.

“The way guys are throwing breaking balls now, there’s a reason, right?” New York Mets outfielder Mark Canha said. “There’s a reason they’re throwing sliders more often. They’re harder to hit.”

Last season, MLB hitters managed a .212/.265/.355 batting line against sliders and sweepers, whiffing on 34.2% of their swings. Against fastballs, their line was .262/.346/.425, and they whiffed only 19.7% of the time.

“There’s a funny saying that a lot of baseball people say,” Canha said. “‘If you want to get better at hitting the slider, hit the fastball.’ And the idea is when they throw a fastball, don’t miss it.”

That tongue-in-cheek advice is getting less applicable every year. And if the trend continues, it might get turned on its head. Sliders — long dubbed “secondary” offerings alongside changeups, curveballs and the like — could theoretically overtake fastballs as the primary piece of the MLB pitching puzzle.

The days of sitting fastball are over

Investigating the beginning of the slider sea change, you could point to Jacob deGrom and Max Scherzer, purveyors of the era’s deadliest sliders. You might look to Chris Archer, Patrick Corbin and a slew of relievers who carved out significant big-league careers by leaning heavily on sliders and other breaking pitches. Or you might look to Clayton Kershaw, who developed a generational slider after he reached the big leagues, then gradually elevated it in his repertoire.

The late-diving pitch, which verges on cutter status, first nudged past Kershaw’s fastball in usage in 2018 and has overtaken it entirely since 2021. This season, the future Hall of Famer has gone slider with 44% of his pitches and fastball with just 38%.

Another prominent ace who might know something about making life difficult for hitters, two-way Los Angeles Angels star Shohei Ohtani, has made the sweeper his go-to pitch. You might remember it striking out Mike Trout at the climax of the World Baseball Classic, and Ohtani is using his sweeper more than 42% of the time in 2023 MLB action.

No success goes long without mimicry, so it’s fitting that a huge swath of the league has gradually chosen to give their breaking balls more play. This season, seven entire teams have fastball rates under the 45% mark, with the San Francisco Giants persistently encouraging pitchers to use their sliders. In fact, 32.5% of all Giants pitches have been sliders or sweepers, with starter Anthony DeSclafani using his slider more often than his two fastballs combined.

Maybe the most striking evidence of the mentality change is how pitchers behave when they need a strike the most. Fastballs are not just being filtered out of advantageous two-strike counts. They are being replaced on first pitches, in pivotal 1-1 and 3-2 counts, and when the pitcher is behind. As recently as 2017, 60% of those moments led to fastballs. In 2023, it’s almost a coin flip, with 51.2% of those counts drawing heaters.

To many hitters and hitting coaches, such as the Philadelphia Phillies’ Kevin Long, the idea of “sitting fastball” as a go-to strategy is already a thing of the past.

“We know it's a big part of what's happening,” Long said of the slider surge. “So we prepare more for offspeed. We do more breaking ball stuff than we've ever done before. And we try to do the best we can to make sure we're in tune.”

Right now, preparing hitters for breaking balls means using specialized machines — such as the high-tech, pitch-imitating Trajekt Arc — to get a handle on visualizing what might be coming. It also means studying a more variable playbook of tendencies.

“They're well aware of what pitches they're gonna go to, when they're going to use it, how they're going to use it,” Long said of his hitters’ understanding of opposing pitchers. “Do they throw it for strikes, and where are they locating it? Where does it need to start? Where does it need to finish? All those things are part of their process and what they're throwing into their computer system.”

Still, knowing that a barrage of sliders is coming helps only to a point.

“It’s difficult, especially when you’re talking about not just more sliders are being thrown,” Canha said. “They’re better sliders.”

And therein lies the reason for the pitch’s unabated momentum.

How much further could sliders go?

Given his druthers, Driveline Baseball’s Chris Langin would start every pitcher’s toolkit with an electric four-seam fastball. That’s what put wind in the sails of epic careers for Justin Verlander, Gerrit Cole and the like, and it might do the same for young pitchers such as Zac Gallen and Spencer Strider.

“If you can get a guy with a good four-seam fastball — and when I say good, I mean, like, at least a 60-plus-grade four-seamer,” Langin said, using the 20-80 scale scouting parlance for significantly above average, “that guy generally can go a really long way in this game.”

“Even though that pitch frequency has gone down so much, the ability to have that specific pitch and throw it frequently just leads to a lot,” he said. “It seems to be the biggest propeller of your outlier-esque pitchers, those guys who put up 4-plus WAR seasons, they just almost always seem to at least have that four-seamer.”

Problem is, that’s just not happening for most pitchers.

The vast majority of professional athletes don’t have a Verlander fastball or anything approaching Kershaw’s pitches. But they do have some of the same basic decisions to make. What can they throw that gives them the best opportunity to get to the majors, stay in the majors and succeed in the majors?

A lot of them are finding that fastballs, even if they are the best answer, simply aren’t their best answer.

At the same time, access to data and video has helped pitchers and coaches master the spin necessary to create great breaking balls. More pitchers have been learning how to throw sliders and how to amplify their sliders by studying the best ones already out there. In doing so, they’ve gained confidence that pitching, even as a starter, doesn’t have to be a color-by-numbers exercise of mostly fastballs, a few breaking balls and an occasional changeup.

In part through working with MLB front offices and a cavalcade of envelope-pushing private instructors at facilities, such as Driveline and Tread Athletics, professional hurlers are better than ever at assessing their own stuff and seeking improvements. The slider’s ascent toward primary pitch status is just one of the ripple effects of that process, broadly known as pitch design.

Longer-term, the question is whether the fastball, baseball’s constant standby, will eventually fall behind its flashier cousin and take on a supporting role.

‘The last piece of the puzzle’

The Pittsburgh Pirates, who have sent a number of pitchers to Tread, are among the teams throwing the fewest fastballs in the game. Many of their starters have leaned into a slider-heavy approach under pitching coach Oscar Marin. Having seen some of the positive results, manager Derek Shelton said he is ready to believe that sliders will one day overtake fastballs.

“What we're seeing in the industry is throw your best pitch as many times as possible to get as many outs as possible, and then we'll figure it out after that,” Shelton said.

That approach sometimes creates starters who pitch in a fashion you might associate with closers — hard fastball, bendy breaking ball, mix them evenly, repeat. Other times, it simply opens up more avenues. Deemphasizing the need for a single, winning fastball helped the Pirates’ Mitch Keller make the leap from inconsistent former top prospect to young ace.

Tyler Zombro — a Tread instructor who is making a comeback as a pitcher in the Rangers organization following a terrifying injury — said his process for helping a pitcher build his plan has shifted away from fastballs and toward movement-based pitches. He will often begin by trying to locate a pitcher’s natural talents, such as how they release the ball or what type of movement their max-effort throws have.

“So from that standpoint, I look at the fastball as really the last piece of the puzzle,” Zombro said, “where I would say, you know, three, four years ago, that used to be the first thing I was looking at.”

Zombro used Keller as an example of a pitcher who opened up his options by adding a cutter. Pure fastballs — four-seamers and sinkers — almost always run to the pitcher’s arm side on their way to the plate. Cutters, by contrast, tend to have what is designated as “zero” horizontal movement, which still looks like abnormal movement toward the glove side to hitters conditioned on regular fastballs. With the cutter in his mix alongside a sweeper and a sinker, for instance, Keller can follow Zombro’s advice to “own the zero line.” He can throw the hitter off by veering pitches to either side of that line, whereas a pitcher trafficking mostly in a typical fastball will have pitches flying off in only one direction.

How we categorize and tally cutters might decide the answer to the fastball vs. slider question on a technicality, but the function of that pitch — essentially half-fastball, half-slider — hints at a deeper understanding of why this conversation is happening at all.

There are examples all over the place of pitchers finding more success with cutters, either as go-to fastballs or as complementary options. Corbin Burnes, the Milwaukee Brewers’ ace who won the 2021 NL Cy Young, famously scrapped his four-seamer for a cutter en route to stardom. The Rangers’ Nathan Eovaldi has posted his best seasons, perhaps not coincidentally, when he has utilized his cutter the most — keeping hitters off a fast but arrow-straight four-seamer.

Langin, Driveline’s director of pitching, pointed to the San Diego Padres’ Joe Musgrove as another example of a pitcher who needed something to use, like a fastball that didn’t act quite like a fastball.

“He's found a route to more or less throw these hybrid type of pitches that aren't considered traditional fastballs, but you can use a cutter in an extremely, extremely similar fashion as a replacement for a four-seam fastball,” Langin said.

What do you throw under pressure?

With the cutter included in the slider category, Zombro sees a future in which fastballs take a back seat, though he worries about the health question that tags along behind any pitching innovation. He and his Tread colleagues put a lot of effort into aligning pitchers with strategies that fit their bodies’ natural movements. Without having seen it in action, it’s tough to say whether boosting slider usage even higher carries more risk.

“I can't just tell a guy who's got a really good slider to throw 80% sliders because you don't know how his body is going to respond to that,” Zombro said. “So there's — and this is where I think the game is gonna grow a lot — there's a lot of physiological components that come into this.”

Still, Langin and many other eager proponents of pitch design believe fastballs will continue to reign for different, strategic reasons. The surge of breaking pitches will plateau, they theorize, because most pitchers can’t quite maintain the necessary balance of steady strikes and dastardly punchout pitches without a certain fastball dosage.

Baltimore Orioles starter Dean Kremer was part of the wave of pitchers who added a sweeper, in part because it gave him a swing-and-miss offering for right-handed hitters. Yet he said fastballs were an important baseline.

“I think although there are a lot more offspeed being thrown now,” Kremer said, “the fastball is still necessary for a lot of guys.”

One of the reasons goes back to the matter of what to throw under pressure. Right now, sliders are still more likely than not to be used as chase pitches, bait for hitters to make bad decisions. If sliders were a bigger part of the equation, could the typical pitcher get ahead in the count with offerings usually thrown at the very edge of the strike zone or out of it entirely? Could they command the movement of those whirling, diving darts enough to beat a discerning hitter on 3-2?

Teams are working hard to help pitchers funnel their best bat-missing stuff into the strike zone, confident that the difficulty of catching up to it and making solid contact will be enough to win the day. But Langin questioned whether pitches that often succeed by dropping out of a hitter’s reach could really carry the load if forced to land in the strike zone far more often.

“A lot of those outcomes are dependent on guys swinging out of the zone. And you've got to be able to kind of set up that pitch in some manner,” Langin said. “If you actually go through an actual simulation of what happens if guys throw 60% or 70% sliders, it just seems highly improbable that you'd be able to throw that pitch in the zone under 40% of the time or whatever and sustain the chase rates and the whiff rates.”

So far in 2023, fastballs are thrown in the strike zone about 54% of the time, while cutters are in the zone 51% of the time, and sliders and sweepers have a 45% zone rate.

The data on those in-zone pitches — the ones that would likely be called a strike even without a swing-and-miss — can’t give us the full picture of the hypothetical but does give us a sense of how close the pitches might be to diminishing returns. Right now, in-zone cutters wind up with almost identical numbers to the more traditional fastballs, while sliders and sweepers still earn markedly better outcomes.

So wherever you think the lines might stop, don’t expect the fastball’s slide to end quite yet.