Are Pitchers Being More Aggressive With the New Ball?

On Friday night, Jacob deGrom pitched brilliantly against the Nationals in what was the best start of his career: nine innings, just two hits allowed, no runs, no walks, and 15 strikeouts. By traditional game score, his outing was the finest pitching performance of the young 2021 season, at a cool 98.

There’s a lot to marvel at with deGrom. He has a 0.31 ERA and 0.85 FIP through four starts and is leading all of baseball in WAR. What’s interesting to me, though, is how he attacks the strike zone. His 4.5-point increase in overall Zone% might look modest on the surface, but that’s not always the best measure of aggression: Pitchers frequently want hitters to go fishing. What’s more interesting is that his four-seam fastball zone rate has jumped by 10 points year-over-year, good for the second-largest increase in baseball. deGrom dares hitters to hit his 100 mph fastball, and, well, they just can’t.

This is not solely an investigation into deGrom; he is just emblematic of what might be a league-wide trend. Through games played on April 24, 55% of four-seam fastballs were in the strike zone — a figure that, if it holds, would mark the highest four-seam Zone% of the pitch tracking era. Though more four-seam fastballs have been thrown in the strike zone in recent years, 2021 represents a statistically significant tick up, with a p-value of practically zero.

League-wide four-seam fastball Zone% had been extraordinarily stable over the last eight years: From 2013 through ’20, it never rose above 53.7% and never fell below 53.1%. Even looking at this rate on a month-to-month basis, it did not vary significantly: Since 2018, the highest monthly four-seam Zone% was 54.6, in August of that year, which is the only month that falls within the 95% confidence interval for the “true” 2021 four-seam Zone% thus far.

There could be all sorts of reasons why we’re seeing this league-wide shift — and we can’t totally rule out noise — but my first thought was that pitchers are feeling more comfortable attacking the strike zone with the new baseball. Indeed, we’re seeing this uptick mostly occur among fastballs thrown at least 95 mph. If you’re worried less about giving up the long ball, why not pour more hard fastballs in the zone? After all, pitchers are throwing harder again this year, so if fewer non-squared-up baseballs are going for extra-base hits, then they may feel as if they have much more margin for error and can throw more pitches in the zone.

In fairness, we won’t be able to answer the question; I can only offer trends and hypotheses. But what I can evaluate are hitters’ results on the fastball, demonstrating that it has become “safer,” so to speak, for pitchers to throw their fastballs in the zone. That still might not be the reason why they are doing so, if there is a reason at all, but it’s certainly a plausible inference.

First, consider this: In 2020, when a four-seam fastball thrown at a velocity of at least 95 mph was put in play, batters posted a .403 wOBA; in 2021, that’s down to .374. A 30-point wOBA difference is helpful for pitchers by orders of magnitude. Think about it this way: deGrom faced 804 hitters in 2019, our most recent full season. A 29-point change in wOBA is worth roughly 18.5 runs over 800 plate appearances. From a pure runs-to-wins standpoint, that’s almost a two-win improvement!

This raw year-over-year look is skewed, though. It’s April, and the cold weather prevents batted balls from flying as far as they do during the middle of the summer. We didn’t even have baseball last April. So to analyze more accurately what we’re seeing now, let’s compare April 2021 to past Aprils:

While it’s hard to project out what this year’s full-season wOBA will be, the average increase from the April figure is about 15 points. That would suggest that 2021’s ultimate mark will fall somewhere around .389, though we’ve seen April-to-full-season jumps anywhere from five to 23 points. If we assume the average 15-point increase, though, that resulting .389 wOBA would represent a 14-point drop from 2020 and a 17-point drop from ’19. That’s still the difference of roughly 10 runs per 800 batters faced.

The difference is even more pronounced if you move deeper into deGrom territory. On fastballs thrown at least 98 mph, we’ve seen hitters produce at a wOBACON (.292) that is 24 points lower than their April 2019 mark (.316). We begin to get into some small sample weirdness here, but, if it holds, a 24-point improvement on these types of events has enormous implications for pitchers of the hardest-throwing variety.

The best way to see if there’s something different happening is to look at the underlying quality-of-contact numbers. Things are a bit funky here early on, likely due to the new ball: As both Ben Clemens and Justin Choi covered, it has a propensity to yield higher exit velocities without traveling as far, creating a huge disparity between the league-average wOBA and league xwOBA. This difference is particularly notable on batted balls.

Thus, I adjusted the disparity between the league-wide wOBA and league-wide xwOBA on batted balls to account for any year-to-year differences in the data. So while it seems rather outrageous that hitters are underperforming their quality of contact by 50 points on four-seam fastballs thrown at or above 95 mph, the adjusted figure still suggests that these types of events are significantly better for pitchers than they were in our most recent full season’s worth of data. This holds true even when comparing the data to April 2019.

While the average exit velocity on these events has increased notably, the overall performance on these pitches is way down, which would make it much more advantageous for pitchers to dare hitters with a plethora of fastballs in the strike zone. Indeed, when we consider all four-seam fastballs at 95 mph (not just those which have yielded contact), we find that the total run value on these pitches has spiked dramatically:

Given the increase in four-seam zone rate, it seems as if pitchers are already catching on to this huge increase in value, and it makes sense why they would. In an era where the three true outcomes dominate, a slightly more deadened baseball would prove to be a huge boon to pitchers, given that strikeouts are almost entirely independent of a change in the ball itself. In the early part of the 2021 season, that seems to be the case so far, and it seems like pitchers like deGrom have already adjusted.