How to prevent SB? It's about much more than a strong arm

Major League Baseball's well-received round of rule changes in 2023 aimed to bring the stolen base back to prominence. It has, and then some.

The last three seasons have seen the three lowest caught stealing rates in history; approximately eight of every 10 steal attempts are successful. The rate of successful steals per game in 2025 is roughly equivalent to what they were in 1987, generally considered a running-happy time in the sport’s history.

If that’s good news for runners, it’s obviously going to be bad news for those charged with trying to contain them: catchers. It's now more difficult for them to throw out runners attempting to steal, and many individual catchers have seen their caught stealing rates decline since 2023. Not all of them, though.

Toronto’s Alejandro Kirk isn’t just treading water under the new rules. He’s throwing out more runners than ever, despite them.

Kirk, caught stealing rate

• 2020-23 // 20%
• 2024-25 // 30%

Yet Kirk, among 70 qualifying catchers over the last three seasons, rates just 54th in average arm strength. If throwing arm power matters -- and it does, as you'll see shortly -- this mostly isn't what's behind Kirk's success. (Numbers below are as of Friday.)

For Kirk, it is success, to be clear, whether viewed via the traditional caught stealing rate, or something more advanced like Statcast’s Caught Stealing Above Average (a metric which takes into account the situations the catchers are presented with, like runner speed, lead distance, and pitch position). Over the last three seasons, Kirk is the eighth-best catcher -- and over the last two, he's tied for third-best.

Caught Stealing Above Average, 2024-25

• 12: Patrick Bailey, Giants
• 12: Will Smith, Dodgers
• 9: Kirk, Blue Jays
• 9: Luis Torrens, Mets

If that all sounds surprising – strong stolen base prevention without a strong arm – that’s because it is. While simple caught stealing rate has a lot to do with how well pitchers hold the runners on base, a metric like Caught Stealing Above Average tries to account for that issue, and even with that in mind, there’s a pretty obvious (if noisy) relationship between “throw ball hard” and “be good at nailing base stealers.”

Kirk isn't exactly where you'd expect to be in this chart, which shows average arm strength on the vertical axis and Caught Stealing Above Average on the horizontal axis:

Ideally, you’re in the top right, like Christian Bethancourt (a catcher so strong-armed that he’s pitched 11 times) or Shea Langeliers. You really don’t want to be in the bottom left, where weak-armed catchers Austin Barnes and Yasmani Grandal have struggled badly with the running game, even if they excel in other skills.

And then there’s Kirk (and well-traveled backup Austin Wynns, currently with the Reds), pretty much all alone down there in the bottom right, preventing steals without elite arm strength, standing as the only two who are simultaneously below 78 mph in arm strength and also at least +5 in steals prevented above average.

If you’re that good at throwing out base stealers, and you haven’t flashed an elite throwing arm, you better be outstanding at a few other things – and that really is going to be the trick.

How is that happening – and how has Kirk improved?

Using Statcast data, we can break down the various skills that go into steal prevention, assigning value for each skill that's added or subtracted based on the play outcome and the performance of each skill on that play. You won’t at all be surprised to find out that for Kirk, the throw speed has been a negative (-7 bases) that he has had to overcome on his way to career +9 steals above average.

1) It’s the fast exchange (worth +7 steals prevented).

What do you do when you can’t necessarily throw it as fast as everyone else? You can, at least, get rid of it quickly. That’s what an ‘exchange’ is, measuring the time in seconds between the catcher receiving the ball and getting rid of it.

This is a clear area of improvement for Kirk, who went from a below-average 0.71 seconds in his first three years in the Majors to just 0.63 in his last three years – the three in which the new rules have been in effect. Over the last three seasons, the value he’s provided in simply getting the ball out of his glove is tied for sixth-most, as Boston’s speedy Ceddane Rafaela learned last September when Kirk flashed the quickest exchange of his career, just one-half a second from receiving to releasing.

2) It’s the accuracy (worth +8 steals prevented).

It’s nice to get the ball out fast, but you would probably do well to put it on target, too. It’s hard to do that any better than Kirk did when he threw out Jazz Chisholm Jr. earlier this year.

Each throw can be graded for accuracy – if you really want to see how it works, Kirk’s least-accurate throw ever certainly looked the part, back in 2023 when he sidearmed a wild throw against Minnesota – and so we can see that his most accurate throw came when he stopped then-Ray Wander Franco that same year.

The accuracy has always been part of Kirk’s game; if you go back to 2020, despite how sparingly he played those first few years, Kirk is in a three-way tie for second place behind the Rangers' Jonah Heim for the most bases saved simply on accuracy alone. But we can make it a little more accessible, too, if you think about a box, stretching 4 feet off the ground and 4 feet down the first-base line from second base. That, for the most part, is where catcher throws matter.

Qualified catchers hit that box around 37% of the time. Kirk, in 2021, did so only 29% of the time. The next year, it was 44%. The last two seasons? He’s up to 51%, which would place him in the top six. (No one can touch Torrens, the Mets' current backup, who rates at an elite 71% of throws hitting the ‘ideal’ box.)

Simply throwing the ball accurately, to be clear, isn’t good enough, in the same way that you wouldn’t be impressed by an expert strike-throwing pitcher who topped out at 75 mph. But in the sense of “is he throwing it where it needs to be,” then Kirk is excellent at that.

But there’s one more thing going on, still.

3) Bonus: Maybe there’s a strong arm here, too.

Against all odds, Kirk was doing just fine despite a throwing arm that rated as below average in terms of strength. Until, of course, it didn’t.

Kirk, arm strength on steal attempts

• 2022 // 77.3 mph
• 2023 // 76.8 mph
• 2024 // 76.6 mph
• 2025 // 79.4 mph

In the offseason of 2023-24, as Sportsnet recounted last July, Kirk began a concerted effort to improve his throwing, working with weighted baseballs in an attempt to build arm strength – and if you think that sounds exactly like what pitchers have been doing for the last decade, it sure does. The men on the mound aren’t the only ones with arms. ("There's always room for improvement and we identified that the real area to improve was his throwing," said Luis Hurtado, Toronto’s bullpen catcher.)

This year, Kirk’s arm strength rates 29th of 60 catchers; not elite, exactly, but squarely in the “solid” range. When we said earlier that his poor arm strength was worth -7 bases cost … that’s down to 0 this year, or average. Nothing gained, nothing lost. If that doesn’t sound elite, it’s not, but it’s better than being a detriment, isn’t it?

If we focus only on stolen base attempts – we’re excluding wild pitch throws, because moving to get a loose ball and throwing from a standing position isn’t exactly in the spirit of what we’re doing here – then the four hardest throws of Kirk’s career have all come in the first month of 2025.

If it’s more difficult than it’s ever been to throw out a base stealer, then any catcher who’s managed to tread water in caught stealing percentage is doing something right – and any catcher who has actually managed to improve is doing something really right. Kirk was already an excellent blocker and an elite framer. With the improved ability to throw out runners, as well, he might just be the best defensive catcher in the American League, too.