Now that Statcast™ technology is active in each Major League park, we have the ability to measure things we've never been able to before -- like outfield throwing arms.
Last winter, for example, we used it to identify Houston as having 2015's best collection of strong outfield arms, and in May, we were able to point out that even though Miguel Sanó's transition to the outfield had been rocky, his arm was for real.
Now that we're more than halfway through the second year of Statcast™, we can begin to answer a different question. Which outfielder improved his throwing arm the most since last season? Or to put it another way, is it even possible for an outfielder improve his arm strength noticeably? If so, how?
Let's get right to the leaderboard before we get to the explanation. We found 93 outfielders, roughly three per team, who had at least 40 tracked throws, both this year and last. We used our "competitive throws" definition from the Astros story, which allowed us to discard the unimportant lobs back to the infield and focus only on throws that were shown to be at least 90 percent or more of a player's full displayed arm strength.
What we found was that there was a clear leader, and he can be found playing right field in St. Louis. (As well as on Thursday's free MLB Plus broadcast on MLB.TV, where Carlos Martínez and the Cardinals face Jon Lester and the Cubs.)
Most improved throwing arms, 2015 to 2016 (through Aug. 7)

1. +7.7 mph, Stephen Piscotty
   
2. +4.9 mph, Jay Bruce
   
3. +4.5 mph, Juan Lagares
   
4. +4.2 mph, Corey Dickerson
   
5. +3.8 mph, Aaron Hicks
   Minimum 40 throws in both years
   That's a nearly nine percent improvement for Piscotty, from last year's 86.8 mph to this year's 94.5 mph, and it's by far the largest. (Bruce, Lagares and Dickerson all showed improvements of just over five percent; the overwhelming majority of players were within three percent, in either direction, compared to last year.)
   Gif: Stephen Piscotty Throws Out Man At Second
   When you run a comparison like this for the first time, you wonder how it's going to turn out. Would it be random noise? Would it show arm strength isn't a repeatable skill, as unlikely as that sounds? Fortunately, the top of this list contains players with some obvious reasons for improvement. Bruce, for example, seems to finally be over the knee injury that sabotaged nearly two years. We know that Lagares dealt with a nagging elbow issue for all of 2015, one that limited his previously strong arm. Heath, or lack of it, would be an obvious issue at play here.
   And Piscotty? Well, we know exactly what he did, because MLB.com's Cardinals beat reporter Jen Langosch pointed out way back in early April that improving arm strength was a focus of Piscotty's last spring. As she wrote, Piscotty changed his throwing mechanics when new teammate Mike Leake "identified a flaw that was costing the outfielder some strength," then worked with fellow outfielder Randal Grichuk on improving his arm action. Two days into the season, Piscotty showed off his new skills by hitting 94.8 mph while throwing out Andrew McCutchen at the plate.
   Four months later, that increased velocity has more than held up. Consider these career numbers:
   • Of the 58 times Piscotty has reached 75 mph on a throw, 51 of them have come in 2016.
   • Of his 30 hardest tracked throws, 29 have come in 2016.
   • Of Piscotty's five career assists, four have come in 2016.
   • Of the 93 qualified outfielders, he was tied for 69th in throwing velocity last year, and he's 11th this year, just behind Kole Calhoun and Adam Eaton.
   It's worth pointing out that Piscotty has already played about twice as many innings this year as he did last year, so he's had more opportunities in the field. Still, that's not nearly enough to explain the massive changes we're seeing.
   No, what we have here is a clear cause (Piscotty changed his throwing motion with help from Leake and Grichuk) and effect (he's had the largest improvement in velocity by far). But what we may not have is definitive proof that an outfielder can suddenly create velocity where there was none before, because we've seen Piscotty show off his arm in the past. Back in college, he threw 43 innings for Stanford, with reports indicating that he had a "mid-90s fastball." 
   That being the case, is this "new" velocity, or is this just unlocking the physical gifts Piscotty already possessed? Perhaps a little of both, and it should be noted that a stronger arm doesn't always mean a more valuable arm, as Piscotty still ranks slightly below average in terms of preventing runners from advancing to the next base. But the larger point here is that for all the effort expended on fixing pitching mechanics, you rarely hear about the same with outfielders. Maybe it's time that we do. After all, the spring work has made enormous differences for Piscotty.