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Second-half Statcast storylines to watch

Fastball velocity, grounder rates and elite velocity among most compelling angles

When Jeff Samardzija fires his first pitch in Chicago against the Royals on Friday afternoon, the "second half" of the 2015 season will finally be underway. There's no shortage of stories to follow -- non-waiver Trade Deadline deals, pennant races, etc. -- but from an analytics perspective, what should we be looking for? Here are a few notable numbers to follow.

1. How well will the Statcast™ leaders persist?

We have a half-season of Statcast™ data, and the results have been impressive. But what we don't know, just yet, is how long it takes some of these rates to stabilize. For example, previous research has shown that a hitter's grounder or fly-ball rate will stabilize after about 80 plate appearances, which is to say that you aren't likely to see major in-season fluctuations after that point. For something like batting average, it can take far longer, up to nearly 1,000 plate appearances. (Case in point: Dee Gordon hit .403 through his first 170 plate appearances and .287 in his next 208.)

You can probably safely assume that when healthy, Giancarlo Stanton will continue dominating the exit velocity charts. But, for example, does first-half spin rate inform second-half spin rate? Will the current leaders in route efficiency be the same at the end of the year? Do we have enough data to truly know who has the best catcher pop time?

In some cases, the answer is yes. In others, not so much. When we look back at the end of the year to see if Garrett Richards, Charlie Morton and Jesse Hahn are still atop the curveball spin leaderboards, or if Rajai Davis and Christian Yelich are still the kings of route efficiency, we'll have a much better idea of how much data is needed to be meaningful.

2. Will big league pitchers keep throwing historic heat?  

We have reliable data on pitch velocity pre-dating Statcast™ all the way back to 2008, and the trend is unmistakable:

Tweet from @mike_petriello: Pitchers just keep on throwing harder and harder every single year, up 1.4 mph from 2008.

Every year, pitchers as a whole are throwing a little harder than they did the year before. In 2015, pitchers are throwing about 1.4 mph harder than they did in 2008, and if you're looking for a reason why run scoring is still down, look no further. According to Fangraphs, for every mph a starter loses on his fastball, his ERA goes up by 0.28. For relievers, it jumps 0.54. Velocity isn't everything, but it matters.

Pitchers are throwing harder partially because they're just better conditioned and trained than they've ever been, but also because there are more of them coming into games for short stints. In 2001, 453 pitchers appeared in at least one game in relief. In 2015, we've already seen 452 -- and the season is only about 55 percent complete. Last year's record of 538 seems well in reach.

3. Will anyone challenge Aroldis Chapman on the velocity leaderboards?

Because it's been a continued trend over many years, it's not really fair to attribute the velocity increase entirely to Chapman. It sure does seem like a simple explanation, though, because what Chapman is doing seems unprecedented, even if we have only one year of Statcast™ data to pull from.

If you look at the "fastest pitches" leaderboard, you'll see Chapman at No. 1, with a 103.9 mph laser to Brian Dozier on May 29. He's at No. 2, also. And 3. Five. Ten. Twenty. Fifty. You have to go all the way down to No. 70 to find Pittsburgh's Arquimedes Caminero and his "mere" 101.62-mph offering to Yunel Escobar on June 21. From there, you'll see other names pop up -- a Trevor Rosenthal here, a Bruce Rondon there -- but of the top 200 fastest tracked pitches this year, 161 belong to Chapman.

In retrospect, that was the wrong question. Of course no one's topping Chapman, not even Carter Capps, who leads in perceived velocity only because of his best-in-class extension. What we should be asking is, "can anyone crack the top 50?"

4. Can hitters find a way to stop hitting grounders?

For the last several years, hitters have been putting more balls on the ground:

Tweet from @mike_petriello: The grounder rate has been rising, too. Along with the velocity increase, no wonder it's so hard to hit these days.

A grounder is most pitchers' best friend: It won't land in the seats, and it can turn into a double play. It's somewhat easier to get a hit on a grounder than on a fly, but much rarer to get an extra-base hit. It's how pitchers who don't get a ton of strikeouts (like Brett Anderson and Dallas Keuchel) can succeed, and how those who can combine both (like Felix Hernandez) are legends. Since hitters keep breaking strikeout records every year and hit more grounders, it's easy to see why runs are down.

Why are there more grounders, though? Some teams -- like the Pirates, D-backs and Astros -- specifically select pitchers who can induce grounders. All across the sport, pitchers have become aware of the lower strike zone, and when balls thrown lower in service of trying to get those strikes are put into play, they're often turned into grounders. It's why two-seamers with particularly low spin rates -- think Anderson, Wily Peralta, Mark Buehrle -- can be so effective.

Mike Petriello (@mike_petriello) is an analyst for
Read More: Aroldis Chapman