In the first year of Statcast™, we showed pretty well that for hitters, exit velocity has a nice relationship with production, and that shouldn't come as a surprise. The guys who hit the ball the hardest -- stars like Giancarlo Stanton, Jose Bautista and Miguel Cabrera -- are the guys who put up the biggest numbers. Nineteen of the top 20 leaders in exit velocity had above-average seasons, and the 20th, Marcell Ozuna, offers plenty of hope for a rebound. This is all as it should be.
But Bryce Harper proved to be something of a sticking point to that idea. Despite a historic offensive season, the unanimous National League MVP Award winner came in only 39th in exit velocity among hitters with 50 tracked batted balls at 92.3 mph, right between noted non-sluggers Jonathan Villar and Aaron Altherr. Even his percentage of balls hit at 100 mph or more of 27.6 percent was very good (league average was 16.2 percent) but not necessarily elite (Danny Valencia and Pedro Alvarez were among those with higher marks).
So how did Harper put up a season like that with exit velocity that was above average, but not spectacular? We considered a few options, including the possibility that he's not human and normal rules don't apply. The most fruitful idea seemed to be that Harper hit more softly-struck balls around his blasts than you'd expect, dragging down the overall average. That's pretty close to the truth -- and it may help answer how pitchers can attack him going forward.
Let's explain: Yes, even baseball's deadliest hitters can have weaker areas. Remember all the stories a year ago about how Mike Trout had to work on improving against high fastballs? This is something like that. For Harper, it's all about pitching him inside. A quick look at his exit velocity by hitting zone (displayed from the catcher's viewpoint) easily shows that while he destroys pitches in the middle and outside parts of the zone, the inside parts aren't quite so deadly.

Now, "weakness" is a relative term, because even those lighter red areas up and in are still around league average. But let's take those six left blocks of the zone -- the three in the middle and the three to Harper's outside corner -- and measure some performances.
If we set the minimum at 100 balls in play, we get some stunning numbers among lefties:
Exit velocity: 96.3 mph (5th)
Batting average:  .442 (1st)
Slugging: .942 (1st)
Clearly, a pitch down the middle or to the outside to Harper is going to end poorly for the pitcher.
Gif: Bryce Harper 3 HR Swings and Curtain Call WSH
But it's a little different if you look at the inner portion of the plate, particularly low and in, as Harper admitted "I hate that pitch" in an interview with FanGraphs in May. Let's look at those inner three squares, along with inside balls, lowering the minimum to 75 due to the smaller area, and again looking only at lefty hitters:
Exit velocity: 84.8 mph (16th)
Batting average: .264 (26th)
Slugging: .479 (17th)
Still good enough, but not exactly superhuman, and that explains a lot of how Harper's overall exit velocity sank somewhat. As you can see, the overwhelming majority of his extra-base hits were in the middle or outer thirds of the plate, too:

So, between the evidence that he doesn't hit the ball as hard inside, and Harper's own words backing up his dislike for the pitch, did he actually see more inside pitches as the season went on? Well, no. He actually saw fewer. In the first half of the season, 29.3 percent of the pitches Harper saw were to the inside third or further. In the second half, that was just 26.1 percent, likely because pitchers were wary of being able to hit that spot and not finding themselves entering the danger zone over the middle. This falls under the category of "easier said than done," apparently.
Of course, exit velocity isn't everything. Harper more than doubled his walk rate from 9.6 percent to 19 percent, he decreased his strikeout rate from 26.3 percent to 20 percent, and of course there's an entirely different conversation to be had about his launch angle. Anyone as great as Harper is going to have more than a few reasons supporting that stature.
But just maybe, pitchers have something to work on in 2016, targeting Harper low and inside. It's risky; then again, it's not like what they did in 2015 worked out that well. Anything they can do to force Harper to have to adjust might be worth their effort.