Checking in on Aroldis Chapman’s Fastball

The Aroldis Chapman experiment enters its fourth (full) season in Cincinnati this spring, and if this season goes anything like the first three, the Reds can rest easy knowing the back end of their rotation will be one of the best in baseball in 2014. Since his debut on August 30, 2010, the ‘Cuban Missile’ owns a 2.40 ERA, 1.02 WHIP, 40.9% strikeout rate and 77 saves – enough for seventh, fifth, second and sixth-best among relievers with 198 innings since that date. Now that Homer Bailey is sewn up through 2020, general manager Walt Jocketty now turns his attention to extending Chapman to solidify the backend of his bullpen for the prospective future.

Yet while Chapman has been one of the most dominant relievers in the game since his debut, his game hasn't come without a few shortcomings. Chapman's first three full seasons have been rather inconsistent, at least from a statistical perspective. Let's take a quick look at the numbers.

Chapman has been brilliant at racking up strikeouts, but his rocket arm has also cost him a good number of walks. The league average strikeout rate for relievers with at least 100 innings since 2011 is 22.9%, so we knew Chapman is elite in terms of punching batters out. However, his fluctuating walk rate is concerning, as the league average mark for those same relievers is 8.6%, and as we can see, Chapman has only one full season to his credit (2012) in which he posted a walk rate lower than that mark. He took several steps backward last season, adding another earned run to his 1.51 ERA from 2012. Opponents had much more success against his stuff in the meantime, posting a career-high .544 OPS against him. Consistency is king for closers, and though Chapman has been elite, there's room for improvement.

So, what's the problem? Chapman is becoming too reliant on his fastball. In his first full season with the Reds in 2011, Chapman tossed his fastball at a 79.4% clip -- fourth-highest among relievers with at least 50 registered innings that season and well above the 49.1% league average mark. During his best season to date in 2012, Chapman increased his heater use to 81.6% -- fourth-highest in the league once more and again noticeably higher than the 48.3% league average use. The 6-foot-4, 205-pound southpaw went to his fastball at a career-high 82.6% rate last season, however, which was third-most among lefty relievers with at least 60-innings.

Consequences of More Fastballs

While Chapman's fastball has maintained a steady (if not slightly increasing) velocity over the last three season, his fastball simply isn't generating the 'elite' type of results that we'd expect. His ground ball rate has plummeted incrementally from 42.9% in 2011 (compared to the league average mark of 38.4%) to 35.5% last season, which was actually below the league average mark of 35.5%.

And while opponents are putting fewer of his fastballs in play than ever before (21.5% in-play rate last season), they're doing more with those balls they do put in play, shown by a 2013 HR/FB ratio of 12.8% -- highest among relievers who threw at least 800 fastballs last season. Could Chapman's increase in zone% have anything to do with his ground ball decrease? Absolutely. Since 2008, the trend with relievers is that when you throw more fastballs in the zone, your ground ball rate tends to decrease roughly three percent with every five percent increase in fastball use.

When we think about relievers, we tend to think about the development of their secondary (i.e. non-fastball) offerings at a young age, particularly in the minor leagues (which Chapman didn't spend much time in). Often times, development of these pitches proves critical later in their careers, since fastball velocity tends to wane with age and young pitchers can't blow past batters with their heaters. Chapman seems to be going in the opposite direction in this respect; relying too heavily on his fastball, which has hampered the offering's ability to generate easy outs in critical late-game situations.