Aroldis Chapman made his name by doing something better than any man in baseball history, and that would be throwing fastballs.  I mean FASTballs.  There were a couple of instances where the lanky lefty was clocked at 105 mph.  You did not read that wrong.  Since his entry into Major League Baseball with the Cincinnati Reds in 2010, he’s had 77 saves (38 each of the last 2 seasons), and struck out 324 hitters in just 198.2 innings.  Doing the math, that’s alot.  But there has been talk of changing his role with the team, and removing him from the bullpen, where he’s dominated hitters with mind-blowing velocity, and put him in the starting rotation. 

The thought process at first glance is simple.  He’s good one inning at a time, let’s give him 5-7 innings every 5 days, and see if he can replicate those outstanding numbers.  He was originally thought to be a starter when he signed with the Reds, and has been in the bullpen since. 

But it’s not that simple.  Relief pitchers give 100% of their night’s effort into that one inning they pitch.  That is what allows the late inning guys to pump their fastballs at the high 90’s, to maybe breaking triple digits.  Starting pitchers, on the other hand, have to divide that 100% of energy into every inning they pitch, and conserve their arm to be able to pitch later into games. 

Joba Chamberlain, for example, came on the scene in 2008, and was dominating hitters with a 98 mph fastball, and a complimentary slider that just disappeared (much like Chapman’s skill set, but with less 100 mph-ness).  The Yankees moved him to the rotation, and his velocity slipped to the low 90’s, and his effectiveness exponentially fell. 

Compare this to Chapman.  Chapman likely will have to tone his fastball down from averaging 98 mph roughly, to possibly low-mid 90’s.  That’s a huge difference.  His mechanics, as explained by Sports Science’s John Brenkus, can’t possibly be repeated 100+ times a night. 

Tim Lincecum has similar mechanics, and his effectiveness died down over the last couple of years, as his body caught up to him.  So too with Chapman, that motion is extremely taxing on the body.  As simple as he makes it look, a human arm isn’t meant to throw a baseball 100+ mph.  The tremendous strain it causes on his arm and shoulder would cause his arm to collapse if he tries overthrowing his fastball too many times. 

Also, part of the reason Chapman is so effective, is that at that velocity, his control doesn’t need to be pin-point.  Batters have less time to see where the ball is going, and will wildly hack at it, trying to hit it.  If Chapman’s fastball is more pedestrian, say at 94 mph on average, hitters have more of a chance to decide not to hit it, and can pick and choose rather than guess on the fastball.  That would increase his walks, hits, and home runs, and decrease his strikeouts. 

While Chapman is in the bullpen, his quasi-legendary velocity is unleashed, reaching 100 mph on average.  But if he’s installed in Cincinnati’s rotation, his effectiveness will hit a sharp decrease.  The transition the opposite way has shown success, but it’s incredibly rare to find a reliever make his way to being a number one starter overnight.  I’m sure he’d be effective, but I doubt it would work in the Reds’ benefit.  Keeping him in the bullpen optimizes his talents, and allows him to use his full potential every outing.