Young hitters tend to gain power as they age. The norm is a peak around age 27-32 in which the player displays the most power and records the most home runs per season. This much is known. But gaining this edge in power can come sooner depending on when the player matures at the plate and how much muscle he gains at certain ages. Jason Heyward could be an example of gaining power sooner.
As a 20-year-old rookie in 2010, Heyward hit 29 doubles, five triples and 18 home runs in 623 plate appearances. That includes a .456 SLG and .179 ISO.
As a 21-year-old player in his second season, Heyward is slugging .525 with a .263 ISO. He already has seven home runs in 113 plate appearances, so he's 11 home runs away from his total last season while just a month into this season.
It may seem abnormal for a hitter at 21 years old to have such a power gain, but Heyward is not a normal hitter. I think we can all agree with this. But is his power spike in April an indication of a true power gain this early in his career?
Heyward is hitting more fly balls, from 27.2% to 33.8%, while hitting fewer ground balls. His HR/FB has increased from 16.8% to 25.9%. So Heyward is getting more lift on the ball while hitting fewer of those famous backspin line drives, as well as fewer choppers to second base. When Heyward gets lift on the ball, things happen. Good things, normally.
He's being thrown more fastballs and curveballs with fewer sliders. More fastballs is always good, and I'm not sure why pitchers would do such a thing. But perhaps the biggest change is fewer cutters, which is a pitch he couldn't handle last season when thrown in on the hands. Heyward doesn't seem to be getting as many low and in pitches, and he is able to connect on pitches over the plate and away. He also seems to be covering the plate much better so far, and as a result he is striking out less at 19.2%.
Heyward is swinging at more pitches in the zone and isn't being quite as patient, which may have something to do with a slight decrease in walks. His zone contact percentage is up to 93%, which compares very well to a league-average 87.6%. So he isn't missing as much on pitches in the zone, which likely has something to do with recognizing pitches better the second time around.
These numbers show Heyward is seeing and recognizing pitches better as a sophomore in the majors. While his walks may be down some, his strikeouts are also down and he's making more contact in the zone. This is a possible link to an increase in power as Heyward makes better contact and is getting more lift on those balls.
Also worth mentioning is the fact that Heyward had six home runs through April of last season and 10 through May. His thumb injury sapped his power in July, resulting in zero home runs for the month. He had just seven total after the injury. So the injury certainly led to a decrease in power for Heyward.
So while it's unusual for a hitter to have a power increase at the age of 21, Heyward is recognizing pitches and making better contact, and combined with more fly balls, leads to more home runs. A healthy Heyward is also helping the power gain.