Power hitters face steeper aging curve

If you haven't already heard, the Cardinals are expected to pursue a "big bat" who would–in addition to such noble deeds as ending world hunger and stopping all wars–resurrect an offense that struggled to a paltry wRC+ ranking of...fourth among National League teams in 2017.

Yes, I believe the "big bat" narrative to be dramatized. While I do think the Cardinals could stand to consolidate their current roster, the goal should always be to acquire value in the most cost-efficient manner rather than "land a 100-RBI bopper". Granted, right field appears to be one of the club's weakest positions. With rumors swirling around, among others, J.D. Martinez and Giancarlo Stanton, it appears that a power hitter could very well be the medium the Cardinals use to add value.

Resources are finite, and St. Louis will need to weigh the longer-term value of potential acquisitions. Much of this is contingent on the way a player ages, and we already know that different types of players age differently. But how do we define different player archetypes?

For the sake of data collection and research, I classified all hitters from 2006 onward (when modern drug testing began) who in any season finished in the highest quartile for both isolated power and strikeout percentage as power bats. I should also note that a seasonal 200 plate appearance minimum was required. This is by no means a perfect method, but it should give us an adequate look at how players who derive most of their value from their power evolve as their careers progress.

Once I had isolated the group of players who satisfied the listed criteria, that left my bucket of "other" players to serve as the control. I began by looking at the year-to-year changes in fWAR, scaled per 600 plate appearances.

All stats in this article were calibrated so each group begins with 0 at age 20, or no change in that metric. From there, we can see the differing career arcs develop as the cumulative WAR/600 totals (relative to the beginning of those same players' big league careers) increase or decrease. While the non-sluggers performed best upon arrival, the power hitters didn't peak until their mid-to-late twenties. After that, however, we notice a much sharper post-peak decline for the power bats.

I divided the above graph into four sections: age 20-26, 26-29, 29-33, and 33-38. Here are the average changes in WAR/600 from the first year of those timeframes to the final season.

While the decline of power hitters is delayed, their regression after falling on the wrong side of 30 is much more pronounced than the typical player. What is to blame for this falloff? To answer that question, I broke down the changes by each of the components of WAR for position players: batting value, fielding value, positional adjustment, and baserunning value (for this, I turned to BsR, FanGraphs' all-inclusive baserunning metric).

While a loss of prowess with the bat appears to be what sinks the value of power hitters in their early 30s, it's their shortcomings in the field that hurt them in the twilight of their careers. These problems are only worsened by relegations to positions that are lesser defensively, namely corner outfield spots, first base, and designated hitter.

How does all this tie back to the Cardinals? Needing to accrue six years of Major League service time to become a free agent, players generally don't hit the open market in their younger years. This results in albatross contracts being handed to 4-win "big bats" who will decline at a much sharper rate than the average 4-win position player. Potential trade targets like Josh Donaldson, who turns 32 next month, simply can't be expected to produce at the high level they did in the past. That's not to say that these types of sluggers should be avoided at all costs, but a more stern regression discount needs to be applied to them when evaluating their value going forward.

It bears repeating: the end goal here is to acquire as much value as possible given your resources. Don't fall prisoner to the narrative that the Cardinals absolutely need to fix their sights on a "big bat". A one win upgrade is a one win upgrade. What the Cardinals need are improvements to their 25-man roster; what form they take is irrelevant for all intents and purposes.