THE MOMENT YOU think you've seen it all in baseball, the game delivers something you've never encountered. Consider the decline of Albert Pujols (age: 34; OPS: .810). Everyone's stats fall off sooner or later, but Pujols' skills have decayed in a way that is unique in baseball history -- and that shows how the latest advanced metrics let us gauge the physical and even mental struggles of a waning ballplayer.
On average, as baseball players age, they lose speed, then power, but maintain or even improve their strike-zone judgment well into their 30s, even 40s. Willie Mays, for example, led the NL with a .425 on-base percentage at the age of 40 in 1971, the only time in his 22-year career that he drew more than 100 walks in a season.
Not so with Pujols, whose annual walk totals since his superhuman peak in St. Louis in 2008 read like this: 115, 103, 61, 52, 40 and 42 (through Aug. 22) .. Has there been any other player, ever, whose free passes cratered after he established himself as the most fearsome hitter on the planet?
We can answer that by using the tools available to today's sabermetricians. Let's begin with a list of players who started out similarly to Pujols, then see how their careers proceeded. Baseball-Reference.com is great for this; the site provides comparables for every MLB player at any age, and you can use its Play Index to construct lists based on your own criteria.
Baseball Reference's top-10 list for players similar to Pujols through 2010 (the last time he was an All-Star) is a fantastic club that includes seven top-shelf Hall of Famers, like Frank Robinson and Lou Gehrig, plus Miguel Cabrera and Ken Griffey Jr. Through the age of 30, these great hitters averaged 380 homers with an OPS of .978 and walked in 12.5 percent of their plate appearances. Pujols was actually better than that, with 408 homers, a whopping 1.050 OPS and a 13.5 percent walk rate.
After they turned 30, the supergroup's walk rate edged up to 13.3 percent, almost enough to make up for the declining batting averages over the rest of their careers. But Pujols has taken a sharply different path -- since 2011, his walk rate has plunged to 8.4 percent, and his OBP has declined by 82 points. Where did all the walks go? We can look for changes in Pujols' batting approach at Fangraphs.com, whose plate-discipline statistics record how often hitters swing at and make contact with various kinds of pitches. Early in his career, Pujols swung at less than 20 percent of pitches outside the strike zone; that proportion, which Fangraphs calls his O-swing percentage, has leaped to more than 30 percent during his years with the Angels. That's a much bigger change than his swings inside the strike zone or his contact rates.
Digging deeper, we can turn to BrooksBaseball.net, where a team led by Dan Brooks, a Brown University neuroscientist, slices, dices and serves up PITCHf/x data in every imaginable way -- like heat maps showing how a batter performs against pitches in different locations. It turns out that in 2012 and 2013, Pujols' power stats declined dramatically in particular regions: in the middle of the strike zone and along the outside of the plate.