The good old days, when strike zones were smaller and biceps were bigger. (Frank Franklin II/AP photo)

If it feels like baseball teams are scoring fewer runs these days, that's because they are.

The average number of runs scored per team, per game has fallen in 10 of the last 14 years — most recently, from 4.14 runs per game in 2013 to 4.06 this most recent season. In all, the average team scores more than a run less than it did in 2000 (when baseball teams scored what now seems like an impossible 5.14 runs per game).

To a casual fan, baseball's disappearing offense might seem natural, what with the recent crackdown on performance enhancing drugs. The league began to enforce considerably stricter penalties for both steroid and amphetamine use in 2006, after a string of players — many of whom were significant run producers — were caught juicing up.

And there's certainly some truth to that. By virtually every metric — hits, doubles, triples, home runs, runs batted in, average — batters are performing worse at the plate today than they did in 2006 when the league started cracking down. In some cases, hitters are more crippled than they have been in more than twenty, thirty, or even forty years. In 2014, the league-wide batting average was the lowest since 1972.

But there might be something else — something a bit more nuanced — at play here that has less to do with bat-swinging, and more to do with strike-calling. Umpires, as it turns out, are getting a lot more giddy about calling strikes, and that's making it a bit harder for hitters to perform like they used to, according to a recent paper by Brian Mills, an assistant professor at the University of Florida.

By Mills' analysis, the strike zone has expanded by more than 30 percent for both left and right-handed batters since 2007 (the Wall Street Journal has a fantastic visualization of that expansion, for those who are interested). But it's not as though umpires are suddenly calling more balls as strikes — rather, they're simply calling strikes more accurately. The rate of accurate strike calls is up by almost 10 percent since 2007, according to Mills, thanks in large part to technological innovations in the sport that have promoted a more consistent strike zone.

Exacerbating the effects of umpires' sharper eyes is that fact that the expansion has mostly included the lower half of the strike zone, which has long been under-called, and is a particularly cumbersome area for hitters. Hitters are nearly 30 percent less likely to get a hit on pitches in the bottom-most bit of the strike zone by the batter's knees. It's of little wonder that teams struck out more per game this year—7.7 times per game, on average—than ever before.

So sure, the muscles hiding beneath baseball uniforms might not bulge like they did a decade ago, but the strike zone all the scrawnier hitters have to deal with today is swelling, and that might have just as much to do with why baseball teams aren't putting runs on the board like they used to.