For the first couple months of the season, Aroldis Chapman was putting up video game numbers. Using the power of selective endpoints, let's look at what he did over his first 24 appearances...
29 innings, seven hits, 52 K, 9 BB, 0.00 ERA.
That's, er, quite a line.
But then Aroldis Chapman Fever grinded to a halt, as he became human over the course of seven games in mid-June, allowing nine hits (three homers) and eight runs in 6.1 innings. By the end of that stretch, Chapman -- once an unstoppable force of pitching dominance -- had a 2.04 ERA. Shiny, but...back to earth. And so the talk of his crazy-good season settled down quite a bit.
But fear not -- he's continued on a record pace, as he's in position to finish the season with the highest strikeout rates (both K% and K/9) ever in a single season.
Through 45.2 innings, Chapman has now struck out 86 of the 173 batters he's faced. For reference, 86 is half of 172, so one batter has made the difference between Chapman having a 50% strikeout rate. In any event, he has a 49.7% -- still pretty decent, if I do say so myself. He's also struck out opposing hitters at a rate of 16.95 per nine innings.
The single-season record for K% (minimum 40 innings)? Eric Gagne circa 2003, at 44.8%; the single-season record for K/9 (minimum 40 innings)? Kenley Jansen circa 2011, at 16.10 K/9. How Jansen didn't generate more hype while striking out 16 batters every nine innings is beyond me. Anyway, as you can see, Chapman has a pretty strong grip on both these records at the moment, so even if he slows down considerably as the season wears on, he's got a very good shot at holding onto the #1 spots in the record books.
Of course, it's no surprise that a pitcher is setting strikeout records this seasons, just as one did last season. The league average strikeout rate is way, way higher than it's ever been before. That's a bona fide recipe for broken records. And it also means that, in context, Chapman's accomplishments are less impressive. (That's why metrics like OPS+ and FIP- exist.)
So, how do the strikeouts look in context? Without making any adjustments of any sort (park factors, et al.), Chapman's K% is 254% of the league-average (19.6 K%). That is, in fact, not a record, as Eric Gagne's 2003 season was 273% of the league average. I'm not sure if there are other seasons that rank higher, but I wouldn't be surprised if that's the case.
So in the context of the time frame he's pitching in, Aroldis Chapman's strikeout rate isn't quite as shocking (it's "merely" outstanding); regardless, he'll very likely finish with the highest K/9 and K% of all time, and that's no frivolity.