In a previous post on the four-man rotation, I wondered if there are more arm injuries today than in the past. This Baseball Prospectus piece doesn’t address that issue, but it makes a compelling case that pitchers must work harder each inning than they did 20 or 30 years ago.
If a pitcher never gets an easy out, and needs to bear down on every pitch, and throw more breaking pitches, that could explain why there are more injuries (if there are more — and I would expect there to be more, if this is truly the case).
I’m not sure, though, if these stats make the idea of a four-man rotation a bad idea. The recovery time of throwing 120 pitches in 7 innings should be about the same as throwing 120 pitches in nine innings, except, of course, you’ve had to throw 120 higher-effort pitches today than 120 pitches 30 years ago.