It was once not unheard for pitchers to pitch both games of a doubleheader, even to throw back-to-back complete games:
The most distinguished doubleheader pitching achievement was that of “Big Ed” Reulbach of the Cubs, who did it only once, but nobody did it better. His big day was September 26, 1908, when he threw a pair of shutouts in Brooklyn, allowing a total of just eight hits.
I had always assumed this was because pitchers in the old days didn't throw all the fancy pitches they do today. But the New York Times indicates otherwise:
For the past 12 years, a team led by Dr. Carl Nissen, a surgeon, has been doing research into the stresses inherent in pitching. At one point, Major League Baseball gave the center a grant to research why so many pitchers were being injured. Because the results contravened accepted wisdom — that certain pitches are more harmful than others, for example — Nissen believes that M.L.B. is unlikely to fund further studies there. “We were myth-busting,” Nissen said. “Beliefs handed down over time with no science behind them. It’s not the curveball that’s damaging to the elbow, as everyone says. It’s the fastball, thrown again and again over time. Pure physics.”
So what's different? Do we just know better these days? Or was pitching not as hard on the pitcher's arm long ago?