How could pitchers once pitch both games of a doubleheader?

Question

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?

Answer 1

This is basically one of the million dollar questions in sports today:

Why are pitchers getting hurt more now than ever before?

Nobody knows the answer, but here are a couple theories:

1. Athletes are bigger and stronger than ever today. They lift more weights, maintain better nutrition, and have access to technology and knowledge that has never existed before. Increased strength and size allow pitchers to put more stress on their muscles and bones than ever. Throwing a baseball is an extremely unnatural motion, and this is even more the case with today's stronger athletes. Some people say that pitchers are so strong today that they're damaging their own bodies with the force their muscles can create (and the repetitive training they endure as they strive to apply such forces).

2. Kids today spend far more time pitching and training at a young age than ever before. As the sports industry grows (along with the money it offers) the incentive to succeed grows with it. Youth sports today is more specialized than ever, with coaches expecting their athletes to focus on a single sport. Today's young pitchers throw hundreds of pitches per week starting at a young age and continue through adulthood. One of the leading theories about the cause of today's pitching injuries is that kids are throwing their arms out at a young age.

3. Doctor's are better at detecting injuries today. My guess is Nolan Ryan pitched through dozens of injuries during his career that would sideline pitchers today. Professional pitchers have also become hundred-million dollar assets in today's sports, so doctors have more incentive than ever to protect them. However, some people would argue that this protective approach could also be one of the causes of today's injury epidemic...

4. After writing #3, I did a quick google about Nolan Ryan (hoping for a good quote from the legend) and found another theory: that pitchers today are over-protected and get "babied" too much by their managers. You can read more about that here.