Originally Posted by RedsManRick
I would hypothesize that we're still in a place where we're over-genericizing things. While training, nutrition and bio-mechanical analysis have improved, the human physique is still essentially what is has always been. I would imagine that pitching is much like any physical activity and repetitive use injuries (even those that culminate in a traumatic event) are in large part of a function of the specifics of a given pitcher's body -- it's "design", it's recovery rate, etc.
One thing not taken in to account is the selection effect. If pitchers in the 60's were expected to throw 300 innings, the ones who were particularly susceptible to repetitive use injuries were more likely to get injured via overuse early in their baseball careers, lose effectiveness and fail make/establish themselves in the major leagues to begin with.
Would Rich Harden or Mark Prior have "survived" long enough to make the majors in 1966? It's possible that the pool of major league pitchers 40 years ago had already been filtered of it's most injury prone pitchers, making it appear that the average pitcher had greater ability to give innings. It's also quite possible that a number of pitchers today could be 300 inning guys were they given the opportunity to do so.
Add in the lack of the slider, an extremely stressful pitch to throw, and the increase of power throughout the lineup requiring a more sustained effort of the pitcher and it's easy to see the variety of explanations adding up.
I agree that breaking balls are used at a much higher rate than ever before. Not only are they thrown more often, they are thrown harder and there are more varieties.
Pitchers also throw their fastballs much harder than in the golden age. Pitchers are much taller and stronger than they used to be. The average fastball velocity has been rising every year. Throwing harder puts more stress on the shoulder and elbow.
Pitching is more difficult than ever because the fences are closer, the balls are bouncier, the bats are denser, the strike zone is smaller, the mound is lower, and the hitters are bigger, stronger and better than ever before. The days of facing only two or three real threats in a batting order are long gone. The pitcher has to throw with max effort and concentration on every pitch, making the strain on his arm much more severe.
Since the turn of the millenium, smarter handling of pitchers along with highly effective modern surgical and rehabilitation procedures have kept many star pitchers on the mound that would otherwise have been replaced with lesser pitchers. This is the major reason why scoring has been declining since the year 2000. In the 80's and 90's all the changes to the game favored the hitters, but in recent years the science of pitching has evolved so much that it has offset most of the offensive advantages. Improved defensive metrics have increased the understanding and value of good defense, which has also contributed to the recent decline in scoring even though players are bigger and stronger now than they were during the steroid era.