Consider this part two to my study on pitch counts and pitchf/x.

The first time through a lineup, pitchers traditionally throw fastballs, and then switch to off-speed pitches when facing batters a second time. In order to isolate the effects of pitch counts on a pitcher's stuff as opposed to his pitch selection, I had to classify a whole lot of pitches. That was fun.

There were about 5,000 games in which a pitcher threw 100 pitches during the pitchf/x era. These pitchers performed admirably to have lasted that long into a game, so this sample won't be representative of all, or even most, starters. To illustrate the point that pitchers mix up their repertoire over the course of a game:

Six pitches are regularly thrown throughout any given game. The four-seam fastball (F4) belongs in most every pitcher's repertoire, though some sidearmers or sinkerball specialists will only throw fastballs of the two-seam variety (F2). These two pitches are often difficult to distinguish from one another, be it by the human eye, or by the detailed pitchf/x data. Cut fastballs (FC) are also difficult to make out at times from four-seamers and sliders at times. Sliders (SL), curveballs (CB), and changeups (CH) increase in usage over the course of the game. Knuckleballs and splitters are thrown only one or two percent of all pitches, so I won't include them in this study, and I made no attempt to classify screwballs, shuutos, or gyroballs, since I'd guess they compose about .001% of pitches in the last three years.

Perhaps some pitches are more useful later in the game than others. In theory, all pitch types should have the same effectiveness. Game theory would dictate that if a pitcher's curveball is better than his fastball, he should throw his curveball so often that batters come to expect it. Therefore his fastball gains value. Eventually, the two pitches become equal in terms of overall effectiveness. For one reason or another (maybe there is credence to the notion of the "out pitch"), this theory does not hold true for many pitchers, or at a league-wide level. The run value of fastballs is higher than the run value of breaking balls, which would signify that pitchers are under-using their secondary pitches. (Keep in mind, the main advantage to using run values is that they take the count into account.) As you will see in the below image, this trend narrows, but still exists, even as pitchers use more off-speed offerings deeper into the game.

All run values per 100 pitches.The high points and low points in the graph represent the high points and low points in the opponent's batting order.

It seems to me that changeups are ineffective pitches at the start of the game, but gain effectiveness later in the game. This makes sense intuitively. The graph also lends merit to the manager's decision to leave these pitchers in for 100 pitches, as the sample of pitchers is clearly above average through 90 pitches. However, these pitchers were also undoubtedly lucky. They would not make it to 100 pitches if they gave up runs. That's where my metric for measuring a pitcher's stuff based on a pitch's physical characteristics comes into play.

First, the two least impressive types of pitches in terms of stuff: the sinker and changeup.

As you'll see with each of these charts, there's something funky going on in the first several pitches of the ballgame. I'm not even going to attempt to form a guess as to why changeups appear to have a better StuffRV as the game goes on. The success of changeups is obviously not built on how "nasty" they are.

Again, for some reason, we should disregard the first dozen points or so. Pitchers throw fastballs an inordinate amount of time on the first pitch, and apparently, anything they throw lacks in stuff. They're warming up or something. Maybe they know batters tend to not swing at the first pitch of the game. I don't know. But you see that with all three types of fastballs, from the tenth pitch to the hundredth, a pitcher loses about a 10th to a 20th of a run in StuffRV per 100 pitches.

Finally, breaking balls.

So, even pitchers who have successful games lose a significant amount of stuff over the course of a game. Since this sample represents an above average group of pitchers, I'd imagine lesser ones deal with inferior durability. I would be comfortable saying that the quality of a generic starting pitcher's stuff decreases by at least .05 runs per 100 pitches from his first pitch to his last.