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September 12, 2012

Manufactured Runs

Searching for Fatigue in Stephen Strasburg

by Colin Wyers

Tonight in New York is the “not” heard round the world: the game Stephen Strasburg would have been pitching if the Nationals hadn’t shut him down ahead of schedule, due to problems “mentally concentrating” that the Nationals blame on the level of media attention to the team’s plans to shut him down.

The Nationals have a strong lead in the NL East, so they are unlikely to miss his performance in one game, or for the rest of the regular season, very much. The larger issue surrounding Strasburg is the impact of losing him for the postseason. When the Nationals instituted their plan for Strasburg at the beginning of the season, it made a lot of sense for a young team with slim hopes of making the playoffs to protect one of their most valuable (and most fragile) players from injury. With the Nationals heavily favored to make the playoffs, though, some Nationals fans are likely to be disappointed if their team’s ace isn’t available for a single game of the postseason.

Clearly, the Nationals know that Strasburg is a better pitcher than whomever they will replace him with in the playoff rotation, just as everyone else does. However, the public is at a disadvantage when discussing a pitcher’s condition, since teams simply know more about how he’s doing than fans and the media do. They have scouts, trainers, doctors, and even the player themselves to shed light on how the pitcher’s body is reacting to his workload.

It’s difficult for anyone not affiliated with a major-league team to get that sort of feedback on a player. The next-best thing we can do is look at the pitches themselves to see if we notice anything that we may be able to link back to the pitcher’s workload.

A look at the PITCHf/x data reveals that Strasburg’s four-seam fastball (his primary pitch) has been steadily declining in velocity over the course of the season. This graph is going to be relative to his average fastball speed for the season:

The data is noisy, but there’s a definite trend there: according to the best-fit line, Strasburg has lost nearly a mile per hour off his four-seam fastball since the beginning of the season. Knowing this by itself doesn’t tell us much of anything, without knowing about how other pitchers fare over the course of the season. Again, this is relative to the average fastballs of all pitchers: