Do relief pitchers suffer from pitching back-to-back days?

Every baseball fan has heard the excuse that a relief pitcher has been overworked or underworked after he pitches badly in a game. How much of a difference does it make if a reliever has to come out and pitch back-to-back days? Does he lose some velocity on his fastball? Does his sinker have more bite? What happens when he pitches three days in a row? What about when he hasn’t pitched in a while?

For this article, I am using the nearly 300,000 pitches thrown last year that were tracked by PITCHf/x. Only data where the PITCHf/x system was in effect will be used, so if a pitcher threw back-to-back days but the first day wasn’t tracked, then none of the data are used. Same if a pitcher was the starter for either day, or if a pitcher threw both days of a double header either of the days. A similar setup will be used when looking at pitchers who had exactly one day of rest or more than one day of rest, etc. Here fastballs will be considered either four-seam fastballs or two-seam fastballs (sinkers).

To start, let’s compare fastball speed for pitchers depending on days of rest.

                           Speed of fastball in mph
Back-to-back days                91.0
Exactly one day of rest          91.6
Exactly two days of rest         91.4
At least one day of rest         91.5
More than two days of rest       91.1
More than five days of rest      91.1
Three straight days              89.2

So when pitchers throw on back-to-back days they lose half a mile per hour on their fastball compared to having at least one day of rest. If a pitcher is asked to throw three straight days, his fastball goes down nearly a mile and half per hour. Exactly one or two days of rest appear to be the ideal situation. When a relief pitcher doesn’t get into a game in a while, it appears his fastball suffers slightly, but whatever rust there is it doesn’t appear to get worse if he goes more than five days without pitching.

Now as John Walsh pointed out yesterday, none of these differences should be enough alone to cause large problems. Still, if the velocity of these pitches is changing, there clearly is some effect. How days off affect command is another story. Pitchers’ command is one of the hardest things to track with PITCHf/x. If the pitcher throws a curveball in the dirt, was that intentional or did he miss his spot by two feet?

How does rest affect a pitcher’s sinker? Let’s make a similar table looking at how much sink sinkers have depending on days of rest

                       Vertical movement of sinkers compared to a pitch thrown without spin, in inches
Back-to-back days                0.6
Exactly one day of rest          2.3
Exactly two days of rest         3.1
At least one day of rest         3.2
More than two days of rest       3.3
More than five days of rest      3.4

Remember, less is better. While the goal of a sinker is to produce downward action, most sinkers still “rise” slightly compared to a ball thrown without spin. The league average is actually around 4.7 inches of “rise” so the relievers here are always doing better than the league average.

Sadly, no data are available for three straight days of work for sinkers alone. Here you can see a stark contrast: When a reliever throws on back-to-back days, he actually is getting incredible movement on his sinker. When he has exactly one day of rest he still produces good results but anything more than that and the reliever suffers. If he can control his sinker, regular work looks advisable.

Okay, so sinkers seems to sink more with regular work. What about sliders? Many relievers who use a sinker regularly are sinker/slider pitchers almost exclusively.

                       Vertical movement of sliders compared to a pitch thrown without spin, in inches
Back to back days                3.0
Exactly one day of rest          3.1
Exactly two days of rest         3.2
At least one day of rest         3.4
More than two days of rest       3.3
More than five days of rest      3.4

Again, not enough data are available to measure the effect of three days straight. Here the effects of pitching regularly are much less pronounced. There appears to be some benefit, but almost certainly not enough to make a difference to the pitcher.

While this study appears to generally conform to common knowledge, it still is nice to look at. While pitchers appear to lose a little on their fastball, their sinkers appear to have more bite while their sliders remain relatively unchanged.

This study really is just the tip of the iceberg as to what could be done with these data. How a pitcher’s control is affected is the most important thing not touched on, but also things like pitching three out of four days or having long periods of inactively in a row may be important. This study could be modified to look at starters and how starting on short rest or long rest affects a starter as well.

Also, how does rest affect different pitchers? You can probably think of a reliever or two you won’t want to throw back-to-back games, but studying how much more that pitcher is affected might go a long way to getting to the heart of the matter and finding out how and why rest affects pitchers.