Jonathan Broxton’s Declining Fastball

Jonathan Broxton, known to Dodger fans as “The Ox,” lost his closer role in August this past season after a 7.45 ERA and 1.76 WHIP in July. After a 2.26 FIP in 2008 and a 1.97 FIP in 2009, Broxton fell dramatically in 2010 with a miserable post-All Star performance. His home run rate remained relatively unchanged, while his walk rate increased by a little less than one walk per nine innings. Of the rate statistics, Broxton’s strikeout rate changed the most, decreasing from 13.5 K/9 in 2009 to 10.54 K/9 in 2010. But the largest difference in Broxton’s pitch results was how often he got hit (and how hard he got hit). Batters hit .270/.351/.367 off Broxton in 2010 (as opposed to .203/.276/.289 in 2007-2009).

The decline in Broxton’s fastball speed was properly publicized throughout the summer, and I wanted to take a look at how much that decline was. Broxton’s fastball averaged 97.5 mph in 2009 compared to 95.3 mph in 2010. I made a density plot of Broxton’s fastball speeds in 2009 compared to that of 2010 to show how drastic of a change this was. Take a look at Broxton’s fastball speed distribution below:

I plotted the first and third quartiles of each density plot, represented by the dotted lines. The first quartile is the 25 percentile mark which is larger than 25% of the sample, while the third quartile is the 75 percentile mark. Here are the values of those values along with the medians of each sample.

2009 fastball speeds:
1stQ: 96.6 mph
Med: 97.7 mph
3rdQ: 98.6 mph

2010 fastball speeds:
1stQ: 94.2 mph
Med: 95.5 mph
3rdQ: 96.5 mph

What I find particularly significant is that the first quartile of the 2009 sample (96.6 mph) is greater than the third quartile of the 2010 sample. Essentially, a 96 mph fastball would have been in the bottom fourth of Broxton’s 2009 fastballs, but faster than about 75% of his 2010 fastballs.

Broxton actually started off hot in 2010, holding a 0.00 ERA in April and striking out 35 in 23.1 innings over the first two months. But what’s confounding is that Broxton’s fastball speed was also diminished during that time frame, averaging 95.6 mph on his fastballs over those two months.

Although not nearly as significant, Broxton’s slider speeds also showed a decrease. Here’s his slider speed distribution for both seasons:

The bulk of Broxton’s sliders in 2010 were under 88 mph (about 75% of them), which matches the median speed of his sliders in 2009. Broxton mixes about 75% four-seam fastballs and 22% sliders though. In general, a rule of thumb is to keep sample size in mind when looking at any reliever’s secondary pitch. Still, there’s clearly diminished speed in Broxton’s slider, which is usually accompanied by a change in breaking movement. Broxton’s changes in distributions of slider horizontal movement and slider vertical movement suggest that his 2010 slider was more of a breaking ball with less cutter-like spin than his 2009 slider.

Frankly, Broxton wasn’t the same pitcher in 2010, and it showed up pretty blatantly in both of his main pitches as well as his performance. There are all sorts of reasons why this may have happened whether it’s conditioning, a change in mechanics, or an injury. What’s clear is that, fundamentally, Broxton has been a different pitcher, particularly in his four-seam fastball. If come late March, reports of Broxton’s fastball speeds indicate no improvement, expect his struggles to continue into 2011.