How much did CC Sabathia's new knee brace have to do with his late 2015 success?

CC Sabathia acknowledged to reporters this past week that despite his initial reluctance to don a knee brace on the mound, he would likely wear one while pitching throughout the remainder of his career. The need for the brace stems from Sabathia's pitching with bone-on-bone arthritis, which will most likely require knee replacement surgery in retirement.

Sabathia first pitched with the brace on September 9, and in his five final games of the 2015 season he enjoyed his most successful stretch of the year. Over that five game span Sabathia threw 29 innings, allowing seven earned runs, a batting average against of .224, and an OPS of .647.

While only time will tell whether Sabathia's success with the brace in 2015 will translate over to the 2016 campaign, a dive into pitch selection data from Brooks Baseball can provide us with some indication as to the source of his improved September performance.  More specifically, the pitch selection data can facilitate our distinguishing between whether a) Sabathia had the same approach in terms of pitch selection with and without the brace, but improved his execution with the brace, b) he modified his pitch selection in some way that helps to account for his success with the brace, or c) a little bit of both.

The table below displays the frequency with which Sabathia deployed each of his pitches pre and post-brace.

Although the sample size for Sabathia's post-brace stretch is relatively small, the difference that is most readily apparent is in the utilization of his fourseam fastball. Sabathia threw his fourseamer about 22% more often with his knee brace than without, and in doing so, chose to go to his changeup and his slider less often.

What is notable about this uptick in fourseamers is that improved stuff cannot account for Sabathia's decision to use this pitch more frequently.  In his first 24 starts, Sabathia's average velocity with his fourseam fastball was 91.4 mph, compared to 90.6 mph in his final five starts with the brace.  The PitchF/X velocity and horizontal movement numbers are also almost virtually identical between his starts with the brace and his starts without it.

In addition, Sabathia's fastball control with the brace did not improve.  He threw 33% of his fastballs for balls without the brace, and 37% for balls without it.

However, Sabathia's command of his fourseamer did appear to benefit over the season's final five games.  During that stretch, batters fouled or whiffed on swings against the fourseamer 65% of the time (versus 57% of the time pre-brace), they hit more ground balls for every fly ball (2.2 post-brace versus 1.9 pre-brace), and fewer fly balls or line drives turned into home runs (8.3 HR/(FB+LD) post-brace versus 12.5 HR/(FB+LD) pre-brace).

While this evidence is far from conclusive, it does suggest that Sabathia's pitch location post-brace did make it more difficult for hitters to square up his fastball.  It is also possible that his other pitches manifested similar improvement with the brace, even if Sabathia chose to use them slightly less frequently.

Ultimately the most prudent hypothesis for Sabathia's improved performance with the brace is choice C above: a little bit of both better execution and a modified approach.  Sabathia did not drastically alter his pitch mix with the brace but did throw more fourseam fastballs, which can potentially be attributed to improved command.

No Yankees pitcher threw more innings than Sabathia's 167.1 last year, and if the Yankees make another playoff run in 2016, it is likely that Sabathia will play a central role in that story.  And while hoping for Sabathia to duplicate his September success over a full season is most certainly setting expectations too high, fans can hope that there is some truth to the results he realized while wearing the brace. There is no doubt that Sabathia knows how to pitch and compete on the mound, and if it can help to make his command more consistent, CC's knee brace is poised to become a fan favorite this season.