Pence, 31, told the FOX broadcasters before Game 1 of the NLCS that he has Scheuermann's Disease, a spinal disorder that he likely developed as an adolescent.
The condition affects the vertebrae, and the way Pence described it, he has no flexibility in his thoracic spine.
"That's why he throws the way he does," Giants head athletic trainer Dave Groeschner said. "He has no rotation in the mid-spine.”
Nevertheless, Pence has consistently rated among the best outfielders in baseball since he broke into the majors in 2007. According to FanGraphs, only five outfielders have been more valuable than Pence (29.2 WAR) since the beginning of the ’07 season: Matt Holliday (42.1), Andrew McCutchen (33.9), Ryan Braun (33.1), Jose Bautista (30.7), and Jayson Werth (29.9).
Pence has reached base in all six of the Giants’ post-season games thus far.