Board Thread:Questions and Answers/@comment-25322628-20151220092919/@comment-24930713-20151220102737

Both? They originally thought, because of how slow he moved and how much armor covered his body, that his joints were covered as well. In his, if I'm remembering correctly, second fight with Eren, they realize that suddenly he's moving a lot faster. Based on previous history; like medieval armor, Hange makes the connection that to allow a soldier to move, specific parts needed to be kept uncovered. Hange correctly assumed that he had some level of control over whether his joints were or weren't hardened. At least, that's how I understood it.