In the present, Finch shows signs of significant spinal trauma. He has a severe limp and his neck and upper back are generally held very rigidly. He'll turn his whole torso, or sometimes even his entire body, rather than just rotate his head to look at something.

However, in most flashbacks we've seen him working fairly normally. In one scene, he's even jogging on a treadmill.

Has there been an explanation yet, as to what injuries Finch suffered to bring him into his present condition? I'm pretty sure I'm fully up to speed on the series, but don't recall any explanation thus far - is there something I've missed or forgotten?