I can't remember if I have something specifically addressing chastisement itself, but I do remember having written about it. Consider how Hebrews refers to this chastisement - as a father dealing with his son, and not as with a bastard. A father (at least understood by the Hebrews) had his son's DEVELOPMENT in view ... not his shortcomings. Now, when he saw his son wandering into insanity he might deal with him IN the situation, but it was ACCORDING TO his heart's desire.
Our father sees us according to the reality of Christ He has made us alive in, WE are the ones who have a problem recognizing it. Just because His dealings may SEEM to line up with injuries or trials or what-have-you, we are the ones who will try to figure out what GOD might be doing by things that may have no connection whatever!!
Do your back injuries really have to be considered as chastisements at all? Could they not just be a weakness and/or an injury? Then again, might this incident not end up being a gift as our father works all things together for your good? I think we have been trained to be very, very superstitious in our "Christian" upbringing.