Your muscles may release too much myoglobin and overwhelm the kidneys, which will be working hard anyways. Only you know how much exercise would be tolerated by your fasting body, and only if you had done it a few times previously. Paul Bragg exercised while fating, but after years of 4x 10-days fasts a year, numerous shorter fasts, together with nearly impeccable diet in between. Secondly, the priorities of a fasting body are independent from one's intentions, i.e. whether fat breakdown and burn will go toward energy, toward repair of some old conditions, a recent injury or something else is up to your body's autonomous wisdom, not up to you or what someone else here would tell you. I hope that helps.
DL

Check out Leangains or eatstopeat

dot com ...those guys do intermittent fasting and weight lift and they are huge