Even after stretches, i can't keep my back straight during a squat. i've slipped disk. has my back lost/never had this ability? should i avoid it? my back aches after 5 sets of 6 reps or 10 min of using hammer on flooring nailer - no pain to peers.

Back pain: See your provider and get assessed. If indeed you do have a slipped disc then you should be referred initially to physical therapy. Disc problems tend to resolve within a 6 week period and there are specific exercises that help. Now if you are having specific weakness or loss of bowel or bladder then you may need more immediate therapy.