Least weight or benefit from additional weight

Taking a quick lunch break...  I think about that decision with each item in my gear:  less weight or the benefit that comes from the weight.  With some people, reducing the weight of their gear is part of the fun, some weird goal to see how little they can carry.  I guess they could refuse to carry a backpack at all, not eat unless they can find some pine cones or berries, not drink until they can scoop some up in their hands out of a stream and sleep on the bare ground hoping they don't freeze.  Other people add items to improve the experience of hiking.  They add each extra item they can carry for a reason.  They want higher quality pictures so they bring a better but heavier camera.
My situation is a little different, with each item I add and the associated weight involved the risk increases of hurting myself while hiking.  With me, I won't be hurting myself while I'm putting on the pack.  I will be hurting myself from bearing the weight over hours and hours of hiking.  During the hours and hours of hiking, my back and abdomen muscles get more and more tired from supporting the weight of my upper body and whatever gear I'm carrying.  As the muscles get tired, they carry less and less of the weight, depending on the bone structure of my vertebrate to make up the difference.  My problem is my lower back is held together by pins and screws.  I need to keep my gear weight as low as possible, reduce the length of time I am hiking and improve the strength of my abdomen and back muscles.  I think it would also help to reduce the weight of my torso through improved diet and eating habits.
Ultralight for me includes strengthening my core and losing fat in my upper body.  That will save my back and knees.