it seems really not much of a problem here.
when underweight, eat more, exercise more, get fit, strength
training, muscle training.
when low back pain, avoid carrying any heavy loads and objects,
and strengthen your body core muscles by simple and very low
intensity slow exercises such as some yoga/ pilates movements.
if people ask you to carry heavy things, simply refuse to do so
and show them in the face the medical report, MRI report and any
specialist memo, and say to them that if you get injuried, who
is going to take full responsibility since you had already
sound out beforehand?
The problem is did MO give threadstarter excuse heavy weight
permanent?