squatting on a barbell and raising the weight up using your legs can help strengthen the back, provided it is done properly. https://www.nhs.uk/live-well/exercise/exercises-for-back-pain/ Doing deadlifts with lower back rounded excessively will compress the vertebral discs and cause back pain worse.
going from laying flat to sitting upright puts strain on the spine particularly if the core muscles is weak. For someone with little or no strong core muscle to do sit-ups will put a strain on the spine and this will exacerbate back pain.
- Toe touches
if you do toe touches in order to stretch the back of the legs, there is good chance that you will round your back too much and put pressure on the your spine.