I vary between bench pressing with feet up and feet on the floor. I'm an older guy (50) and short (not easy to plant feet flatly on the ground) with arthritis in my SI joint and a history of back pain. Spasms. Sciatica. When I plant my feet my back arches and doesn't feel comfortable. But I see the legs up method dissed by most experienced and qualified lifters. Yesterday I was on my 1st working set with feet planted and suddenly felt a bad pain in the lower spine. I have been lifting for a couple years and studied good form. But I'll be the first to admit I don't always execute correctly. By morning I was in so much agony I could barely get out of bed. I'm thinking I triggered spasms and not much I can do but rest and pain killers. But any advice on benching with these issues? Fwiw legs up makes my back flat and feels much better.