I need advice for tightening my stomach but have metal from my T11-L3 therefore I cannot bend my back for sit ups. Advice on any other way?
You can actually tighten and relax the abs in a sitting position. A few sets of that would help out. A whole sit-up is just wasted energy anyhow.
Lay flat on your back and lift your legs, then move your feet in small motions up and down. It's really hard on your belly!
flex belt or ab contour belt
Try Isometric's. They might have some vidio's on utube.