hi, so i'm a paraplegic in a wheelchair and i also have a rod in my back due to a major surgery to curb scoliosis. that being said, let's talk about what wheelchair bound people who can not bend their spine do to excercise:
1. someone mentioned it - eat in moderate portions. cut out the regular cokes and go for the diet ones if you have to drink colas. no discussion of chemicals in diet sodas please! :)
2. i work out in the gym for 30 min three times a week. twice a week, i wheel just fast enough to get my heart rate up around the neighborhood for 30 min. my workouts aren't long or a big process b/c i don't have much time.
if i can't do either, i'll do some contracting of muscles. therein lies the resistance that weights or pushing around provides. ie, contract stomach muscles.
hopefully, this helps someone.