text: if i work out 3 times a week at a gym, what excercises are best (bad back) for firming tummy? i used to be a runner and hope to get back into it.  i know if i strengthen my abs, i can maybe help my back not ache or suffer damage from running, but where do i start?  i do some sit ups, but they aren't great for my back either.  what about walking or an eliptical machine? i would recommend you focus on abs just once a week any type of crunch should do it. what you have to keep in mind is that the key to slim tummy is diet and aerobic workouts...so you are right on the money, eliptical machine should do it...good luck