I can do isometrics on any part of my body whenever I want to.........walking, lying in bed, sitting in the car, standing at the sink... I don't like to do crunches, so many times a day, I'll suck and hold in my stomach for ten seconds. I do the upper and lower part of my stomach seperately.
I have reduced the size of my waist considerable by doing isometrics. I now also stand taller, it has helped my posture.