When I do my ab work, and a machine isn't available I do "planks". Get into position like you're going to do a push-up - arms, legs and back straight and then just stay in that position for as long as you can. I usually wait 'till my arms are shaking before relaxing, but that's just me. If it's too much at first, put your knees on the floor. Repeat 3 or 4 times and track the time you are in that position. Works as good as crunches or ab machine. Will NOT burn fat, but will tighten up your core for sure!