Consider investing in a fitness ball if you want to do effective conditioning of the trunk area, say Mayo Clinic experts.
Fitness balls (also known as Swiss or therapy balls) may look more like a children's toy, but they're a powerful new fitness tool. Researchers at the Mayo Clinic suggest using a slightly deflated ball of 55 to 65 centimetres diameter.

The balls work by constantly changing your body's stability base - so you have to work your muscles to keep your balance. A workout using the ball will challenge your trunk muscles - that is, the muscles of the abdomen, chest and back. Strengthening this area makes your whole body look more toned, as well as preventing back problems. You can use the ball to do abdominal curls, push ups and lots of other exercises. But if you haven't used one before, it's wise to get advice from a fitness instructor to make sure you use the right technique.