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Ab belts may cause muscle contractions if used properly. But the belt alone cannot produce six-pack abs. The real challenge to ab definition is the layer of fat that lies right under the skin, hiding muscle tone.
Another example of an ab exercise that works the hip flexors is any leg-raising exercise done in a supine (laying face up) position. Again, this movement works the hip flexors far more than the abs and shouldn't be done until you have good abdominal strength.