The crunch is the simplest direct ab exercise — a short spinal flexion that trains the rectus abdominis, the muscle behind the 'six-pack' look.
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Get FormTwin — free on the App StoreCrunches work the rectus abdominis — the front sheet of ab muscle that forms the 'six-pack' — with some oblique involvement. They train spinal flexion specifically.
No exercise burns fat from one spot. Crunches build the ab muscle underneath, but seeing it requires lowering overall body fat through nutrition and total activity.
Crunches train the abs by moving the spine (flexion); planks train them to resist movement (stability). Both are useful — many programs include the two.
This guide is general fitness information, not medical advice. Muscle involvement varies with form, grip and individual anatomy. Consult a professional before starting a new program.