If you've ever wondered (mid-crunch, most likely) exactly how many sit ups you should be doing every day to optimise your fitness, then you're in the right place - this is exactly the type of question ...

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When you buy through links on our articles, Future and its syndication partners may earn a commission. Credit: Getty Images Bored of planks and crunches? Why not try mixing things up with a killer ab ...

Take a 30-second break, and try it again. Its a lot to handle, so let Marthaler take you through proper sit-up technique: Keep that chest up, he says, abs tight, and slowly peel off and come up.