Research from the Biomechanics Lab at San Diego State University, commissioned by the American Council on Exercise.
This study measured activation of the abdominals muscles - the rectus abdominus (6-pack muscle) and obliques - for 13 different exercises.
The three most effective in terms of activating muscles are the bicycle, captains chair (leg raises) and stability ball crunches.
Some equipment, like the ab rocker, was far less effective than even traditional cruches.
The study also reported that most people are unable to separately trigger activation of the upper and lower ab muscles - suggesting that the muscles act together and not independently.
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