The purpose of the core muscles is to resist movement to protect the spine and support the visceral organs, rather than concentrically flex, extend and rotate the spine. For that reason, we can design programs that work the core isometrically.
The most effective core exercises are those that apply force that the rectus and/or obliques must resist. Anti-extension and anti-rotation exercises force the musculature to resist either movement. Plank variations are the best way to accomplish this.
The Banded Plank Fly will work your obliques more than anything you've tried before. Using a small band, perform a Plank with the band around your wrists and your feet spread wide. Lift one arm and fly out to the side, stretching the band and fighting the urge to rotate your hips. See the video above for a demonstration.
Not only does gravity on the side of the lifted arm activate the obliques, but the resistance from the band makes it even more challenging, hitting all four obliques at once! Depending on your shoulder strength, you may initially feel it more in the shoulders, but your obliques will never be sorer from the double anti-rotation. An additional benefit is the work done by the rear delts, traps, and rhomboids to fly against the resistance of the band.
Start with 10 reps on each arm. One set may be enough, but shoot for three sets. If you're really feeling brave, pair it with the Stir-the-Pot exercise in a compound set.
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