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The core refers to a group of muscles around the midsection. Core is a marketing term. And there is no real rigid definition of what muscles constitute the core. And anatomy books don’t have a chapter titled ‘Core’.
But here’s why narrowing the function of your core to planking or a handful of exercises is probably short sighted.
A loose definition of what we think of ‘quality’ movement has a few traits:
Quality movement be it picking up things, walking, squatting, cycling, swimming, running, jumping, pushing, pulling, carrying, bending, straightening or any movement really does require the body to stabilize itself.
When you run, your midsection helps your upper half stay in sync with the lower half. When you push or pull a heavy object, your midsection helps the upper body generate momentum by maintaining a strong link with the lower half.
When viewed through this prism, a heavy squat or single leg squat requires your mid section to balance many moving parts to produce movement. And this is as potent a dose of exercise for your ‘core’ as any plank, hollow hold or crunch.
Rotating, bending, straightening, extending and fighting all those muscle patterns is necessary for a strong ‘core’.
Broaden your view of the core to include the muscles of the back, upper back, hips, abdomen and glutes. All these muscles and more contribute to quality movement. Carrying things, squatting, deadlifting, glute hams are all required exercises for a strong core.
All movement will require your mid-section to contribute to stabilization. As you make the movement more challenging with load, you learn to stabilize more.
If the explicit goal is to improve the appearance of your mid-section, the abdomen can be trained like any other muscle group. Using a variety of movement in the right doses with load and make those muscles stronger. And all of this works only if you are carrying sufficiently low body fat levels. But remember the primary goal for a useful core is to support quality movement.