Sit-Ups, Crunches, Planks, Knee-Ups: The Evolution of Core Fitness

Core strength testing has come a long way from the straight-legged, twisting sit-ups used before World War II.

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3 Easy Ways to Do Knee Crunches - wikiHow

Crunches Vs. Sit-Ups—Which Exercise Is Better?

That powerful connection between exercise and teens' brain development

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Planks or Crunches: Which is Better?

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Sit-Ups: Correct Way To Do, 8 Variations, Benefits

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