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.

Are sit-ups really the best way to tone your abs and what exercises would be better?

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

Fitness

Beginner Ab Workout Without Crunches - Postpartum Ab Exercises - Diastasis Recti Safe Workout

What are the differences between squats, lunges, and push-ups and crunches/ sit-ups and leg raises/planks? - Quora

Come plank! It engages more muscles than crunches or sit ups. Plus it burns more calories!!

ACE - ProSource™: February 2014 - Reality Check: Are Planks Really the Best Core Exercise?

Fitness

The Best Sit-Up Variations for a Strong Core

Can you do sit-ups while pregnant?