Two exercises keep proving to be gold standards of core strengthening: the crunch, which firms the more superficial abs-the rectus abdominis down the center and the obliques along the sides-and the plank, which works the deep, corset-like transverse abdominis. (Try these plank variations to torch your core from all angles.)

The crunch is so effective at activating these superficial muscles because their fibers all have a vertical orientation that lets them be in sync with the exercise's straight-up motion, says Martin Eriksson-Crommert at Örebro University in Sweden. His research found that women who place their hands behind the head-instead of atop the chest or reaching forward-get the biggest activation. Speeding up the pace of crunches can double the activation of the rectus, other research has shown. Add a twist to further engage obliques.

Now, about those planks. The variations shown here-plus the core-stability challenge of the double-leg stretch-elicit transverse activation that goes off the charts, according to lab research by Shape Brain Trust member Michele Olson, Ph.D., a senior clinical professor of sport science at Huntingdon College in Alabama who has tested ab exercises for over 20 years. Work them in with your crunches or any of the ab-focused routines and tips in this story for your fittest, firmest core. (And it’s not just about getting a six-pack; here’s why it’s so important to have a strong core.)

Routines with Built-in Core Tighteners

Certain workouts double as strength sessions for your abs. "When you increase your intensity or load during total-body movements, your ab muscles naturally brace," Olson says. "That action really works the transverse." Here's the way to multitask.


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