Day 30: Stay Still for a Six-Pack

Isometric exercises (like the plank) are the most effective way to train your core, according to a recent study by scientists at the University of Waterloo. Why? Because not only does holding a rigid position extend your muscle’s time under tension (a key growth trigger) but it also reinforces your core’s primary job: Stabilizing your spine.

Plank: Assume a push-up position, but with your elbows bent and your weight on your forearms. Hold for as long as you can with your abs braced, your glutes clenched, and your body straight and rigid from head to heels. Hold for 30 to 90 seconds. Rest for one minute, then repeat.

For more 31 Days of Fitness tips, check here for a new one every day in December.