Ripped abs are reason enough to add some core moves to your routine. But a strong torso can also give you the foundation you need to lift more weight with less risk of injury. "Crunches alone won't get the job done, though," says Todd Durkin, C.S.C.S., author of The IMPACT! Body Plan." You need to train the two dozen muscles between your hips and shoulders the way they function."

That means doing moves that bend and stabilize your core. The exercises in this weekend's challenge -- the running man, side ups, plank to pushup, and cross-body mountain climbers -- do both of those things. "They'll test your core from every angle," Durkin says. "The better you are at these, the stronger you'll be in everything else you do." (Think you have the pushup down? Think again. Here are 5 Body-Weight Exercises You're Doing Wrong.)

Here's how it works: Grab a stopwatch. Perform 20 reps of each exercise in the following order: running man, sideup (right side), plank to pushup, sideup (left side), and cross-body mountain climber. Todd's time, and your goal: 1 minute and 48 seconds. Let us know your score in the comments below.

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We found 3 simple tricks to perfect your form and protect your shoulders: The Best Way to Do Pushups.