The 4-Minute Gut and Guns Challenge 

Dec 12 / Build Muscle

This might become your favorite weekend challenge to date. The reason: It hammers your biceps and abs—two muscle groups that most guys love to pay special attention to. 

The challenge is comprised of only biceps curls and knee-tuck situps. While the moves aren't complicated, you must complete a ton of reps and you must do them with perfect form. That means absolutely no body English, says Todd Durkin, C.S.C.S., author of musclemorphosis.com. 

"When you do an exercise with strict form, you're forcing the working muscle to do all of the work," he explains. "Gravity and momentum can't help you cheat, so you'll maximize your results."

Ready to try it? Here's how it works: Grab a pair of 25-pound dumbbells. Perform 25 strict bicep curls. Without resting, put down the dumbbells and perform 25 knee-tuck situps. That's 1 round.

Perform 3 rounds total as quickly as you can. If your form slips, stop and take a break. You can pick up where you left off, but you must keep the clock running the entire time. 

Durkin completed the challenge in just 4 minutes and 19 seconds. Try to beat his time and let us know how you did in the comments below.