A weight-free weekend challenge may sound easy -- until you try the Bodyweight 300. It's composed of just three exercises -- the squat, the pushup, and the situp -- but requires that you crank out 300 reps in as little time as possible. It's a fast way to challenge every muscle in your body and skyrocket your heart rate when you don't have the luxury of spending hours in a gym. (Think you have the pushup down? Think again. Here are 5 Body-Weight Exercises You're Doing Wrong.)

"This is the ultimate do-anywhere, zero-excuse metabolic circuit," says Todd Durkin, C.S.C.S., author of The IMPACT! Body Plan. "You can do it anywhere, anytime -- outside, in your garage, or even your bedroom. Your body weight is all you need to crush fat, break a sweat, and build total-body muscle."

Perform 100 reps of each exercise in any order as quickly as you can without rest. You don't have to do all 100 reps in a row -- you might do 20 squats, move on to 10 pushups, do 30 situps, do another 20 squats, and so on.

Watch the video above to see how to perform the exercises. Durkin completed this challenge in 5 minutes and 47 seconds. Did you beat his time? Let us know in the comments below.

Add these 4 Ways to Burn More Fat to your regimen, and that extra belly roll will be gone soon.

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