In gambling-speak, “to sandbag” is to deceive. In workout-speak, the sandbag may be just as sly. A duffel bag filled with beach dust? How challenging can that be? Very.

For starters, the sand is constantly shifting inside, so you (your core, your legs, your whole body) have to work harder to stabilize that resistance. Then you’ve got all the handles attached to the bag. “The variety gives you endless grip options and movement possibilities,” says certified strength and conditioning specialist and physical therapist Dan Ogborn, Ph.D., who created the workout here. “When you can play around with the way you hold and move weight, you engage new muscles and challenge them in alternative planes of motion, which is key to accelerating your results.”

Plus, thanks to its size and shape, this training tool also better mimics real-life loads—say, a bag of laundry, a 30-pounder of dog food, an overstuffed weekender—helping you develop more functional strength that will translate to any effort, in or out of the gym.

(The Slim, Sexy, Strong Workout DVD is the fast, flexible workout you’ve been waiting for!)

Two or three times a week, complete the following circuit: Perform each exercise in order as directed, moving from one to the next without resting or dropping the sandbag. Repeat twice for a total of three sets. No sandbag at your gym? You can buy one for $60 at sklz.com.