If you really want to work your abs [1], you should start using the TRX Suspension Trainer [2]. The TRX was invented by a Navy Seal in 1997, and since then it's become a staple piece of equipment in the fitness world. It can be used for total-body workouts [3], or you can isolate each body part for an intense workout. Fire up every muscle in your abs with the TRX knee tuck.

Here's how to do it:

• With the straps at mid-calf length, place your feet into each of the foot cradles.
• Lift your knees off the ground, coming into a plank position with your hands directly underneath your shoulders. Your back should be flat, and your core should be engaged (there should be no arch or dip of the torso visible).
• Bring both knees in toward your elbows at the same time. Extend your legs, returning to your starting position. This completes one rep.
• Repeat for a total of 15 reps.