How to do it: Place both feet on a bench, and assume a pushup position. Brace your core. Then, without dropping your hips or moving your feet, make a full revolution around the bench by "walking" your hands all the way around it. Do 4 revolutions, resting 60 seconds in between revolutions.
Benefit: The stabilizing muscles in your abs, upper and lower back, and shoulders work harder than they would in a traditional plank.
How to do it: Attach a rope to the low pulley of a cable station and lie on your back alongside the stack. With both hands, hold the rope in front of your chest. Perform a crunch, arms locked, and don't let your torso rotate (even though the cable pulls you sideways). Pause, and return to the starting position. Do 15 reps, switch sides, and repeat. That's 1 set. Do another set of 12, and a final set of 10. Rest 45 seconds between sets.
Benefit: You're boosting the intensity and efficiency of a traditional crunch, and your abs have to work harder to prevent rotation.
Side Plank with Low-Cable Row
Replaces: Side plank
How to do it: Attach a handle to the low pulley of a cable station and lie on your side, facing the stack. Grab the handle with your top hand. Raise up into a side plank: knees straight, upper body propped on your elbow and forearm. Pull the handle to your rib cage; keep your hips pushed up and forward. Slowly extend your arm. Do 10 to 15 reps, switch sides, and repeat. Do 2 more sets, resting 60 seconds between sets.
Benefit: By engaging your lats, the fan-shaped muscles in your back, you increase torso stability and muscle endurance.
Back Extension with Alternating Dumbbell Row
Replaces: Back extension
How to do it: Holding a dumbbell in each hand, position yourself in a back-extension machine so that your torso is parallel to the floor. Without rounding your back, row the dumbbell in your right hand toward your rib cage. Lower, and repeat with your left hand. That's 1 rep. Complete 3 sets of 8 reps, resting 60 seconds between sets.
Benefit: As your entire body works to stabilize your spine, you're boosting endurance in often-neglected muscles: spinal erectors, glutes, and hamstrings.
Replaces: Swiss-ball pike
How to do it: Assume a pushup position, but with your feet on a medicine ball. Keep in a straight line from your head to your ankles. Without bending your knees, roll the medicine ball toward your hands by raising your hips as high as you can and rolling the ball with your feet. Pause, and reverse back to the starting position without letting your hips sag. Complete 3 sets of 15 reps, resting 45 seconds between sets.
Benefit: You improve balance and engage your hip flexors and rectus abdominis—your six-pack muscles