Save your back and round up a six-pack.

The Swiss ball is most commonly used to make crunches harder, but that doesn’t take full advantage of its greatest offering—instability. Putting yourself in a position where the ball is free to roll, or even encouraging that rolling, requires your abs to clamp down on your spine twice as hard to prevent losing balance. Swap out your other core work for Swiss ball elbow circles and you’ll see how well it works.

1. GET INTO A PLANK POSITION with your forearms on the ball. Your elbows should be directly under your shoulders, and your body must form a straight line.

2. BRACE YOUR ABS and begin moving your elbows in a circular motion, clockwise, as if you were stirring a big pot. Continue for 30 seconds. Perform 2–3 sets, alternating the direction each set.

Quick Tip: Swiss ball elbow circles strengthen the lower back, and, unlike situps and crunches, are safe to use if yours is injured.

http://www.muscleandfitness.com/workouts/abs-exercises/stir-stronger-core