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The Seated Torso Rotation Over-Twist
This machine is designed to shape the obliques and eliminate love handles, making it popular with gym-goers looking to whittle their waists. However, it requires you to lift a stack of weights by twisting only your upper body and keeping your pelvis facing forward (try it in your chair; awkward, isn't it?). Isolating the back in this way puts an excessive amount of rotational force on the spine. Over time, it can result in a disk-related injury, says Matthews. Some devotees of this machine have found themselves hobbling to the chiropractor with nerve damage that causes shooting pain in the legs.
Be safe: Protect your spine and work your obliques with wood chops, says Matthews. Stand facing forward with your left foot slightly in front, and hold a medicine ball at hip height with both hands. Rotate your torso to the left as you straighten your arms to bring the medicine ball up over your left shoulder. Now bring it down diagonally across your body to the outside of your right hip (you'll feel it in your core). Repeat on the opposite side with the right leg forward.