The weighted Russian twist improves the rotational ability of the midsection while targeting your abs, obliques, and hip flexors.
Lie with your back on a Swiss ball. Feet should be flat on the floor and bent at 90 degrees. Clasp your hands together around a dumbbell or medicine ball, and hold it in front of your chest with straight elbows.
Carefully rotate your arms to one side, keeping your feet planted and arms parallel to the ground. Rotate back to the starting position, and then repeat on the opposite side.
Do not rotate too far. Only rotate as far as you can maintain tightness.
Make sure you pick a weight suitable to your strength level.