The Weighted Russian Twist is a core strengthening exercise that primarily targets the internal and external obliques. The addition of a medicine ball in your hands will give your abdomen an extremely challenging workout.
To begin, select a medicine ball of moderate weight. I light barbell plate or kettlebell will also suffice. Sit down on a comfortable mat with your torso straight but leaning back at around a 45-degree angle. The weight should be held out in front of you with both hands and elbows bent.
Lift your legs a few inches off the ground with your knees bent and your feet crossed together. At this point, you are balanced entirely on your glutes. Start to twist your torso and arms in unison to your right side so the medicine ball descends almost to the floor. Your eyes should be following the movement of the ball across your body and your feet should stay off the ground.
At the bottom of the twist, reverse back up and now twist over to your left side. Return to starting position. This completes one rep. Do three sets to failure.
This is a tricky movement to get right at first. You can practice with your feet flat in the ground and zero weight in your hands until you have mastered the necessary balance to stay upright only on your buttocks.