I don't think I've ever used one heavier than 2 or 3 kg. I draw the line between "challenging" and "suicidal."
Another interesting core exercise is a drill in which you get into push-up position in front of a step, then lift one hand to the step, then the other, so you're now in push-up position with your hands on the step. Then you put one hand back on the floor, then the other.
Then you repeat a few times, alternating the hand you lift up first.
You aren't moving your feet throughout this drill, so you're changing your center of gravity every time you move your hands.
Like the medicine-ball toss, you're integrating the muscles stabilizing your shoulder girdle with the muscles supporting your hips and spine. Everything needs to be firing to do this while maintaining perfect spinal posture.
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