Weighted Ball Side Bend

Muscles worked

Filled = primary · Outlined = secondary

Source: Free Exercise DB (public domain)

To begin, lie down on an exercise ball with your left side of the torso (waist, hips and shoulder) pressed against the ball.
Your feet should be on the floor while your legs are crossed and hanging from the ball. Hold a weighted plate with your right hand directly to the right side of your head. Tip: Make sure the smooth side of the plate is resting against your head.
Place your left arm across your torso so that your palm is on your obliques. There should be a right angle between your left forearm and upper arm. This is the starting position.
Raise the side of your torso up by laterally flexing at the waist while exhaling.
Hold the contraction for a second and slowly lower yourself back down to the starting position while inhaling.
Repeat for the recommended amount of repetitions.
Switch sides and repeat the exercise.