The side bend is a simple but effective lateral trunk flexion exercise.
Stand straight and hold a weight in one arm at your side (kettlebell, dumbbell, plate or farmer handle). Keeping your entire trunk and glutes tight and making sure your hips and shoulders remain squared off facing forward, bend to one side and then to the other with a controlled tempo. Switch hands and repeat after the desired number of reps.
Lateral trunk flexion is usually neglected in favor of forward flexion, but the sides of the trunk provide an important stabilizing function for the trunk and spine. The side bend is a very simple but effective method of strengthening the lateral trunk.
The side bend can be done for 3-5 sets of 10-15 reps/side. Use a weight that allows full range of motion.