Let's look at some simple but effective trunk movements. My first suggestion is to execute free movements on your own two feet. As stated in previous tips, the trunk is constantly stabilizing the body during free motion. Stabilizing means to contract the muscles to ensure stability in your stance and more importantly, your spine.
Squats, deadlifts, push ups, pull ups, bent-over rows, and so many others hit the abdominals as a consequence of performing that primary movement. Not only does this remove the need to spend an extra 50 minutes in a summer abs class, it teaches your abdominals, glutes, and lower back to communicate with the rest of your body. Who wants to throw out their back while picking up a pencil off of the floor?
With this in mind, you really only have a few specific trunk movements to include in your arsenal. The trunk rotation with a resistance band affixed to a point at any level will allow you to rotate in the opposite direction. The wood chop, reverse wood chop, and horizontal rotation hit the entire trunk.
Bands allow the resistance to start opposite the movement. Simply doing rotations with a weight in your hands loads your shoulders more than the trunk because the gravitational pull of load is vertical, thus making this less effective. However, with the origin to your left and rotating to the right, for example, you more effectively target the trunk. A cable pulley machine with multiple height adjustments can also be used in the same manner for trunk rotations.
But, how about the lower back and glutes? While you rotate against an opposite resistance, your feet pivot and your hips rotate. You naturally engage the lower back and glutes to facilitate this strong rotation. Imagine a tennis stroke, golf swing, and baseball swing...the lower back and glutes always support abdominal action.
A little long for a quick tip, but I hope you get the idea behind following the human movement principles to train the entire YOU. Aren't you worth it? :)
Make it a great day!