It includes your back and abdomen. Sometimes, you need to twist your trunk to one side or another while keeping your lower body stationary. This general motion is a trunk rotation.

You should feel a mild stretch in your lower back. Repeat the rotation 10 times, holding each stretch for 1–3 seconds before slowly moving out of the rotation. Repeat steps 1–6 on your left side.

Another reason it's a must-do exercise, in my opinion, is because rotational movements are often bypassed but are important to prevent excessive rotation in the lumbar spine (your low back).

The side-lying internal rotation exercise relieves tight shoulders and improves range of motion. Here's how to do it, and the benefits of stretching the shoulders.