Here are some common exercises I teach for upper trunk stability:
Stick Up: Stand with your back to the wall and try to hold your shoulder blades flat against the wall throughout this activity. Bend elbows and put hands into the air as if someone told you, “This is stick up!” While keeping your shoulder blades flat against the wall, slowly raise arms overhead, then back down again. Repeat x 15.
With your back against the wall, and feet out 12″ from the wall, pull theraband at diagonals back all the way to touch the wall with top and bottom arm. Keep your arms straight throughout the exercise. If you bend the elbows, you will work the wrong muscles! Complete 15 reps in each direction.