Scapula retraction sitting arm supported (Scapular retraction/depression strengthening isometric, arm on bench, sitting)

Sit up straight on a chair.
Do not lean onto the back of the chair.
Place your weaker arm onto a raised surface to your side so that your arm is just below shoulder level.
Place your other arm on your lap.
Use a small movement to bring your shoulder blades back and down.
You should not move from your back during this exercise
Hold this position.