Final answer:
The middle deltoid is responsible for abduction of the arm at the shoulder joint. If the middle deltoid is not functional, the ability to lift the arm away from the body in a lateral direction will be impaired.
Step-by-step explanation:
If the middle deltoid is not functional, the action at the shoulder that will be impaired is abduction of the arm. The middle deltoid is responsible for lifting the arm away from the body in a lateral direction. Without the middle deltoid, the arm will have reduced ability to move out to the side, resulting in limited shoulder mobility.