Answer:
The answer to the question: What thoracic muscle is the prime mover of shoulder flexion, would be, A: Pectoralis minor.
Explanation:
The movement, and stability of the shoulder joint, and the arm itself, depends mostly on many muscles and tendons that keep it from coming apart with movement. While all of them work together to protect this joint and permit arm movement, some of them will aid in a certain type of movement, while others will antagonize these first. The important thing here is that as your joint moves, these muscles activate to ensure stability and movement. During flexion, which is literally the movement of bringing the arm closer to the body, there are several muscles that act as synergists, meaning, they act together to ensure correct movement and stability. One such muscle is the pectoralis minor, which arises from the ribs, and then inserts into the coracoid process of the scapula. In conjunction with other muscles, the pectoralis minor will be prime in shoulder flexion, and will be aided by others.
