Final answer:
The sternoclavicular joint, linking the sternal end of the clavicle with the manubrium of the sternum, is vital for shoulder mobility. The degrees of movement in this joint, along with the clavicular portion of the pectoralis major muscle, facilitate various shoulder movements.
Step-by-step explanation:
The sternoclavicular joint is an important articulation between the manubrium of the sternum and the sternal end of the clavicle. It forms the only bony attachment between the pectoral girdle of the upper limb and the axial skeleton. This joint allows for considerable mobility in several directions including the upward/downward and anterior/posterior movements which are essential for shoulder function.
The clavicular portion of the pectoralis major, which is one of the nine muscles that move the humerus, originates on the clavicle. This portion is instrumental in facilitating shoulder movement, including rotation and flexion. The degrees of movement allowed by the sternoclavicular joint depend on the range of motion of this muscle as well as the joint itself.
The sternal end of the clavicle, as mentioned in the question, articulates with the manubrium of the sternum, forming part of the sternoclavicular joint, and it plays a crucial role in the overall function of the shoulder and upper limb.