Final answer:
Retraction is the posterior movement of body parts like the scapula towards the vertebral column and the mandible backward. It is essential for maintaining posture, performing certain exercises, and enabling complex jaw movements for speech and chewing. Protraction, elevation, and depression are the opposite or complementary movements to retraction in these body parts.
Step-by-step explanation:
Retraction refers to the movement where parts of the body are pulled posteriorly. In the shoulder, retraction occurs when the scapula moves towards the vertebral column, often referred to as squeezing the shoulder blades together. This movement is essential for various functions, such as maintaining good posture or in exercises that target the back muscles. In the context of the jaw, retraction is the movement of the mandible backwards, contributing to the complexity of movements necessary for speech and mastication. The masseter muscles are involved in jaw retaction movements, particularly during chewing.
These motions are counteracted by protraction, where the shoulder is moved forward, as in pushing movements, and the jaw is thrust forward. In addition to protraction and retraction, the shoulder and jaw can also elevate and depress. Elevation involves raising these body parts, such as shrugging the shoulders or closing the mouth, while depression is the lowering of them, like when opening the mouth to yawn.