The human body is composed of a central core and appendages that extend from it. The trunk, or torso, forms the main mass of the body, providing a central axis. From this core extend the extremities, which include the two arms and two legs. They work in a coordinated fashion to allow for complex movements and interaction with the environment.
Anatomy of the Trunk
The trunk serves as the body’s central pillar, extending from the base of the neck down to the pelvic region. It is broadly divided into the chest, also known as the thorax, and the abdomen. These regions are separated internally by the diaphragm, a large muscle that facilitates breathing. The back forms the posterior aspect of the trunk, encompassing the vertebral column.
This central region is structured around the axial skeleton, which includes the vertebral column and rib cage. The vertebral column, composed of 33 vertebrae, runs through the center of the back, providing both flexibility and support.
Within the thoracic cavity, shielded by the rib cage, are the heart and lungs. The abdominal cavity contains the majority of the digestive organs, such as the stomach, liver, and intestines, as well as the kidneys. The lowest part of the trunk, the pelvis, holds the reproductive organs and the urinary bladder.
The Upper and Lower Extremities
The extremities, or limbs, are attached to the trunk and are specialized for interaction and movement. They are structured around the appendicular skeleton, which consists of the bones of the limbs.
The upper extremities, the arms, are connected to the trunk at the shoulder. Each arm consists of the upper arm, forearm, wrist, and hand. This structure allows for a high degree of dexterity for manipulating objects and interacting with the surrounding environment.
The lower extremities, the legs, are attached to the trunk at the hip. Each leg is composed of the thigh, lower leg, ankle, and foot. Their primary functions are locomotion and supporting the entire weight of the body. The bones and muscles of the legs are correspondingly larger and stronger than those of the arms to accommodate these demands.
How the Trunk and Extremities Work Together
Movement and stability arise from the integrated function of the trunk and the extremities. The arms connect to the trunk through the shoulder girdle, while the legs connect through the pelvic girdle. This connection provides a stable base from which the limbs can operate effectively.
The generation of force for powerful movements often originates in the trunk and is transferred through the limbs. For example, the act of throwing a ball begins with the rotation of the trunk, creating momentum that flows through the shoulder and into the arm. A stable core, provided by the muscles of the trunk, is needed for this efficient transfer of power.
Similarly, effective locomotion relies on this partnership. During walking or running, the muscles of the trunk stabilize the pelvis, allowing the legs to move efficiently and powerfully. Without a stable core, movement would be less controlled and more prone to injury.