The Appendicular Skeleton

By LadyofHats Mariana Ruiz Villarreal (i did it myself) [Public domain], via Wikimedia Commons

The term Appendicular Skeleton refers to the set of bones associated with the limbs of the body. This is primarily the legs and arms and the support structures associated with these appendages. The name Appendicular Skeleton derives from the notion that legs and arms are appendages to the body.

The diagram at right provides an overview of the bones associated with the Appendicular Skeleton. It also provides common names for various regions of the skeleton.

The Appendicular Skeleton is primarily responsible for movement. This includes locomotion capabilities largely provided by the legs and feet, and the grasping and manipulative capabilities provided by the arms and hands.

Most of the bones in this system function as levers. A muscle pulls against two bones to cause at least one of them to move. For example, your bicep muscles attached to the scapula at one end and the radius bone in the forearm on the other. When the muscle contracts the forearm moves, pulling toward the scapula.

We will cover the muscular system in a future belt curriculum, but you should appreciate that directed bone movement is caused by muscles pulling against bones, which function as simple levers. As we will discuss in a future section on this topic, various tendons, ligaments, and cartilages are involved in this process to ensure proper joint movement and to facilitate this movement in a nearly frictionless and painless manner.

For now, understand that the Appendicular Skeleton consists of the body’s appendages and the support structures to which major muscles attach to enable the bones to function in their role as levers.

The appendicular skeleton is divided logically into the upper and lower appendicular skeletons. We will discuss each in the following sections.

Upper Appendicular Skeleton

The upper appendicular skeleton consists of the arms and hands and the core structures that support their movement. A separate article, referenced at the end of this page, provides detailed information about the skeletal structure of the hands and the feet.

The bones of the arm consist of the humerus or upper arm bone as well as the radius and ulna bones found in the forearm. The humerus rests between the shoulder and the elbow. The radius and ulna are situated between the elbow and the wrist.

The radius bone articulates with both the humerus and the ulna at the proximal end of the bone. It rotates in contact with the humerus and rotates around the ulna. This combination allows the radius to move around the ulna which allows the forearm to rotate in supination and pronation.

At the wrist the radius and ulna bones both are able to rotate about the wrist enabling the wrist to turn in supination and pronation. Through these motions the radius bone moves around the ulna, allowing the wrist to invert between palm up and palm down orientations. This arrangement is demonstrated in the article on the hands and feet.

The arms are supported in the trunk of the body by the scapula and clavicle. These two large bones, together with the humerus bone, form the multi-faceted shoulder joint. The humerus articulates with the scapula via a ball and socket joint. Major muscles connect between the bones in the arm and the scapula and clavicle providing the shoulder and forearm with their large range of motion and overall strength.

The picture at right shows the left shoulder viewed from the front. The head of the humerus is bound tightly to the scapula by the large number of ligaments that help form a joint capsule. The tendon that connects the biceps to the scapula is shown at the front of the humerus. It extends over the head of the humerus and then attaches to the scapula. We will cover these muscle attachments in more detail in a later belt.

Lower Appendicular Skeleton

The lower appendicular skeleton consists of the legs and feet and the core structures that support their movement.

Legs and Feet

The bones of the leg include the femur, tibia, fibula, and patella. The femur articulates with the hip via a ball and socket joint. It articulates with the knee via a hinge joint providing extension and retraction mobility of the lower leg.

The tibia and fibula, like the ulna and radius, provide for rotation of the ankle joint and lower leg, but the degree of rotation is much more limited than that of the lower arm and wrist.

The primary purpose of the patella or knee cap is to increase extension forces applied to the tibia. The patella is a sesamoid bone embedded within the tendon that attaches the quadriceps to the tibia. The patella raises the tendon above (anterior to) the knee joint and thereby increases the angle at which the tendon connects to the tibia, allowing the tendon to exert greater pulling forces on the tibia.

Sometimes the portion of the tendon that extends distally from the patella to the tibia is referred to as a ligament since this is a bone-to-bone connection. But in reality this is simply an extension of the tendon in which the patella has grown. Additional strong ligaments surround the knee to strictly limit its movement. This helps ensure the knee can only move in a hinge fashion. If the knee could move in other ways you would likely find it very difficult to balance yourself upright.

The function of the patella is a major contributing factor in the power that can be generated with a kick such as Mae Geri.

Pelvic Girdle

The pelvic girdle supports the legs and serves to connect the lower appendicular skeleton to the axial skeleton. This area has multiple bones that are held tightly together via ample high strength ligaments. This area contains bones from both the appendicular skeletal system and the axial skeletal system. The appendicular skeletal components are shown in green in the diagram at right.

The head of the humerus fits into a socket called the acetabulum forming the hip joint. The acetabulum is formed at the junction of the ischium, pubis, and ilium. As an individual reaches adulthood these three bones fuse together into a single bone. The top medial side of the ilium forms what is commonly called the hip bone which can be felt just below the skin on either side of the body at the waist. The large flat bone surface provide a strong foundation for attachment of ligaments and tendons that function to both maintain rigidity in this part of the anatomy and provide strong muscle attachment points.

Appendicular Skeleton Bone Diagram

Here is a more detailed diagram detailing the major bones of the entire skeleton. The bones of the appendicular skeleton are shaded in yellow.

See Also

Skeletal System – Hands and Feet

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