# 13.66: Joints

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## What allows running?

Running. A means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. The knees, which connect one part of the leg to the other, have to allow the legs to move. The knee is a joint, the part of the skeletal system that connects bones.

## Joints

A joint is a place where two or more bones of the skeleton meet. With the help of muscles, joints work like mechanical levers, allowing the body to move with relatively little force. The surfaces of bones at joints are covered with a smooth layer of cartilage that reduces friction at the points of contact between the bones.

### Types of Joints

There are three main types of joints: immovable, partly movable, and movable.

• Immovable joints allow no movement because the bones at these joints are held securely together by dense collagen. The bones of the skull are connected by immovable joints.
• Partly movable joints allow only very limited movement. Bones at these joints are held in place by cartilage. The ribs and sternum are connected by partly movable joints.
• Movable joints allow the most movement. Bones at these joints are connected by ligaments. Movable joints are the most common type of joints in the body, so they are described in more detail next.

### Movable Joints

Movable joints are also known as synovial joints. This is because the space between the bones is filled with a thick fluid, called synovial fluid, that cushions the joint (see Figure below).

There are a variety of types of movable joints, which are illustrated in Figure below. The joints are classified by how they move. For example, a ball-and-socket joint, such as the shoulder, has the greatest range of motion, allowing movement in several directions. Other movable joints, including hinge joints such as the knee, allow less movement.

### Summary

• Joints are places where two or more bones of the skeleton meet.
• With the help of muscles, joints allow the body to move with relatively little force.
• Some joints can move more than others.

### Review

1. Define immovable joint, and give an example of bones that are connected by this type of joint.
2. Describe a synovial joint.
3. Describe the movement of a pivot joint, such as the elbow.

### Explore More

Use this resource to answer the questions that follow.

Describe the motion of the following joints:

1. shoulder
2. knee
3. neck
4. wrist
 Image Reference Attributions [Figure 1] License: CC BY-NC [Figure 2] Credit: Laura Guerin Source: CK-12 Foundation License: CC BY-NC 3.0 [Figure 3] Credit: Zachary Wilson, using skeleton by User:GregorDS/Wikimedia Commons Source: CK-12 Foundation (skeleton available at commons.wikimedia.org/wiki/File:Human_skeleton_diagram_trace.svg) License: CC BY-NC 3.0; Skeleton: Public Domain

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