Anatomy of the Back: Why Do Injuries Occur?

In order to understand why back injuries are so common, you have to understand a little bit about the anatomy of the back and the physical forces that may come into play.

The Spine

The human spine (or backbone) is made up of small bones called vertebrae.

Smspine.gif (9063 bytes)

The Spine: Courtesy of
the Medical Multimedia Group

The vertebrae are stacked on top of each other to form a column. Between each vertebra is a cushion known as a disc. The vertebrae are held together by ligaments, and muscles are attached to the vertebrae by bands of tissue called tendons.

Openings in each vertebra line up to form a long hollow canal. The spinal cord runs through this canal from the base of the brain. Nerves from the spinal cord branch out and leave the spine through the spaces between the vertebrae. This cross-section of the spine shows how the spinal nerves and spinal cord are protected.

The lower part of the back holds most of the body's weight. Even a minor problem with the bones, muscles, ligaments, or tendons in this area can cause pain when a person stands, bends, or moves around. Less often, a problem with a disc can pinch or irritate a nerve from the spinal cord, causing pain that runs down the leg below the knee, called sciatica. Every time you bend or move, these disks compress (.avi animation 642K) with the motion of the spine.

Types of Injuries

Every time you bend over, lift a heavy object, or sit leaning forward, you put stress on the components of your back and spine. Over time, they can start to wear out and become damaged.

Many of the problems that cause back pain are the result of injury and degeneration of the intervertebral disk. Degeneration is a process where wear and tear causes deterioration, like when your favorite jeans get old. The disk is subjected to different types of stress as we use our backs each day.

Eventually, disks can collapse or herniate(.avi animation 277K); vertebrae can shift; bone spurs can develop.

Acute or immediate injuries to the back can be caused by tearing or straining ligaments and muscles. Muscles can also spasm due to stress or tension.

The Forces Involved

The amount of force placed on your back under certain conditions can be surprising. Anytime you bend or lean over to pick something up, you put tremendous pressure on your lower back.

fulcrum1.gif To demonstrate this, think of your back as a lever. With the fulcrum in the center of the lever, how many pounds would it take to lift a ten pound object?

A.  5 pounds
B. 10 pounds
C. 15 pounds

Fulcrum3.gif (2936 bytes)

When you add in the 105 pounds of the average human upper torso, you see that lifting a ten pound object actually puts 1,150 pounds of pressure on the lower back.

And if you've gained weight...

Given these figures, it is easy to see how repetitive lifting and bending can quickly cause back problems. Even leaning forward while sitting at a desk or table can eventually cause damage and pain.

Contributing Factors

Some things may contribute to your risk of injuring your back:

Poor physical condition

Poor posture

Extra weight


Overdoing it

Common causes of back injuries

[Intro] [Why?] [Causes] [Prevention]  [Quiz]

Graphics courtesy of the Medical Multimedia Group; description of spinal anatomy courtesy of the NIAMS