The Back and Why It Hurts

chapter

4

CONTENTS

1 The Spine 2 The Back in Distress 3 Risk Factors 4 Lifting and Other Forceful Movements 5 Work Postures and Conditions 6 Tool Belts and Back Belts 7 Ergonomics and Other Safety Measures

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INTRODUCTION

The construction industry has the highest rate of back injuries of any industry except the transportation industry. Every year, these injuries causes 1 in 100 construction workers to miss anywhere from 7 to 30 days of work. Most of the back problems occur in the lower back. There is a direct link between injury claims for lower-back pain and physical activities such as lifting, bending, twisting, pushing, pulling, etc. Repeated back injuries can cause permanent damage and end a career. Back pain can subside quickly, linger, or can reoccur at any time. The goal of this chapter is to expose risks and to prevent back injuries.

OBJECTIVES

Upon successful completion of this chapter, the participant should be able to:

1. Identify the parts of the spinal column.

2. Explain the function of the parts of the spinal column.

KEY TERMS

compressive forces forces, such as gravity or the body’s own weight,

that press the vertebrae together

3. Define a slipped disc. 4. Discuss risks of exposure

disc tough, fibrous tissue with a jelly-like tissue center, separates the vertebrae

to back injuries.
5. Select safe lifting

horizontal distance how far out from the body an object is held

procedures.

spinal cord nerve tissue that extends from the base of the brain to the tailbone with branches that carry messages throughout the body

vertebrae series of 33 cylindrical bones, stacked vertically together and separated by discs, that enclose the spinal cord to form the vertebral column or spine

vertical distance starting and ending points of a lifting movement

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1 The Spine

The spine is what keeps the body upright. The spine’s basic functions include providing support to other body parts, protecting the spinal cord, and providing flexibility to allow bending and rotating. The spinal column, from the base of the skull to the end of the tailbone, is made up of 33 bones

called vertebrae. See Figure 1.

Vertebrae Discs

The 33 vertebrae of the spine are divided into four sections, as shown in Figure 2.
1. The neck is composed of 7 vertebrae. The neck has quite a wide range of movement: up, down, and side to side. A broken neck occurs in this part of the spine.

figure 1

Vertebra and discs

2. The upper back is composed of 12 vertebrae. The upper back is much more rigid than the neck. It supports the upper body.
3. The lower back is composed of 5 vertebrae. The lower back supports most of the body weight. It has a good range of motion and bears most of the compressive forces. Compressive forces squeeze or press the spine together.
4. The tailbone is composed of 9 bones. In adults, these last bones of the spine have fused together to make the tailbone rigid.

Neck

Each vertebra is separated from the next by a disc. Discs are rings of tough fibrous tissue, or cartilage, with jelly-like center tissue. See Figure 3. Cartilage is the tissue at the ends of bones. It is capable of resisting a lot of pressure and reduces friction between bones. Compression of the spine can damage the shock-absorbing discs between the vertebrae.

Upper Back

A disc serves several important functions:

■■■■■

provides padding between vertebrae acts like a shock absorber to protect the vertebrae gives strength and flexibility to the spine allows for movement between vertebrae cushions the compressive forces that press the vertebrae together

Lower back
Tailbone

Ligaments and muscles hold together the spinal column of vertebrae separated by discs. The spinal cord is made of nerve tissue and extends from the base of the brain to the tailbone with branches that carry messages throughout the body. See Figure 4. The spinal cord comes from the base of the brain and runs through the channel created by the vertebrae. Smaller nerves branch out between each vertebra to carry messages around the body. The spinal column ends at the tailbone.

figure 2

Four sections of the spine

52 Ergonomics

Spinal cord

Vertebra
Disc

Vertabrae
Nerve

Muscle
Disc

figure 3

figure 4

Disc function

Spinal cord and nerves

SELF CHECK

1. The spinal column is made up of

bones,

called vertebrae.

2. In adults, the last nine bones of the spine fuse together to form the
.

3.

act like shock absorbers to protect the vertebrae and give strength and flexibility to the spine.

4. The

is made of nerve tissue and runs through the channel created by the vertebrae.

When normally aligned, the spine forms 3 natural curves, sometimes called the S-curves. These occur at the neck (cervical), upper back (thoracic), and lower back (lumbar). Maintaining these natural curves keeps the spine in balance. Human bodies are designed to move, bend and flex. Body posture changes to fit the task. Static posture leads to discomfort and lower productivity.

Just as there is a neutral position for wrists, keeping wrists straight and not bent or twisted, so also the spine has a neutral posture. When in a neutral posture, the body will find its natural balance. Neutral postures include the following:

■

sitting posture

■

head balances naturally over shoulders instead of protruding in front of body

■■■■■

shoulders relaxed rather than hunched up forearms and thighs parallel the floor body sits back in the chair for support, not on the front edge chair back adjusted for support feet set on the floor or footrest

The Back and Why It Hurts 53

■

standing posture

SAFETY TIP

■■■■■

spinal column aligned in its natural S-curves one foot propped on a stool to reduce stress in lower back change or shift posture often stretch frequently throughout the day body kept flexible, not rigid or fixed

Maintaining a neutral body posture while working can help prevent an injury.

Maintaining the body in a static posture becomes uncomfortable and decreases productivity. Do not force the body to conform to its workspace. Adjust the work site to fit before beginning the task. Habitually poor posture will cause increased aches and pains. Feeling discomfort or pain is an indication that something is wrong. Heed the signs. Combinations of awkward posture, force, repetitions, and insufficient rest periods are a set-up for injury. Combat this risk by taking more frequent “micro-breaks” before becoming fatigued. Also, become aware of mounting stresses, aches, and pains.

2 The Back in Distress

The Center to Protect Workers’ Rights reports that 1 in 4 construction injuries are back injuries, mostly from manual materials handling. According to the Bureau of Labor Statistics (BLS), back injuries account for 1 of every 5 injuries and illnesses in the workplace. Of these injuries, 80% occur to the lower back and are associated with manual materials handling. According to the BLS, the back was the body part most frequently affected by injuries involving days away from work in 2001. These injuries accounted for over 24% of the total 1,537,567 injuries in private industry.

Re-aggravation of a previous injury almost always results from a new incident, such as a slip, twist, trip, or extended reach. Lifting-related injuries include sprains, strains, and other injuries that can affect any part of the body, but the majority occur to the lower back.

Most people will, at some time, feel muscle soreness and fatigue in the lower back. Often, it is impossible to pinpoint a specific cause. Back problems that seem to “appear overnight” may have been building up slowly over a period of time.

Sprains and Strains and Chronic Back Pain

Ligaments attach one vertebra to another and help support the spine. A sprain is an injury or tear to a ligament. A strain occurs in muscles that have been stretched or used too much. Strained muscles and sprained ligaments both irritate the muscles around them. This adds to the pain and discomfort.

When muscles, tendons, and ligaments in the lower back are stressed and stretched, a lot of chronic back pain can result. If exposed to risk factors

54 Ergonomics

at work while experiencing chronic back pain, the body may not be able to recover. Pain, stiffness, and muscle weakness, even permanent damage, may result.

Disc Disease

The discs are the gel-like shock absorbers between the vertebrae of the spine. In degenerative disc disease, the discs break down. This usually happens in the lower back where the compressive forces are greatest. Over time, high stress on the back can cause tiny tears in the discs. If the stress continues, the gel gets pushed out of its regular position in the spinal column. This glob of disc gel then presses against one of the nerves that branch off from the spinal column.

The nerves that branch off from the lower spine extend down the legs. That’s why pain from a “slipped” disc is often felt in the legs, below the pinch point. The symptoms of a pinched nerve include numbness, pain, and weakness. The symptoms are similar to those in the hand when the nerve in the wrist’s carpal tunnel is compressed.

Degenerative disc disease develops slowly over time. A UBC member won’t know it is happening until the disc begins to shift. Repetitive awkward postures and lifting, even light loads, can cause the problem.

Slipped Disc

A slipped disc is a disc that ruptures or bulges out of place. See Figure 5. The rupture frequently occurs during a slip, trip, fall or while lifting. But the underlying problem is usually that the disc has already been damaged by degenerative disc disease.

figure 5

Spinal cord

Slipped disc

Disc gel slipped and presses on nerve

Disc
Nerves

Vertebrae

3 Risk Factors

Back care is very important. Back pain is the largest cause of pain in the United States and the major reason people go to their doctors. According to the U.S. Public Health Service, one in five workers reported back pain lasting at least a week. After experiencing a back problem, the chances of having it happen again are three times higher than someone who has never had a back problem. Using ergonomics and other techniques to prevent injuries are discussed elsewhere in Ergonomics.

The lower back takes most of the compressive forces, which act on the back during most of a person’s waking hours. Changing position from lying down to standing up doubles the compressive forces on the lower back! Stability is provided by the two cords of back muscles on each side of the spine. They work as guy lines to keep the upper body centered above the hips. As a side note, medical studies have shown that without the support of back muscles, the spines of cadavers buckled under just 5 pounds of pressure.

Standing up is twice as stressful for the spine as lying down. But there’s more to life than sleeping. When standing, about 50 to 60 pounds of compressive force press on the spine. The force comes from the effects of gravity and the weight of the upper body. Standing most of the day compresses the spine about an inch. When sleeping, the pressure reduces and the compressed inch returns.

Some work activities can lead to back problems. Actions and activities that can increase the risk of developing back problems are risk factors. Awareness of the risk factors helps to change some practices or equipment, reduces risks, and extends the ability to work and play free from pain.

PRODUCTIVITY TIP

Fewer injuries mean better productivity and lower project costs.

Work-related risk factors include the following:

■■■■■■■

lifting and forceful movements bending, twisting, and unequal lifting repetitive movements work postures workplace conditions strength, size and fitness tool belts and back belts

Notice that forceful movements, static work postures, repetitive work, and poor workplace conditions are also risk factors for CTDs of the upper extremities.
“I’m 39 years old, and I’ve had both knees operated on, both hands, and severe back pain. I’ve seen a lot of jobs with a forklift or crane sitting right there, and the old timers will want you to lift something by hand.” B. G., 39 years old, 20 years as pile driver, carpenter, millwright

56 Ergonomics

SELF CHECK

1.

is the effect of gravity on the body.

2. When normally aligned, the spine forms 3 natural
-curves.

3. When seated, keep forearms and thighs to the floor.

4 Lifting and Other Forceful
Movements

Lifting is a forceful movement requiring energy and muscle effort. It stresses muscles, tendons, and ligaments and increases the compressive forces on the spine. This is true even when a UBC member is lifting properly. Bending from the waist at the same time as lifting an object increases compressive forces on the spine even more. See Figure 6. This is also true when reaching, twisting, or holding the object away from the body while lifting. One arm lifting or lifting and carrying using one shoulder or one hip also places extra and uneven stress on the spine.

The load on the spine is a combination of the weight of the object being lifted and the weight of the upper body. About 70 percent of body weight is in the upper body. Bending over from the waist instead of lifting with a straight back adds the weight of the upper body to the lift, along with the object being lifted.

Have materials delivered close to where they will be used to minimize manual material handling and save time.

figure 6

Working bent over

Factors Affecting the Stress of Lifting

Back injuries from overexertion are rarely the result of a single event or accident. In some cases, an accident may have resulted in a pulled muscle. But the muscle really didn’t become bothersome until after several weeks or months of repetitive lifting or awkward work postures. In other cases, months or years of repetitive lifting, pushing, pulling, and carrying didn’t become noticeable until a single lift produced significant pain from a bulging or ruptured disc. See Figure 7. Several factors could influence stress from lifting:

■

the weight, shape, and size of the object

■

horizontal distance, how far away from the body the object is held with the arms extended

■■■

vertical distance, where the lift starts and where it ends lifting and twisting or bending at the same time how much lifting is performed

Heavy objects require more energy to lift than lighter ones. However, the shape of the object greatly influences how difficult it will be to lift. For example, a box of nails weighs more than a bundle of insulation. The compact size of the box makes it easier to lift and carry. A squat lift can be used (bent legs, straight back). The box fits between the knees and can be held close to the body. Because of the shape, the insulation is large, cumbersome, difficult to grasp and because it itches, it is not pleasant to hold close to the body.

Lifting an object that has to be held away from the body (horizontal distance) can increase the compressive forces on the back substantially. If the object is too big or bulky, the squat lift won’t work. The stress on the lower back increases by three times when bending from the waist to lift an object compared to lifting with a straight back.

figure 7

Spinal cord

Bulging/ruptured disc

Disc gel slipped and presses on nerve

Disc
Nerves

Vertebrae

58 Ergonomics

The lower back is especially susceptible to injury due to the mechanics of the human body and the type of tissue and structures that make up the spine. The upper body can be thought of as a lever arm and the low back as the fulcrum, or pivot, point around which the trunk rotates. See Figure 8. For this reason, the compressive forces on the spine are the greatest at the fulcrum point and consequently can cause the most damage to the discs that sit between each vertebra.

For instance, lifting a 20-pound bag of flour 20˝ away from the body produces approximately 400 pounds. of compressive force on the discs at the fulcrum point. This is 20 times the weight of the actual object lifted! In this example, it is not only the distance of the sack of flour from the body that contributes to the large compressive force, but also the weight of the body’s trunk as it bends forward. The muscles in the back have to work to support the weight of both the flour sack and the upper body. For this reason, even if a person is not lifting an object, large compressive forces are produced just to maintain the trunk in a forward bent posture.

Try to keep the vertical distance of lifts between knuckle and shoulder height. See Figure 9. The lift should not start below knuckle height nor end above shoulder height. Lifting from below knuckle height puts stress on the legs and knees as well as on the back. Lifting above shoulder height puts stress on the upper back, shoulders, and arms.

figure 8

Upper body mechanics

400 lbs of compressive force
20 lbs

Fulcrum
20 in lever arm
2 in

Resistance arm (distance from back muscles to fulcrum)

figure 9

Storing lumber

Frequent lifting of even light loads can cause localized muscle fatigue or whole body fatigue. Fatigue increases the risk of accident and injury. Frequent lifting also puts stress on the discs, tendons, and ligaments of the back.

SAFETY TIP

Store materials so the vertical distance of lifts is between knuckle and shoulder height.

Other Forceful Movements

Pushing, pulling, tugging, and sliding objects are examples of other forceful movements. These movements require a lot of effort. They put strain on the lower back. They also stress the muscles, tendons and joints of the shoulders, arms, upper back and legs. How much force these movements require depends on the amount of force used to pull or push the object. Pulling, which stresses shoulders and arms, is worse than pushing. Pushing allows the use of the entire body weight.

Body posture while pushing or pulling is also a factor. In an awkward posture, more force must be exerted to move the object. Friction between an object and the surface also requires more force to move the object. Using a screw gun or pneumatic nailer are examples of pushing. Stress on the back can be reduced by supporting the upper body with your other hand as shown in Figure 10.

Pushing or pulling an object above shoulder height or below waist height is very stressful on the body. The awkward posture requires more force. See Figure 11. The problems encountered by the upper extremities are also transferred to the back. Lifting an object vertically is difficult because it’s

figure 10

Supported upper body

60 Ergonomics

figure 11

figure 12

Pushing above shoulder height
Arching backward

moving against gravity. Objects that shift or fall cause the body to jerk, and in response to the sudden change in weight and resistance, that movement can cause injury.

Bending sideways or twisting the trunk also puts stress on the lower back. Reaching backward usually causes the back to arch. See Figure 12. This increases the amount of force on the lower spine. It also puts stress on the upper back, shoulders, and arms. Avoiding these awkward body positions may be as simple as moving the ladder, working surface, or repositioning your body.

Using mechanical means, such as a forklift or board cart, reduces the material handling done and increases productivity.

Forward reaches that are longer than arm’s length and require bending or stretching put stress on both lower back and legs. See Figure 13. Applying force with arms out in front of the body increases the stress on the lower back. An example is reaching forward while using a portable power saw. One way to reduce the stress is to support the upper body with one hand. Avoid bending the wrist backwards.

figure 13

Reaching forward

Carrying an object using only one shoulder, arm, hand, or hip puts unequal stress on the spine. See Figure 14. The more reaching, twisting, sideways bending, and unequal lifting or carrying a UBC member does, the greater the risk of back injury. Alternating the load between shoulders or dividing the load for a two-arm carry is better for the body. Even simply balancing the load reduces stress.

The more a UBC member is required to bend, lift, twist, or reach, the greater the chances of developing back problems. It is like the tires on a car—the more driving done, the faster the tires wear out. Repeated stresses on the back can also speed up normal wear and tear.

figure 14

Carrying lumber on one shoulder

SELF CHECK

1. Standing up is

as stressful for the spine as lying down.

2. How far away from the body the object is held is the distance.

3. Keep the

and shoulder height. distance of lifts between knuckle

5 Work Postures and Conditions

If much of the daily work involves bending over, leaning forward, and working with the arms above shoulder height, it is likely that the body may be in one posture for long periods of time. These postures put stress on the lower back and shoulders. Static work activities are extremely tiring. See Figure 15. When one posture is maintained without moving or changing position, muscles remain contracted. Blood flow to the contracted muscles is reduced; so muscles tire quickly.