However, before we dive into a discussion of these three disorders and their relationship to the workstation, let's fill in some background.
By Dr. Mark Vettraino
Interiors & Sources Continuing Education Series articles allow design practitioners to earn continuing education unit credits through the pages of the magazine. Use the following learning objectives to focus your study while reading this issue's article. To receive credit, see the series of questions and follow the instructions.
After reading this article, you should be able to:
- Identify three common types of upper body injuries caused by repetitive stress.
- Discuss the causes of these injuries and identify those causes that are impacted by decision made by workplace designers.
- identify the important considerations to be made when specifying work station furniture.
- provide an analysis of the important relationship between furniture design and primary-zone ergonomics.
What causes ergonomic injuries?
All ergonomic injuries are result of at least one of three factors:
Force is not a big deal in offices. When a full file cabinet needs to be moved, most of us are smart enough to get maintenance to come in with a refrigerator dolly.
Repetition, on the other hand, is a big menace for office workers. We don't have a choice. We have deadlines. Getting the work out involves typing, using a mouse and many other kinds of repetitive movements. Office jobs may not be as repetitive as those of assembly-line workers, but they are close; taking scheduled breaks may be the remedy.
Posture is the other factor in office-worker injuries, but unlike repetition, it is one we can influence. Generally speaking, it takes two of these three factors to cause a cumulative stress injury. Office workers are commonly subjected to both repetition and postural problems. However, if we can correct posture, we can do some good.
In our last article we discussed the issue of posture as it relates to spinal health. But posture is a major issue with the extremities, as well. I see three major categories of posture-related injuries in office workers: tendonitis, carpal tunnel syndrome and thoracic outlet syndrome. Let's address them one at a time.
Tendons are the cord-like tissues that attach muscle to bone. When a muscle flexes, it pulls the tendon across a joint, causing friction. All tendons have a covering called a tendon sheath, the function of which is to protect and lubricate the tendon while it is in motion. The body's first reaction to any insult is inflammation. When the tendon inflames, a condition known as tendonitis occurs (the suffix "itis" means inflammation.) Likewise, when the tendon sheath inflames, a condition known as tenosynovitis occurs. In either case, the result is discomfort or pain in the associated tendon, because an inflamed tendon swells, making it more difficult to slide through the protective sheath.
Long-term irritation will dry out the lubricant, causing more friction, discomfort and pain. Eventually, a condition known as stenosing tendonitis occurs, in which the sheath becomes completely dried out, causing restriction or total loss of movement in the affected area.
I have seen such injuries in factories where workers used power tools like screw guns—tools that require the worker to pull a trigger hundreds or thousands of times a day. Eventually, the tendons in the trigger finger just wear out. I've seen people who could no longer operate a screw gun at all, because they could no longer pull the trigger.
In Figure 1, you see diagrams of various hand positions. The outer columns show postural deviations. The center column shows different views of the neutral position. Add repetition to the positions in the outer columns and you have a formula for trouble. The further away from neutral the movement is, the greater the risk of injury.
||Figure 1 - Postural Deviations|
If you hold your hand straight out from your forearm, you can wiggle your fingers all day and not irritate any tendons. However, if you bend the wrist while working with your fingers, friction is created at the joint, leading to inflammation of the tendon or tendon sheath, or both.
The key to preventing tendonitis is good posture—that is, working with the joints in a neutral position so that the tendons are not stretched across bones. If we control posture, we control the condition.
||What does ergonomics really mean?|
Ergonomics is the science of fitting the physical aspects of the work environment to the human body. It is based on anatomy and the merging of man and machine according to the height and size of the body. This isn't just a matter of making things convenient. It's a vital aspect of workplace health, and in fact, injuries resulting from poor office workstation design—or poor use and set-up of its elements—are common and costly.
When the physical environment isn't suited to the physical capacity of the person to perform required tasks, musculoskeletal disorders (MSDs) can result. MSDs are injuries and disorders of the muscles, nerves, tendons, ligaments, joints, cartilage and spinal discs. Examples of MSDs are carpal tunnel syndrome, tendonitis, sciatica, herniated discs and low-back pain. The effects of these conditions have significant impact on worker productivity and cause financial losses to organizations in terms of lost time, employee efficiency and utilization, and insurance and workman's compensation costs.
On the other hand, a solid understanding of applied ergonomics can eliminate most of these injuries and improve worker morale and productivity. Designers need to understand how and why the body wears out, why certain postures cause changes in the body—for instance, the head constantly looking down. Additionally, a critical aspect of the problem is that workers need to take responsibility for their own health. They need the training and advice to understand what constitutes the right workstation set-up and the motivation to carry it out.
Years ago, I realized in my practice as a physician that whatever I was prescribing for a patient spending five hours a week in therapy was counterbalanced by the 40 hours a week he or she spent at work. The big benefits come when we modify the workstation to make it work properly.
Carpal Tunnel Syndrome
By the mid-1980s to 1990s, carpal tunnel syndrome had become such a "popular" disorder that it actually became a frequent misdiagnosis for other problems. There is a long list of diagnostic entities that can mimic carpal tunnel syndrome. Among these is anything that causes a high degree of water retention in the body, such as high-dose contraceptives, pregnancy, obesity or high salt intake. Rheumatoid arthritis and the side effects of certain medications can also mimic carpal tunnel syndrome according to a study done at the University of Michigan in the early 1990s.
In fact, carpal tunnel syndrome is an irritation of the median nerve in the wrist. This nerve begins in the neck at the C6-C7 nerve root (i.e., between the sixth and seventh cervical vertebrae.) The nerve runs from the neck through the shoulder and arm into the wrist, then branches off into the hand and innervates (controls) the thumb, the first and second finger, and half of the third.
The carpal tunnel is a region in which the median nerve passes between the wrist bones: the trapezium, trapezoid scaphoid, hamate, capitate, pisiform, triquetrum and lunate and the transverse carpal ligament (ligaments are tissue somewhat like tendons, except that they attach bones to bones, helping to hold joints together.) The more the wrist is bent back into extension, the more the medial nerve is exposed or brought closer to the skin. Add a sharp edge, such as the edge of a desk, and that will make the condition worse.
So, carpal tunnel syndrome is caused by some combination of repeatedly bending the wrist backward and rubbing the bottom of the wrists on the desk surface or edge. The solution to warding off carpal tunnel syndrome is to ensure that the wrists are held in a neutral position while typing, using the mouse, or performing other repetitive tasks, and to keep the wrists out of contact with the desk. More on this later.
Thoracic outlet syndrome
To understand thoracic outlet syndrome, we need to look at the circulatory system, in particular the arteries, which carry outgoing, oxygenated blood to the body's tissues, and veins which return blood to the heart and lungs to trade carbon dioxide for oxygen.
||Figure 2 - Good Workstation Ergonomics|
The finer points of proper work station ergonomics are quite technical, but here are the four essential guidelines to assure minimal physical stress while keying on a laptop or desktop:
- With the roof of mouth parallel to the ground, the eyes should be looking at the top third of screen.
- The wrist should be a natural extension of the forearms, no deviation from normal. The mouse should be at the same level as the keys or below. The top row of the keyboard should be no further away than the knees to assure that you are keying in the primary zone.
- The center mass of the skull should be at or behind the hip joint, with the torso reclined at an angle of 91 to 112 degrees.
- The knees should be no more than three inches above or below the hip joint.
There are also two main muscles in the chest—the pectoralis major and minor. These muscles originate in the sternum and insert into the arm just below the deltoid muscle and the head of the humerus, the upper arm bone.
I hate to criticize designers, especially the designer of the human body, but there is a design flaw in the way that the main artery and vein that serve the arm fit under the tendon that attaches the pectorals to the humerus. The further the arm gets away from the body, the more you pinch that artery and vein. That's why a person's arm goes to sleep if it is in wrong position for too long.
Having one's arm go to sleep for a while is not harmful. However, bad posture—at the workstation, for example—can cause problems relating to inadequate blood supply to the arms over long periods, especially when combined with the nervous-system problems also caused by bad posture.
The nerves that serve the arms come from the spine out through the neck, then join, then separate again. This structure is called the plexus of cervical nerves. They travel from the neck through the shoulder to the arms. If you turn your head to one side as far as possible, you pull and stretch those nerves, causing irritation. These nerves can also be irritated when the head is bowed forward for a long time. Moreover, if you bring the arms up and away from body, the roots of these nerves can be stretched.
Another group of muscles called scalenes also relates to some nerve-irritation problems. Scalenes are the muscles in the front of the neck that run down to the clavicle, or collarbone. Nerves run through these muscles and are adversely affected by bad posture of the head, neck and upper arms.
In short, there are postural issues that can affect both the nerves and the vascular supply to the arms, causing neuro-vascular problems.
The most common of these neuro-vascular problems is called thoracic outlet syndrome—thoracic outlet referring to the area in the shoulder where nerves and blood vessels pass from the thorax (chest) into the arms. It is an inconsistent disorder with symptoms ranging from pain to paresthesia—a numbness or tingling in the extremities. When thoracic outlet syndrome affects the hands, it is sometimes confused with carpal tunnel syndrome.
Setting up the workstation for good upper-body ergonomics
In my first article, I discussed the three work zones in a typical workstation. They are:
1. Primary Zone: everything within one's reach when the elbows are at one's side. The keyboard and mouse should be within this zone to prevent injury.
2. Secondary Zone: everything within arm's reach—planner, telephone, etc.
3. Reference Zone: outside of arm's reach—heavy reference material, files, etc.
It is mostly because of upper body issues that we have defined these work zones. If we pay attention to the three work zones and postural issues, then we can relieve most of the physically damaging stresses in the work environment.
Some specialized jobs may require fine-tuning of the workstation, but the general parameters I discussed in my first article are fairly universal. Let's review them with an eye to their effect on upper body health.
The finer points of proper workstation ergonomics are quite technical, but here are the four essential guidelines to assure minimal physical stress while keying on a laptop or desktop (see Figure 2):