Occupational Health & Safety

Dangerous Postures

Worker education plays a crucial role in getting the greatest advantage from ergonomic seating.

CHAIRS are a common point of debate in today's workplace. The questions most frequently debated derive from the differences between an engineering approach and a therapeutic approach to work injury prevention.

The therapeutic approach considers most important how the body works and how the worker uses his or her body to do the job, whereas a strict engineering approach relies on how the workstation fits a specific worker or group of workers. Industrial and workplace therapists are experts on how the human body does work, how the body wears out, and how the body repairs itself. This article will address seating from the therapeutic approach.

The Effect of Sitting
With the perspective for this article on seating understood, we must look at sitting as one of the worst occupational hazards known today. Sitting can severely damage the lower back and neck due to the accumulation effect of prolonged time spent in abnormal positions. This accumulation effect, combined with how the worker cares for his or her body, results in an imbalance of damage and repairs to working tissues that occurs on a daily basis.

The results of this damage don't always appear as low back pain or neck problems. Many times, injured workers have discomfort and debilitating pain that manifests itself as some type of repetitive motion disorder, which often can be traced back to a problem that started with sitting--a common static posture that not only causes back problems, but also restricts blood flow to working tissues.

The mechanical effect of sitting on the low back is a forward bent posture of the lower lumbar vertebra and a forward flexed posture with the hips. The forward posture of the vertebra places pressure on the front of the discs, forcefully projecting the nucleus in the disc toward its back wall, a back wall that is much weaker and much less protected that the front wall of the disc. This would not be too much of a problem if we didn't spend too much time sitting. But workers whose jobs require sitting and work the typical 40-hour week spend approximately 1,665 hours per year in this dangerous, forward flexed posture. This alludes to the problem with sitting being the amount of time spent in the chair; no matter how much you spend on an ergonomic chair, you won't change the fact that the low back and hips must flex to remain seated.

Proper Posturing in the Chair
Proper posturing of the body while sitting in a chair, perfect use of the human body while functioning, and perfect care of the working body throughout the day are critical components to worker comfort. These factors will determine whether or not workers go home with fatigue or energy; go home with a back, neck, or shoulder ache; or go home with the capacity to enjoy their personal lives.

In addition to the static forward bend of the lower lumbar vertebra and hips, another of the most common postural problems we see with sitting is forward head posture. The accumulation effect of this posture can lead to a multitude of problems, such as upper back ache (the one you get between your shoulder blades), headache, thoracic outlet syndrome, TMJ, tendinitis, and carpal tunnel syndrome. Correct posturing of the head and neck assures the correct alignment of the cervical curve of the spine. The easiest way for a worker to assess his own head posture is for his to maintain the position of his ears directly over his shoulders.

Perching is another abnormal sitting posture. This is the position where a worker sits on the front edge of the seat pan, thus not sitting with her bottom and low back resting comfortably against the backrest of the chair. Sooner or later, the postural muscles holding our spine erect will fatigue, causing this worker to sit with a forward slouch, and thus a forward head posture.

The worker should allow her feet to rest firmly on the floor. This will allow the weight of the thighs to be supported by her feet and the seat pan of the chair to properly support her natural center of gravity. As the worker sits with her bottom and low back comfortably against the backrest of the chair with the feet firmly planted on the floor, there should be a greater than 90-degree angle between the upper torso, hips, and thighs. This open angle will allow blood to flow to the lower extremities with minimal restriction. The length of the seat pan should not be so long as to make contact with the legs behind the knee. This contact with the leg soon becomes uncomfortable and workers will assume various--unhealthy--sitting postures (such as sitting on their legs) that will create other problems.

The shoulders, arms, and hands should be held in a neutral posture. That is, the shoulders should be allowed to relax, dropping the elbows to a comfortable position next to the side of the body. The elbows, arms, and hands should be maintained at a 90-degree angle while working. At this point, armrests of the chair should be raised to contact the bottom surface of the arm. There may be an inward/outward adjustment to the armrests, and if so, they should be adjusted inward to touch the side of the body. Some chairs may have armrests that also swivel inward and outward. If so, they also should be swiveled inward while a worker performs keyboarding tasks. The ultimate purpose of the armrest adjustment is for them to be positioned where they will support the weight of the arms, thus releasing the neck and shoulder muscles from the sustained contraction required to hold the arms up while they are in a working position.

Additional Features and Chair Adjustments
There are countless "ergonomic" chairs available today. The differences lie in certain features and luxuries of the chair, such as built-in lumbar support, height of the backrest, adjustability, and upholstery.

Manufacturers will try to sell you many other features, but the most important ones will be mentioned here. Rest assured that an ergonomic chair need not be pricey, and, above all, the chair should not be considered--by itself--as the answer to injury and pain problems.

A very important feature of a correct ergonomic chair is adjustability of the backrest. The entire backrest should have the capacity to adjust inward and outward. As the worker sits on the seat pan, with room at the front edge of the seat pan so there is no contact with the back of the knee, the backrest should be adjusted inward to contact and support the low back. It is important not to confuse this inward/outward adjustment of the chair with a recline feature: They are different. The primary position for lumbar support is--most often--directly behind the navel. Some chairs have built-in lumbar support, which is good, but the worker must sit with his mid back and shoulders against the backrest; otherwise, forward head posture and slouching will be encouraged. For this reason, built-in lumbar support may be a disadvantage.

It is up to the worker to know the potential stressors, to know the features of the chair and how to use these features correctly. Once proper lumbar support is attained, and the worker knows the advantages and disadvantages of the chair's features, the height of the backrest is an option, not a necessity. A backrest that ascends the entire length of the spine is useful only when the worker sits back against this backrest. Otherwise, the head and neck will be in forward head posture, and this will eventually lead to a slouch. Proper head posture can be attained without the backrest rising so high, but this is up to the worker, not the chair.

A backrest of a chair also may recline, and this is satisfactory for the many adjustments to posture a worker should make throughout the day. However, tasks such as keyboarding or some other type of focused work while seated with the backrest in a reclined position do not work. The worker performing focused work will sit forward either with forward head posture, a slouch, or both, thus not utilizing the reclined backrest. It may even be safe to say that the reclined seat position will force the worker into this unhealthy posture when he might have utilized the backrest had it not been reclined.

Another feature offered with some chairs is an angle adjustment of the seat pan. This, too, is satisfactory for the many position changes that are suggested throughout the day, and there may be an advantage with this feature for combining seat height adjustment with tilt adjustment to fit a worker into a workstation where the work surface is at a fixed height. However, here, too, there is a risk that must be known. When a worker tilts and locks the seat pan in a backward position, the angle we are looking for at the hips, torso, and thigh tips upward and forces blood to flow against gravity, thus restricting blood flow to the lower extremities.

The last feature to mention is the height adjustability of the chair. Almost all chairs utilized for work have this feature. The key to appropriate height adjustment is creating the proper angle at the hips, torso, and thighs. The height of the chair may be raised to fit the worker to a fixed-height work surface. However, if this height adjustment raises the feet off the floor, a footrest will be necessary to support the feet, and thus the weight of the lower extremities.

Concluding Comments
There are several important issues from the therapeutic perspective on ergonomic seating. Remember that an expensive ergonomic chair will not make a difference when it comes to solving pain and injury problems. The chair can make a big difference, however, when the worker has intimate knowledge of the features of the chair, how these features operate, and how chair adjustments affect the working human body.

The worker also must understand just having a fancy ergonomic chair doesn't mean she will sit in it properly. The amount of money spent on ergonomic seating doesn't mean the worker will sit in the chair with correct posture, assure the worker will utilize the most appropriate mechanics to do the work, or assure workers will take perfect care of their bodies before, during, and after work. The most important aspect of ergonomic seating is educating the worker about the physical stressors of sitting, how to avoid these stresses, and how to repair damage done throughout the day. Workers must be motivated to sit in the chair correctly, to use the body perfectly, and to take care of their working body with work, home, and leisure activities.

Above all, the spine and extremities like movement. Encourage workers to take frequent breaks, to rise from the seated position, and to perform a brief stretch routine that will repair the damage done to working tissues. They also should make regular adjustments to their chairs throughout the day to vary static postures and to change stressful postures frequently.

This article originally appeared in the January 2003 issue of Occupational Health & Safety.

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