In sedentary occupations, our upper bodies are movable, actively engaged agents that propel our workday, our activities and our successes. The upper body includes our face, neck, shoulders, arms, hands and wrists. Technically, the rib cage and thoracic spine are included, as well.
Neutral Posture Body Awareness
The clavicle bone runs horizontally across the top of the ribcage. A good analogy would be to imagine the shoulder girdle as a yoke worn across the top of the torso. The arms are suspended from that yoke on each side. The neutral posture for arms is to hang down.
Injury Prevention
The goal for the upper body in computer ergonomics revolves around:
- . Sustaining neutral posturing of the shoulders, arms and wrists.
. Minimizing the force used in striking a keyboard.
In my Computer Related Injuries article I mentioned Cumulative Trauma Disorders as a broad category that includes computer related injuries. Ergonomist and researcher Dennis Ankrum outlines causes of Cumulative Traumatic Disorders:
A.) Holding and non neutral postures.
B.) Local pressure.
C.) Forceful movements.
D.) Overexposure to cold or vibration.
The Value of Armrests
When we consider ergonomic seating, we need to know the value of armrests. Ergonomic research studies have shown that arm rests:
- . help to lighten key striking force
. allow for a relaxed shoulder position.
These functions of armrests address two of the causes of computer injury prevention listed above (A. and C.).
Holding Non-Neutral Postures
When holding (especially non-neutral postures), the constriction of blood flow occurs. Muscles involved in holding will stay contracted for long periods of time. This tension pressures the blood vessels present within the muscle, blocking the flow of conduction. This is called ischemia. Ischemia causes, among many other things, muscle fatigue, pain and tenderness.
Sitting without arm support is the best way to end up with tight shoulder muscles. This is because unsupported arms can only be in a non-neutral position. At least one researcher has found that armrests help minimize the tight shoulder muscles, and says that the mere presence of arm rests is a more powerful factor in injury prevention than the actual position of the arms.
If your arms are held forward of your body all day, this will automatically cause your shoulder joints to experience rotation, a non-neutral posture. This rotation is the beginning of the tightening of the shoulder muscles, the constricting of the range of the shoulder movements, fatigue and injury.
Typing Tension
In neutral postures, the bones center freely in their joints. There is no need for help from the musculature, because the body�s natural alignment is in integrity and all parts are properly connected to one another. The pieces fit. Because neutral postures will use the least amount of muscle tension, when typing, there will be less tension translated from the shoulders and arms to the wrists, fingers and onto the keyboard. The muscles do not have to hold, so they won�t stiffen, and therefore won�t cause pain.
However, typing at a keyboard with unsupported arms is a deviant from that neutral posture. This position consists of the holding of the arms forward of the body, the automatic shoulder rotation described above and resultant muscle tension. Therefore, armrests can be a very relevant consideration.
Choosing Armrests
A review of the research done about armrests by Rani Lueder, C.P.E. with Paul Allie finds that there is no one type of armrest that is recommendable for all. They are as individual as the workers who use them.
Armrests should allow access to:
. Workspace
. Tools
. Performance of tasks
The armrest adjustments are:
. Height
. Length
. Distance between armrests
Height
The armrest height helps the worker to avoid non-neutral positions of the arms and shoulders. Workers should be able to sit in a number of different positions without raised shoulders (armrests too high) or having to lean to one side (armrests too low).
Length
The length of the armrest should allow users to sit close enough to the work surface to perform their tasks while maintaining contact with the backrest.
Distance between armrests
The distance between armrests should allow the user to avoid shoulder lifting or rotating arms outward. Also, the inside distance between the armrests should allow the user to easily enter and exit the chair. The hips should comfortably fit between the armrests or supports.
