Conversely, arm rests that are too high compress the wrist tendons because they cause a prolonged upward bend to allow the hands to reach the keyboard. Generally, carpal tunnel syndrome results from either prolonged stretching or compression of the wrist tendons within the fibrous sheath surrounding them. The other keyboard issue is that some computer users have a tendency to turn their hands outward in the horizontal plane. This also triggers chronic carpal tunnel problems.
To avoid keyboard-related wrist problems, keyboard trays must allow a neutral wrist position in both the vertical and horizontal planes. Sometimes, this requires the use of a split keyboard that allows ergonomic wrist positioning for users who have difficulty reaching keyboard keys without excessive outward bending of the wrists.
The computer mouse can also cause ergonomic-related problems when computer users must reach for them or alter the ergonomically correct arm positioning to reach the computer mouse. Even a properly positioned keyboard tray will not solve arm and shoulder problems when computer users position the mouse on the desk. To avoid having to reach for the mouse, the keyboard tray should also have a mouse support that allows the user to use it without changing the ergonomically correct arms position.
Ideally, the mouse support should be separately adjustable from the keyboard tray to maximize adaptability to each users unique requirements.
Prolonged sitting at work stations can trigger ergonomic problems, particularly those involving lower back or lumbar strain. First, ergonomic seating requires lower back support; second, ergonomic practices require taking periodic breaks to allow increased blood circulation and stretching of the back muscles and the spine. Prolonged uninterrupted sitting also compresses the nerves and blood vessels to the legs; this is another reason that frequent rest breaks and stretching are recommended.
Just as avoiding wrist, shoulder, and neck issues requires proper relative height of the hands, wrists, and arms, the same holds true for ergonomics issues caused by improper seating height. Generally, the proper ergonomic seating is that which allows the user to maintain a 90-degree angle at the knees while seated in the working position. Some users, particularly shorter users, may require foot support to avoid ergonomically unsound seating positions and angles in order for the soles of their feet to rest flat on the floor..