Vision And Posture Interact
The location of the monitor often determines the range of comfortable postures we can assume. Sometimes we are forced to compromise between postural and visual comfort.
For example, if the monitor is to our left or right, we will most likely twist our necks and/or trunks into uncomfortable positions in order to see clearly.
Vision and Posture Problems with Eye-level VDTs
In the past, some VDT guidelines recommended placing the top of the monitor at or slightly below eye-level. The reasons for this included reducing the load on the neck muscles by preventing the user from tilting the head too far back or forward. This attempt to encourage a neutral posture can actually produce the opposite results. As Ankrum and Nemeth (1995) explain, the eye level monitor position, when combined with an upright trunk posture, restricts the range of comfortable head and neck postures a user can assume.
When the head-erect posture becomes tiring with an eye-level monitor, we have three alternative neck postures we can assume.
One option is to tilt our heads back. This posture increases muscular tension in the neck and back muscles.
The second option is what is called the "forward head posture." Keeping our head erect, we jut it forward from the trunk. With this position, we attempt to relieve muscle tension from contracted neck muscles. Unfortunately, it is associated with cervical headaches, increased fatigue and thoracic outlet syndrome.
The third option with an eye level monitor is bending the neck forward (flexion). Chaffin (1973) found that sustained neck flexion of 15 degrees caused no subjective discomfort or EMG changes after six hours with ten-minute breaks each hour.
While neck flexion may be comfortable, try bending your neck down and looking out of the top of your eyes at something close. Since it's not comfortable on the eyes, most people refuse to do it.
The net result is that eye-level monitors allow for only one healthy head/neck posture. When you get tired of maintaining that posture, the alternative positions are unhealthy.
Eye level has often been determined with the user "sitting tall." But is this the way we usually sit? In normal upright sitting (without a visual target), subjects studied by Hsaio and Keyserling (1991) tilted their heads an average of 13 degrees forward from the "upright" position. In fact, hardly anyone sits in the "upright" posture often shown in some guidelines.
If the monitor is set to eye level as described in the preceding paragraph, the user is presented with a choice: either assume a more erect head/neck posture than preferred, or look up at the monitor. Most people find it uncomfortable to look out of the top of their eyes at something close. Neither is a good solution.