Reading a computer screen with background glare can affect muscles not only in the eyes but also the neck, according to researchers.
Their randomised controlled trial “indicates that exposure to direct glare affects the trapezius muscle, possibly by an interaction between the visual system, sympathetic nervous system, and head-stabilising muscles,” wrote Randi Mork of the University College of Southeast Norway in Kongsberg, Norway, and colleagues.
The finding, published in Optometry and Vision Science
, could help in the design and organisation of offices and work stations.
Previous studies have established that computer work can cause discomfort in the eyes, neck, and shoulders, and a correlation among these symptoms.
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Relative to viewing objects far away, looking at a computer screen creates a higher workload for both smooth and cross-striated muscles in and around the eyes.
And, when natural visual scanning changes from dynamic motions of vergence and accommodation into static activity, the risk of ocular fatigue increases. Likewise, muscles in the neck take part in active gaze stabilisation.
Glare affects the accommodation response, and can increase visual discomfort and affect reading performance.
To see how glare affects muscles in the neck and scapular area, the researchers recruited 15 healthy students with normal vision from the Department of Optometry and Visual Science at University College of Southeast Norway.
Twelve of the students were female. They ranged in age from 19 to 25 years, and had used computers for a mean of 11 years.
The researchers excluded people with chronic neck pain, dyslexia, chronic disease, and regular use of medications affecting circulation, pain sensation, vision, or visual comfort. The patients’ mean LogMAR distance visual acuity was -0.16. The eight who had prescription contacts or glasses wore them for the study.
The researchers assigned the patients to read a text on a computer screen during two different conditions: 30 minutes in an optimum workplace environment and 30 minutes exposed to direct glare. The order of these sessions was determined with the flip of a coin.
Each session started with a 1-minute rest and was followed by a 5-minute break afterward.
The researchers asked the patients to sit in a normal upright position and optimised their sitting positions according to international recommendations. To keep the trapezius muscle as relaxed as possible, the patients rested their forearms on supports. The table height was above the elbow height when the paients were seated.
The computer task was done on a 24-inch liquid crystal display with 1920 by 1200 pixels resolution and a mean refresh rate of 69.5 Hz. The screen had an anti-reflective coating. It was connected to a laptop computer. At a distance of 60 cm, the vertical viewing angle of the letters was 0.24 degrees.
To encourage the patients to concentrate, the researchers told them they would have to answer questions about the reading.
During rest sessions, the subjects could stand up but not walk around because they were attached to monitoring cables. The computer screen and glare source were turned off during these breaks.
The researchers created glare with two large, flat luminaries about 70 cm behind the computer screen, intended to simulate the placement of windows behind the computer screen and to ensure symmetrical exposure of glare to both eyes.
They constructed the luminaries out of translucent acrylic diffusing fronts with six 60 W fluorescent tubes. The luminaries’ intensity varied from 2800 to 5100 cd/m2, which is similar to the luminance from a window on an overcast day.
During the optimal condition, the luminance levels were 155 cd/m2 in the working field, 75 cd/m2 on the desk top, and 46 cd/m2 in the background, values that fall within the recommended luminance ratio of 5:3:1 for a workplace.
During the glare condition, the luminance was 155 cd/m2 in the working field, 590 cd/m2 on the desk top, and 4268 cd/m2 in the background. This ratio of 1:4:28 is considered excessive background light.
Patients rated their discomfort or tiredness for various anatomical regions on a visual anatomical scale.
The researchers used electromyography to measure the patients’ muscle activity in the m. trapezius and m. orbicularis oculi.
They measured muscle blood flow in the right m. orgicularis and left non-dominant m. trapezius, and heart rate, using photoplethysmography, a non-invasive optical technique that can detect flood volume changes in the microvascular bed of muscle tissue.
None of the symptoms registered at baseline were significantly different between the glare and optimal conditions. But there was significant overall higher incidence of eye pain when the patients were exposed to glare than when they were in optimal lighting at 10, 20, and 30 minutes of reading. This difference was statistically significant (p=0.011).
Patients reported greater symptoms of dry eyes, blurred vision, photophobia, and headache in the glare condition compared with the optimal condition after 30 minutes of reading.
Eye tiredness was greater in the glare condition as well, but the difference was not quite statistically significant (p=0.069) as was neck pain (p=0.077).
The eye pain and tiredness and neck pain increased with time in both the glare and optimum conditions.
The researchers detected more muscle activity in the orbicularis muscle during the glare condition compared with the optimal condition. Trapezius blood flow was also higher in the glare condition than in the optimal condition.
The researchers found a correlation between orbicularis oculi muscle activity and trapezius blood flow and neck pain during both exposure to direct glare and optimal lighting.
Both psychological stress and bright light exposure have been shown in previous studies to increase heart rate, but in this study there was no significant difference in heart rate between the glare and optimal conditions.
“The eyes and neck are known to be closely connected and this indicates an important role of neck muscles in normal visual performance and gaze stabilisation,” the researchers concluded. “Further, the notion that proprioceptive information from ocular and craniofacial muscles may influence somatic motor activity cannot be dismissed as a possible explanation for the relationship we see.”