It has been taken as a given.
And now, a recent Hong Kong study confirms that angling the computer monitor 35 degrees to the right or left requires greater neck muscle exertion and promotes operator discomfort compared to a central placement of the monitor.
Surface electromyography (sEMG) revealed that when the screen is placed to the right or left, there is a significant increase of muscle activity within the ipsilateral cervical erector spinae and contralateral upper trapezius muscle groups. When compared to a central monitor placement position, 20 asymptomatic research subjects reported a discomfort level:
- over two times greater when the screen was placed to the right
- nearly four times greater when the screen was placed to the left
Despite these findings, there was no statistically significant relationship identified between muscle activity level and the report of discomfort. Authors Szeto and Sham felt this may have been due to their use of healthy, pain-free volunteers in this study.
In all three monitor positions, the mean muscle activity level was highest in the cervical erector spinae.
Table 1 reveals the changes in neck muscle activity level relative to monitor positioning.
Sequential level of EMG mean muscle activity (from highest to lowest)
Table 1: Study findings of the relationship between monitor positioning and muscle activity
The Bottom Line – How This Applies To Ergonomists
At most contemporary office workstations, the computer monitor is placed in direct alignment with the keyboard. However, it is not uncommon to see the monitor to the left/right of the keyboard when:
1) a computer is used on office furniture never intended for a computer (i.e., old wood/metal desks or credenzas)
2) multiple computers are operated on one work surface
3) a telecommuter creates a home office
The recurrent theme is insufficient desk space.
This research confirms the importance of monitor alignment positioning in containing neck muscle exertion and discomfort.
The ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations standard recommends that monitors should not be positioned greater than 35 degrees to the left or right. Szeto’s and Sham’s findings suggest the standard guidance may be too liberal.
From a university setting, 10 male and 10 female students volunteered for the study. Subjects did not have a history of neck/upper limb injury, pain or discomfort during the previous 6 months. For at least the prior 2 to 3 years, they used the computer a minimum of 1 to 2 hours per day. Further, they had to have the skill of using 4 digits in each hand when keyboarding. No subject wore bifocal glasses.
Surface EMG recording devices were applied to the subject’s right and left cervical erector spinae and upper trapezius muscles.
All subjects used the same computer workstation which was equipped with a chair (height adjustable, swivel, with backrest, no armrests), keyboard tray (adjustable, slide-out model with a wrist rest) and a LCD screen (15 inch diagonal). The equipment was adjusted by the volunteer such that a comfortable position was assumed.
The monitor horizontal distance was determined by the subject (based on comfort) when the LCD screen was placed in the central position. The vertical position of the monitor was placed such that the topmost of the screen was at the subject horizontal eye height.
In a random fashion, the monitor was placed in one of three positions: 35 degrees angled left, central or 35 degrees angled right. A 20 minute typing task was performed while the monitor was in the select position. At the 5th, 10th, 15th, and 20th minute, 30 seconds of EMG data was recorded. Following the task, the subject verbally reported subjective discomfort on a scale of 0 to 10 for 10 upper back/neck/upper extremity regions.
After a 10 minute rest, the monitor was placed in a second position and the same typing task/discomfort survey was completed. When 10 minutes of rest had occurred, the same procedure was applied with the monitor in the third position.
There was great variation in specific muscle group activity for each monitor position.
In their literature review, the authors identified the dissonant research outcomes involving the association between muscle activity revealed by sEMG and musculoskeletal symptoms. These include:
- No difference in sEMG data between those with and those without musculoskeletal discomfort
- A weak relationship between high sEMG findings and musculoskeletal symptoms
- When performing computer tasks, increased sEMG findings associated with workers who suffer from chronic neck/shoulder pain
- When performing functional or manual tasks, increased sEMG findings associated with subjects who have chronic neck pain
Article Title: The effects of angled positions of computer display screen on muscle activities of the neck-shoulders stabilizers
Publication: International Journal of Industrial Ergonomics, 38, 9-17, 2008
Authors: G P Y Szeto and K S W Sham
This article originally appeared in The Ergonomics Report™ on 2008-09-10.