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In: Peper, E., Harvey, R., and Shumay, D. (1997). How to use applied
psychophysiology/biofeedback in the prevention and assessment of upper extremity
musculoskeletal disorders. In: Salvendy, G., Smith, M. J. and Koubek, R.J. (eds). Design
of Computing Systems: Cognitive Considerations. New York: Elsevier, 551-554.
Erik Peper, Richard Harvey and Dianne Shumay
Institute for Holistic Healing Studies, San Francisco State University,
1600 Holloway Avenue, CA 94132
1. BACKGROUND
Painful musculoskeletal and other disorders associated with computer
use are becoming increasingly common in the workplace.1,2 Risk factors for
computer related disorders (CRD) include incorrect ergonomics, psychosocial stress,
absence of somatic awareness, high physiological reactivity and inappropriate workstyle.3
The most common interventions to prevent CRD focus on ergonomics, while ignoring the
role of other important risk factors and fail to address the underlying tension patterns
which are often still present.4 Even if employees are placed in correct
ergonomic positions, it does NOT mean that they are as relaxed as possible during task
performance. This paper reports on applied psychophysiological investigations for studying
1) muscle awareness at the computer and 2) applied psychophysiology to identify ergonomic
positions.
2. MUSCLE AWARENESS AT THE COMPUTER STUDY
This study explored the degree to which computer operators were
aware of changes in muscle tension induced by different keyboard locations.
2.1. Subjects
Twenty-three subjects (six men and seventeen women) volunteered to
participate in the study. The participants' mean age was 32.9, ranging from 21 to 46
years.
2.2. Equipment
Electromyographic activity (EMG) was recorded from the dominant forearm
and two areas of the shoulder (trapezius and deltoid areas) with surface electrodes using
the J&J I-330 Physiodata system. The subjects performed word-processing at a standard
ergonomically adjusted computer station consisting of a height-adjustable chair, computer,
keyboard on a movable tray, monitor and word-processing software. The keyboard was placed
on a tray that could be moved forward or back and locked into position. This tray was
marked with four positions at two inch intervals. Each two inch interval was equal to an
approximate 2.5 degree change in the shoulder angle.
2.3. Procedure
The subjects were monitored during two sequential tasks: task one --
hands on keyboard at home row without typing, and task two -- typing. Each task consisted
of five 48 second periods. During the first period, the subject's hands rested in his or
her lap. For periods two through five, the keyboard was moved into positions two through
five in a random order. At the end of each period the subject rated his or her muscle
tension in the shoulders and forearm on a scale from one through five (1 most relaxed, 5
most tense).
2.4. Results
No significant relationship was found between subjective measure of
shoulder tension and the trapezius and deltoid sEMG activity in any of the conditions.
Between subjective measure of forearm tension and flexor/extensor during typing, a
positive correlation of 0.57 was observed. A representative subject’s physiological
recording with subjective ratings is shown in figure 1.
Figure 1. Mean sEMG during typing of a subject with poor awareness of
muscle tension. Note the absence of correlation between sEMG activity and subjective
rating.
2.5. Discussion
This investigation illustrates that subjects’ subjective ratings
did not correspond to the objective sEMG of their shoulders and forearms when their arms
are in different positions due to slight changes in keyboard position. The increase in
trapezius sEMG when hands are at the keyboard is often not noticed. This supports our
observations that many subjects were surprised to learn that their shoulder muscles showed
an increase in sEMG activity when their hands were at the keyboard and this could be a
contributing factor in the development of CRDs.
3. APPLIED PSYCHOPHYSIOLOGY TO IDENTIFY ERGONOMIC POSITIONS
This study investigated muscle tension levels and subjective awareness
during mouse and trackball use (a standard keyboard and a "Natural" ergonomic
keyboard), and two postural variations, (leaning forward and backward at the waist and
looking 45 degrees right and left).
3.1. Subjects
Nineteen (fourteen women and five men) volunteer experienced computer
users participated in the study. The participants’ mean age was 29.76 years (range 20
to 55).
3.2. Equipment
A simulated computer workstation consisted of a standardized mouse,
mousepad, trackball, and keyboard placement. Electromyographic activity was recorded with
surface electrodes from the sternocleidomastoid, posterior deltoid, upper trapezius and
lower trapezius/rhomboid muscles with a Thought Technology FlexComp physiological data
acquisition system.
3.3. Procedure
Subjects were ergonomically positioned and surface EMG sensors were
attached. After baseline recordings, they drew their name for 1 minute with a centrally
positioned trackball, or a mouse to the side of an extended keyboard or a Microsoft
"Natural" keyboard while looking straight ahead or to the side at 45 degrees.
They rated their muscle tension on a 1-5 scale (1 for least, 5 most muscle tension). All
trials were repeated twice in an order-balanced sequence.
3.4. Results
Mouse use with the arm positioned to the right of a Microsoft
"Natural" keyboard resulted in significantly higher overall muscle tension
measures (p<.001, df = 1,18) compared to trackball use positioned at the center
of a body(see figure 2).
Figure 2. Average arm and shoulder sEMG activity during mouse use.
Significantly higher sEMG levels (p<.001, df =1,18)
occurred in the upper trapezius, lower trapezius, and posterior deltoid when subjects used
the mouse to the right of the extended and Microsoft "Natural" keyboard. Leaning
forward resulted in significantly higher sEMG levels (p<.001, df =1,18) in the
upper trapezius, lower trapezius, and posterior deltoid compared to leaning back Group
data of subjective awareness of muscle tension showed no significant correlation with sEMG
measures. No microbreaks were observed during mouse use.
3.5. Discussion
Mouse use to the right of an 18" extended keyboard, and more so
for the 20" Microsoft "Natural" keyboard, led to significant increases in
muscle tension in the upper shoulder, back, and arm, while lower muscle tension was
observed with central trackball use. Most importantly, elevated muscle tension occurred
without awareness and without the occurrence of mircobreaks. The sEMG data showed that an
ergonomic keyboard, designed to reduce ulnar rotation by splitting and widening the
keyboard more than a typical extended keyboard, forced mouse use even further from the
center of the body. This type of keyboard, while reducing the risk of wrist and hand
injury may lead to a significant increase in upper and lower trapezius and deltoid muscle
tension during mouse use.
4. CONCLUSION
Without sufficient awareness, regardless of appropriate ergonomic
adjustments and optimum keyboard position, individuals may unknowingly engage in
unnecessary muscles tension when carrying out tasks and may not be able to take beneficial
steps to prevent discomfort. Based on these findings, we strongly recommend 1) to train
employees to increase internal awareness of muscle tension during keyboard and mouse use
and to learn appropriate work/rest patterns, 2) to use sEMG assessment for determine the
impact of keyboard and pointing device use on the potential for CRD, 3) to use applied
psychophysiology for assessment, diagnostic analysis of worksite locations and worker
interactions with computer equipment, and 4) to offer preventative biofeedback training
for the prevention of CRD.
REFERENCES
1. Fine, L. J. (1996). Musculoskeletal disorders in office work. In:
Moon, S. D. and Sauter, S. L. (Eds). Beyond Biomechanics. London: Taylor
& Francis, 295-305.
2. Faucett, J. and Rempel, D., (1994). VDT-related musculoskeletal
symptoms: Interactions between work posture and psychosocial work factors. American
Journal of Industrial Medicine, 26, 597-612.
3. Peper, E., Wilson, V.S., Taylor, W., Pierce, A., Bender, K., &
Tibbetts, V. (1994). Repetitive Strain Injury. Prevent computer user injury with
biofeedback: Assessment and training protocol. Physical Therapy Products. 5(5), 17-22.
4. Peper E. & Harvey, R. (1996). The role of applied psychophysiology in
ergonomics, assessment and prevention of computer-related disorders. Proceedings of the
International Congress on Stress and Health. Sidney, Australia: The University of Sydney
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