How to Use Applied Psychophysiology/Biofeedback in the Prevention and Assessment of Upper Extremity Musculoskeletal Disorders
Erik Peper, Richard Harvey and Dianne Shumay
Institute for Holistic Healing Studies, San Francisco State University, 1600 Holloway Avenue, CA 94132
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.
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.
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.
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).
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 subjects 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.
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).
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).
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.
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.
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.
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.
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.
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
Last Updated: 10/05/99