An analysis of posture, muscle activity and keyboard dynamics in computer users with and without work-related neck and upper limb disorders
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Computer technology has advanced rapidly in the past few decades and computers have become a very important and powerful tool in our everyday lives. Prolonged computer use by office workers has been reported to result in an increased risk of developing Work-related Neck and Upper Limb Disorders (WRNULD) (Bernard et al.. 1994: Faucett & Rempel. 1994: Tittiranonda et al.. 1999). The occupational risk factors associated with prolonged computer use include static posture and the speed and force of keyboard operation. Past studies have examined different aspects of these risk factors through measuring muscle electrical activity (EMG), kinematics and keyboard forces. However, most of these studies have been conducted on healthy painfree subjects and even the few Case-Control studies have not clearly established any direct relationships between the risk factors and WRNULD. The present research project consisted of a series of three studies aimed at investigating whether there were intrinsic differences among different individuals in response to different physical stressors. These intrinsic differences may have important implications to help explain why some individuals would develop WRNULD while others do not. The individuals' responses to the demands of three physical stressors: static posture, speed and force of keyboard operation were assessed. The internal exposure measures of kinematics, EMG, keyboard dynamics and subjective discomforts were used to evaluate the inter-individual differences. Study I was a field investigation comparing the neck-shoulder kinematics between symptomatic ("Case", n=8) and asymptomatic ("Control", n=8) office workers. Results showed trends for consistently greater head tilt and neck flexion angles, and greater ranges of movements in the Case Group than the Control Group.The Case Group also exhibited a trend for increased acromion protraction compared to the Control Group. The Case Group also reported significantly greater discomfort scores compared to the Control Group. Neither the discomforts nor the kinematics displayed any significant changes over a working day. Study 2 was a laboratory study comparing the responses of Case and Control Groups in terms of EMG, kinematics and subjective discomforts, while a standardised computer task was performed continuously for one hour. The responses of Case (n=23) and Control (n=20) Groups were compared to examine the effects of static posture. The results showed similar trends to those in Study I, with increased neck flexion mean angles and ranges of movements in the Case Group compared to the Control Group. In terms of EMG results, there were trends for EMG amplitude differences in the right upper trapezius (UT) and cervical erector spinae (CES) muscles between Case and Control Groups. These trends became statistically significant when the Case subjects were sub-divided into the High (n=15) and Low (n=8) Groups based on their mean discomfort scores. Study 3 was also a laboratory study to compare the Case (n=21) and Control (n=20) Groups when they were challenged by the physical stressors of speed and force of keyboard operation. In this study, each subject's EMG and discomforts were examined in three typing conditions of normal speed and force, increased typing speed and increased typing force. The Case Group showed trends for higher increases in both UT and CES muscle activities than the Control Group, and when divided into the High-Low Groups, the High Group (n=8) showed trends for much higher muscle activities in all three conditions.Beside muscle activity changes, the High Group subjects also demonstrated a trend for much higher within-subject Speed and Force Variabilities in their keystroke performance, compared to the Low Group and the Control Group. This result implied that the High Group subjects had a more erratic motor control of the keystroke actions. Based on these results, conceptual models were developed to describe the relationships among the physical stressors, internal exposure responses and discomforts. The Altered Motor Control Model refers to the programmed changes in motor control strategies involving muscle recruitment and joint movement patterns, and these changes were closely related to the subjects' musculoskeletal discomforts. The Heightened Sensitivity Model describes the higher sensitivity levels of individuals with more severe discomforts, in response to the demands of physical stressors. These models are closely related and heightened sensitivity may be an 'effect-modifier" of the motor control mechanisms and the perception of discomforts or pains within the individual. In conclusion, the present research has identified important differences between individuals on the basis of their motor control strategies which may contribute to the development of WRNULD. While the present research has mainly examined the individual responses to three physical stressors, it is possible that the models developed may be applicable to other physical stressors. These findings may also have important implications for future ergonomic research, emphasising the need to address interindividual differences in ergonomic interventions to workers. Further research should be directed towards better understanding of these intrinsic individual differences in both physical and non-physical factors that contribute to the development of WRNULD.
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