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The effect of prolonged sitting on mental task performance
Computcraind. EngagVoL 31, No. 112,pp. 499- 502,1996 Copyright@ 1996 I~xbUJh~lby E1msvierScim~ LM P~nted in Grest Britthl Alh~lhts mmn~ed
Pergamon S0360-8352(96) 00184-2
o36o-8352/96$15.00+ o.oo
The Effect of Prolonged Sitting on Mental Task Performance Michelle S. Pitman and Celestine A. Ntuen
Departmentof IndustrialEngineering North Carolina Agricultural and Technical State University 419 McNair Hall Greensboro, North Carolina 27411
ABSTRACT Due to changes in technology, people are frequently relegated to the role of system monitor. This has created a more sedentary workplace, where the person commonly assumes a traditional seated position for prolonged periods of time. The seated position causes both bkanechanical and physiologic stressors which can lead to discomfort over a prolonged period. R was hypothesized that the discomfort associated with prolonged sitting will degrade performance of a system monitoring task. A pilot study was conducted testing this hypothesis.
1.0 INTRODUCTION Sitting appears to have ori~nated some 5000 years ago as a means for the leader or chief of a group of people to sit with his head higher than the commoners. Over time, sitting took on a meaningfid status. Historical coincidences and social definitions have created the traditional seated position that remains today, a 90 degree bend at the waist and the knees. In today's workplace, improvements in technology have created an increase in mental demands and what seems to be a decrease in physical demands. As people are required to monitor and reslxaXL or perform vigilance tasks, a by-product of this work transformation is "postural fixity" (Grieco, 1986), commonly the traditional seated position. The pilot is a sedentary worker that is relegated to the seated position, frequently for prolonged periods of time. Automation has particularly captured the interest of the Aviation Industry where the pilot is being delegated to the role of system monitor rather than the decision maker for which be was previously selected and trained. 2.0 PROLONGED SITTING
A review of the literature on prolonged sitting shows the effects of both biomechanical and physiologic stressors. The normal shape of the spine is a compound curvature and occurs when a person is standing erect. The shape of the spine is altered when a person sits which increases pressure on the intervertebral discs. Studies by Nachemson & Morris (1964), Okushima (1970) and others have confirmed that intervertebrai disc pressure is 35% lower when standing versus sitting. Reasons include an increase in the trunk load moment and the deformation of the disc caused by lumbar spine flattening (Andersson, 1974). Physiologically, Calliet (1973) showed that the spinal alterations during sitting also create a process of isometric contraction of the paravertebral muscles. Prolonged isometric contmotion causes endomuscular pressure which restricts blood flow. The resulting ischemia inhibits the subsequent energy requirements. Not only do muscles become fatigued, stress may be transferred to other soft tissues, i.e., 499
500
19th International Conference on Computers and Industrial Engineering
tendons and ligaments. A physiologic slow down is also created by sitting. Intra-abdominal pressure on the internal organs both reduces the supply of oxygen to the brain, resulting in a feeling of fatigue, and restricts disc metabolism which may inhibit nutritional exchange with adjacent tissues(Kraemer, Kolditz and Godwin, 1985). 3.0 HUMAN PERFORMANCE Bailey (1989) defines human performance as the result of a pattern of actions carried out to satisfy an objective according to some standard. Mental performance is generally viewed as part of a closed-loop process m which a stimulus or cause is a prior condition which elicits a response. Within this simplified model of a system, there are opportunities for error from both external and internal sources. In 1931, Bills demonstrated the theory of"blocking" or "blocks" to describe interruptions in information processing. Long-duration mental effort mvokes periodic interruptions of information which increase in both frequency and duration as the effort continues. Broadbent (1958) found that various environmental and circadian stressors increase the frequency of blocks and caused a further decline in performance. Research studies by Mackworth (1948) showed that when performing vigilance tasks, subjects' ability to detect critical signals declines rapidly from 25 to 35 rain. into the vigil. Consistent research iUustratmg this decline in vigilance performance has been called the vigilance decrement (Davies and Parasuraman, 1982). 4.0 SITTING AND PERFORMANCE Stress is often viewed as a force that degrades performance capability (Hancock and Warm, 1989). Stressors can be environmental (i.e, heat, noise, work-rest schedule) or cognitive (i.e, boredom and pressure). The fatigue and discomfort associated with prolonged sitting is considered an aspect of the environment. As the level of fatigue or discomfort increases with periods of prolonged sitting, there may be a shifang of attention from task pefforrmmce to the mitigation of discomfort, i.e., attempts to change posture within the seating constraints, or "restlessness". It would seem that these environmental demands have the potential to degrade performance as demands for attention are shifted away from the task at hand. Previous studies of stressors on task performance relating to the physical environment examine the effects of such extremes as heat, humidity, and whole-body vibrations (see Davies and Parasuranmn, 1982, for a review). Bhatnager, Drury and Schiro (1985) studied the effect of posture on performance in an industrial inspection task. In this study comparisons were made between various postural configurations. Kopardekar and Mital (1994) looked at the optimal work-rest schedules for directory assistance operators at computer workstations. The intent of the study was to determine a work-rest schedule to optimize operator performance. These studies have not considered the effect of prolonged seated posture as a stressor which can effect performance. The work-rest schedule studies allowed for unrestricted postural changes during the rest breaks allowed under the test conditions, thus allowing for both physical and mental recovery. The current research proposes that prolonged sitting may be associated with a decline in performance when performing a vigilance task. It was hypothesized that reaction time in responding to critical signals within the system would increase during sequential trails of a system monitoring task when body posture is constrained to the seated position.
5.0 METHOD 5.1 Aooaratus Subjects monitored a V G A video displayterminalcontrolledby an IBM compatible computer. The screen displaywas the Multi-AttributeTest battery(MAT) designed by NASA, Langley Research Center. The test presents multiple tasks such as system monitoring, pursuit tracking, and resource allocation which simulate those tasks performed by the pilot m the cockpit. Subjects responded by striking keys on a standard IBM-compatible enhanced keyboard. Subjects were seated in a chair with adjustable back height (lumbar support) and seat height.
19th International Conference on Computers and Industrial Engineering
501
5.2 Task All subjects participated m 4 sequential 30 minute vigils with a 10 minute break between trials. During the breaks the task was removed, however, subjects were required to remain seated. The task consisted ofthe system monitoring portion ofthe MAT. Subjects monitored two light displays and four floating scales. Subjects responded to a green fight, default value equal to on, when the light went off. Subjects responded to a red light, default value equal to off, when the light came on. The four scales were monitored for any fluctuations outside of a range of +1 gradient. 5.3 Procedure On arrival, subjects were given a brief overview of the experiment. They were given instructions in the operation of chair adjustments and allowed an opportunity to make adjustments to their own comfort level. Instruction in the performance of the system monitoring task were then read to the subject. They were given a 10 minute practice session to become familiar with the task and its requirements. Subjects were given an opportunity to walk, stretch, etc. prior to commencing the actual experiment. All subjects were given the same system monitoring task. 5.4 Pilot Study Reaction times (RT) were measured for lights and ganges during each trial. The results of the pilot study are shown in Figure 1. RT values do appear to have an upward trend across trials suggesting that performance did decline over the period of the experiment. Further trials are required to fully test this hypothesis. There also appears to be a difference in the RT~,. as opposed to RT,~, with RT~. being greater. Previous research on attention (Wiekens, 1987) suggests this may be attributed to the signal rates of change. The scales provided continuous movements throughout the experiment, however, changes in light displays occur approximately once each minute. Subject scanning techniques may have focused more on the continuous movement of the scales. 6.0 POTENTIAL APPLICATIONS The results of this research may have applications m the design of both the work environment and task allocation. A large body of research and design effort has focused on optimal seating designs. This study implies the need for an emphasis on flexibility in seated positions as well as other postural options. Work-rest scheduling options should consider operator performance capability and effects of body posture. Finally, the efficiency of the human as a
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v
2
........
1
2
3
Lights
4
Trial
Figure 1. Mean Reaction Times From Pilot Study
system monitor is questioned. In the design of systems, the human should be considered for those tasks for which it is best suited, those requiring flexibility and decision making.
502
19th International Conference on Computers and Industrial Engineering
REFERENCES
Andersson, G. B. (1974). On mYoelectri¢ back muscle activity and lumbar disc pressure m sitting posture. Doctoral dissertation, University of Gothenberg, Sweden. Bailey, R. W. (1989). Human _nerf0rmance~ ~ : Using human factors/ergonomics to achieve conm__uter system usabilitv (2rid Ed.). Englewood Cliffs, NJ: Prentice Hall. Bhamager, V., Drury, C. G., & Schiro, S. G. (1985). Posture, postural discomfort, and performance. Human Factors~ 27(2), 189-199. Broadbent, B. E. (1958). Percention and communication. London: Pergamon Press. Calliet, R. (1973). II dolore cervice-brachiale. Rome: Leonardo Ed. Scientifiche. Chaffm, Don B. and Andersson, G. B. (1991). Occupational Bi0mechanics, (2nd ed.). New York: John Wiley & Sons. Davies, D. R., and Parasuraman, R. (1982). The psychology of vigilance. London: Academic. Grieco, A. (1986). Sitting posture: An old problem and a new one. Ergonomics~ 29, 345-362. Hancock, P. A and Warm, J. S. (1989). A dynamic model of stress and sustained attention. Human Factors, 31(5), 519-537. Kopardekar, P. & Mital, A. (1994). The effect of different work-rest schedules on fatigue and performance of a simulated directory assistance operator's task. Ergonomics, 37(10), 1697-1707. Kraemer, J., Kolditz, D., and Cnxiwin, R. (1985). Water and electrolyte content of human intervertebral discs under variable load. Seine. 10. 69-71. Mackworth, N. H. (1948). The breakdown of vigilance during prolonged visual search. Quarterly Journal of Experimental Psvchology. 1.6-21. Nachemson, A., and Morris, J. M. (1964). In vivo measurements of intradiscal pressure. Journal of Bone $oint S ~ . 46A. 1077. Olmshima, H. (1970). Study on hydrodynamic pressure of lumbar intervertchral disc. Arch. Jap. Chir, 45.
Wickens, C. D. (1987). Attention. In P.A. Hancock (Ed.), Human Factors Psychology (pp. 29-80). Amsterdam: Elsevier Science Publishers B.V.
Pergamon S0360-8352(96) 00184-2
o36o-8352/96$15.00+ o.oo
The Effect of Prolonged Sitting on Mental Task Performance Michelle S. Pitman and Celestine A. Ntuen
Departmentof IndustrialEngineering North Carolina Agricultural and Technical State University 419 McNair Hall Greensboro, North Carolina 27411
ABSTRACT Due to changes in technology, people are frequently relegated to the role of system monitor. This has created a more sedentary workplace, where the person commonly assumes a traditional seated position for prolonged periods of time. The seated position causes both bkanechanical and physiologic stressors which can lead to discomfort over a prolonged period. R was hypothesized that the discomfort associated with prolonged sitting will degrade performance of a system monitoring task. A pilot study was conducted testing this hypothesis.
1.0 INTRODUCTION Sitting appears to have ori~nated some 5000 years ago as a means for the leader or chief of a group of people to sit with his head higher than the commoners. Over time, sitting took on a meaningfid status. Historical coincidences and social definitions have created the traditional seated position that remains today, a 90 degree bend at the waist and the knees. In today's workplace, improvements in technology have created an increase in mental demands and what seems to be a decrease in physical demands. As people are required to monitor and reslxaXL or perform vigilance tasks, a by-product of this work transformation is "postural fixity" (Grieco, 1986), commonly the traditional seated position. The pilot is a sedentary worker that is relegated to the seated position, frequently for prolonged periods of time. Automation has particularly captured the interest of the Aviation Industry where the pilot is being delegated to the role of system monitor rather than the decision maker for which be was previously selected and trained. 2.0 PROLONGED SITTING
A review of the literature on prolonged sitting shows the effects of both biomechanical and physiologic stressors. The normal shape of the spine is a compound curvature and occurs when a person is standing erect. The shape of the spine is altered when a person sits which increases pressure on the intervertebral discs. Studies by Nachemson & Morris (1964), Okushima (1970) and others have confirmed that intervertebrai disc pressure is 35% lower when standing versus sitting. Reasons include an increase in the trunk load moment and the deformation of the disc caused by lumbar spine flattening (Andersson, 1974). Physiologically, Calliet (1973) showed that the spinal alterations during sitting also create a process of isometric contraction of the paravertebral muscles. Prolonged isometric contmotion causes endomuscular pressure which restricts blood flow. The resulting ischemia inhibits the subsequent energy requirements. Not only do muscles become fatigued, stress may be transferred to other soft tissues, i.e., 499
500
19th International Conference on Computers and Industrial Engineering
tendons and ligaments. A physiologic slow down is also created by sitting. Intra-abdominal pressure on the internal organs both reduces the supply of oxygen to the brain, resulting in a feeling of fatigue, and restricts disc metabolism which may inhibit nutritional exchange with adjacent tissues(Kraemer, Kolditz and Godwin, 1985). 3.0 HUMAN PERFORMANCE Bailey (1989) defines human performance as the result of a pattern of actions carried out to satisfy an objective according to some standard. Mental performance is generally viewed as part of a closed-loop process m which a stimulus or cause is a prior condition which elicits a response. Within this simplified model of a system, there are opportunities for error from both external and internal sources. In 1931, Bills demonstrated the theory of"blocking" or "blocks" to describe interruptions in information processing. Long-duration mental effort mvokes periodic interruptions of information which increase in both frequency and duration as the effort continues. Broadbent (1958) found that various environmental and circadian stressors increase the frequency of blocks and caused a further decline in performance. Research studies by Mackworth (1948) showed that when performing vigilance tasks, subjects' ability to detect critical signals declines rapidly from 25 to 35 rain. into the vigil. Consistent research iUustratmg this decline in vigilance performance has been called the vigilance decrement (Davies and Parasuraman, 1982). 4.0 SITTING AND PERFORMANCE Stress is often viewed as a force that degrades performance capability (Hancock and Warm, 1989). Stressors can be environmental (i.e, heat, noise, work-rest schedule) or cognitive (i.e, boredom and pressure). The fatigue and discomfort associated with prolonged sitting is considered an aspect of the environment. As the level of fatigue or discomfort increases with periods of prolonged sitting, there may be a shifang of attention from task pefforrmmce to the mitigation of discomfort, i.e., attempts to change posture within the seating constraints, or "restlessness". It would seem that these environmental demands have the potential to degrade performance as demands for attention are shifted away from the task at hand. Previous studies of stressors on task performance relating to the physical environment examine the effects of such extremes as heat, humidity, and whole-body vibrations (see Davies and Parasuranmn, 1982, for a review). Bhatnager, Drury and Schiro (1985) studied the effect of posture on performance in an industrial inspection task. In this study comparisons were made between various postural configurations. Kopardekar and Mital (1994) looked at the optimal work-rest schedules for directory assistance operators at computer workstations. The intent of the study was to determine a work-rest schedule to optimize operator performance. These studies have not considered the effect of prolonged seated posture as a stressor which can effect performance. The work-rest schedule studies allowed for unrestricted postural changes during the rest breaks allowed under the test conditions, thus allowing for both physical and mental recovery. The current research proposes that prolonged sitting may be associated with a decline in performance when performing a vigilance task. It was hypothesized that reaction time in responding to critical signals within the system would increase during sequential trails of a system monitoring task when body posture is constrained to the seated position.
5.0 METHOD 5.1 Aooaratus Subjects monitored a V G A video displayterminalcontrolledby an IBM compatible computer. The screen displaywas the Multi-AttributeTest battery(MAT) designed by NASA, Langley Research Center. The test presents multiple tasks such as system monitoring, pursuit tracking, and resource allocation which simulate those tasks performed by the pilot m the cockpit. Subjects responded by striking keys on a standard IBM-compatible enhanced keyboard. Subjects were seated in a chair with adjustable back height (lumbar support) and seat height.
19th International Conference on Computers and Industrial Engineering
501
5.2 Task All subjects participated m 4 sequential 30 minute vigils with a 10 minute break between trials. During the breaks the task was removed, however, subjects were required to remain seated. The task consisted ofthe system monitoring portion ofthe MAT. Subjects monitored two light displays and four floating scales. Subjects responded to a green fight, default value equal to on, when the light went off. Subjects responded to a red light, default value equal to off, when the light came on. The four scales were monitored for any fluctuations outside of a range of +1 gradient. 5.3 Procedure On arrival, subjects were given a brief overview of the experiment. They were given instructions in the operation of chair adjustments and allowed an opportunity to make adjustments to their own comfort level. Instruction in the performance of the system monitoring task were then read to the subject. They were given a 10 minute practice session to become familiar with the task and its requirements. Subjects were given an opportunity to walk, stretch, etc. prior to commencing the actual experiment. All subjects were given the same system monitoring task. 5.4 Pilot Study Reaction times (RT) were measured for lights and ganges during each trial. The results of the pilot study are shown in Figure 1. RT values do appear to have an upward trend across trials suggesting that performance did decline over the period of the experiment. Further trials are required to fully test this hypothesis. There also appears to be a difference in the RT~,. as opposed to RT,~, with RT~. being greater. Previous research on attention (Wiekens, 1987) suggests this may be attributed to the signal rates of change. The scales provided continuous movements throughout the experiment, however, changes in light displays occur approximately once each minute. Subject scanning techniques may have focused more on the continuous movement of the scales. 6.0 POTENTIAL APPLICATIONS The results of this research may have applications m the design of both the work environment and task allocation. A large body of research and design effort has focused on optimal seating designs. This study implies the need for an emphasis on flexibility in seated positions as well as other postural options. Work-rest scheduling options should consider operator performance capability and effects of body posture. Finally, the efficiency of the human as a
~ilili~iii!iJii!?!~!i!~!i i i! i lli~!i! !!~!!~:! !:! ! !:!:~:i:!:~:i:~:~;i:~:~
v
2
........
1
2
3
Lights
4
Trial
Figure 1. Mean Reaction Times From Pilot Study
system monitor is questioned. In the design of systems, the human should be considered for those tasks for which it is best suited, those requiring flexibility and decision making.
502
19th International Conference on Computers and Industrial Engineering
REFERENCES
Andersson, G. B. (1974). On mYoelectri¢ back muscle activity and lumbar disc pressure m sitting posture. Doctoral dissertation, University of Gothenberg, Sweden. Bailey, R. W. (1989). Human _nerf0rmance~ ~ : Using human factors/ergonomics to achieve conm__uter system usabilitv (2rid Ed.). Englewood Cliffs, NJ: Prentice Hall. Bhamager, V., Drury, C. G., & Schiro, S. G. (1985). Posture, postural discomfort, and performance. Human Factors~ 27(2), 189-199. Broadbent, B. E. (1958). Percention and communication. London: Pergamon Press. Calliet, R. (1973). II dolore cervice-brachiale. Rome: Leonardo Ed. Scientifiche. Chaffm, Don B. and Andersson, G. B. (1991). Occupational Bi0mechanics, (2nd ed.). New York: John Wiley & Sons. Davies, D. R., and Parasuraman, R. (1982). The psychology of vigilance. London: Academic. Grieco, A. (1986). Sitting posture: An old problem and a new one. Ergonomics~ 29, 345-362. Hancock, P. A and Warm, J. S. (1989). A dynamic model of stress and sustained attention. Human Factors, 31(5), 519-537. Kopardekar, P. & Mital, A. (1994). The effect of different work-rest schedules on fatigue and performance of a simulated directory assistance operator's task. Ergonomics, 37(10), 1697-1707. Kraemer, J., Kolditz, D., and Cnxiwin, R. (1985). Water and electrolyte content of human intervertebral discs under variable load. Seine. 10. 69-71. Mackworth, N. H. (1948). The breakdown of vigilance during prolonged visual search. Quarterly Journal of Experimental Psvchology. 1.6-21. Nachemson, A., and Morris, J. M. (1964). In vivo measurements of intradiscal pressure. Journal of Bone $oint S ~ . 46A. 1077. Olmshima, H. (1970). Study on hydrodynamic pressure of lumbar intervertchral disc. Arch. Jap. Chir, 45.
Wickens, C. D. (1987). Attention. In P.A. Hancock (Ed.), Human Factors Psychology (pp. 29-80). Amsterdam: Elsevier Science Publishers B.V.