PROCEEDINGS of the HUMAN FACTORS A N D ERGONOMICS SOCIETY 39th A N N U A L MEETING-1995
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RECOVERY FROM STRAIN UNDER DIFFERENT WORK/REST SCHEDULES
Wolfram Boucsein and Michael Thum
Physiological Psychology, University of Wuppertal
Wuppertal , Germany
Measures of psychophysiological recovery were used to evaluate two rest break
schedules; 7.5 minutes of rest after every 50 minutes of work versus 15 minutes of
rest after every 100 minutes of work. Eleven examiners using a prototype computer
system in the European Patent Office worked under both workhest schedules.
Electrodermal activity , heart rate, respiratory frequency, pulse wave transit time ,
neck electromyogram, and gross body movements were continuously recorded.
Measures of emotional well-being and body comfort were obtained eight times per
work day. Heart rate variability was significantly higher under the short break
schedule, indicating decreased mental strain. Break duration and time of measurement
interacted significantly for electrodermal responses, indicating that emotional strain
was reduced under the short break schedule until mid-day, and under the long break
schedule in the afternoon. The results indicate that a switch to longer breaks in the
afternoon may be favorable during highly demanding computer work. Furthermore, it
could be demonstrated that psychophysiological measures are useful for the evaluation
of worldrest schedules, even if performance data are not available.
INTRODUCTION
The optimization of performance without
increasing strain has always been a major interest
in both, human engineering and industrial
psychology. Stress-strain processes during work
do not only impair performance but also
subjective and objective well-being, Measures of
performance quality and quantity have been
frequently used as indicators of strain. Their main
advantage is that they can be gathered without
interrupting actual work. Recordings of subjective
strain, on the other hand, cannot be gathered
without interrupting the work process if more than
a global post hoc measure is considered.
Physiological recordings have been used as
objective measures of strain as well since they can
not be faked, and their recording does not
interrupt the work flow.
In former times, stress-strain processes
during work had primarily physical origins.
Nowadays, with widespread use of computers in
the workplace, mental strain has become the focus
of stress-strain research. However, emotional
strain may also occur during computer work as
well. For example system response times (SRTs)
that cause involuntary breaks of unpredictable
length can result in inadequate temporal structures
in human computer interaction that cause
emotional strain (Shneiderman, 1987). Although
mental strain is more easily tolerated by workers,
and is sometimes experienced as being pleasant,
emotional strain should be avoided as much as
possible since it increases the risk of developing
psychosomatic disorders in the long term. Various
physiological measures have been successfully
used to distinguish between mental and emotional
strain (Boucsein, 1993).
By means of a multivariate psychophysiological approach which includes the
simultaneous acquisition of subjective,
physiological and performance data, stress-strain
processes resulting from inappropriate work
scheduling in human computer interaction were
studied by the first author’s research group. These
studies began in 1982 and focussed on SRTs
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PROCEEDINGS of the HUMAN FACTORS AND ERGONOMICS SOCIETY 39th ANNUAL MEETING-1995
caused by both, hardware and software features of
the computer system which were problematic at
that time. Interestingly, it was not only too long
SRTs but also very short SRTs that had have
negative consequences for both performance and
well-being. Since the results of these earlier
studies on SRTs led to the hypotheses of recent
studies on break conditions, a short overview of
these findings will be given.
An easy-to-learn detection task was used in
most of the laboratory-based SRT-studies.
Participants were asked to identify a space
surrounded by identical letters in a row of random
letters and spaces presented in the middle of a
computer screen. SRTs were varied systematically
between 1.5 and 8 seconds. Additional time
pressure was induced by using incentives for
speed and accuracy. Keystroke output and error
rate were collected as performance measures.
Electrodermal activity (EDA), blood pressure
(BP), and heart rate (HR) were also recorded. In
addition, subjective ratings of mood and physical
comfort were collected.
An increase in mental strain could be
observed under short SRTs with additional time
pressure, mainly reflected by an increase in
cardiovascular reactivity. Furthermore, error rate
markedly increased and subjective complaints
commonly associated with intensive computer
work were reported. When no time pressure was
imposed, no specific physiological and subjective
strain reactions emerged with short SRTs.
However, the subjects spontaneously increased
their working speed, showed a tendency for
higher error rates, and reported more subjective
excitement and physical discomfort. These
unexpected negative effects of short SRTs were
confirmed in several studies.
When SRTs were long, the subject’s work
style seemed to be relaxed under time pressure at
first glance. The error rate decreased and the
subjects worked more carefully since error
correction was much more time consuming.
However, the subjects described their working
situation as uncomfortable, and EDA increased,
indicating increased emotional strain (Boucsein,
1992). Results without time pressure were similar
with respect to performance. Subjective well-
being was reported at the beginning, but
emotional tension developed as indicated by an
increasing frequency of spontaneous electrodermal
reactions during SRTs (Kuhmann, 1989).
Based on these results it is not
recommended that SRTs are made as short as
possible in every instance. Instead, optimal SRTs
should be determined for the task in question by
using psychophysiological methods (Kuhmann,
Boucsein, Schaefer, & Alexander, 1987). Best
performance, lack of high cardiovascular strain,
low EDA, and low subjective discomfort can
occur when SRTs are an optimal length. The
optimal SRT was determined to be 5 s for the
specific task used in these studies.
The rest break standard for computer work
that has been adopted in the majority of German
work places, consisting of 10 minutes of break
after every hour of continuous computer work,
was designed for short-cycle, repetitive tasks.
Since computer work has become less repetitive
and more complex, this standard may no longer
be appropriate. One reason is that the workflow is
at risk of being seriously disrupted by frequent
breaks. A second reason is that physiological
strain during more complex computer work may
require longer recovery times.
Based on findings in the research studies
described above, a field study was conducted to
compare two different workhest schedules for
complex computer-mediated work; breaks that
occurred after every 50 minutes of work versus
breaks that occurred after every 100 minutes of
work. Physiological measures along with
subjective ratings of well-being were used to
evaluate the relative success of the two break
schedules.
METHOD
Field site and participants
The study was conducted at the European
Patent Office. Subjects (Ss) were 11 examiners
who were members of a prototyping group.
Instead of using paper documents, these
examiners have access to patents on laser disk
which can be displayed on a 24 inch computer
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PROCEEDINGS of the HUMAN FACTORS AND ERGONOMICS SOCIETY 39th ANNUAL MEETING-1995
screen. Based on a search through granted
patents, the examiners write a report about the
novelty of the current application.
Each of the examiners performed hidher
work under the long break schedule (15 minutes
break after 100 minutes of work) on one day and
under the short break schedule (7.5 minutes break
after 50 minutes of work) on another day. The
order of break conditions was counterbalanced.
The total break time on each day was 82.5
minutes (including the lunch break) under each
break schedule.
Experimental measures
Physiological responses. Physiological
recordings of HR, EDA , electromyogram (EMG,
neck lead), respiration, pulse wave transit time,
and gross body movements were taken
continuously using the VITAPORT ambulatory
monitoring system. The Ss indicated the
beginning and the end of each break with the
marker channel of this device. The Ss were
instructed to sit relaxed in their chair during the
first 2 minutes of each break and during the first
2 minutes after each break in order to obtain
artifact-free physiological recordings of the
recovery during the break.
Subjective responses. S s rated 20 items
related to emotional well-being and 20 items
related to physical comfort at eight times during
each work day.
Perjiomzance. No performance variables
were acquired. This was because the keyboard
action did not correlate with either task difficulty
or performance quality. While the only indicator
for performance quality was the quality of the
final report about the patent application, this
could not be measured objectively during our
study since objections against the report can be
filed up to years after the report was written.
Analysis
For each physiological variable, a change
score was calculated between relaxation at the end
of the break versus relaxation at the beginning of
the break. These scores indicated the extent of
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recovery over the break period and were
calculated for three times of measurement; an 11
AM score was obtained after both groups had 15
minutes total break time, a 3 PM score after 67.5
minutes total break time, and a 5 PM score after
all breaks were taken. In addition, the subjective
data acquired after the end of the 11 AM, 3 PM
and 5 PM break were selected for the analysis.
RESULTS
The analyses of the subjective data
revealed a significant main effect of "Time of
measurement" for perceived exhaustion (F(2,18) =
7.1; p < .05) and musculoskeletal discomfort
(F(2,18)= 6.88; p < .05), both of which
increased significantly over the work day. In
addition, perceived performance motivation
progressively decreased over the work day
(F(2,18)= 12.29; p < .01). While the break
schedules had no differential impact on these
ratings, subjective well-being was significantly
better under the short break schedule (F( 1,9)=
8.28; p < .05).
The analysis of the neck EMG also yielded
a significant main effect of "Time of
measurement" (F(2,18)= 4.70; p < .05). While
EMG activity increased during the 11 AM and 3
PM breaks, a decrease could be observed during
the 5 PM break. A significant main effect of the
factor "Break duration" (F(1,9)= 9.17; p < .05)
indicated increased heart rate variability (HRV)
under short breaks pointing to a more effective
reduction of mental strain under the short break
schedule. Furthermore, a trend of "Time of
measurement" (F(2,18)= 2.93; p = .08) indicated
that HRV decreased steadily during the course of
the work day under both break schedules. This
can be interpreted as reflecting the workers'
increased efforts to struggle against fatigue. These
results were not caused by differences in HR,
since no significant effects for HR were found.
The analysis of the frequency of
spontaneous changes in EDA resulted in a
significant interaction "Break duration x Time of
measurement" (F(2,18)= 4.57; p < .05). While
an increase of the frequency of spontaneous EDA
during the 11 AM and 3 PM breaks and a
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PROCEEDINGS of the HUMAN FACTORS AND ERGONOMICS SOCIETY 39th ANNUAL MEETING-1995
decrease during the 5 PM break could be
observed under the long break schedule, the
reverse pattern was found under the short break
schedule; namely a decrease in the morning and
an increase in the late afternoon.
These response patterns indicate that the
long breaks increased emotional strain at 11 AM
and 3 PM, which is in accordance with what we
found in earlier research on SRTs that were too
long. In contrast, the longer break schedule
helped reduce emotional strain at 5 PM.
DISCUSSION
The results reported here demonstrate that
workhest schedules have a considerable impact on
stress-strain processes during complex computer
tasks. Psychophysiological methods can be used
to investigate this impact even without the
simultaneous acquisition of performance quality
indicators. Since performance quality is
influenced by stress-strain processes, the use of an
appropriate workhest schedule can be expected to
contribute to the maintenance of high performance
quality. At this stage of our evaluation the
following tentative recommendation for the design
of break schedules during complex mental work
can be given:
In order to provide adequate opportunity
for recovery from strain during the course of the
work day, break times in the late afternoon should
be prolonged. Since the 50 minutes work/ 7.5
minutes break schedule was associated with higher
ratings of well-being, and also associated with
less emotional strain during the 11 AM and 3 PM
reading as indicated by the frequency of
spontaneous EDA, a short break schedule should
be applied until mid-day. However, a more
effective reduction of fatigue and strain symptoms
under the long break schedule during the last
point of measurement, as indicated by the
increase of HRV and the decrease of the
frequency of spontaneous EDA, gives evidence
that fewer but longer breaks should be taken in
the afternoon to ensure performance quality and
to prevent the development of psychosomatic
disorders over the long term.
In summary, psychophysiological
recordings appear to be useful tools for
investigating recovery from strain resulting from
inadequate temporal structures in human computer
interaction, both in laboratory and in field
settings. This has been shown in the evaluation of
relatively short intervals such as SRTs, and in the
evaluation of long intervals such as the duration
of breaks during a whole work day when
performing complex computer tasks. Our next
step is to extend this approach to determine
appropriate break schedules during overtime
work, an activity which is very common among
highly skilled computer workers.
REFERENCES
Boucsein, W. (1992): Electrodeml
Activity. New York: Plenum Press.
Boucsein, W. (1993). Psychophysiology in
the computer workplace - goals and methods. In
H. Luczak, A. Cakir, & G. Cakir (Eds.), Work
With Display Units '92 (pp. 135-139).
Amsterdam: North-Holland.
Kuhmann, W . , Boucsein, W., Schaefer,
F., & Alexander, J. (1987). Experimental
investigation of psychophysiological
stress-reactions induced by different system
response times in human-computer interaction.
Ergonomics, 30, 933-943.
Kuhmann, W. (1989). Experimental
investigation of stress-inducing properties of
system response times. Ergonomics, 32, 271-280.
Shneiderman, B. (1987). Designing the
user interjace. Strategies for eflective humancomputer interaction. Reading: Addison-Wesley .
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