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Physical work load in physical education teachers
Applied Ergonomics 30 (1999) 435 } 442
Physical work load in physical education teachers HeH le`ne Sandmark *, Christina Wiktorin , Christer Hogstedt , Eva-Karin Klenell-Hatschek , Eva Vingas rd Department of Occupational Health, Karolinska Hospital, S-171 76 Stockholm, Sweden National Institute for Working Life, S-171 84 Solna, Sweden Received 11 June 1997; accepted 4 August 1998
Abstract The main objective was to measure and quantify exposure to physical work load in physical education teachers. A further aim was to establish the reproducibility of a three-week test}retest interval of self-reported physical load and to evaluate the agreement between registered and self-rated physical load. Thirty teachers, both female and male, volunteered to participate in the study. The physical load on the lower extremities, as well as the back and the cardio-vascular strain was recorded during one working day. The results indicate that the physical work load in this occupational group is considerable due to the load on the lower extremities. The demands of the cardio-vascular system are also relatively high in comparison to other occupational groups. The teachers could assess their activity quite well regarding heavy lifting and time spent sitting. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomechanical load; Lower extremity; Self-reported work load
1. Introduction Swedish teachers of physical education report a high rate of dysfunction and injuries to the musculoskeletal system, and very few of them are able to work actively in the profession until retirement (HaK ndel and RunsjoK , 1985; LindeH n, 1988; O$cial Statistics Sweden, 1994). High physical load is an important aetiologic factor for musculoskeletal dysfunction (Hagberg and Wegman, 1987; RiihimaK ki, 1991; Hagberg,1992; Vingas rd et al, 1992 Winkel and Westgaard, 1992), and motivated this study of physical work load for those teachers. Occupational titles have often been used to approximate the exposure to physical work load, but the physical load within the same occupational group may vary considerably. Other methods to assess exposures are questionnaires, interviews, logbooks and direct measurements. The validity of these instruments are seldom reported. Performing direct measurements is time-consuming and expensive. Usually just a small fraction of one working day is covered and the total load is extrapolated from this, which is questionable.
*Corresponding author. Tel.: #46 8 729 33 06; Fax: #46 8 33 43 33; e-mail: [email protected].
The Stockholm MUSIC I study group has validated methods for assessment of physical load in epidemiological studies (Wiktorin, 1995). This study continues this research by using the developed methods for measuring physical work load during entire working days, and focusing on certain occupational risk groups reporting frequent dysfunctions in the musculoskeletal system. The overall purpose of the present project was to measure the exposure to physical load and its relation to the development of musculoskeletal dysfunctions in teachers of physical education. This has been carried out in two studies. The present and "rst part is aimed at exposure measurements and the second part at epidemiological studies on the relationship between dysfunction and physical load in this occupational group. In most epidemiological studies questions on retrospective exposure are asked and it is crucial to know the precision and accuracy in such estimations. In particular if there is a relationship between outcome status and the exposure estimation. The main objective of the present study was to measure and quantify exposure to physical load of teachers in physical education during an entire working day, to establish the reproducibility for a three-week test}retest interval of selfreported physical work load in this occupational group, and to evaluate the agreement between self-reported and objectively registered physical load in the same group.
0003-6870/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 9 8 ) 0 0 0 4 8 - 9
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2. Subjects and methods 2.1. Subjects Twenty female and ten male teachers of physical education volunteered to participate in the study. The number of females and males re#ects the gender distribution in this occupation. The subjects were all living in the Stockholm area, and working in di!erent schools in di!erent areas. We contacted the schools and if there was more than one teacher working there all teachers of physical education participated. Invitations were sent out with the aim of studying teachers of di!erent ages and sex, working at di!erent stages in the school system. Fourteen teachers worked in the upper secondary school where the students age range from 17 to 19 yr, and the rest in the nine-year compulsory school system where the pupils are from 7 to 16 yr of age. Ninety percent of the teachers were working full-time and the rest part-time. On average they had been working as teachers of physical education for 15 yr (range 1}32 yr). Background data of the subjects are presented in Table 1. 2.2. Methods for assessing exposure The physical load on the lower extremities, the back and the general physical strain during work was assessed through continuous registrations of postures, movements, and heart rate over an entire working day. The measurement devices were either applied directly to the body, or indirectly used, in systematic and computerised observations. All the registrations were performed by two experienced ergonomists. The walking distance was registered with a pedometer (Fitty 3 Electronic, Germany) (Selin and Winkel, 1994). Each subject's footstep was estimated by measuring the distance when he/she had walked "ve steps. The distance was then divided by "ve and the step length was entered into the pedometer. Thus the number of footsteps during the registration period indicated the walking distance.
Table 1 Characteristics of the study group (n"30). Female teachers n"20 Age (yr) mean (SD) range Height (cm) mean (SD) range =eight (kg) mean (SD) range
Male teachers n"10
40 (9) 24}57
39 (11) 26}56
167 (6) 156}178
178 (3) 173}183
61 (7) 48}78
75 (9) 65}95
The pedometer has been validated, and it was found that the walking distance was overestimated when a subject was jumping or dancing, since the distance is calculated from the length of an ordinary footstep (Selin and Winkel, 1994). On the other hand the number of steps and thus heelstrikes was correct. The registrations of the pedometer gave a rough estimate of the true walking distance, as jumping and dancing and other similar activities were included in the studied subjects. Duration and frequency of jumps ("leaps, springs, skips), light (30}903 knee #exion) and deep ('903 knee #exion) knee-bends, and lifting and carrying were registered using a hand-held computer (PSION) and software (PEO) for entering frequency and duration of these exposure parameters (Fransson-Hall et al, 1995). The validity and reliability of the PEO method have been evaluated and it was concluded that the method is applicable in investigations of most occupations and work tasks (Fransson-Hall et al, 1995). The duration and frequency of carrying and lifting were divided into di!erent weight-classes, 1}5, 6}10, 11}15, 16}20 and '20 Kg. The same classi"cation of weights has been used in other studies (Vingas rd et al, 1991). The weights were usually the equipment used in the training and several of the items had the weight marked or the weight was known by the teacher. Weights over 20 Kg occurred when the teachers lifted the students in the apparatus gymnastics. We experienced some di$culty in registering the frequencies correctly on the hand-held computer in periods of intensive registration and high frequencies, for example in jumping. Too low frequencies registered during these periods underestimated the magnitude of the physical load. The trunk #exion was measured using an inclinometer (Fig. 1), a trunk position analyser, which registers the time spent in 5 intervals of 203 ranging from slight extension of the trunk (!203) to an upright position (03) and to deep trunk #exion (803) (Ericsson, 1994). The inclinometer registers the sagittal angle between the trunk and the reference trunk position. The sensor and the box for data collection were attached to the back at Th5}Th10 level. When the registration were "nished the collected data were transmitted to and analysed in a computer using a specially developed software program (Ericsson, 1994). The twist of the trunk was not measured. The inclinometer has been found to be valid and reasonably precise (Ericsson, 1994). A similar device has been used in earlier investigations (Nordin et al, 1984; Wiktorin et al, 1993; Karlqvist et al, 1994). For six subjects in our study the inclinometer did not register properly and these recordings were rejected. In one of these the device broke when the teacher fell during an exercise. The subjects rated what e!ect the measurement devices had on movements and postures during the time of the registrations. Six of the subjects reported that the device did
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442
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Fig. 1. The inclinometer during registering. The device is attached to the thoracic part of the spine.
not a!ect them at all. To what extent it hindered certain postures and movements in the others depended on what was intended to be carried out. The subjects who reported that they were hindered commented on not being able to do exercises lying on their backs because the inclinometer was attached to the thoracic part of the spine. Also when the exercises included jumping or aerobics the inclinometer slightly hindered the activity. From the comments it seems as if the inclinometer is the instrument that a!ects the positions and movements the most. It can also be concluded that there was an underestimation of some usual activity because the inclinometer did not allow the subjects to lie down on their backs. The general physiological strain was assessed through registrations of the circulatory stress using a heart rate recorder (Sport tester PE 3000). The Sport tester gives a cordless transmission of the heart rate from the electrodes applied to the chest of the subject to the data logger applied around the wrist. The heart rate was registered during the whole day at work including the lunch break. The recorded heart rate was stored as means for each recorded minute. The mean heart rate, and the highest and lowest values during the day were registered in each subject. Heart rate monitors with chest electrodes, like the one used in this study, have been tested for validity and stability and found to be applicable for monitoring heart rate (LeH ger and Thivierge, 1988). The transmission was cordless, and we found that the receiver was sensitive when the subjects wore a watch very close to it. This can cause disturbances in the transmission.
2.3. Questionnaire Eight questions on frequency and duration of sitting, walking distance, jumping, lifting, knee bending, trunk bending during work on the day of registrations, were asked at the end of the day and also after three weeks. The question on sitting had an open answer, and for lifting and carrying the question on exposed or non-exposed was dichotomised; there were also questions on frequencies of di!erent weights lifted. For the remaining questions there were "ve alternative answers to each question. The same or similar design of questions has been used in earlier studies (Wiktorin et al, 1996). None of the subjects knew the results of the recordings when they answered the questions. A 15-graded scale (RPE), graded 6}20, was used for ratings of the perceived exertion and thus a physiological stress indicator for the day (Borg, 1990). Three weeks later the Borg scale and the eight questions about the recording day were sent by mail to each subject. The subjects still did not know the results of the objective recordings when answering the same questionnaire once more. The day on which the recordings were made included on average 4.4 classes (range 3}6). Most classes lasted for 60 min, and a few for 45 and 80 min, respectively. The shorter classes were in the primary school where the schoolchildren are still very young, and the 80 min classes are in the highest grades. The entire length of the working hours during the day of recording was on average 325 min and ranged from 192 to 432 min. This is according to the PEO registrations which covered the entire working day exactly. The time of recording varied slightly for each recording
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device, depending on when the device was applied and turned on. Only one instrument could be applied at a time. We investigated if the day of the registrations had been more or less strenuous compared to other working days. Fourteen subjects rated it as &like any other day', and "ve as more strenuous but not to an extreme. Eleven rated it as less strenuous, and one of them as much less strenuous. 2.4. Analysis The total registration period, the heart rate measurements, and the characteristics of the study group are calculated as mean value, standard deviation and range. The results of frequency and duration of the recorded parameters, except the heart rate measurements, are expressed as median value and range. Median value is used, as the recorded values are not normally distributed. The heart rate values are presented as mean value, standard deviation and range, while the values are normally distributed and thus allowed direct comparisons with other studies. In the analysis we show separate results in the heart rate measurements and accordingly in the ratings on the RPE scale for men and women respectively, as there were gender di!erences in the heart rate frequency. In the remaining variables, including the correlations, we did not see any gender di!erence in quantity of physical exposure or in ability of reporting physical exposure. The duration of jumping, knee bending, lifting and carrying was very short and were therefore not analysed. The frequency of the recorded parameters was also calculated as number per working hour, since the hours di!ered between individuals. The agreement of objective registrations and the individual's own assessments of duration and frequency of the parameters the same day and after three weeks, and the test}retest reproducibility of the self-reports the same day and after three weeks were, evaluated from calculations of a non-parametric rank order correlation (Spearman's rho).
3. Results The activities and the physical load during the working day di!ered from subject to subject, and the interindividual variation was considerable in most of the variables. The most common activity was warming up exercises, and the second most common was di!erent kinds of indoor ball games where the teachers mostly acted as referees and rarely participated in the game. The teachers actively participated in warming up activities, aerobics, dancing, #oor exercises and when the lesson was taught by a student. In apparatus gymnastics the teachers supported the students and in circuit training they assisted and helped them.
3.1. Frequency and duration of postures, movements and lifts The time spent sitting was 24 min or 8% (median value) of the day of recording (Table 2). Three of the teachers did not sit at all during this day. The variation of frequency of jumps and light and deep knee bends was considerable (Table 2), mostly due to whether the teachers joined in the exercises or not. The teachers walked 4956 ms (median value) during the working day (Table 2). Also for this variable there was a high degree of variation between individuals. The longest distances walked were when the classes had outdoor activities and when the teacher participated in the exercises when a student taught the lesson. Fourteen of the teachers spent 3}32 min in di!erent awkward positions like kneeling on both knees or on one knee, mostly when supporting the students in apparatus gymnastics. The median value for time in awkward positions was 6 min. Twenty-nine of the teachers were exposed to carrying and lifting (Table 2). Supporting the students in apparatus gymnastics produced the heaviest lifts, but the frequency for di!erent teachers varied considerably. The results from the measurements of trunk extension and #exion are presented in Table 3. The subjects were in an almost upright position for 78% (!203 to 203) of the registered time (median value) and the interindividual variability was large.
Table 2 Exposure of physical load during work. Frequency and duration of movements, postures and lifts in the study group during the day of registration Exposure
Sort of exposure
Median
Sitting Sitting
%/d min/d
8 24
0}65 0}136
Jump Jump
number/d number/h
29 6
0}339 0}78
Knee bend 303}903 Knee bend 303}903
number/d number/h
74 13
14}236 3}76
Knee bend'903 Knee bend'903
number/d number/h
46 9
5}186 0}58
Walking distance Walking distance
m/d m/h
Lifting 1}5 (kg) 6}10 (kg) 11}15 (kg) 16}20 (kg) '20 (kg)
number/d
4956 843 4 3 0 0 2
Range
742}10 525 242}1460 0}20 0}22 0}13 0}25 0}150
(50% is unexposed. Note: The mean value of recorded time for the computerized observations (PEO) was 325 min. The mean value of the recordings of foot-steps (pedometer) was 346 min.
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442 Table 3 Median time and range spent in each 203 interval of trunk extension and #exion (n"24) x Interval
Table 4 Heart rate (beats/min) during work (n"30). Mean
Range
SD
86 99 95
66}103 73}118 66}118
12 13 14
% of recording time Median
Extension !203}0 Flexion 0}203 21}403 41}603 61}803 '80
439
Range
17
1}62
61 13 3 1 1
26}93 1}26 1}14 1}6 0}5
Men (n"10) Women (n"20) All (n"30)
Note: The recording, lasted 342 min (mean value).
Note: For six subjects the inclinometer broke or failed during the recoding period. The registering time was 356 min (mean value).
3.2. Circulatory stress and rates of perceived physical exertion The heart rate measurements are presented in Table 4. Nineteen female teachers had a heart rate over 100 beats per minute on an average of 42% (range 1}92) of the registered time. Seven of the male teachers had a heart rate over 100 beats per minute during 25% (range 5}60) of the time. The activities which placed the highest demands on the circulatory system were warming up activities, aerobics, dance, #oorhockey, and circuit training. In these activities the teachers partially participated and instructed and helped the students. When the teachers participated in the warming up exercises or the aerobics, or supported the students in the apparatus gymnastics there were heart rate peaks of 150 beats or more per minute in 22 (3 men, 19 women) subjects. The ratings of perceived exertion (Borg scale) of the day as a whole gave values for women from 9, where the verbal description is &very light' to 16 that is described as &strenuous', and the median value is 13 &somewhat strenuous' exertion. For men the exertion was rated from 7 &very, very light' to 13 and the median value is 12. 3.3. Agreement of registered and self-reported movements, postures, lifting and carrying The correlation between registered and self-reported exposure the same day and after three weeks is shown in Table 5 and in Figs. 2 and 3. The best correlations were found in lifting and carrying more than 20 kg, jumping and in light knee-bending rated the same day as the measurements (Table 5, column A). The correlation between the measured and the self-reported physical exposure three weeks later yielded lower values than the assessments performed at the end of the registration day (Table 5, column B). The highest correlations were in lifting 20 kg or more and in sitting (minutes).
Fig. 2. The rated (y-axis) vs. registered (x-axis) time (minutes) spent sitting during work.
Fig. 3. An example of the classi"cation and correlation of exposure in "ve classes. The registered number of knee bendings ('903) during the recording day are on the x-axis and the ratings from the same day on the y-axis. The dark squares represent full agreement.
When asking the subjects three weeks later whether they had been lifting or not ("dichotomised question) during the observation day, 28 of 29 rated with full agreement. The three-week test}retest on the self-reported physical exposure, the reproducibility of answering the questions, showed the best correlations in the sitting variable, (in percentage and in minutes) and in deep knee bends (Table 5, column C).
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Table 5 Correlation between measured and self-reported physical activity at the end of the recoding day (A), and between measured and self-reported activity three weeks later (B), and between the self-reports on the recoding day and three weeks later (C)
and the self-reported physical exposure as a methodological issue in epidemiological studies.
Exposure group
It is usually not possible to apply an instrument directly to body segments or tissues to measure the exact internal mechanical loading in static and dynamic activities, except in advanced laboratory investigations (Nordin and Frankel, 1989; Winkel and Mathiassen, 1994; Burdorf, 1995). To approximate the proportions and variability of physical load, registrations of external physical load, expressed in frequencies, durations and amplitudes can be performed by using devices suitable for "eld studies in working life or in sports activities such as those used in this study. We have used the median value to show the results for the whole group because the results were not normally distributed. Others have usually reported mean values which makes comparisons di$cult. The teachers spent less time sitting compared to other occupational groups. Waiters (n"8) were found to sit 9% of their working hours on average (Iverga rd, 1982). In the Stockholm Music I study, eight furniture removers sat 18% of the working day, and 12 medical secretaries 68%. In the referent populations in the same study females and males had mean values of 49% and 45%, respectively (Karlqvist et al, 1994). These recordings were carried out during an entire working day, as in this study. In the same study of two occupational groups and two referent groups the walking distance was also registered. The mean value for the furniture removers was 7.3 km during one day with 934 m/h, 1.7 km with 212 m/h for medical secretaries, 3.7 km (458 m/h) for the referent male population and 2.6 km (328 m/h) for the female referent population. These recordings were during one full working day. In the study of waiters there was a walking distance of 6.3 km during a shift (Ivergas rd, 1982). The teachers in our study walked as much as the most exposed groups, and the metres per hour (843/h) were also the second highest compared to the results in the Stockholm Music I study (Karlqvist et al, 1994). The teachers' working days are, however, shorter than eight hours. Metres per hour allow a direct comparison of the frequency between di!erent studies. As we do not know whether long periods are more or less harmful than short peak periods both ways of expressing the exposure have been reported. The exposure to carrying and lifting was frequent and 17 teachers were exposed to burdens over 20 kg when supporting the students in apparatus gymnastics. These lifts render a considerably heavier load on the teachers than 20 kg and this is the work task which the teachers refer to as the most strenuous. They not only have to lift the students, but they are also exposed to additional horizontal forces when the students take a run before they do a handspring and similar exercises. This causes high-speed dynamic forces which
Sitting %/d min/d Jumps number/d Knee-bends 30}903 number/d Knee-bends'903 number/d =alking distance metres/d ¸ifting 1}5 kg 6}10 kg 11}15 kg 16}20 kg '20 kg
A
B
C
0.63 0.75
0.50 0.64
0.70 0.75
0.84
0.38
0.47
0.78
0.24
0.40
0.47
0.30
0.67
0.45
0.17
0.60
0.42 0.52 0.61 0.56 0.92
0.40 0.40 0.40 !0.20 0.68
0.28 0.59 0.60 0.01 0.59
Note: The correlation is calculated as a rank order correlation (Spearman's rho).
Table 6 Three week test-retest reliability of rated exertion on the RPE scale (n"29) Correlation coe$cient (Spearman's rho)
Full agreement Overestimation Underfreq. (%) freq. (%) estimation freq. (%)
0.77
13 (45)
10 (34)
6 (21)
The correlation of the three week test}retest of the rated perceived exertion is 0.77 (Table 6). The measurements of self-reported time spent in di!erent degrees of trunk #exion and the registered time showed no correlation.
4. Discussion Our study has demonstrated that the physical work load for teachers of physical education is considerable due to the load on the lower extremities and that the variability in physical exposure is substantial. The subjects could assess their activity quite well in most of the parameters at the end of the working day compared to the measurements. The agreement on the measured physical exposure and the assessments three weeks later showed lower values. The following discussion consider the proportions of biomechanical and cardiovascular load in the teachers in comparison with other occupational groups,
4.1. Biomechanical load
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442
could cause extensive and sudden internal mechanical load to the teachers and an increased risk of accidents. The demands on the circulatory system were increased in the teachers during this work task. The registrations of trunk extension and #exion indicate an upright, slightly extended or slightly #exed, position most of the registration time and are more limited compared to previous investigations of other occupational groups (Nordin et al, 1984; Burdorf and Laan, 1991; Karlqvist et al, 1994). In the study of Nordin and co-workers (1984) and Burdorf and Laan (1991) the registrations only took place during one hour and twenty minutes, respectively, and the results are di$cult to compare with our whole day measurements. Trunk extension has not been reported in earlier studies and this could depend on extension not having been registered before, on di!erences in technique when calibrating the instrument or it could be a true e!ect. It could indicate that this occupational group generally tends to have a more extended and upright trunk posture compared to other individuals. The registered jumping and light and deep knee bends cannot be compared to other investigations, as there is a lack of reference data on frequencies of these activities, which put additional physical load on the lower extremities 4.2. Cardiovascular load The heart rate mean values can be compared with data from the Stockholm Music I study, where the mean value for the furniture removers was 109 beats/min, while the other three categories had lower mean values compared to the teachers (Karlqvist et al, 1994). Older employees in municipal occupations in Finland had mean heart rates similar to the Swedish referent population (SuurnaK kki et al, 1991). The cardiovascular demands among the subjects were comparable with values from subjects in occupations with high physical demands. The most demanding work tasks have been identi"ed and they occur practically every day for all individuals in this occupation. The teachers' ratings of perceived exertion give an indication of an underestimation of the circulatory demands. There were peak loads around 150 beats/min in most of the subjects and they occurred in intensive physical activities. The same peaks have been found in earlier investigations in subjects in the construction industry, iron and steel industry, in shipyard work and in other demanding jobs in industry (SuurnaK kki et al, 1991). Previous investigations have shown that heart rate peaks are related to low capacity and age (SuurnaK kki et al, 1991). It should be emphasised that factors such as mental stress and heart rate peaks could contribute to perceived physical strain (Kilbom, 1971; Theorell et al, 1991). The teachers were exposed to mental stress while working with the
441
students, which was frequently observed although not formally registered. 4.3. Validity and reliability of self-reported exposure The answers to the questions in the questionnaire at the end of the recording day showed a good, and sometimes very good correlation with the measured data. In the ratings three weeks later the correlation was lower in most variables. The scales in the questionnaire were detailed and it might be easier but still su$cient to remember the amount of physical exposure over a longer time period than one day and by using a less detailed scaling of the questions. The highest test}retest values of the reproducibility values were in the assessments of the perceived exertion, sitting, and the dichotomised question of lifting and carrying. Similar results have been shown in other studies (Wiktorin et al, 1996). In a study by Viikari-Juntura and coworkers (1996) the agreement between rated duration of sitting was high. In the same study the correlation between self-reported and observed walking distance was similar to our result. The number of subjects in our study group in the present study was limited. Only two or three outliers can a!ect the values substantially. The outcome when testing the subjects' ability to assess their physical exposure could depend on the their training and occupation, and also on di!erences in study design, and on how the questions were put. In our study we focused on teachers of physical education. They seem to be aware of duration of classes and, as it is included in their profession, they probably have an interest and awareness within the area of movements and postures. This could make it easier for them to report more correctly and provide better ratings of physical exposure than individuals in many other occupations. We did not "nd di!erences between the female and male teachers in terms of ability to assess and to remember exposure to biomechanical load. The number of subjects studied was limited, but no gender di!erences in this respect were found in other, larger studies (Punnet et al, 1988; Wiktorin, 1995). Higher accuracy has been reported in men than in women, however, with respect to self-reported physical exertion and activity (Lamb and Brodie, 1990; Wigaeus Hjelm et al, 1995). The correlation between the measured and the self-reported exposure three weeks later was low and this lack of validity for self-reported exposure should be taken into account in epidemiological studies. 4.4. Concluding remarks It could be concluded that questionnaires can be used when assessing physical load in this occupational group on a dichotomised level or with pooled classes regarding walking distance, jumping, light and deep knee-bending and
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lifting lighter loads. For questions on exposure to heavy lifting and sitting, these subjects could be asked on a more detailed level. Retrospective assessment of exposure after three weeks was poorer. This must be taken into account in epidemiological studies. Our study indicates that the biomechanical as well as cardiovascular work load in teachers of physical education is considerable compared to other occupational groups and that occasional high loads of short duration appear.
Acknowledgements The authors are grateful to Monica Mortimer M.Sc. for initial help with the data collection, and to Eva Bernmark M.Sc. for technical support. We also thank the National Union of Teachers in Sweden, the Swedish Society of Teachers of Physical Education, and the University of Physical Education, Stockholm, for help and support.
References Borg, G., 1990. Psychophysical scaling with applications in physical work and the perception of exertion. Scand. J. Work Environ. Health 16 (Suppl. 1), 55}58. Burdorf, A., 1995. Reducing random measurement error in assessing postural load on the back in epidemiologic surveys. Scand. J. Work Environ. Health 2, 15}23. Burdorf, A., Laan, J., 1991. Comparison of methods for the assessment of postural load on the back. Scand. J. Work Environ. Health 17, 425}429. Ericsson, M.O., 1994. Portable inclinometer system for measurement of trunk postures. Royal Institute of Technology, Stockholm. ISSN 11042656, Trita-IMA 31. Fransson-Hall, C., Gloria, R., Karlqvist, L., Wiktorin, C., Winkel, J., Kilbom, As ., 1995. Stockholm MUSIC I Study group. A portable ergonomic observation method (PEO) for computerized on-line recording of postures and manual handling. Appl. Ergon. 26, 93}100. Hagberg, M., 1992. Exposure variables in ergonomic epidemiology. Am. J. Ind. Med. 2, 91}100. Hagberg, M., Wegman, D.H., 1987. Prevalence rates and odds ratios of shoulder neck diseases in di!erent occupational groups. Br. J. Ind. Med. 44, 602}610. HaK ndel, G., RunsjoK , A., 1985. Vart tog de vaK gen? Hur laK nge ha ller en idrottslaK rare? Rapport fras n Svenska GymnastiklaK rarsaK llskapet (in Swedish). Ivergas rd, T., 1982. TroK tthets- och stressproblem foK r serveringspersonal. Ergonomilaboratoriet AB- Ergolab. Rapport 15 (in Swedish). Karlqvist, L., Winkel, J., Wiktorin, C., 1994. Direct measurements and systematic observations of physical work load among medical secretaries, furniture removers and male and female reference populations. Appl. Ergon. 25, 319}326.
Kilbom, A> ., 1971. Physical training with submaximal intensities in woman - e!ect on adaptation to professional work. Scand. J. Clin. Lab. Invest. 28, 331}342. Lamb, K.L., Brodie, D. A., 1990. The assessment of physical activity by leisure-time physical activity questionnaires. Sports Med. 10, 159}180. LeH ger, L., Thivierge, M., 1988. Heart rate monitors: validity, stability and functionality. Physician and Sports Med. 16, 143}151. LindeH n, K., 1988. FoK rstudie foK r att foK rebygga idrottslaK rares belastningsskador. HoK gskolan i OG rebro (in Swedish). Nordin, M., OG rtengren, R., Andersson, G.B.J., 1984. Measurement of trunk movement during work. Spine 9, 465}69. Nordin, M., Frankel, V.H., 1989. Basic Biomechanics of the Musculoskeletal System, 2nd ed. Lea and Febiger, Philadelphia, London. Occupational risks 1990}91, 1994. Occupational accidents and diseases. O$cial Statistics of Sweden, National Board of Occupational Safety and Health Statistics Sweden, Stockholm. Punnet, L., Armstrong, T.J., Ketner, P.E., 1988. The validity of worker's ergonomic assessment of hand tools and tasks: a pilot study. In: Hogstedt, C., Reuterwall, C. (Ed.), Progress in Epidemiology. Proc. 6th Int. Symp. on Epidemiology in Occupational Health. Amsterdam, New York, Oxford, Excerpta Medica, pp. 223}226. RiihimaK ki, H., 1991. Low-back pain, its origin and risk indicators. Scand. J. Work Environ. Health 17, 81}90. Selin, K., Winkel, J., 1994. Stockholm MUSIC I Study group. Evaluation of two instruments for recording sitting and standing postures and number of footsteps. Appl. Ergon. 25, 41}46. SuurnaK kki, T., Nygas rd, C-H., Ilmarinen, J., 1991. Stress and strain of elderly employees in municipal occupations. Scand. J. Work Environ. Health 17 (Suppl. 1), 30}39. Theorell, T., Harms-Ringdahl, K., Ahlberg-HulteH n, G., Westin, B., 1991. Psychosocial job factors and symptoms from the locomotor system * a multicausal analysis. Scand. J. Rehabil. Med. 23, 165}173. Vingas rd, E., Hogstedt, C., Alfredsson, L., Fellenius, E., Goldie, I., KoK ster, M., 1991. Coxarthrosis and physical work load. Scand. J. Environ. Health 17, 104}109. Viikari-Juntura, E., Rauas, S., Martikainen, R., Kuosma, E., RiihimaK ki, H., Takala, E-P., Saarenmaa, K., 1996. Validity of self-reported physical work load in epidemiological studies on musculoskeletal disorders. Scand. J. Work Environ. Health 22, 251}259. Wigaeus Hjelm, E., Winkel, J., Nyga rd, C-H., Wiktorin, C., Karlqvist, L., 1995. Stockholm MUSIC I study group. Can cardiovascular load in ergonomic epidemiology be estimated by self-reports? J. Occup. Med. 37, 1210}1217. Wiktorin, C., 1995. Occupational physical load in epidemiological studies}development and evaluation of methods. Thesis. Karolinska institute, Stockholm. Wiktorin,C., Karlqvist, L., Winkel, J., 1993. Validity of self-reported exposures to work postures and manual materials handling. Scand. J. Work Environ. Health 19, 208}214. Wiktorin, C., Wigaeus Hjelm, E., Winkel, J., KoK ster, M., 1996. Stockholm MUSIC I study group. Reproducibility of a questionnaire for assessment of physical load during work and leisure time. J. Occup. Med. 38, 190}201. Winkel, J., Mathiassen, S.E., 1994. Assessment of physical work load in epidemiological studies: concepts, issues and operational considerations. Ergonomics 37, 979}988. Winkel, J., and Westgaard, R., 1992. Occupational and individual risk factors for shoulder-neck complaints: part II-the scienti"c basis (literature review) for the guide. Int. J. Ergon. 10, 85}104.
Physical work load in physical education teachers HeH le`ne Sandmark *, Christina Wiktorin , Christer Hogstedt , Eva-Karin Klenell-Hatschek , Eva Vingas rd Department of Occupational Health, Karolinska Hospital, S-171 76 Stockholm, Sweden National Institute for Working Life, S-171 84 Solna, Sweden Received 11 June 1997; accepted 4 August 1998
Abstract The main objective was to measure and quantify exposure to physical work load in physical education teachers. A further aim was to establish the reproducibility of a three-week test}retest interval of self-reported physical load and to evaluate the agreement between registered and self-rated physical load. Thirty teachers, both female and male, volunteered to participate in the study. The physical load on the lower extremities, as well as the back and the cardio-vascular strain was recorded during one working day. The results indicate that the physical work load in this occupational group is considerable due to the load on the lower extremities. The demands of the cardio-vascular system are also relatively high in comparison to other occupational groups. The teachers could assess their activity quite well regarding heavy lifting and time spent sitting. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomechanical load; Lower extremity; Self-reported work load
1. Introduction Swedish teachers of physical education report a high rate of dysfunction and injuries to the musculoskeletal system, and very few of them are able to work actively in the profession until retirement (HaK ndel and RunsjoK , 1985; LindeH n, 1988; O$cial Statistics Sweden, 1994). High physical load is an important aetiologic factor for musculoskeletal dysfunction (Hagberg and Wegman, 1987; RiihimaK ki, 1991; Hagberg,1992; Vingas rd et al, 1992 Winkel and Westgaard, 1992), and motivated this study of physical work load for those teachers. Occupational titles have often been used to approximate the exposure to physical work load, but the physical load within the same occupational group may vary considerably. Other methods to assess exposures are questionnaires, interviews, logbooks and direct measurements. The validity of these instruments are seldom reported. Performing direct measurements is time-consuming and expensive. Usually just a small fraction of one working day is covered and the total load is extrapolated from this, which is questionable.
*Corresponding author. Tel.: #46 8 729 33 06; Fax: #46 8 33 43 33; e-mail: [email protected].
The Stockholm MUSIC I study group has validated methods for assessment of physical load in epidemiological studies (Wiktorin, 1995). This study continues this research by using the developed methods for measuring physical work load during entire working days, and focusing on certain occupational risk groups reporting frequent dysfunctions in the musculoskeletal system. The overall purpose of the present project was to measure the exposure to physical load and its relation to the development of musculoskeletal dysfunctions in teachers of physical education. This has been carried out in two studies. The present and "rst part is aimed at exposure measurements and the second part at epidemiological studies on the relationship between dysfunction and physical load in this occupational group. In most epidemiological studies questions on retrospective exposure are asked and it is crucial to know the precision and accuracy in such estimations. In particular if there is a relationship between outcome status and the exposure estimation. The main objective of the present study was to measure and quantify exposure to physical load of teachers in physical education during an entire working day, to establish the reproducibility for a three-week test}retest interval of selfreported physical work load in this occupational group, and to evaluate the agreement between self-reported and objectively registered physical load in the same group.
0003-6870/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 3 - 6 8 7 0 ( 9 8 ) 0 0 0 4 8 - 9
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2. Subjects and methods 2.1. Subjects Twenty female and ten male teachers of physical education volunteered to participate in the study. The number of females and males re#ects the gender distribution in this occupation. The subjects were all living in the Stockholm area, and working in di!erent schools in di!erent areas. We contacted the schools and if there was more than one teacher working there all teachers of physical education participated. Invitations were sent out with the aim of studying teachers of di!erent ages and sex, working at di!erent stages in the school system. Fourteen teachers worked in the upper secondary school where the students age range from 17 to 19 yr, and the rest in the nine-year compulsory school system where the pupils are from 7 to 16 yr of age. Ninety percent of the teachers were working full-time and the rest part-time. On average they had been working as teachers of physical education for 15 yr (range 1}32 yr). Background data of the subjects are presented in Table 1. 2.2. Methods for assessing exposure The physical load on the lower extremities, the back and the general physical strain during work was assessed through continuous registrations of postures, movements, and heart rate over an entire working day. The measurement devices were either applied directly to the body, or indirectly used, in systematic and computerised observations. All the registrations were performed by two experienced ergonomists. The walking distance was registered with a pedometer (Fitty 3 Electronic, Germany) (Selin and Winkel, 1994). Each subject's footstep was estimated by measuring the distance when he/she had walked "ve steps. The distance was then divided by "ve and the step length was entered into the pedometer. Thus the number of footsteps during the registration period indicated the walking distance.
Table 1 Characteristics of the study group (n"30). Female teachers n"20 Age (yr) mean (SD) range Height (cm) mean (SD) range =eight (kg) mean (SD) range
Male teachers n"10
40 (9) 24}57
39 (11) 26}56
167 (6) 156}178
178 (3) 173}183
61 (7) 48}78
75 (9) 65}95
The pedometer has been validated, and it was found that the walking distance was overestimated when a subject was jumping or dancing, since the distance is calculated from the length of an ordinary footstep (Selin and Winkel, 1994). On the other hand the number of steps and thus heelstrikes was correct. The registrations of the pedometer gave a rough estimate of the true walking distance, as jumping and dancing and other similar activities were included in the studied subjects. Duration and frequency of jumps ("leaps, springs, skips), light (30}903 knee #exion) and deep ('903 knee #exion) knee-bends, and lifting and carrying were registered using a hand-held computer (PSION) and software (PEO) for entering frequency and duration of these exposure parameters (Fransson-Hall et al, 1995). The validity and reliability of the PEO method have been evaluated and it was concluded that the method is applicable in investigations of most occupations and work tasks (Fransson-Hall et al, 1995). The duration and frequency of carrying and lifting were divided into di!erent weight-classes, 1}5, 6}10, 11}15, 16}20 and '20 Kg. The same classi"cation of weights has been used in other studies (Vingas rd et al, 1991). The weights were usually the equipment used in the training and several of the items had the weight marked or the weight was known by the teacher. Weights over 20 Kg occurred when the teachers lifted the students in the apparatus gymnastics. We experienced some di$culty in registering the frequencies correctly on the hand-held computer in periods of intensive registration and high frequencies, for example in jumping. Too low frequencies registered during these periods underestimated the magnitude of the physical load. The trunk #exion was measured using an inclinometer (Fig. 1), a trunk position analyser, which registers the time spent in 5 intervals of 203 ranging from slight extension of the trunk (!203) to an upright position (03) and to deep trunk #exion (803) (Ericsson, 1994). The inclinometer registers the sagittal angle between the trunk and the reference trunk position. The sensor and the box for data collection were attached to the back at Th5}Th10 level. When the registration were "nished the collected data were transmitted to and analysed in a computer using a specially developed software program (Ericsson, 1994). The twist of the trunk was not measured. The inclinometer has been found to be valid and reasonably precise (Ericsson, 1994). A similar device has been used in earlier investigations (Nordin et al, 1984; Wiktorin et al, 1993; Karlqvist et al, 1994). For six subjects in our study the inclinometer did not register properly and these recordings were rejected. In one of these the device broke when the teacher fell during an exercise. The subjects rated what e!ect the measurement devices had on movements and postures during the time of the registrations. Six of the subjects reported that the device did
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442
437
Fig. 1. The inclinometer during registering. The device is attached to the thoracic part of the spine.
not a!ect them at all. To what extent it hindered certain postures and movements in the others depended on what was intended to be carried out. The subjects who reported that they were hindered commented on not being able to do exercises lying on their backs because the inclinometer was attached to the thoracic part of the spine. Also when the exercises included jumping or aerobics the inclinometer slightly hindered the activity. From the comments it seems as if the inclinometer is the instrument that a!ects the positions and movements the most. It can also be concluded that there was an underestimation of some usual activity because the inclinometer did not allow the subjects to lie down on their backs. The general physiological strain was assessed through registrations of the circulatory stress using a heart rate recorder (Sport tester PE 3000). The Sport tester gives a cordless transmission of the heart rate from the electrodes applied to the chest of the subject to the data logger applied around the wrist. The heart rate was registered during the whole day at work including the lunch break. The recorded heart rate was stored as means for each recorded minute. The mean heart rate, and the highest and lowest values during the day were registered in each subject. Heart rate monitors with chest electrodes, like the one used in this study, have been tested for validity and stability and found to be applicable for monitoring heart rate (LeH ger and Thivierge, 1988). The transmission was cordless, and we found that the receiver was sensitive when the subjects wore a watch very close to it. This can cause disturbances in the transmission.
2.3. Questionnaire Eight questions on frequency and duration of sitting, walking distance, jumping, lifting, knee bending, trunk bending during work on the day of registrations, were asked at the end of the day and also after three weeks. The question on sitting had an open answer, and for lifting and carrying the question on exposed or non-exposed was dichotomised; there were also questions on frequencies of di!erent weights lifted. For the remaining questions there were "ve alternative answers to each question. The same or similar design of questions has been used in earlier studies (Wiktorin et al, 1996). None of the subjects knew the results of the recordings when they answered the questions. A 15-graded scale (RPE), graded 6}20, was used for ratings of the perceived exertion and thus a physiological stress indicator for the day (Borg, 1990). Three weeks later the Borg scale and the eight questions about the recording day were sent by mail to each subject. The subjects still did not know the results of the objective recordings when answering the same questionnaire once more. The day on which the recordings were made included on average 4.4 classes (range 3}6). Most classes lasted for 60 min, and a few for 45 and 80 min, respectively. The shorter classes were in the primary school where the schoolchildren are still very young, and the 80 min classes are in the highest grades. The entire length of the working hours during the day of recording was on average 325 min and ranged from 192 to 432 min. This is according to the PEO registrations which covered the entire working day exactly. The time of recording varied slightly for each recording
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device, depending on when the device was applied and turned on. Only one instrument could be applied at a time. We investigated if the day of the registrations had been more or less strenuous compared to other working days. Fourteen subjects rated it as &like any other day', and "ve as more strenuous but not to an extreme. Eleven rated it as less strenuous, and one of them as much less strenuous. 2.4. Analysis The total registration period, the heart rate measurements, and the characteristics of the study group are calculated as mean value, standard deviation and range. The results of frequency and duration of the recorded parameters, except the heart rate measurements, are expressed as median value and range. Median value is used, as the recorded values are not normally distributed. The heart rate values are presented as mean value, standard deviation and range, while the values are normally distributed and thus allowed direct comparisons with other studies. In the analysis we show separate results in the heart rate measurements and accordingly in the ratings on the RPE scale for men and women respectively, as there were gender di!erences in the heart rate frequency. In the remaining variables, including the correlations, we did not see any gender di!erence in quantity of physical exposure or in ability of reporting physical exposure. The duration of jumping, knee bending, lifting and carrying was very short and were therefore not analysed. The frequency of the recorded parameters was also calculated as number per working hour, since the hours di!ered between individuals. The agreement of objective registrations and the individual's own assessments of duration and frequency of the parameters the same day and after three weeks, and the test}retest reproducibility of the self-reports the same day and after three weeks were, evaluated from calculations of a non-parametric rank order correlation (Spearman's rho).
3. Results The activities and the physical load during the working day di!ered from subject to subject, and the interindividual variation was considerable in most of the variables. The most common activity was warming up exercises, and the second most common was di!erent kinds of indoor ball games where the teachers mostly acted as referees and rarely participated in the game. The teachers actively participated in warming up activities, aerobics, dancing, #oor exercises and when the lesson was taught by a student. In apparatus gymnastics the teachers supported the students and in circuit training they assisted and helped them.
3.1. Frequency and duration of postures, movements and lifts The time spent sitting was 24 min or 8% (median value) of the day of recording (Table 2). Three of the teachers did not sit at all during this day. The variation of frequency of jumps and light and deep knee bends was considerable (Table 2), mostly due to whether the teachers joined in the exercises or not. The teachers walked 4956 ms (median value) during the working day (Table 2). Also for this variable there was a high degree of variation between individuals. The longest distances walked were when the classes had outdoor activities and when the teacher participated in the exercises when a student taught the lesson. Fourteen of the teachers spent 3}32 min in di!erent awkward positions like kneeling on both knees or on one knee, mostly when supporting the students in apparatus gymnastics. The median value for time in awkward positions was 6 min. Twenty-nine of the teachers were exposed to carrying and lifting (Table 2). Supporting the students in apparatus gymnastics produced the heaviest lifts, but the frequency for di!erent teachers varied considerably. The results from the measurements of trunk extension and #exion are presented in Table 3. The subjects were in an almost upright position for 78% (!203 to 203) of the registered time (median value) and the interindividual variability was large.
Table 2 Exposure of physical load during work. Frequency and duration of movements, postures and lifts in the study group during the day of registration Exposure
Sort of exposure
Median
Sitting Sitting
%/d min/d
8 24
0}65 0}136
Jump Jump
number/d number/h
29 6
0}339 0}78
Knee bend 303}903 Knee bend 303}903
number/d number/h
74 13
14}236 3}76
Knee bend'903 Knee bend'903
number/d number/h
46 9
5}186 0}58
Walking distance Walking distance
m/d m/h
Lifting 1}5 (kg) 6}10 (kg) 11}15 (kg) 16}20 (kg) '20 (kg)
number/d
4956 843 4 3 0 0 2
Range
742}10 525 242}1460 0}20 0}22 0}13 0}25 0}150
(50% is unexposed. Note: The mean value of recorded time for the computerized observations (PEO) was 325 min. The mean value of the recordings of foot-steps (pedometer) was 346 min.
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442 Table 3 Median time and range spent in each 203 interval of trunk extension and #exion (n"24) x Interval
Table 4 Heart rate (beats/min) during work (n"30). Mean
Range
SD
86 99 95
66}103 73}118 66}118
12 13 14
% of recording time Median
Extension !203}0 Flexion 0}203 21}403 41}603 61}803 '80
439
Range
17
1}62
61 13 3 1 1
26}93 1}26 1}14 1}6 0}5
Men (n"10) Women (n"20) All (n"30)
Note: The recording, lasted 342 min (mean value).
Note: For six subjects the inclinometer broke or failed during the recoding period. The registering time was 356 min (mean value).
3.2. Circulatory stress and rates of perceived physical exertion The heart rate measurements are presented in Table 4. Nineteen female teachers had a heart rate over 100 beats per minute on an average of 42% (range 1}92) of the registered time. Seven of the male teachers had a heart rate over 100 beats per minute during 25% (range 5}60) of the time. The activities which placed the highest demands on the circulatory system were warming up activities, aerobics, dance, #oorhockey, and circuit training. In these activities the teachers partially participated and instructed and helped the students. When the teachers participated in the warming up exercises or the aerobics, or supported the students in the apparatus gymnastics there were heart rate peaks of 150 beats or more per minute in 22 (3 men, 19 women) subjects. The ratings of perceived exertion (Borg scale) of the day as a whole gave values for women from 9, where the verbal description is &very light' to 16 that is described as &strenuous', and the median value is 13 &somewhat strenuous' exertion. For men the exertion was rated from 7 &very, very light' to 13 and the median value is 12. 3.3. Agreement of registered and self-reported movements, postures, lifting and carrying The correlation between registered and self-reported exposure the same day and after three weeks is shown in Table 5 and in Figs. 2 and 3. The best correlations were found in lifting and carrying more than 20 kg, jumping and in light knee-bending rated the same day as the measurements (Table 5, column A). The correlation between the measured and the self-reported physical exposure three weeks later yielded lower values than the assessments performed at the end of the registration day (Table 5, column B). The highest correlations were in lifting 20 kg or more and in sitting (minutes).
Fig. 2. The rated (y-axis) vs. registered (x-axis) time (minutes) spent sitting during work.
Fig. 3. An example of the classi"cation and correlation of exposure in "ve classes. The registered number of knee bendings ('903) during the recording day are on the x-axis and the ratings from the same day on the y-axis. The dark squares represent full agreement.
When asking the subjects three weeks later whether they had been lifting or not ("dichotomised question) during the observation day, 28 of 29 rated with full agreement. The three-week test}retest on the self-reported physical exposure, the reproducibility of answering the questions, showed the best correlations in the sitting variable, (in percentage and in minutes) and in deep knee bends (Table 5, column C).
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H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442
Table 5 Correlation between measured and self-reported physical activity at the end of the recoding day (A), and between measured and self-reported activity three weeks later (B), and between the self-reports on the recoding day and three weeks later (C)
and the self-reported physical exposure as a methodological issue in epidemiological studies.
Exposure group
It is usually not possible to apply an instrument directly to body segments or tissues to measure the exact internal mechanical loading in static and dynamic activities, except in advanced laboratory investigations (Nordin and Frankel, 1989; Winkel and Mathiassen, 1994; Burdorf, 1995). To approximate the proportions and variability of physical load, registrations of external physical load, expressed in frequencies, durations and amplitudes can be performed by using devices suitable for "eld studies in working life or in sports activities such as those used in this study. We have used the median value to show the results for the whole group because the results were not normally distributed. Others have usually reported mean values which makes comparisons di$cult. The teachers spent less time sitting compared to other occupational groups. Waiters (n"8) were found to sit 9% of their working hours on average (Iverga rd, 1982). In the Stockholm Music I study, eight furniture removers sat 18% of the working day, and 12 medical secretaries 68%. In the referent populations in the same study females and males had mean values of 49% and 45%, respectively (Karlqvist et al, 1994). These recordings were carried out during an entire working day, as in this study. In the same study of two occupational groups and two referent groups the walking distance was also registered. The mean value for the furniture removers was 7.3 km during one day with 934 m/h, 1.7 km with 212 m/h for medical secretaries, 3.7 km (458 m/h) for the referent male population and 2.6 km (328 m/h) for the female referent population. These recordings were during one full working day. In the study of waiters there was a walking distance of 6.3 km during a shift (Ivergas rd, 1982). The teachers in our study walked as much as the most exposed groups, and the metres per hour (843/h) were also the second highest compared to the results in the Stockholm Music I study (Karlqvist et al, 1994). The teachers' working days are, however, shorter than eight hours. Metres per hour allow a direct comparison of the frequency between di!erent studies. As we do not know whether long periods are more or less harmful than short peak periods both ways of expressing the exposure have been reported. The exposure to carrying and lifting was frequent and 17 teachers were exposed to burdens over 20 kg when supporting the students in apparatus gymnastics. These lifts render a considerably heavier load on the teachers than 20 kg and this is the work task which the teachers refer to as the most strenuous. They not only have to lift the students, but they are also exposed to additional horizontal forces when the students take a run before they do a handspring and similar exercises. This causes high-speed dynamic forces which
Sitting %/d min/d Jumps number/d Knee-bends 30}903 number/d Knee-bends'903 number/d =alking distance metres/d ¸ifting 1}5 kg 6}10 kg 11}15 kg 16}20 kg '20 kg
A
B
C
0.63 0.75
0.50 0.64
0.70 0.75
0.84
0.38
0.47
0.78
0.24
0.40
0.47
0.30
0.67
0.45
0.17
0.60
0.42 0.52 0.61 0.56 0.92
0.40 0.40 0.40 !0.20 0.68
0.28 0.59 0.60 0.01 0.59
Note: The correlation is calculated as a rank order correlation (Spearman's rho).
Table 6 Three week test-retest reliability of rated exertion on the RPE scale (n"29) Correlation coe$cient (Spearman's rho)
Full agreement Overestimation Underfreq. (%) freq. (%) estimation freq. (%)
0.77
13 (45)
10 (34)
6 (21)
The correlation of the three week test}retest of the rated perceived exertion is 0.77 (Table 6). The measurements of self-reported time spent in di!erent degrees of trunk #exion and the registered time showed no correlation.
4. Discussion Our study has demonstrated that the physical work load for teachers of physical education is considerable due to the load on the lower extremities and that the variability in physical exposure is substantial. The subjects could assess their activity quite well in most of the parameters at the end of the working day compared to the measurements. The agreement on the measured physical exposure and the assessments three weeks later showed lower values. The following discussion consider the proportions of biomechanical and cardiovascular load in the teachers in comparison with other occupational groups,
4.1. Biomechanical load
H. Sandmark et al. / Applied Ergonomics 30 (1999) 435}442
could cause extensive and sudden internal mechanical load to the teachers and an increased risk of accidents. The demands on the circulatory system were increased in the teachers during this work task. The registrations of trunk extension and #exion indicate an upright, slightly extended or slightly #exed, position most of the registration time and are more limited compared to previous investigations of other occupational groups (Nordin et al, 1984; Burdorf and Laan, 1991; Karlqvist et al, 1994). In the study of Nordin and co-workers (1984) and Burdorf and Laan (1991) the registrations only took place during one hour and twenty minutes, respectively, and the results are di$cult to compare with our whole day measurements. Trunk extension has not been reported in earlier studies and this could depend on extension not having been registered before, on di!erences in technique when calibrating the instrument or it could be a true e!ect. It could indicate that this occupational group generally tends to have a more extended and upright trunk posture compared to other individuals. The registered jumping and light and deep knee bends cannot be compared to other investigations, as there is a lack of reference data on frequencies of these activities, which put additional physical load on the lower extremities 4.2. Cardiovascular load The heart rate mean values can be compared with data from the Stockholm Music I study, where the mean value for the furniture removers was 109 beats/min, while the other three categories had lower mean values compared to the teachers (Karlqvist et al, 1994). Older employees in municipal occupations in Finland had mean heart rates similar to the Swedish referent population (SuurnaK kki et al, 1991). The cardiovascular demands among the subjects were comparable with values from subjects in occupations with high physical demands. The most demanding work tasks have been identi"ed and they occur practically every day for all individuals in this occupation. The teachers' ratings of perceived exertion give an indication of an underestimation of the circulatory demands. There were peak loads around 150 beats/min in most of the subjects and they occurred in intensive physical activities. The same peaks have been found in earlier investigations in subjects in the construction industry, iron and steel industry, in shipyard work and in other demanding jobs in industry (SuurnaK kki et al, 1991). Previous investigations have shown that heart rate peaks are related to low capacity and age (SuurnaK kki et al, 1991). It should be emphasised that factors such as mental stress and heart rate peaks could contribute to perceived physical strain (Kilbom, 1971; Theorell et al, 1991). The teachers were exposed to mental stress while working with the
441
students, which was frequently observed although not formally registered. 4.3. Validity and reliability of self-reported exposure The answers to the questions in the questionnaire at the end of the recording day showed a good, and sometimes very good correlation with the measured data. In the ratings three weeks later the correlation was lower in most variables. The scales in the questionnaire were detailed and it might be easier but still su$cient to remember the amount of physical exposure over a longer time period than one day and by using a less detailed scaling of the questions. The highest test}retest values of the reproducibility values were in the assessments of the perceived exertion, sitting, and the dichotomised question of lifting and carrying. Similar results have been shown in other studies (Wiktorin et al, 1996). In a study by Viikari-Juntura and coworkers (1996) the agreement between rated duration of sitting was high. In the same study the correlation between self-reported and observed walking distance was similar to our result. The number of subjects in our study group in the present study was limited. Only two or three outliers can a!ect the values substantially. The outcome when testing the subjects' ability to assess their physical exposure could depend on the their training and occupation, and also on di!erences in study design, and on how the questions were put. In our study we focused on teachers of physical education. They seem to be aware of duration of classes and, as it is included in their profession, they probably have an interest and awareness within the area of movements and postures. This could make it easier for them to report more correctly and provide better ratings of physical exposure than individuals in many other occupations. We did not "nd di!erences between the female and male teachers in terms of ability to assess and to remember exposure to biomechanical load. The number of subjects studied was limited, but no gender di!erences in this respect were found in other, larger studies (Punnet et al, 1988; Wiktorin, 1995). Higher accuracy has been reported in men than in women, however, with respect to self-reported physical exertion and activity (Lamb and Brodie, 1990; Wigaeus Hjelm et al, 1995). The correlation between the measured and the self-reported exposure three weeks later was low and this lack of validity for self-reported exposure should be taken into account in epidemiological studies. 4.4. Concluding remarks It could be concluded that questionnaires can be used when assessing physical load in this occupational group on a dichotomised level or with pooled classes regarding walking distance, jumping, light and deep knee-bending and
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lifting lighter loads. For questions on exposure to heavy lifting and sitting, these subjects could be asked on a more detailed level. Retrospective assessment of exposure after three weeks was poorer. This must be taken into account in epidemiological studies. Our study indicates that the biomechanical as well as cardiovascular work load in teachers of physical education is considerable compared to other occupational groups and that occasional high loads of short duration appear.
Acknowledgements The authors are grateful to Monica Mortimer M.Sc. for initial help with the data collection, and to Eva Bernmark M.Sc. for technical support. We also thank the National Union of Teachers in Sweden, the Swedish Society of Teachers of Physical Education, and the University of Physical Education, Stockholm, for help and support.
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