- Email: [email protected]
Is there a reduction in postural load when wearing progressive lenses during VDT work over a three-month period?
: &
Applied
UTTERWORTH EINEMANN
Ergonomics Vol 26, No. 3, pp. 165-171, 1995 Copyright 0 1995 Elsevier Science Ltd Pnnted in Great Britain. All rights reserved 0003.6870/95 Sl0.W + 0.00
00034870(95)ooo12-7
Is there a reduction in postural load when wearing progressive lenses during VDT work over a threemonth period? Gunnar Horgen Kongsberg College of Engineering, Department of Optometry, Norway
Arne AarQs Alcatel, Oslo, Norway
Hans Fagerthun and Stig Larsen Medstat, Str@mmen, Norway
In an initial study, 18 VDT operators were found to have a higher muscle load in the neck/ shoulder region when using progressive lenses compared with single-vision lenses, while working on a VDT unit. For single-vision lenses, a higher flexion angle in the neck was also discovered. The present study investigates whether this higher muscle load is reduced when a progressive correction is worn for a three-month period. Adaptation in terms of reduced muscle load measured by electromyography (EMG) was not discovered. Head flexion angle was smaller when using progressive correction as compared with single-vision lenses. Some practical implications for occupational optometry are discussed. Keywords:
progressive
lenses,
VDT work,
electromyography
period of time leads to an adaptation that it is possible to measure by a reduction in the trapezius load. Whether the longer wearing time leads to a change in the neck angle is also investigated.
There are many reports of discomfort among VDToperators (e.g. Knave et al, 1985; World Health Organization, 1987). Important contributors to such discomfort are assumed to be work posture, work task and visual strain (Daae et al, 1984; Bjorseth, 1985; Knave et al, 1985; Aarh 1987; Daum et al, 1988). Work posture and postural load of the neck and shoulder muscles during VDT work when correcting presbyopia with different types of multifocal lens were studied by Horgen et al (1989). The results from the earlier study showed that, when performing data entry work, single-vision lenses produced less traoezius load than that oroduced bv the types of multifocals tested. Subjects wearing singlevision lenses also had a significantly larger head flexion angle, but a smaller range of head movements than subjects wearing multifocals. However, the subjects had not worn the multifocal lenses for more than approximately 4 h (Horgen et al, 1989). In practice it is generally accepted that clients need a period of time in order to ‘get used to’ a new optometric correction (Borish, 1975; Michaels, 1980; Ball, 1982). After the adaptation period, any reported symptoms usually decrease. The present study investigates whether wearing multifocal lenses over a longer
Material and methods Subject population
The subjects were 10 females and 8 males. Their mean age was 55.7 years, with a range from 51 to 61 years. They were chosen according to the following criteria: (1) All were experienced VDT workers, and their daily work situation involved using a VDT screen. (2) They were presbyopic, with a need for reading additions ranging from +1.25 dioptres to +2.50 dioptres. The criterion for the reading addition was x= l/Y - %Z, where X = reading addition (dioptres), Y = working distance (metres) and Z = amplitude of accommodation (dioptres) (Borish, 1975). (3) They all wore spectacles in their daily activity, for both distance and near vision. Their lens power for distance correction ranged from -3.75 dioptres to
165
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
166
+3.00 dioptres spherical power, no one being astigmatic by more than -3.00 dioptres cylinder. (4) The participants had a binocular visual acuity of 1.0 (20/20) or better (Snellen); this value was obtained through their distance correction. Vision analysis
At the beginning of the earlier study, the subjects underwent a vision analysis. Retinoscopy and subjective refraction with binocular control were tested. Oculomotor balance was measured with the Cover test, Graefes dissociated test and Malletts fixation disparity test. The Mallet fixation disparity test was used only for the near-work situation. Amplitude of accommodation was measured with a near-point ruler (RAF ruler), and the dioptric power needed in the lenses for near work was calculated (see ‘Corrective lenses’). Sterescopic acuity was assessed with a Titmus fly test. Ocular motility, visual fields (Donders), pupil reflexes and ocular inspection were tested at the same time. The corrective lenses
Two types of lenses were chosen from the first study for this prolonged experiment (see ‘Experiment’): one progressive lens that was specially made for VDT work (Datacomfort), and one ordinary single-vision lens. The Datacomfort lens has an increasing dioptric power from distance to near point, with a zone between the distance and the near point that has a power of 0.6 times the full near power. This zone is meant for VDT work. A single-vision lens has the same dioptric power all over the lens. Details of the lenses are given in Horgen et al (1989). The workplace
The experimental set-up was designed for a data entry VDT task. The workdesk and the VDT were individually adjustable, with a document holder mounted between the screen and keyboard. This had an automatic ruler device, controlled by the foot, which prevented the subject pointing with their fingers to find the correct information on the document. Pointing with the fingers would influence the load on the trapezius muscle; therefore this had to be avoided. The workplace was configured so that the viewing angle below the horizontal was 15” to the centre of the screen. This angle was approximately the same as that which workers used for data entry work at Alcatel-STK (Horgen, 1985). The viewing angle magnitude is also in agreement with the recommendations of Grandjean and Vigliani (1980), Shahnavaz (1982) and Cakir et hl (1980). The distances from the subjects’ eyes (spectacle plane) to the centre of the screen, to the document and to the keyboard were adjusted to the values shown in Table I. These values were the same as in the earlier experiment (Horgen et al, 1989). The spectacle lens powers were modified to meet these specifications, and they were also tested directly on the actual workplace before the prescriptions were finally decided. The working task
The
subject
typed
Table 1 Distances from the subjects’ eyes Mean (cm)
Screen Document Keyboard
from
a hard
copy
63.00
54-70
57.50 50.50
52-65 46-55
document into a database. There were two test periods, one for each lens. Each test period lasted for 30 min, and the measurements were taken continously during this time. To ensure that the subjects looked both at the screen and at the keyboard, the format of the screen was made different from the hard copy in two ways. First, the location of the different items displayed on the screen was made different from the location they had on the hard copy and, second, the sequence of information was made different from that on the hard copy. To prevent the subjects from memorizing the screen layout, the format of the screen was altered every 5 min throughout the test period. Measurements
of load on the musculoskeletal system
Postural loads of the neck and shoulder muscles were quantified by electromyography (EMG) using surface electrodes (Aaras and Westgaard, 1987). The load on m. trapezius (descending part) was selected as an indicator of the load on the shoulder and neck area, as this muscle provides the main lift for the shoulder girdle, and is important for the stabilization of the scapula during arm movements. Also, many subjects reported symptoms located in the body and tendons of the trapezius muscle, which were confirmed during medical examination (Aaras, 1990). Postural load was also assessed by recording the movements of the upper arm in the shoulder joint (glenohumeral joint) in terms of flexion/extension and abduction/adduction (Figure I) as well as flexion/extension of the head and back, using inclinometers (Figure 2).
adductlon ____----
Figure
information
Range (cm)
1 The
the upper
flexionlextension arm in the shoulder
and abductionladduction joint
of
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
Figure 2 The ‘Physiometer’, with transducers attached to the head, upper arm and back
The physiometer has four channels for EMG recordings, and six channels for postural angles. These postural angles were measured in terms of deviation from a reference body position, defined as a sitting position with well-balanced neutral upright head and trunk posture, relaxed shoulders and both arms hanging relaxed down the body. The quantitative EMG technique and the recording of postural angles, along with the limitations of the methods, are discussed elsewhere (Aarls, 1987; Aaris and Stranden, 1988; AarHs et al, 1988; Westgaard,
167
1988). However, the main steps in the analysis of the signals representing the EMG activity and postural angles are detailed below. Figure 3 shows the quantification of the EMG signal from a 280 s recording from the descending part of m. trapezius. The RMS (Root Mean Square) of the electrical signal was calculated over 0.1 s intervals, converted to a percentage of maximal voluntary contraction (%MVC), giving a measure of the average muscle force developed by the trapezius muscle. The relationship between the electrical RMS signals and the force was obtained by a calibration procedure with simultaneous recording of force and electrical signal from the muscle (Aarls and Westgaard, 1987). Thus the points in Figure 3a represent the muscle force over 0.1 s intervals. A quantitative analysis of muscle load during this recording was obtained by ranking the interval estimates (0.1 s duration) according to muscle force (Figure 3b) to produce an amplitude probability distribution function (APDF) as shown in Figure 3c (Jonsson, 1982; Aaris and Westgaard, 1987). The muscle force given by this curve indicates the time fraction of the recording period with the force lower than, or equal to, a given level. ‘Static force level’ is defined as the level of muscular contraction corresponding to a probability level of 0.1; that is, the muscle load is higher than this level for 90% of the working period. Peak load is similarly defined as the load corresponding to a probability level of 0.9; that is, the muscle has a lower
A
4min 40s
B
C
0
MUSCLE
FORCE
( % MVC 1
50
MUSCLE
FORCE
( % MVC I
Figure 3 (a) Muscle load on the upper trapezius. Each point in the recording gives the mean load in 0.1 s intervals, as the percentage of maximum voluntary contraction (% MVC). (b) Points (0.1 s duration) are ranked according to muscle force (% MVC) on the x axis. The number of points is given on they axis. (c) The cumulative amplitude probability distribution function of the recording
168
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
force level for 90% of the recording period. A probability level of 0.5 defines the median level of contraction. The quantitative analysis of postural angle was carried out by the same procedure as for the EMG signal. For the development of musculoskeletal illness, both the muscle load, quantified in terms of the amplitude probability distribution function, and the number and distribution of time periods with low levels of muscular activity may be important factors. Therefore, an analysis of low levels of activity was performed by choosing a level of muscle force and counting the number of periods below this level from the RMS value of the EMG signal (Figure 4~). Correspondingly, for an analysis of low levels of postural angles, the number of activity levels within a sector close to the defined reference body position was calculated (Figure 4b) (Aaris et al, 1990). Experiment
The present study was performed to determine whether the effects of an increased postural load and lower neck angle wearing multifocals compared with single-vision lenses, shown in the earlier study, would still be the case when the multifocal lenses were worn for an extended period. The earlier study showed no significant difference between the three multifocal lenses tested regarding muscle load or neck angle. The progressive lens that had been given the best subjective reports from the subjects during that experiment was the Datacomfort from Essilor. This is a progressive lens constructed for VDT work. These lenses were given to the subjects, who were instructed to wear them ‘for as long as possible’ when working at the VDT. All were asked to report whether any serious problems occurred. There was no other communication between the investigators and the subjects in this period. After a three-month period, the subjects were brought into the laboratory and tested again. They were instructed in the same task
as in the first experiment (see ‘Working task’), and given time to get used to the test situation, and also to become relaxed before the test started. Based on the clinical observation that users of progressive lenses need an adaptation time ‘to get used to’ the corrections, we did not want to disturb this habitual situation. Consequently, all subjects were first tested with the progressive lenses, and then the test was repeated with the single-vision lenses. Because singlevision lenses of correct dioptrical power are very unlikely to influence the postural angle, there is no need for an adaptation time for these lenses. Statistical analysis
All results are expressed as mean values with 95% confidence intervals constructed by using the Student’s procedure (Kendal and Stuart, 1979). Differences were considered significant if the p-value was less than or equal to a level of 5%. Comparison of the groups was carried out by using analysis of variance (ANOVA), with repeated measurements (Jones and Kenward, 1990).
Results In the initial study a significant reduction in static trapezius load was recorded when the subjects wore single-vision lenses, compared with the trapezius load when progressive lenses were worn. When measuring the same parameters after three months, the difference between single-vision and progressive lenses was still significant (Figure 5). There was a tendency towards a reduction in trapezius load between the earlier and this study when wearing similar lenses. This is, however, not significant. The reason for this reduction might be that the total test period was much shorter during this second experiment, because only two different lenses were tested. Other influencing factors may have been that the subjects were more familiar with the test situation, and the amount of work per time unit may have differed.
B A 45. FkxDn
0
2 1 n
Exiensim
-5 I
Figure 4 (a) Examples of time periods of muscular activity below 1 and 2% MVC are indicated by broken lines. (b) Two periods of postural angles in the upper arm in the sag&al plane between +5” (flexion) and -5” (extension) are shown by broken lines
169
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
TRAPEZIUS
10 %
(A) KMVC TRAPEZIUS
%MVC h
: Datacomfort
0 : Datacomfort 0
3.5
: Single
50 %
0
7
vision
:
Single vision
6
3.0
2.5
2.0
1.5
1.0
0.5
* EARLIER
THIS
STUDY
STUDY
Figure 5 Static trapezius load as a mean value with 95% confidence interval for the group of workers with single vision (white columns), and Datacomfort (hatched columns). The left two columns illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort for 3 months
EARLIER
THIS
STUDY
STUDY
TRAPEZIUS
(B)
90 %
m : Datacomfort 0 : Single vision
%MVC
The same significant reduction regarding the median trapezius load (i.e. the values for 50% of the recording
time) was found when comparing single vision lenses and Datacomfort in this and the earlier study (Figure 6a).
Looking at peak trapezius load (i.e. the load was below this value for 90% of the recording time), the same significant reduction was found (Figure 6b). The muscle load was also assessed in terms of the duration of time when the trapezius load was below 1,2 and 5% MVC. There was a significant difference between the single-vision and Datacomfort lens in both the earlier and in this study regarding the duration of time when the trapezius load was below 5% MVC. The graphs (Figure 7) indicate a longer total time for the trapezius load being below 5% MVC both for singlevision and Datacomfort in this study compared with the earlier study, but the difference was not significant. There were no significant differences in the duration below 1 and 2% for the two types of lens. There were highly significant larger static (Figure 8a) and (Figure 8b) head flexion angles for the single-vision lenses compared with Datacomfort lenses in both the earlier and in this study. Considering the peak value, there was no such difference. There were no significant differences comparing single-vision lenses from the earlier study with this study. The same results were obtained for Datacomfort lenses.
EARLIER
THIS
STUDY
STUDY
Figure 6 (a) Median
(50%) and (b) peak (90%) trapezius load as mean value with 95% confidence interval for the group of workers with single vision (white columns), and Datacomfort (hatched columns). The left two columns in each figure illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort lenses for 3 months
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
170
TIME BELOW 5%
HEAD FLEXION 10%
(A)
S : Datacomfort
A
0 D : Datacomfort 0 : Single vision
: Single
vision
16-
14-
4-
EARLIER
THIS
STUDY
STUDY
Figure 7 The duration
2-
of time when the trapezius load was
below 5% MVC as a percentage of the total recording time on the y axis. The means with 95% confidence interval are illustrated for single-vision lenses in the white column, and for the Datacomfort lenses in the hatched columns
THIS STUDY
HEAD FLEXION 50%
Discussion In the earlier study it was shown that single-vision lenses produced less postural load in terms of EMG values and postural angles (Horgen et al, 1989). As this study was intended to see whether the subjects would adapt to progressive lenses if they were worn for a prolonged period, we did not want to upset the habitual situation before the test started. Therefore the test always started with the progressive lenses, which the subjects had worn constantly at work for three months. After having completed the task with these lenses, they carried out the same task with the single-vision lenses. One might argue that this design slightly favoured the progressive lenses, in terms of the possible effects of fatigue. The whole test period lasted for no more than approximately 1 h, so this effect should have been negligible. Thus it can be stated with reasonable confidence that multifocal lenses create more postural load than single-vision lenses when working on a VDT screen, and that an adaptation time of 3 months does not alter these results. Several studies have documented a relationship between trapezius load and development of musculoskeletal discomfort in the upper part of the body (Erdelyi et al, 1988; Parenmark et al, 1988; Philipson et al, 1990; Veiersted et al, 1990, 1993; Aaras, 1994; Jensen et al, 1993). Single-vision lenses also lead to a larger head flexion angle, which is usually considered to be an advantage compared with extension of the neck often observed when wearing multifocals (HarmsRingdahl, 1986). Furthermore, this study also shows
EARLIER STUDY
A
D: Datacomfort 0:
I-
Single vision
*
EARLIER
THIS
STUDY
STUDY
Figure 8 (a) Static and (b) median
head flexion angle as a mean value with 95% confidence interval for the group of workers with single-vision lenses (white columns), and Datacomfort (hatched columns). The left two columns in each figure illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort lenses for 3 months
Posture
load when
wearing
progressive
that no significant changes in the trapezius load occurred after the 3 month test period when wearing the progressive correction. Do these results mean that we should in future advise against the prescription of multifocals for VDT work? It should be remembered that a single-vision lens, given as a presbyopic correction, may visually isolate the subject from tasks requiring different viewing distances. This may in turn lead to problems in other areas, such as having to bend forward to see more distant objects clearly, not being able to see persons entering the room, etc. As examples of subjective reactions registered, 3 of the 18 subjects spontaneously reported satisfaction with the Datacomfort lens. They benefited from the possibility of being able to see clearly at different distances. Even these subjects, however, showed the same pattern as the others regarding higher trapezius load when wearing the progressive lenses as compared with single-vision lenses.
Conclusion In conclusion, this study has shown that single-vision lenses should be recommended as the first lens choice for VDT workers. It must be added, however, that the results of this study indicate that multifocal lenses may be used if the visual demands of the work task require both distant must then be and near vision. The workplace ergonomically designed, the power of the near portion of the multifocal lenses must be adjusted for the working distance, and the relative sizes of these portions must be carefully designed. The implication is that it is of importance that occupational optometry should be carried out in close cooperation with other health workers in an organization. The work should always be carried out in the following order: (1) ergonomic evaluation and adjustment of the workplace; (2) evaluation and adjustment of lighting conditions; (3) optometric measurements and care. This will ensure that the final optometric correction is prescribed correctly for an ergonomically designed workplace. If the optometric correction is given before ergonomic evaluation and adjustment of the workplace has been done, the risk exists that the optometric correction will preserve a bad working posture, instead of helping to correct it.
References Aarls, A. 1987 ‘Postural load and the development of musculoskeletal illness’ Stand J Rehab Med Supplement No 18 Aa&, A. 1990 ‘Load related musculo-skeletal illness - is ergonomic workplace design a sufficient remedy’ in Ha&grave,-C. M., Wilson, J. R., Corlett. E. N. and Manenica. I. ledsj Work Desipn ” in Practice Taylor & Francis, London, pp 3&d ’ AarPs, A. 1994 ‘Relationship between trapezius load and the incidence of musculoskeletal illness in the neck shoulder’ Int / hi Ergonomics 14. 341-348
lenses during
VDT
work:
H. Horgen
et al.
171
Aa&, A. and Westgaard, R. H. 1987 ‘Further studies of postural load and musculo-skeletal injuries of workers and an electromechanical assembly plant’ Appl Ergonomics 18 (3), 211-219 Aa&, A., Westgaard, R. H., Stranden, E. and Larsen, S. 1990 ‘Postural load and the incidence of musculo-skeletal illness’ in Dainoff, M., Sauter, S. and Smith, M. (eds) Proc NIOSH Conf Promoting Health and Productivity in the Computerized Office Miami Universitv. Tavlor & Francis Ltd. pp 68-93 Aa&, A. and St&de;, E. 1988 ‘Measurement of postural angles during work’ Ergonomics 31 (6), 935-944 AarBs, A., Westgaard, R. H. and Stranden, E. 1988 ‘Postural angles as an indicator of postural load and muscular injury in occupational work situations’ Ergonomics 31 (6) 915-933 Bj#rseth, H. H. 1985 Workplaces with VTD. Ergonomics, visual and lighting conditions EFI TR nr. 3219 (in Norwegian) Ball, G. 1982 Symptoms in Eye Examination Butterworth, London, pp 37-46 Borish, I. M. 1975 Clinical Refraction Professional Press Inc Cakir, Hart and Stewart 1980 Visual Display Terminals John Wiley, Chichester Daae, K. I., Holgersen, H., Saude, T., Horgen, G., Garaas, T. and Quist, N. 1984 Visual Display Terminals and Visual Ergonomics Kongsberg College of Engineering (in Norwegian) Daum, K., Good, G. and Tijerina, L. 1988 ‘Symptoms in video display terminal operators and the presence of small refractive errors’ J Am Assoc 59 (9) 691-697 Erdelyi, A., Sihvonen, T., Helin, P. and Hanninen, 0.1988 ‘Shoulder strain in keyboard workers and its alleviation by arm supports’ Int Arch Occup Environ Health 60, 119-124 Grandjean, E. and Vigliani, E. 1980 Ergonomic Aspects of Visual Display Terminals Taylor & Frances, London Harms-Ringdahl, K. 1986 ‘An assessment of shoulder exercise and load elicited pain in cervical spine’ Stand J Rehab Med Supplement No 14 Horgen, G. 1985 Visual Demands on a VDT Workplace Kongsberg College of Engineering/Alcatel-STK (in Norwegian) Horgen, G., AarL, A., Fagerthun, H. and Larsen, S 1989 ‘The work posture and postural load of the neck/shoulder muscles when correcting presbyopia with different types of multifocal lenses on VDU-workers’ in Smith, M. J. and Salvendy, G. (eds) Work With Computers; Organizational, Management, Stress and Health Aspects Elsevier Science Publishers, Amsterdam, pp 338-347 Jones, B. and Kenward, M. 1990 Design and Analysis of Cross-over Trials Chapman & Hall, London Jensen, C., Nilsen, K., Hansen, K. and Westgaard, R. H. 1993 ‘Trapezius muscle load as a risk indicator for occupational shoulder-neck complaints’ Int Arch Occup Environ Health 64, 415423 Jonsson, B. 1982 ‘Measurement and evaluation of muscular strain in the shoulder during constrained work’ / Hum Ergo1 11, 73-88 Kendall, M. and Stuart, A. 1979 The Advanced Theory of Statistics Vols 1 and 2, Charles Griffin & Co, London Knave, B., Wibom, R., Voss, M., Hedstrem, L. and Bergquist, V. 1985 ‘Work with video display terminals among office employees’ Stand .I Work Environ Health 11, 457-466 Michaels, D. D. 1980 Visual Optics and Refraction C. V. Mosbv Company, 577 Parenmark, G., Engvall, B. and Malmkvist, A.-K. 1988 ‘Ergonomic on-the-job training of assembly workers’ Appl Ergonomics 19 (2), 143-146 Philipson, L., Serby, R., Larsen, P. and Kaladjev, S. 1990 ‘Muscular load levels in performing musicians as monitored by quantitative electromyography’ Med. Prob. Perform. Artists 5, 79-82 Shahnavaz, H. 1982 ‘Lighting conditions and work place dimensions of VDU operators’ Ergonomics 25 (12), 116.5-1173 Veiersted, K. B., Westgaard, R. H. and Andersen, P. 1990 ‘Pattern of muscle activity during stereotyped work and its relation to muscle pain’ Int Arch Occup Environ Health 6 (62), 31-41 Veiersted, K. B., Westgaard, R. H. and Andersen, P. 1993 ‘Electromyographic evaluation of muscular pattern as a predictor of trapezius myalgia’ Stand J Work Environ Health 19, 284-290 Westgaard, R. H. 1988 ‘Measurement and evaluation of postural load in occupational work situations’ Eur J Appl Physiol 57, 291-304 World Health Organization 1987 Visual Display Units and Workers’ Health WHO Offset Publication No. 99
Applied
UTTERWORTH EINEMANN
Ergonomics Vol 26, No. 3, pp. 165-171, 1995 Copyright 0 1995 Elsevier Science Ltd Pnnted in Great Britain. All rights reserved 0003.6870/95 Sl0.W + 0.00
00034870(95)ooo12-7
Is there a reduction in postural load when wearing progressive lenses during VDT work over a threemonth period? Gunnar Horgen Kongsberg College of Engineering, Department of Optometry, Norway
Arne AarQs Alcatel, Oslo, Norway
Hans Fagerthun and Stig Larsen Medstat, Str@mmen, Norway
In an initial study, 18 VDT operators were found to have a higher muscle load in the neck/ shoulder region when using progressive lenses compared with single-vision lenses, while working on a VDT unit. For single-vision lenses, a higher flexion angle in the neck was also discovered. The present study investigates whether this higher muscle load is reduced when a progressive correction is worn for a three-month period. Adaptation in terms of reduced muscle load measured by electromyography (EMG) was not discovered. Head flexion angle was smaller when using progressive correction as compared with single-vision lenses. Some practical implications for occupational optometry are discussed. Keywords:
progressive
lenses,
VDT work,
electromyography
period of time leads to an adaptation that it is possible to measure by a reduction in the trapezius load. Whether the longer wearing time leads to a change in the neck angle is also investigated.
There are many reports of discomfort among VDToperators (e.g. Knave et al, 1985; World Health Organization, 1987). Important contributors to such discomfort are assumed to be work posture, work task and visual strain (Daae et al, 1984; Bjorseth, 1985; Knave et al, 1985; Aarh 1987; Daum et al, 1988). Work posture and postural load of the neck and shoulder muscles during VDT work when correcting presbyopia with different types of multifocal lens were studied by Horgen et al (1989). The results from the earlier study showed that, when performing data entry work, single-vision lenses produced less traoezius load than that oroduced bv the types of multifocals tested. Subjects wearing singlevision lenses also had a significantly larger head flexion angle, but a smaller range of head movements than subjects wearing multifocals. However, the subjects had not worn the multifocal lenses for more than approximately 4 h (Horgen et al, 1989). In practice it is generally accepted that clients need a period of time in order to ‘get used to’ a new optometric correction (Borish, 1975; Michaels, 1980; Ball, 1982). After the adaptation period, any reported symptoms usually decrease. The present study investigates whether wearing multifocal lenses over a longer
Material and methods Subject population
The subjects were 10 females and 8 males. Their mean age was 55.7 years, with a range from 51 to 61 years. They were chosen according to the following criteria: (1) All were experienced VDT workers, and their daily work situation involved using a VDT screen. (2) They were presbyopic, with a need for reading additions ranging from +1.25 dioptres to +2.50 dioptres. The criterion for the reading addition was x= l/Y - %Z, where X = reading addition (dioptres), Y = working distance (metres) and Z = amplitude of accommodation (dioptres) (Borish, 1975). (3) They all wore spectacles in their daily activity, for both distance and near vision. Their lens power for distance correction ranged from -3.75 dioptres to
165
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
166
+3.00 dioptres spherical power, no one being astigmatic by more than -3.00 dioptres cylinder. (4) The participants had a binocular visual acuity of 1.0 (20/20) or better (Snellen); this value was obtained through their distance correction. Vision analysis
At the beginning of the earlier study, the subjects underwent a vision analysis. Retinoscopy and subjective refraction with binocular control were tested. Oculomotor balance was measured with the Cover test, Graefes dissociated test and Malletts fixation disparity test. The Mallet fixation disparity test was used only for the near-work situation. Amplitude of accommodation was measured with a near-point ruler (RAF ruler), and the dioptric power needed in the lenses for near work was calculated (see ‘Corrective lenses’). Sterescopic acuity was assessed with a Titmus fly test. Ocular motility, visual fields (Donders), pupil reflexes and ocular inspection were tested at the same time. The corrective lenses
Two types of lenses were chosen from the first study for this prolonged experiment (see ‘Experiment’): one progressive lens that was specially made for VDT work (Datacomfort), and one ordinary single-vision lens. The Datacomfort lens has an increasing dioptric power from distance to near point, with a zone between the distance and the near point that has a power of 0.6 times the full near power. This zone is meant for VDT work. A single-vision lens has the same dioptric power all over the lens. Details of the lenses are given in Horgen et al (1989). The workplace
The experimental set-up was designed for a data entry VDT task. The workdesk and the VDT were individually adjustable, with a document holder mounted between the screen and keyboard. This had an automatic ruler device, controlled by the foot, which prevented the subject pointing with their fingers to find the correct information on the document. Pointing with the fingers would influence the load on the trapezius muscle; therefore this had to be avoided. The workplace was configured so that the viewing angle below the horizontal was 15” to the centre of the screen. This angle was approximately the same as that which workers used for data entry work at Alcatel-STK (Horgen, 1985). The viewing angle magnitude is also in agreement with the recommendations of Grandjean and Vigliani (1980), Shahnavaz (1982) and Cakir et hl (1980). The distances from the subjects’ eyes (spectacle plane) to the centre of the screen, to the document and to the keyboard were adjusted to the values shown in Table I. These values were the same as in the earlier experiment (Horgen et al, 1989). The spectacle lens powers were modified to meet these specifications, and they were also tested directly on the actual workplace before the prescriptions were finally decided. The working task
The
subject
typed
Table 1 Distances from the subjects’ eyes Mean (cm)
Screen Document Keyboard
from
a hard
copy
63.00
54-70
57.50 50.50
52-65 46-55
document into a database. There were two test periods, one for each lens. Each test period lasted for 30 min, and the measurements were taken continously during this time. To ensure that the subjects looked both at the screen and at the keyboard, the format of the screen was made different from the hard copy in two ways. First, the location of the different items displayed on the screen was made different from the location they had on the hard copy and, second, the sequence of information was made different from that on the hard copy. To prevent the subjects from memorizing the screen layout, the format of the screen was altered every 5 min throughout the test period. Measurements
of load on the musculoskeletal system
Postural loads of the neck and shoulder muscles were quantified by electromyography (EMG) using surface electrodes (Aaras and Westgaard, 1987). The load on m. trapezius (descending part) was selected as an indicator of the load on the shoulder and neck area, as this muscle provides the main lift for the shoulder girdle, and is important for the stabilization of the scapula during arm movements. Also, many subjects reported symptoms located in the body and tendons of the trapezius muscle, which were confirmed during medical examination (Aaras, 1990). Postural load was also assessed by recording the movements of the upper arm in the shoulder joint (glenohumeral joint) in terms of flexion/extension and abduction/adduction (Figure I) as well as flexion/extension of the head and back, using inclinometers (Figure 2).
adductlon ____----
Figure
information
Range (cm)
1 The
the upper
flexionlextension arm in the shoulder
and abductionladduction joint
of
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
Figure 2 The ‘Physiometer’, with transducers attached to the head, upper arm and back
The physiometer has four channels for EMG recordings, and six channels for postural angles. These postural angles were measured in terms of deviation from a reference body position, defined as a sitting position with well-balanced neutral upright head and trunk posture, relaxed shoulders and both arms hanging relaxed down the body. The quantitative EMG technique and the recording of postural angles, along with the limitations of the methods, are discussed elsewhere (Aarls, 1987; Aaris and Stranden, 1988; AarHs et al, 1988; Westgaard,
167
1988). However, the main steps in the analysis of the signals representing the EMG activity and postural angles are detailed below. Figure 3 shows the quantification of the EMG signal from a 280 s recording from the descending part of m. trapezius. The RMS (Root Mean Square) of the electrical signal was calculated over 0.1 s intervals, converted to a percentage of maximal voluntary contraction (%MVC), giving a measure of the average muscle force developed by the trapezius muscle. The relationship between the electrical RMS signals and the force was obtained by a calibration procedure with simultaneous recording of force and electrical signal from the muscle (Aarls and Westgaard, 1987). Thus the points in Figure 3a represent the muscle force over 0.1 s intervals. A quantitative analysis of muscle load during this recording was obtained by ranking the interval estimates (0.1 s duration) according to muscle force (Figure 3b) to produce an amplitude probability distribution function (APDF) as shown in Figure 3c (Jonsson, 1982; Aaris and Westgaard, 1987). The muscle force given by this curve indicates the time fraction of the recording period with the force lower than, or equal to, a given level. ‘Static force level’ is defined as the level of muscular contraction corresponding to a probability level of 0.1; that is, the muscle load is higher than this level for 90% of the working period. Peak load is similarly defined as the load corresponding to a probability level of 0.9; that is, the muscle has a lower
A
4min 40s
B
C
0
MUSCLE
FORCE
( % MVC 1
50
MUSCLE
FORCE
( % MVC I
Figure 3 (a) Muscle load on the upper trapezius. Each point in the recording gives the mean load in 0.1 s intervals, as the percentage of maximum voluntary contraction (% MVC). (b) Points (0.1 s duration) are ranked according to muscle force (% MVC) on the x axis. The number of points is given on they axis. (c) The cumulative amplitude probability distribution function of the recording
168
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
force level for 90% of the recording period. A probability level of 0.5 defines the median level of contraction. The quantitative analysis of postural angle was carried out by the same procedure as for the EMG signal. For the development of musculoskeletal illness, both the muscle load, quantified in terms of the amplitude probability distribution function, and the number and distribution of time periods with low levels of muscular activity may be important factors. Therefore, an analysis of low levels of activity was performed by choosing a level of muscle force and counting the number of periods below this level from the RMS value of the EMG signal (Figure 4~). Correspondingly, for an analysis of low levels of postural angles, the number of activity levels within a sector close to the defined reference body position was calculated (Figure 4b) (Aaris et al, 1990). Experiment
The present study was performed to determine whether the effects of an increased postural load and lower neck angle wearing multifocals compared with single-vision lenses, shown in the earlier study, would still be the case when the multifocal lenses were worn for an extended period. The earlier study showed no significant difference between the three multifocal lenses tested regarding muscle load or neck angle. The progressive lens that had been given the best subjective reports from the subjects during that experiment was the Datacomfort from Essilor. This is a progressive lens constructed for VDT work. These lenses were given to the subjects, who were instructed to wear them ‘for as long as possible’ when working at the VDT. All were asked to report whether any serious problems occurred. There was no other communication between the investigators and the subjects in this period. After a three-month period, the subjects were brought into the laboratory and tested again. They were instructed in the same task
as in the first experiment (see ‘Working task’), and given time to get used to the test situation, and also to become relaxed before the test started. Based on the clinical observation that users of progressive lenses need an adaptation time ‘to get used to’ the corrections, we did not want to disturb this habitual situation. Consequently, all subjects were first tested with the progressive lenses, and then the test was repeated with the single-vision lenses. Because singlevision lenses of correct dioptrical power are very unlikely to influence the postural angle, there is no need for an adaptation time for these lenses. Statistical analysis
All results are expressed as mean values with 95% confidence intervals constructed by using the Student’s procedure (Kendal and Stuart, 1979). Differences were considered significant if the p-value was less than or equal to a level of 5%. Comparison of the groups was carried out by using analysis of variance (ANOVA), with repeated measurements (Jones and Kenward, 1990).
Results In the initial study a significant reduction in static trapezius load was recorded when the subjects wore single-vision lenses, compared with the trapezius load when progressive lenses were worn. When measuring the same parameters after three months, the difference between single-vision and progressive lenses was still significant (Figure 5). There was a tendency towards a reduction in trapezius load between the earlier and this study when wearing similar lenses. This is, however, not significant. The reason for this reduction might be that the total test period was much shorter during this second experiment, because only two different lenses were tested. Other influencing factors may have been that the subjects were more familiar with the test situation, and the amount of work per time unit may have differed.
B A 45. FkxDn
0
2 1 n
Exiensim
-5 I
Figure 4 (a) Examples of time periods of muscular activity below 1 and 2% MVC are indicated by broken lines. (b) Two periods of postural angles in the upper arm in the sag&al plane between +5” (flexion) and -5” (extension) are shown by broken lines
169
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
TRAPEZIUS
10 %
(A) KMVC TRAPEZIUS
%MVC h
: Datacomfort
0 : Datacomfort 0
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: Single
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7
vision
:
Single vision
6
3.0
2.5
2.0
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Figure 5 Static trapezius load as a mean value with 95% confidence interval for the group of workers with single vision (white columns), and Datacomfort (hatched columns). The left two columns illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort for 3 months
EARLIER
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STUDY
TRAPEZIUS
(B)
90 %
m : Datacomfort 0 : Single vision
%MVC
The same significant reduction regarding the median trapezius load (i.e. the values for 50% of the recording
time) was found when comparing single vision lenses and Datacomfort in this and the earlier study (Figure 6a).
Looking at peak trapezius load (i.e. the load was below this value for 90% of the recording time), the same significant reduction was found (Figure 6b). The muscle load was also assessed in terms of the duration of time when the trapezius load was below 1,2 and 5% MVC. There was a significant difference between the single-vision and Datacomfort lens in both the earlier and in this study regarding the duration of time when the trapezius load was below 5% MVC. The graphs (Figure 7) indicate a longer total time for the trapezius load being below 5% MVC both for singlevision and Datacomfort in this study compared with the earlier study, but the difference was not significant. There were no significant differences in the duration below 1 and 2% for the two types of lens. There were highly significant larger static (Figure 8a) and (Figure 8b) head flexion angles for the single-vision lenses compared with Datacomfort lenses in both the earlier and in this study. Considering the peak value, there was no such difference. There were no significant differences comparing single-vision lenses from the earlier study with this study. The same results were obtained for Datacomfort lenses.
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Figure 6 (a) Median
(50%) and (b) peak (90%) trapezius load as mean value with 95% confidence interval for the group of workers with single vision (white columns), and Datacomfort (hatched columns). The left two columns in each figure illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort lenses for 3 months
Posture load when wearing progressive lenses during VDT work: H. Horgen et al.
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TIME BELOW 5%
HEAD FLEXION 10%
(A)
S : Datacomfort
A
0 D : Datacomfort 0 : Single vision
: Single
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Figure 7 The duration
2-
of time when the trapezius load was
below 5% MVC as a percentage of the total recording time on the y axis. The means with 95% confidence interval are illustrated for single-vision lenses in the white column, and for the Datacomfort lenses in the hatched columns
THIS STUDY
HEAD FLEXION 50%
Discussion In the earlier study it was shown that single-vision lenses produced less postural load in terms of EMG values and postural angles (Horgen et al, 1989). As this study was intended to see whether the subjects would adapt to progressive lenses if they were worn for a prolonged period, we did not want to upset the habitual situation before the test started. Therefore the test always started with the progressive lenses, which the subjects had worn constantly at work for three months. After having completed the task with these lenses, they carried out the same task with the single-vision lenses. One might argue that this design slightly favoured the progressive lenses, in terms of the possible effects of fatigue. The whole test period lasted for no more than approximately 1 h, so this effect should have been negligible. Thus it can be stated with reasonable confidence that multifocal lenses create more postural load than single-vision lenses when working on a VDT screen, and that an adaptation time of 3 months does not alter these results. Several studies have documented a relationship between trapezius load and development of musculoskeletal discomfort in the upper part of the body (Erdelyi et al, 1988; Parenmark et al, 1988; Philipson et al, 1990; Veiersted et al, 1990, 1993; Aaras, 1994; Jensen et al, 1993). Single-vision lenses also lead to a larger head flexion angle, which is usually considered to be an advantage compared with extension of the neck often observed when wearing multifocals (HarmsRingdahl, 1986). Furthermore, this study also shows
EARLIER STUDY
A
D: Datacomfort 0:
I-
Single vision
*
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Figure 8 (a) Static and (b) median
head flexion angle as a mean value with 95% confidence interval for the group of workers with single-vision lenses (white columns), and Datacomfort (hatched columns). The left two columns in each figure illustrate the results from the previous study, and the right two columns represent the results after wearing the Datacomfort lenses for 3 months
Posture
load when
wearing
progressive
that no significant changes in the trapezius load occurred after the 3 month test period when wearing the progressive correction. Do these results mean that we should in future advise against the prescription of multifocals for VDT work? It should be remembered that a single-vision lens, given as a presbyopic correction, may visually isolate the subject from tasks requiring different viewing distances. This may in turn lead to problems in other areas, such as having to bend forward to see more distant objects clearly, not being able to see persons entering the room, etc. As examples of subjective reactions registered, 3 of the 18 subjects spontaneously reported satisfaction with the Datacomfort lens. They benefited from the possibility of being able to see clearly at different distances. Even these subjects, however, showed the same pattern as the others regarding higher trapezius load when wearing the progressive lenses as compared with single-vision lenses.
Conclusion In conclusion, this study has shown that single-vision lenses should be recommended as the first lens choice for VDT workers. It must be added, however, that the results of this study indicate that multifocal lenses may be used if the visual demands of the work task require both distant must then be and near vision. The workplace ergonomically designed, the power of the near portion of the multifocal lenses must be adjusted for the working distance, and the relative sizes of these portions must be carefully designed. The implication is that it is of importance that occupational optometry should be carried out in close cooperation with other health workers in an organization. The work should always be carried out in the following order: (1) ergonomic evaluation and adjustment of the workplace; (2) evaluation and adjustment of lighting conditions; (3) optometric measurements and care. This will ensure that the final optometric correction is prescribed correctly for an ergonomically designed workplace. If the optometric correction is given before ergonomic evaluation and adjustment of the workplace has been done, the risk exists that the optometric correction will preserve a bad working posture, instead of helping to correct it.
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VDT
work:
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et al.
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