PII: S0003—6870(97)00062-8 Applied Ergonomics Vol. 29, No. 3, pp. 217—224, 1998( 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain0003—6870/98 $19.00#0.00

Significance of mat and shoe softnessduring prolonged work in uprightposition: based on measurements oflow back muscle EMG, foot volumechanges, discomfort and groundforce reactions

Lone Hansen1, J+rgen Winkel2—4 and Kurt J+rgensen1*1August Krogh Institute, ºniversity of Copenhagen, DK21000, Denmark2National Institute for ¼orking ¸ife, Solna, Sweden3Department of Occupational and Environmental Medicine, ºniversity of ¸und, Sweden4¸und ºniversity Centre for Research on Humans, ¹echnology and Change at ¼ork, Sweden

(Received 11 February 1997; in revised form 4 July 1997)

The aim of the investigation was to study the significance of mat and shoe softness during prolongedwork in an upright position based on some physiological, biomechanical and comfort measurementsrelated to the lower extremities and the low back. Eight healthy female volunteers performed 2 h ofsimulated standing and 2 h of standing/walking work tasks in the laboratory using four combina-tions of soft shoes, clogs, soft mat and concrete. Thus, each subject performed a total of eight 2 hwork tests. The following parameters were measured pre-experimentally and one or more timesduring 2 h: total foot volume, vascular volume and interstitial volume of the left foot, EMG from thelumbar paraspinals, movement of centre of gravity (only during standing), biomechanical heelimpact (only during standing/walking), perceived discomfort in lumbar back, legs and feet, wholebody oxygen uptake, arterial blood pressure and heart rate. Using soft shoes rather than clogsduring standing/walking work implies approximately a halving of the foot oedema formation andthe heel impact. The effects due to the introduction of the soft mat are negligible. The localcirculatory responses in the feet and the EMG-signs of paravertebral muscle fatigue are largerduring standing compared to standing/walking work. The two investigated work types in this studydiffer regarding exposures as well as responses. Thus, it is recommended to shift between thesepostures and seated work during the working hours to improve job exposures. ( 1998 ElsevierScience Ltd. All rights reserved.

Key words: foot swelling, discomfort, paraspinal muscle fatigue, oedema prevention, electromyography

Introduction

In the Danish working population, the one year pre-valence of low back and feet complaints have been re-ported to be approximately 40 and 12%, respectively("rhede et al., 1992, Biering-S+rensen, 1985). Corre-sponding data have been reported from Switzerland(Buchberger 1993); back problems 39% and leg problems23%. The causes of these complaints may be individualfactors as well as a number of occupational risk factors.

*Author to whom correspondence should be addressed

In particular, low back pain is frequently explained byheavy lifting, poor working postures and whole bodyvibration. An additional risk factor which has been lessemphasized in scientific litterature is prolonged work inupright proper working postures (e.g. Magora, 1972;Ryan, 1989). Correspondingly, disorders in the lowerextremities may be due to prolonged work in uprightposition (standing and walking) (Ryan, 1989).

In the Danish working population almost one-third isworking mainly in an upright working position, witheasy walking and standing, and without physical strain(Groth et al, 1988). "rhede et al (1992) report that 25% ofthe Danish working population works in the standing

217

position almost all of the working day, while 21% arewalking most of the day. From a French national enquiryon working conditions 43% of the salaried women spentmore than 4 h a day standing (Cristofari et al, 1989). Inmany occupations the occurrence of standing and walk-ing work is much higher than for the general population.For example, according to Estryn-Behar et al (1990),84% of the female health care workers are standing formore than 4 h a day. Nemecek and Buchberger(1987)found that in large-scale laundries, 70—80% of the work-ing day consists of standing work.

A few studies show an association between the dura-tion of time spent in the upright position and troublesfrom the lower back and the feet. Magora (1972) reporteda high incidence of low back pain when standing morethan 4 h every day. From a survey carried out in Austra-lia among supermarket employees, Ryan (1989) reporteda positive and significant correlation between proportionof time spent standing and symptoms in the lower limbsand feet. In this survey it is reported that the checkoutdepartment spent more than 90% of the time standing inone place, and these workers had particulary high ratesof symptoms in the lower back, lower limbs and feet,compared to other body areas and to other departments.Lobo et al (1989) reported from a study, evaluatingfoot discomfort in female retail staff, a possible relation-ship between time spent on feet (including walking) anddiscomfort. There appears to be a ‘threshold value’ abovewhich there is a pronounced risk for increase in discom-fort. Buckle et al (1986) indicate that this correspondsto above 30% of the working day while Ryan (1989)shows a value of about 45—50% for regular symptomsin lower limbs and feet, and about 25% for low backsymptoms.

The subjective troubles appear to be related to physio-logical and/or biomechanical aspects such as oedemaformation in the feet (e.g. Dupuis and Rieck, 1980) andfatigue sensations from the tissues of the lower back (e.g.J+rgensen et al, 1993; Voloshin and Wosk, 1982). Know-ledge is available indicating that the development ofacute as well as chronic symptoms are related to theshock-absorbing properties of the ground surface (e.g.Kuorinka et al, 1978). To minimize or alleviate theseproblems, different aids such as mats and shoes withgood shock absorbing qualities have been developedduring the last decade. Their positive effects on the localcirculation in the legs and the contractile pattern of thepostural muscles are often suggested in sales promotionadvertisements. The occurrence of such effects is, how-ever, still ambiguous; moreover it is unknown in whichjob situations the effects of the aids are most important.Thus, the ergonomic criteria for choosing a mat anda pair of soft shoes are far from elaborated. Kim et al(1994) found from EMG recordings, indications of a re-duction in muscle fatigue in the back, with the mostcompressible of two mats compared to a concrete sur-face, but no significant effect on the legs. Rys and Konz(1994) conclude, in their paper on standing workers pos-ture, that mats are more comfortable than concrete andthat mats with 6% compression from the body weight aremore comfortable than mats with 18% compression.Unfortunately, the results from investigations of variousmats and shoes are difficult to compare due to poor andgenerally uncomparable material specifications. In addi-tion, the studied response parameters are rarely identicalbetween studies.

The aim of this investigation was to study the signifi-cance of mat and shoe softness during prolonged work inupright position based on physiological, biomechanicaland comfort measurements related to the lower extremi-ties and low back.

Material and methods

Subjects

Eight healthy women [mean(range)] age 24(21—29) yr,height 1.73 (1.66—1.79) m, weight 63.6 (57.9—70.0) kg]volunteered for the investigation. Informed consent wasobtained. None of the subjects had varicose veins. Thesubjects did not take part in the experiments during thelast week before onset of menstruation, and consumeda standardised amount of food and liquid in the last 24 hbefore each experiment. To avoid the influence of diurnalvariations on the microcirculatory parameters, the sub-jects participated during the same time of day for each ofthe 4 interventions.

Experimental design

Two types of simulated work tasks were investigated inthe laboratory.

(1) Standing work: Performed as letter sorting in frontof a semi-circular sorting shelf at a predetermined pace(one letter per second). The dimensions of the worksta-tion were within the limits of the normal horizontal workarea for standing manual work, outlined by Das andGrady (1983). The sorting shelf was a ‘real-life’ worksta-tion moved to the laboratory, and the 2 h of letter sortingrepresents the early part of the working day for mostDanish postmen. To minimize the exposure time re-quired to obtain sufficient foot oedema formation, anambient temperature of 30°C was chosen. According toWinkel et al (1988) the same effect on the oedema forma-tion during 8 h in 15°C could be found after 2 h in 30°C.

(2) Standing/walking work: This task comprised 20 s ofstanding followed by 10 s of walking (15 m). During thisprocedure, the subjects handled bottles and carried onesmall beer crate with eight emty bottles in each hand,(8 kg total). The crates were carried symmetrically. Thewalking speed and the proportion between the time spentwalking and standing, respectively, corresponds to dataobtained from field studies (Winkel and Ekblom, 1982).Room temperature was kept constant at 25°C (for practi-cal reasons it was not possible to maintain 30°C in theworking area used for this part of the investigation). Eachwork period lasted for 2 h.

A balanced block design with four interventions andwith each block containing the same eight subjects wasused. Each individual acted as her own control. Twoindependent variable were investigated: shoes and sur-face, each in a hard and a soft variant. Each subjectperformed the standardized work 4 times correspondingto the 4 possible combinations.

The subjects were resting supine for 20 min before eachexperiment. During this period pre-experimentalmeasurements were taken.

Independent variables

A porous elastic polyurethane profiled mat (thickness:10#5 mm knobs) was used as a soft floor. In standing

218 Significance of mat and shoe softness during prolonged work: ¸. Hansen et al.

Table 1 Central circulatory parameters and energetic load (% of V0 O2max)

Standing work Standing/walking work

After 10—15 After 100—105 After 10—15 After 100—105

Mean arterial 86 (74—95) 87 (69—104) 84 (74—99) 85 (75—96)blod pressure (MAP) (mm Hg)Heart rate 78 (62—94) 85 (66—105) 76 (63—100) 85 (68—106)(HR) (min~1)Oxygen 0.35(0.31—0.38) 0.34(0.29—0.41) 0.67(0.52—0.85) 0.65(0.53—0.75)intake (V0 O

2) (l )min~1)

Relative 4 14oxygen intake (% V0 O

2.!9)

work, an AMTI forceplate (LG6-4-2000) was usedas the hard floor. In the walking/standing work theforceplate was then integrated in a concrete floor used asa hard floor. The footwear were clogs without a heelsupport as the hard shoe, and sport shoes as soft shoes.The shoe sole consisted of ethyl-vinyl-acetate foam(thickness: 28 mm below the heel and 15 mm below theforefoot).

Dependent variables

The volume of the left foot was measured by hydro-plethysmography (Winkel, 1986), immediately before andafter 2 h of work, the vascular volume of the foot (VV)was estimated by elevation followed by arterial occlusionof the leg (Asmussen et al, 1940), and the reduction in footvolume (FV) was then measured by the plethysmograph.The changes in the interstitial foot volume (the edemaformation) (IFV) was calculated as the change in FVcorrected for change in VV over time. It was expressed as% of the total foot volume before work. The skin temper-ature (¹

4,) of the foot was measured on the dorsal venous

arc. The perceived discomfort was rated by a visual-analog scale (VAS) (0%, no discomfort; 100%; worstconceivable discomfort) (Freyd 1923). The electric activ-ity (EMG) from m. erector spinae (L3-level) wasmeasured bilaterally by surface electrodes. The EMGamplitude was normalized to the EMG amplitude at100% maximal voluntary contraction of the trunk exten-sors (%MVE). The signals were amplified and stored onmagnetic tape for subsequent analysis of root-mean-square (RMS) amplitude and mean power frequency(MPF).

The movement of the subjects centre of gravitywas estimated from the movement of the centre offoot pressure (m]min~1) recorded on the force plate.The forceplate was used to measure the impact causedby heel contact expressed as rate of force increase(N]s~1). In addition, whole body oxygen uptake(V0 O

2), blood pressure (BP) and heart rate (HR) were

measured, to facilitate comparisons with occupationalfield studies.

Protocol

FV and VV were measured before the onset of thework and immediately after the work period. Dis-comfort and ¹

4,was measured before and 30, 60,

and 115 min after the start of the 2 h work. BP, HR

and V0 O2

were measured before the 2 h of work,and 10—15, 65—70 and 100—105 min after the onsetof the work. The MVE of the trunk extensors wasmeasured pre-experimentally. The EMG and the forcep-late measures were registered after 5, 55 and 115 min ofwork.

Statistics

Statistical analysis software (SAS) was used to test thesignificance of difference between the treatments effects(ANOVA: four ways; subjects/time/mats/shoes and threeways; subjects/mats/shoes). Paired t-test was applied totest for significance between standing and standing/walk-ing work.

Results

The main results are given in Tables 1 and 2 and inFigures 1—3.

Standing work

Two hours of standing work increased the discomfort inthe low back from 6 to 30% [F(2,14)"10.8; p"0.002],in the legs from 3 to 14% [F(2,14)"9.5; p"0.003] andin the feet from 8 to 31% [F(2,14)"12.6; p"0.0007](Figure 1a). No effects on discomfort ratings due to shoeor floor softness could be shown. FV increased on aver-age by 3.9% [F(1,7)"120.4; p"0.0001]. This was main-ly due to an increase in IFV (3.0%). The remainingincrease of 0.9% was caused by an increase in VV[F(1,7)"6.7; p"0.04]. Standing work on the hard floorimplied the largest oedema preventing effect in the foot(Figure 2a). Introduction of soft shoes reduced theoedema formation from 3.2 to 2.8% on average[F(1,7)"5.7; p"0.05]. However, using a soft mat in-creased the oedema formation from 2.5 to 3.5%[F(1,7)"7.5; p"0.03] (Figure 2a). ¹

4,increased on

average from 32.1 to 35.5°C during the 2 h of standingwork with the major increase during the first hour. Nodifference could be demonstrated between the treatments(¹able 2). The movement of projection of the centre ofgravity did not vary according to shoes or softness of theflooring. However, a significant 10—15% increase occur-red after 1 h and returned to the initial value by the endof the 2 h experiment (Table 2). The relative paraspinalmuscle strain during standing was 4—6% MVE. The

Significance of mat and shoe softness during prolonged work: ¸. Hansen et al. 219

Table 2 A selection of the dependent variables: Differences between start and end of the experiment and between various combinations ofshoes and flooring: HS5Clogs, SS"sport shoes, HF"hard floor, SF"soft floor

Start-end HSHF/HSSF HSHF/SSHF HSHF/SSSF HSSF/SSHF HSSF/SSSF SSHF/SSSF(mean)

Standing work values for the treatments. p values for paired treatments

EMG amplitude left(%MVE) p values:

4.7—4.50.27

4.7/4.30.56

4.7/4.90.03

4.7/4.00.04

4.3/4.90.01

4.3/4.00.12

4.9/4.00.0001

EMG mean powerfrequency left (MPF)(Hz) p values:

146—140.50.05

145/1370.16

146/1460.95

145/1340.076

137/1460.18

137/1340.67

146/1340.87

Centre of gravitymove (m )min~1)p values:

4.2—4.20.03 *)

4.3/4.10.42

4.3/4.10.53

4.3/4.00.64

4.1/4.10.88

4.1/4.00.73

4.1/4.00.86

Skin temperature(¹

4,*/.) (°C)

p values:

32.1—35.50.0001

35.4/35.50.39

35.4/35.60.72

35.4/35.70.47

35.5/35.60.62

35.5/35.70.89

35.6/35.70.72

Walking/standing work, values for the treatments. p values for paired treatmentsSkin temperature(¹

4,*/.) (°C)

p values:

33.7—36.00.0001

35.4/35.80.59

35.4/36.10.11

35.4/36.60.41

35.8/36.10.28

35.8/36.60.13

36.1/36.60.66

* The p value is based on recordings three times during the experiment. The average value of the second recording was 4.4 m )min~1.

Figure 1 Changes in discomfort during (a) standing work and(b) standing/walking work using by a Visual analog scale (VAS)(0%, no discomfort, 100%, worst conceivable discomfort).Mean values of the four possible combinations of shoe andsurface. The bars indicate 1 standard error of mean

Figure 2 Relative changes in foot volume (%) after 2 h of (a)standing work and (b) standing/walking work. The average footvolume was 1029 ml before work. The bars indicate 1 standarderror of mean

220 Significance of mat and shoe softness during prolonged work: ¸. Hansen et al.

Figure 3 Heel impact (10~3 N ) s~1) at the four possible combi-nations of shoe and surface after 10, 60 and 115 min of stand-ing/walking work. The bars indicate 1 standard error of mean

EMG RMS-amplitude was constant during the 2 h, andthe MPF in the left paraspinalis decreased significantlyfrom 146 to 140.5 Hz [F(2.14)"8.0; p"0.005] (Table 2).No significant correlation coefficient between changes inMPF and RMS could be demonstrated (r"!0.17). TheMPF change was not influenced by shoe or floor soft-ness.

Standing/walking work

The discomfort increased over time from 5 to 27% (lowback: [F(2,14)"12.1; p"0.0009]), 5 to 16% (legs. [F(2,14)"9.1; p"0.003]) and 8 to 27% (feet: ([F(2,14)"18.6; p"0.0001)) (Figure 1b). However, no treatmenteffects due to shoe or floor softness could be shown. FVincreased by 3.4% in average [F(1.7)"57.4; p(0.0001].Both VV and IFV increased by 1.7% during the 2 h[F(1,7)"171.6; p"0.0001] (Figure 2b). The standing/walking condition implied that the oedema formationwas halved compared to standing (p"0.0009). Usingsoft shoes decreased the oedema formation from 2.2 to1.2% [F(1,7)"16.5; p"0.005] (Figure 2b). ¹

4,in-

creased from 33.7—36.0°C (Table 2). The biomechanicalimpact disminished gradually during the working periodfrom an average of 37.8]10~3 N ) s~1 during the initial10 min to 34.3]10~3 N ) s~1 during the last 10 min[F(2,14)"7,6; p"0.006] (Figure 3). Using soft shoescompared to clogs reduced the biomechanical impact atthe end of the working period by about 50%[F(1,7)"77.1; p(0.0001]. However, the soft matcaused an impact reduction of only 15% [F(1,7)"14.0;p"0.007] and only when hard shoes were used (Fig-ure 3) (Shoe/mat interaction: [F(1,7)"15.5; p"0.006]).It was not possible to demonstrate any EMG signs ofmuscular fatigue in the paravertebral muscles of thelower back.

Discussion

The present study shows that introduction of softshoes in standing/walking work implies a pronouncedreduction in heel impact. Further soft shoes reduceoedema formation in the feet in both standing and

standing/walking work. Soft shoes and soft mats haveonly modest additional effects on the investigated para-meters under the present working conditions comparedto the effects related to exposure duration shown pre-viously (e.g. Lobo et al, 1989).

Methodological considerations

Although the ambient temperature was 25°C in thestanding/walking and 30°C in the standing experimentsthe ¹

4,above the dorsal venous arch of the foot reached

approximately the same value under both conditions.This may be explained by the larger local metabolismand blood flow in the lower extremities during stand-ing/walking causing a larger increase in ¹

4,in spite of

a lower ambient temperature compared to the standingexperiment. Similarity in ¹

4,is one important factor

when comparing the volume changes between the twowork situations (Noddeland and Winkel, 1988). Thechange of VV during standing/walking work was, how-ever, twice that of standing. This may partly reflect highertissue temperatures below skin surface during stand-ing/walking conditions. A methodological implication ofthe different changes in VV is that oedema formationshould be estimated with caution solely from FV cha-nges.

Oxygen uptake and heart rate

The whole-body reactions (V0 O2, HR) were in accordance

with results from occupational studies of similar activites(e.g. Karvonen, 1974).

Discomfort and *EMG fatigue+

Previous studies have shown that mats (Redfern andChaffin, 1988; Rys and Konz, 1994) and shoes with softsoles (Zhang et al, 1991) are able to diminish discomfortassociated with prolonged standing work. Further, it wasshown by Kuorinka et al (1978) that comfort during 1 hperiods of standing depends on the softness of thesurface. However, this could not be confirmed in thepresent investigation using healthy female students. Ifthis discrepancy is related to differences in the musculos-keletal health profile or the anthropometrics of the sub-jects or to other factors cannot be concluded from thepresent data.

The paravertebral muscle load and the concomitantdevelopment of EMG signs of muscular fatigue werenot influenced by the flooring and shoe conditions.This is in agreement with Cook et al (1993). Kim et al(1994), however, found less fatigue development in thetrunk extensors (reduction of the median frequencylevel) during a test contraction at 75% MVC after 2 hof standing on an ‘anti-fatigue mat’ with large compress-ibility compared to standing on more stiff mats andconcrete surface. Kim et al (1994) were able to explainthis effect by the increased postural sway leading toa more dynamic contraction pattern of the trunk exten-sors due to larger compressibility of the mat. In ourstudy, however, the postural sway was not influenced bythe surface.

In the present study, a sign of postural muscle fatiguewas observed as a significant fall in MPF from theparaspinal muscles during the 2 h of standing work.This is in accordance with the fact that even low-graded

Significance of mat and shoe softness during prolonged work: ¸. Hansen et al. 221

sustained isometric contractions may cause muscle fa-tigue and a concomitant marked increase in variouspsycho-physical reactions (Sj+gaard et al, 1986; J+rgen-sen et al, 1988). However, the enhancement in the EMGsigns of muscle fatigue seems to be small compared thefour to sixfold increment in the perceived lumbar fatigue.This discrepancy could possibly be explained if themuscle fatigue leads to an increasing segmental instabil-ity causing overloading of the ligaments of the spine andother so-called passive structures of the spine (Pope et al,1992; Seidel, 1996). Additionally, one could speculate thatthe pressure on the intervertebral discs along with thesmall variation in work position may cause a similarcascade of events.

The rate of discomfort change in the lumbar regiontends to be slightly larger in constrained standing com-pared to standing/walking work. This observation co-incides with the observation that no electromyographicsigns of paravertebral muscle fatigue occurred in thelatter working type, probably due to the dynamic con-traction pattern of the paravertebral muscles during gait(e.g.Thorstensson et al, 1982).

According to e.g. Kuorinka et al (1978) and Kim et al(1994) only discrete signs of muscular fatigue are presentin the legs after prolonged standing. It is therefore morelikely that (local circulatory mechanisms) explain the timerelated changes in discomfort from the lower extremities.This is also supported by the marked increase in the IFVduring 2 h work in constrained standing. In the followingsection the changes of foot volumes are discussed in moredetail.

Foot volumes

It is often claimed that local circulatory factors play akey role in the explanation of discomfort in the depen-dent legs during prolonged standing and sittingwork (reviewed by Winkel, 1985). In spite of this, themarked oedema-preventing effect of soft shoes whilestanding on a hard surface was not reflected in thediscomfort ratings.

This discrepancy is not clear; one possible explanationis the limited duration of the exposure time in this experi-ment. The pronounced oedema-preventing effect ofsoft shoes during standing/walking may be explainedby the mechanism of the venous pump (Pegum andFegan, 1967a, b); it is influenced by, e.g. step frequency(Noddeland et al, 1983), the degree of plantar flexion ofthe foot (Edfeldt, 1991) and the dorsiflexion of the meta-tarsophalangeal joints (Bojsen—M+ller, 1983, Bojsen-M+ller and Lamoreux, 1979). Since the rigid soles ofclogs reduce the dorsiflexion of the metatarsophalangealjoints as well as the stride length (Bendix et al, 1986),it is likely that this implies a shorter relaxation period(‘diastole’) of the soleus muscle during a gait cycle witha likely reduction of the efficiency of the venous pump(Bojsen-M+ller, 1983; Bojsen—M+ller and Lamoreux,1979).

Even during standing work soft shoes were demon-strated to have an oedema-preventing effect. Ourmeasurement of the movement of the centre of gravityshowed that ‘standing work’ actually implies a certainamount of postural sway and minor movement of the feet(Table 2). These movements are, in turn, expected tocause dynamic squeezing of the foot tissues including thevenous plexus in the plantar and the lymph vessels. Using

a soft shoe rather than clogs will probably minimizeinterference with such matural movements. Althoughthey are modest compared to the powerful movementsduring walking they may imply a significant oedema-preventing effect (Winkel and J+rgensen, 1986, Winkelet al, 1988). Alternatively, one may speculate if the softshoes, fitting the feets tightly compared to the clogs, havea support stocking effect on the circulation in the foot.However, visual inspection of the feet after the experi-ments did not support this.

The oedema-promoting effect of a soft mat, as shownin the present study, may be due to a subtle reduction inmovement pattern which is not reflected by our centre ofgravity and oxygen uptake measurements.

Heel impact

The heel impact is determined by the elasticity, plasticity,compliance and the geometry of the shoe heel as well asthe flooring (J+rgensen et al, 1993b). In addition, themotor control may contribute significantly to the heelimpact by modulating the shock-absorbing character-istics of the muscle—tendon system in the legs (Simonsenet al, 1995, 1997).

Our biomechanical heel impact data are comparableto data from walking experiments using leather shoes(50]10~3 N ) s~1) (Light et al, 1980). The pronouncedimpact reduction effect of soft shoes and the less pro-nounced effect of a soft mat may be explained by differ-ences in the shock-absorbing characteristics and thethickness of the various materials.

The heel impact is transmitted up to the trunk(Voloshin and Vosk, 1982; Radin et al, 1973); the dam-pening effect of soft shoes is thought to influence theimpact value in the spine. Thus, Edfeldt (1991) showeda larger ‘creep effect’ of the spine when using clogs com-posed to soft shoes, indicating a larger mechanical spineexposure in the former case.

Comparison between standing and standing/walking work

The development of discomfort in the legs, feet andlower back was about identical. For the legs and feetthis corresponds to data from Winkel (1985) showingthat seated work without any movement of the lowerextremities and walking work elicited almost the samediscomfort, whereas work allowing large variation inthe activity of the feet and legs elicited the smallestdiscomfort. One may therefore suggest that the discom-fort in the legs in the present experiments is causedby various mechanisms. In standing work, the oedemaformation is one obvious reason for the discomfort,while in the standing/walking work the oedema forma-tion (IFV increase) may play an insignificant role.It is therefore possible that biomechanical impactfactors play a role for development of discomfort duringstanding/walking work if the exposure time is sufficientlylong (cf. Buckle et al, 1986; Ryan, 1989). Regardingthe spine, different causes for the discomfort may bepresent in standing and standing/walking respectively.In the former situation paravertebral muscle fatigueis one possible causative factor, while in the latter,spinal transmission of heel impact is an suggested alter-native explanation (Voloshin and Wosk, 1982; Radin,1973).

222 Significance of mat and shoe softness during prolonged work: ¸. Hansen et al.

Conclusions

f Constrained standing work during 2 h causes EMGsigns of muscle fatigue and pronounced discomfort inthe lower back as well as oedema formation in the feet.

f For both standing and standing/walking work thelargest oedema-preventing effect occurs with the com-bination of soft shoes and hard floor.

f Using soft shoes rather than clogs during stand-ing/walking work half the oedema formation whilea soft mat has negligible effects.

f The shoe and mat effects on the investigated responseparameters are marginal compared to the time-depen-dent effects.

Some ergonomic considerations

f The two investigated work types in this study differregarding some significant exposures as well as re-sponses. Thus, it is recommended that changes aremade between standing, walking and seated work dur-ing working hours in order to improve job exposures.

f At work premises with hard floor it is recommended tointroduce soft shoes rather than soft mats.

f The pronounced time-dependent changes in the inves-tigated response parameters suggest that the worksituation should be organized with time limits forwork in an upright position.

Acknowledgement

The funds for this investigation was granted by theDanish Work Environmental Fund.

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