Endurance, pain and resumption in fully flexed postures

Applied Ergonomics 32 (2001) 501–508

Endurance, pain and resumption in fully flexed postures

Linda Rosea,b,*, Roland .OOrtengrena, Mats Ericsonb,c

aDepartment of Human Factors Engineering, Chalmers University of Technology, G .ooteborg, SwedenbDepartment of Industrial Economics and Management, The Royal Institute of Technology, S-100 44 Stockholm, Sweden

cDepartment of Human Resources, Management and Environment, Mid Sweden University, .OOstersund, Sweden

Accepted 26 February 2001

Abstract

In this study effects of low loads in fully flexed postures were investigated. Thirteen men who were unused to the posturesparticipated. Thirteen professional construction workers with long experience of suchlike postures were also studied. Pain reactionsduring and after loading were observed, as well as endurance time and the recovery process, here by studying the resumption time.

Endurance and resumption times differed little from those given by models used for more common postures. Pain from the legs andnot from the back limited the working ability in 86% of the endurance tests. Thirdly, the construction workers had significantlylonger endurance time and shorter resumption time. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Endurance; Recovery; Pain; Fully flexed postures

1. Introduction

Occupational risks among construction workers arehigh compared to those in other occupations. Forconcrete reinforcement workers, (rodmen), the reportedoccupational accidents and diseases in Sweden were88.7/1000 employees in 1990, compared to a mean of39.3/1000 for all occupations (SCB, 1994). Constructionwork is physically demanding, often with high repeti-tiveness. Some working tasks are physically heavy whileothers are carried out in awkward postures. Fully flexedpostures are very common among rodmen, and aredominant when tying reinforcement bars together (Saariand Wickstr .oom, 1978). The back is fully flexed forwards,the knees usually over-stretched and the neck extendedat times (Fig. 1). Tying rods is experienced as one of themost tiring tasks. During the tying the external load isrelatively small, while other tasks such as cutting andbending the bars involve higher loads.The working environment in construction work has

been analysed in several studies, many with focus on theback. A correlation between low-back pain, injury andconstruction work has been reported by e.g. Riihim.aakiet al. (1990) and Nurminen (1997), but has not been

found by others (e. g. Wiikeri et al., 1978; Rothenbacheret al., 1997). In rodmen working the back is not the onlystructure, however, exposed to loads at the end of itsrange of motion (ROM). Other structures may also beof interest to investigate. Statistics reported work-related musculoskeletal disorders (WRMD), i.e. occu-pational diseases caused by overloading, show thatWRMDs are 3.4 times more frequent for rodmen thanfor a mean of all occupations (SCB, 1994). Rodmenhave higher injury rates especially in the back and hip-joint/legs (3.9 and 4.6 times respectively more commonamong them than the mean of all occupations).In other occupations also, e.g. nursing and industrial

assembly a large amount of the working time is spent infully flexed postures. In such postures the work is oftencarried out until the discomfort, fatigue or pain becomesunbearable and a pause becomes necessary. After apause, the work can be resumed.During the pause several recovery phenomena can be

detected. The term ‘‘recovery time’’ is defined differentlyby different researchers. For general fatigue Rohmert(1960a) defined it as the time during periodic work whenthe rise per minute in pulse frequency was below 0.1 fora certain load and holding time. Rohmert presented anequation for the recovery addition in percent of theloading time. Glimsk.aar et al. (1987) used a definition ofrecovery time based on mean power frequency of

*Corresponding author. Tel.: +46-8-790-6170; fax: +46-8-108-377.

E-mail address: [email protected] (L. Rose).

0003-6870/00/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 0 3 - 6 8 7 0 ( 0 1 ) 0 0 0 1 6 - 3

electromyographic (EMG) signals in muscles. In thepresent study the term ‘‘resumption time’’, Tres, (Roseet al., 2000, 2001) has been chosen to avoid confusionwith different recovery time definitions. It describes anaspect of recovery and is defined as the time it takes untilthe discomfort/pain level has decreased to a level atwhich the participant is willing to start working again,i.e. resume the task.In fully flexed postures passive structures are believed

to carry the load in the back while the back muscles arepassively loaded. In most studies of the lumbar backmuscles in such postures no EMG activity has beendetected (e.g. Floyd and Silver, 1955; McGill andKippers, 1994), although contradictory results havebeen presented by Andersson (1996). She found that thedeeply lying erector spinae and quadratus lumborummuscles were active in the back while the superficialerector spinae was almost silent. McGill (1997) sug-gested that rodmen load the posterior passive tissueswhen working in these postures. High tensile forces arethought to persist in ligaments, fascia and collagenoustissue in the back muscles in these postures (Dolan et al.,1994). Several models have been developed to estimatephysical strain in different work situations (Rohmert,

1960a; Hagberg, 1981; Sato et al., 1984; Waters et al.,1993; Dul et al., 1994) and they are mainly based onloading situations where the muscles are contractedactively. When analysing work in fully flexed postures,where the back may be passively loaded, questionsabout the postures’ influence on the loading situation,the working ability, endurance and recovery arise. Ergo-Index (Rose, 1992), is a model where a coupling betweenphysical load, pause need and production time is made.In our work with refining the Ergo-Index model, oneimportant question is whether work in such postures canbe incorporated in a general endurance and resumptionmodel or needs to be dealt with separately. Thereforethe present study was carried through. In the presentstudy, endurance time, Tend is defined as the time untilthe participant asked for the load to be removed.Another topic of interest is why some workers become

injured but not others. In many countries, companiesuse worker selection in attempts to decrease the risk oftheir staff developing injuries. In the constructionindustry there seems to be a natural selection. Manyconstruction workers suffer from work-related injuriesand many quit the trade quite young. The risks ofinjuries seem to increase with long-term exposure (years)but also persons who are young and unused to the workhave higher risks of injury. This raises the question ofwhether there are major differences between personsprofessionally used to a working task and those not.

1.1. Aims

The aims of this study were to investigate, (i)endurance and resumption times for fully flexedpostures with low external loads, (ii) which structureslimit the endurance time in such postures and (iii)whether there are differences as to endurance andresumption times between experienced workers andunskilled persons.

2. Materials and methods

Thirteen men, without experience of work in fullyflexed postures, mean age 32.5 years (S.D. 10.3), werechosen among healthy hospital staff, hereafter calledunskilled participants. For the sake of comparison,thirteen rodmen, mean age 39.6 years (S.D.15.9 years)with a mean of 16.6 years (S.D. 10.8 years), in the jobwere also studied. The rodmen were used to fully flexedpostures. All 26 participants, all without current pain ordisorder from the back, gave their informed consent.Two different tasks were performed by the unskilled

participants while the rodmen participated in one ofthem. In both cases the instruction was to carry on untilendurance time, Tend. Tend and the following resumptiontime, Tres; were recorded. One task was to move nails on

Fig. 1. Tying reinforcement bars at foot level is a common work task

for rodmen.

L. Rose et al. / Applied Ergonomics 32 (2001) 501–508502

a plate with holes, positioned on the floor and this wasdone by all participants. Here the intention was tosimulate the tying of reinforcement bars. A seconds taskwas to hold a 10 kg weight with both hands in theposture given and this was carried out solely by theunskilled participants. On average this load corre-sponded to 12% of the participants maximum liftingstrength (maximum voluntary capacity, MVC) in thatposture. The intention here was to simulate handling,cutting and bending bars. The posture was as fully flexedas possible, and with the back as relaxed as possible.Fig. 2 illustrates the two postures. To measure MVC inthe fully flexed posture a handle, connected to a straingauge and fastened in the floor, was pulled during two3 s max contractions with a minute’s pause betweenthem. The strain gauge output was amplified anddisplayed digitally with an average value every 1 s. Thehighest value was used as MVC value. The MVC was inmean 874N (S.D. 193). During loading the participantswere asked to estimate intensity of discomfort/painevery 15th second using Borg’s CR-10 scale (Borg,1982). The participants were asked to state where theysensed the main discomfort/pain originated. Afterunloading, Borg’s CR-10 scale, was used every 30thsecond until the participant stated he would have startedworking again.Torques on the major joints were calculated with the

computer manikin program for biomechanical model-ling, JACK (2.2, Engineering Animation Inc, 1999).This was done to relate the present torques caused bythe external force and the body segment’s weight to themaximum joint torque strength. The tying simulationwith approximately 1N external load corresponded to22% of this maximum. For the 10 kg loading thecorresponding values were 98N and 34% of max. Theselevels are used when comparing the endurance timesfrom the present study with results gained from otherstudies (see Fig. 5).Eight of the unskilled participants carried out

the experiments on different occasions to study

intra-individual variation. These eight also did endur-ance tests with one repetition, where a second loadinguntil endurance time after a 3min pause was carried out.The intention was to see whether the endurance hadbeen regained after the pause.Each participant was introduced to the procedure

with an introductory test. In total 92 endurance testswere carried out, 13 by the rodmen, and 79 by theunskilled participants.Statistical analysis was performed using a statistical

program (Axum 6.0, MathSoft, 1999). Differences inresults between tests were analysed with non-parametrictests (Mann–Whitney U-test and Wilcoxon signed-ranktest), with two-sided statistical hypotheses. The signifi-cance level in the study was chosen to be p50:05.

3. Results

3.1. Work in fully flexed postures}unskilled participants

The endurance times and resumption times in thedifferent cases are shown in Table 1. In the cases withtwo loadings until Tend with a 3min pause between theloadings the second endurance time was significantlyshorter than the first: 35% shorter for the tyingsimulation and 13% shorter for the 10 kg loading case.For the tying simulation the resumption time was 34%longer (not significant) after the second endurance testthan after the single endurance test. For the 10 kg loadthe second resumption time was significantly (37%)longer. Ratings of discomfort/pain during and after theloadings for the repetitive endurance test of tyingsimulation are shown in Fig. 3. For the 10 kg the ratingpattern was similar. Starting at 0-level, discomfort/painincreased until the participants terminated the loading atendurance time. During the pause the discomfortdecreased, while it increased again during the secondloading and thereafter decreased to a level where theparticipants stated that they were ready to resume thetask. Pain increased faster during the second loading.All thirteen rodmen stated that pain from thighs and

calves limited their working ability, as did ten of thethirteen unskilled participants. Three of the latter gavepain from the back as limiting. In 66 of the 79experiments that the unskilled participants carried out,limiting pain was sensed from the legs (mainly from thethighs). For the other 13, pain sensed from the backlimited the capacity.The rodmen had significantly longer endurance times

(67%) and significantly shorter (83%) resumption thanthe unskilled participants (Table 1). In Fig. 4 the painratings, endurance times and resumption times for theunskilled participants and the rodmen, respectively, areshown.

Fig. 2. The experimental postures: (a) tying simulation, (b) 10 kg

loading.

L. Rose et al. / Applied Ergonomics 32 (2001) 501–508 503

The unskilled participants endurance times variedbetween different days, intra-individual variation aver-aging 37%. In most cases (86%) the participantsreported the same structure as limiting the ability tocontinue the task when the same experiment was carriedout on different days.

4. Discussion

4.1. Endurance and resumption times

The endurance and resumption times in fully flexedpostures were studied to see if such postures need to be

dealt with separately from other postures or if they canbe incorporated in a general model based on morecommon working postures. The mean endurance timewas around 6.5min for the unskilled participants and10.75min for the rodmen. Although the external loadswere low, 1N and 98N, they led to considerable strainlevels (22% and 34% respectively) because of thepostures and the weights of the body parts. The factthat the postures we were studying were extreme mayhave affected our normalisation, where the maximumtorque was calculated with JACK. Normalisation is adifficult concept since it is difficult to determine themaximum strain level. MVC, for example, depends onphysiological factors as well as on motivation. For fully

Table 1

Endurance and resumption times for the unskilled participants (tying simulation and the 10 kg loading) and the rodmen (just the tying simulation)

Unskilled participants Rodmen

Endurance time

(min)

Resumption time

(min)

Endurance time

(min)

Resumption time

(min)

Tying simulation

Single loading: mean value (S.D.)(n ¼ 13þ 13) 6.26 (4.52) 2.66 (0.61) 10.45 (5.66) 2.21 (1.45)

Loading with one repetition: (just unskilled participants)

Loading 1: mean value (S.D) (n ¼ 8) 7.30 (5.99) 3.0a

Loading 2: mean value (S.D.) (n ¼ 8) 4.75 (3.81) 3.82 (2.03)

10 kg loading: (just unskilled participants)

Single loading: mean value (S.D.) (n ¼ 8) 6.38 (4.02) 2.69 (0.65)

Loading with one repetition:

Loading 1: mean value (S.D.) (n ¼ 8) 6.20 (3.64) 3.0a

Loading 2: mean value (S.D.) (n ¼ 8) 5.24 (3.37) 3.69 (1.87)

aA pause of 3min was enforced.

Fig. 3. Borg CR-10 scale rating for estimation of discomfort/pain during loading until endurance time and the following recovery period until

resumption time for tying-simulation. Mean values and ranges (minimum and maximum values) for 8 unskilled participants.


flexed postures it is preferable to analyse normalisedjoint torque rather than normalised force (%MVC),since the contribution from the body segments’ weightcannot be neglected in such postures. In Fig. 5 thepresent results are plotted together with those frommodels developed for situations where muscles are

actively contracted (Rohmert, 1960b; Hagberg, 1981;and Sato et al., 1984), as well as one developed forpassive loading of the elbow (Rose et al., 2000). It can behypothesised that endurance time for the awkwardpostures would be shorter than for more ‘‘normal’’postures. On the other hand, if back and knee structures

Fig. 4. Borg CR-10 scale rating for estimation of discomfort/pain during loading until endurance time and the following recovery period until

resumption time for tying-simulation. (a) for the rodmen (n ¼ 8) and (b) for the unskilled participants (n ¼ 8). Mean values and ranges (minimum

and maximum values).


are passively loaded, this might lengthen the endurancetime compared to more ‘‘normal’’ loading situations.However, the results show small differences from‘‘normal’’. This is somewhat surprising. Rose et al.(2000) found the endurance time to be shorter forpassively loaded elbows than the times given by modelsdeveloped for active muscle loading. One explanation ofthe present results may be that in most cases pain fromthe leg muscles was stated as limiting, and therefore theendurance we found is in fact quite similar to morecommon cases.Tend and Tres change negligibly when the strain level

varies between 22% and 34%. In these postures thestrain due to the weight of the body parts accounts forabout 20% of the maximum strain. The posture, not therelatively small external load, may be the importantfactor in these cases and this may partly explain thesmall variation in the endurance times.In the tests with one repetition, the participants stated

that they had recovered after the obligatory 3-minpause. Yet, during the second loading pain increasedmore rapidly and the second endurance time was shorterthan the first one, indicating that recovery is notcomplete in all aspects after a 3-min pause. This mayin the long run increase the risk of injury. Whendeveloping prediction models for job analysis such risksmay be overlooked.

4.2. Origin of limiting pain, risks in fully flexed postures

In the vast majority of the tests, pain from the legsand not from the low-back limited the working ability.

This is interesting and surprising and adds to ourknowledge of pain reactions in fully flexed postures. Themain focus when studying such postures has been on thelumbar back and the common approach has been toinvestigate pain and injury in the low-back and, in somecases, in the knee.Compression and shear forces on the vertebral discs

are not sensed like muscle strain, and for non-musclestructures in the spine pain may not function as awarning signal. Unlike the response from musclesexposed to heavy strain, prolapsation of intervertebrallumbar discs for example is not preceded by a warningsensation. Rose et al. (2000) found that pain provokedfrom passively-loaded, fully-extended elbows was sensedmainly originating from muscle tissue and not the jointitself. Actively loaded muscles and passively loadedtissues in the legs as well as in the back may be damagedby loading situations such as studied in this paper.However, the fact that pain is sensed mainly from thelegs does not necessarily imply that the legs have ahigher risk of injury than the back. It is important todistinguish between immediate reactions such as painand long-term effects such as chronic pain and injury.However, although WRMD due to overloading of theback is 1.6 times more frequent than that due tooverloading of the leg and hip among rodmen, rodmenhave 4.5 times more frequent WRMDs due to over-loading hip joint and leg than a mean of all occupations(SCB, 1994). The corresponding figures for WRMDsdue to overloading of the back and neck are 3.9 and 2.7respectively. Our finding that in most cases limiting painwas sensed from the legs is in line with this. General

Fig. 5. The relation between normalised load and endurance time Tend, obtained from the present study (,) and obtained according to Rohmert

(1960b) (n). Hagberg, 1981 (&), Sato et al. (1984) (}) and Rose et al. (2000) (*), respectively.


connections between low-back problems and work havebeen studied intensively. This has also been the case forwork in the fully flexed postures described here andwhich are common for rodmen. However, the presentfindings indicate that leg problems also warrant atten-tion. Further studies of problems from the legs and hipsin occupations where fully flexed postures are commonare of interest.

4.3. Differences between the two groups regardingendurance and resumption times

The rodmen, who had years of practice from posturessimilar to the experimental posture, had 67% longerendurance time than the unskilled participants.Although the rodmen spent considerably longer timein the working posture in the view of their longerendurance, their resumption time was 17% shorter thanthe unskilled participants. One explanation may be thatrodmen are trained in their work and that this mayincrease their endurance time. Grant et al. (1996) foundsignificant differences in endurance between elite rockclimbers and non-climbers during specific tasks essentialin climbing. Similar results have been found by otherresearchers (Ferguson and Brown, 1997). Thus, thereseems to be a training effect regarding endurance timesand it is hypothesised that this also decreases theresumption time. Another explanation may be thatreinforcement work is physically demanding and thisleads to a selection of workers from the beginning, asdiscussed by Wickstr .oom et al. (1978). Individuals whodevelop health problems and cannot cope with the highphysical demands leave the occupation, leading to asecond selection. Those who continue are thus indivi-duals who can stand the physical demands and they‘‘train’’ in fully flexed postures during their work.When general models based on studies with partici-

pants unused to the working task and posture are usedto estimate effects of loads in real life applications, theresults may be misleading. Thus knowledge of thedifferences between experienced and in-experiencedworkers should be considered when modelling andusing models in real life.

5. Conclusions

* Endurance times in relation to normalised total loadin fully flexed postures differ little from those in morecommon postures. The results indicate that fullyflexed postures may be assessed by more generalprediction models for endurance.

* Pain was mainly sensed to originate from the thighmuscles (and not from the spinal column or thetissues around it) and limited the endurance for allrodmen and for 84% of the unskilled participants.

* There are considerable differences between enduranceand resumption times for skilled and non-skilledworkers in the postures studied. Skilled workers havelonger endurance and shorter resumption times.

Acknowledgements

The authors wish to thank Arbetsmilj .oofonden (theSwedish Work Environment Fund) and Statens R(aad f .oorByggnadsforskning (the Swedish Council for BuildingResearch), for financing this study. Thanks also toAssociate Professor Bengt Nordgren and Mr. MichaelMattson, Department of Rehabilitation Medicine, Uni-versity Hospital, Uppsala for their valuable contributionto this study.

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Endurance, pain and resumption in fully flexed postures