ORIGINAL ARTICLE

Pascal Madeleine á Birthe LundagerMichael Voigt á Lars Arendt-Nielsen

Shoulder muscle co-ordination during chronic and acuteexperimental neck-shoulder pain. An occupational pain study

Accepted: 10 June 1998

Abstract Little is known about the mechanisms leadingto chronic neck-shoulder musculoskeletal disorders(MSD). The aim of the present study was to investigateand compare motor function during controlled, lowload, repetitive work together with chronic or acuteexperimental neck-shoulder pain. The clinical study wasperformed on workers with (n � 12) and without(n � 6) chronic neck-shoulder pain. In the experimentalstudy, experimental muscle pain was induced in healthysubjects by intra-muscular injection of hypertonic salineinto the trapezius muscle (n � 10). The assessed pa-rameters related to motor performance were: work taskevent duration, cutting forces, surface electromyogram(EMG) activity in four shoulder muscles, displacementof the centre of pressure, and arm and trunk 3Dmovements. For controlled cutting force levels, chronicand acute experimental pain provoked a series ofchanges: a decreased working rhythm and a protectivereorganisation of muscle synergy (experimental study),higher EMG frequency contents which may indicatealtered motor unit recruitment, and greater posturalactivity and a tendency towards increased arm and trunkmovements. These pain-related changes can play a rolein the development of MSD. The present clinical andexperimental study demonstrated similar interactionsbetween motor co-ordination and neck-shoulder pain inoccupational settings. We therefore suggest that thisexperimental model can be used to study mechanismsrelated to MSD. Information on such modulatory pro-cesses may help in the design of new strategies aimed atreducing the development of MSD.

Key words Low load, repetitive movement á Chronicand acute experimental neck-shoulder pain á Sensory-motor interaction á Shoulder muscle co-ordination

Introduction

Muscle pain originating from the neck-shoulder regionis a frequent complaint in many work-related disorders.A recent survey has shown that more than a quarter ofthe total workforce reported painful or tiring positions(for at least half of the working time) and continuouslycarried out short repetitive tasks at work (First Euro-pean Survey on the Work Environment 1991±92 1992).The socio-economic impact of musculoskeletal disorders(MSD) is enormous and prevention of MSD has becomea high priority.

Repetitive arm movements, heavy work, insu�cientrest and a static posture (Sommerich et al. 1993) to-gether with mental stress (Sjùgaard et al. 1995) havebeen shown to be risk factors for the development ofshoulder girdle MSD. Ergonomic e�orts to decrease theoccurrence of MSD are not always suitable. For lowload, repetitive work, the physical risk factors have beenfound to be predominantly the repetitiveness and theworking posture (Sommerich et al. 1993).

The MSD are accompanied by pain located aroundcartilage, tendons, ligaments and muscles. Even if theknowledge regarding the transduction, transmission andprojection of deep pain has increased (Mense 1993), thepatho-mechanisms governing the transition from acuteto chronic muscle pain are not well-established. It hasbeen reported that experimental muscle pain can beelicited by means of intra-muscular injection of hyper-tonic saline (Arendt-Nielsen et al. 1995; Svensson et al.1995; Graven-Nielsen et al. 1997; Madeleine et al. 1998).E�orts have been made to obtain further information onthe in¯uence of muscle pain on the motor system. Gait(Arendt-Nielsen et al. 1995; Graven-Nielsen et al. 1997)and masticatory processes (Svensson et al. 1995) havebeen investigated during experimental or chronic muscle

Eur J Appl Physiol (1999) 79: 127 ± 140 Ó Springer-Verlag 1999

P. Madeleine (&) á M. Voigt á L. Arendt-NielsenCentre for Sensory-Motor Interaction (SMI),Aalborg University, Fredrik Bajers Vej 7 Bldg D-3,DK-9220 Aalborg East, Denmark

B. LundagerClinic of Occupational Medicine,Aalborg Regional Hospital, P.O. Box 561,DK-9100 Aalborg, Denmark

pain. These studies have reported similar functional re-organisations of muscle activities during muscle pain.

One of the ®rst theories developed to explain musclepain has suggested ischaemia as important in eliciting avicious cycle leading to muscle hyperactivity (Travellet al. 1942). Later on the pain adaptation model hasbeen developed to explain the interaction betweenmuscle pain and motor activity. It has been reportedthat during muscle pain, the model predicts decreasedelectromyograph (EMG) activity of the agonist muscle,increased EMG activity of the antagonist muscle, lesspowerful and slower movements (Lund et al. 1991). Allthese e�ects act as active protection of the painfulmuscle. Another hypothesis has been established re-garding the genesis and spread of muscular tension inoccupational muscle pain and in chronic musculoskele-tal pain syndromes. When chemosensitive musclea�erents activate the c-system, it has been found thatthis will increase muscle sti�ness and constitute a posi-tive feedback that can explain the propagation of painfrom one muscle to others (Johansson and Sojka 1991).Recent studies have supported this hypothesis(DjupsjoÈ backa et al. 1994; Pedersen et al. 1997) althoughanother study has shown an inhibitory e�ect of, forexample, myositis on motoneuron activity (Mense andSkeppar 1991).

There is a lack of data regarding the interactionbetween chronic muscle pain, experimental muscle painand motor control in occupational situations, forexample low load, repetitive work. Such studies can beused to evaluate whether experimentally inducedmuscle pain is a relevant tool in occupational settings.Furthermore, they may be valuable for gainingnew knowledge which can be applied to the design ofnew strategies aimed at reducing the occurrence ofMSD.

The aim of the present clinical and experimentalstudy was to measure and compare the e�ects of chronicand acute experimental neck-shoulder pain on motorperformance during standardised, low load, repetitivework tasks.

Methods

Protocol, patients and subjects

Clinical and experimental studies were conducted. The clinicalstudy included a patient group and a control group while the ex-perimental study involved healthy subjects. The studies were ap-proved by the local Ethics Committee, and informed consent wasobtained from all participants. The studies were conducted inconformity with the Declaration of Helsinki.

Only motor recordings (Table 1) were performed in the clinicalstudy. In this experiment 18 male industrial butchers participated.The inclusion criteria (for the patient group) were neck-shoulderpain for a minimal period of 3 months within the last 12 months,no systemic illness, and a minimal score of three on a visual ana-logue scale (VAS) while performing daily work; 0 corresponded tono pain, and 10 to intolerable pain. After interviews and clinicalexaminations of the workers, it was decided that 12 of them shouldconstitute the patient group [mean age 47.4 (SEM 1.84) years] withneck-shoulder musculo-skeletal pain. The 6 remaining workers[mean age 43.8 (SEM 2.75) years] participated as a control group.The purpose of the measurement was to study the interaction be-tween chronic muscle pain and motor control while performingstandardised low load, repetitive work. After a practice period,motor recordings were performed (Table 1).

Sensory and motor recordings (Table 1) were carried out in theexperimental study. In the sensory part, 10 healthy male subjectsparticipated [mean age 28 (SEM 1.4) years]. The volunteers were ingood health and had no history of injuries or neck-shoulder pain.The purpose of the measurements was to obtain information aboutthe pain intensity in order to adjust motor changes to muscle painintensity (during pain measurement part of the experiment). Acuteexperimental neck-shoulder pain was elicited by means of hyper-tonic saline injection in the right trapezius muscle (n � 10).

In the motor part of the experimental study, 10 healthy malesubjects participated [mean age 25.6 (SEM 0.82) years]. Goodhealth and no history of injuries or neck-shoulder pain were re-quired. The aim of the measurements was to study the interactionbetween experimental neck-shoulder pain and motor control whileperforming controlled low load, repetitive work. After a practiceperiod, motor recordings were performed (Table 1). Hypertonicsaline was injected in the right trapezius muscle (n � 10).

Work task

By investigating real work situations in the ®sh and poultry in-dustries, a work task was designed to simulate these work processesin the laboratory. A workbench (Fig. 1a) and a repetitive, time-

Table 1 Schedule of the para-meters recorded in the clinicaland experimental study

Clinical study Experimental study

Subjects 18 workers 20 volunteers12 patients 10 for the sensory recordings6 control subjects 10 for the motor recordings

Interview Yes YesClinical examination Yes YesWork simulation Yes YesHypertonic saline injection No YesSensory recordingsPain intensity No Yes

Motor recordings 3 times 3 min 3 min before pain5 min during pain

Events timing Yes YesCutting force Yes YesElectromyogram Yes YesKinetics Yes YesKinematics Yes Yes

128

paced work scheme was de®ned and a knife, mounted with trans-ducers enabling force measurements, was constructed (Fig. 1b).The simulated cycle consisted of ®ve events during which thesubject should:

1. Take a ®llet from ``conveyor belt 1'', simulated by pressing abutton with the left hand (LH1). The ``conveyor belt 1'' wassituated 875-mm above ¯oor level.

2. Place the ®llet on the workbench (910-mm above ¯oor level).3. Cut the ®llet once in a diagonal direction, backwards from left

to right while applying a force of approximately 30 N along a100-mm long slot (CUT1; Fig. 2a).

4. Cut the ®llet a second time in a parallel direction with thefrontal plane from left to right while applying a force of ap-proximately 30 N along a 100-mm long slot (CUT2; Fig. 2a).

5. Put the ®llet on the ``conveyor belt 2'', simulated by pressing abutton with the left hand (LH2). The ``conveyor belt 2'' wassituated 1245-mm above ¯oor level.

Every second a tone generator was triggered to standardise thesubject's working rhythm allowing events 1±5 to take 1 s each. Thework task rhythm therefore constituted 12 cycles a minute.

Sensory recordings and analysis (experimental neck-shoulderpain)

To obtain cutaneous analgesia, Emla creme (Astra Denmark A/S,Albertslund, Denmark) was applied to the site 1 h before injectionto avoid activation of cutaneous nociceptors by the hypertonicsaline. Intra-muscular injections of 0.75-ml sterile hypertonic saline(6%) were performed only on healthy subjects, and the bolus wasinjected over 15 s. A 27G ´ 1-� cannula was inserted into the

muscle belly (1.5±2 cm) of the right trapezius pars descendensmuscle (2-cm laterally from the mid-distance between cervicalvertebra C7 and acromion). Immediately after the bolus injection,the subject began the work task (predominantly using the righthand, see work task part) and rating the pain intensity (with the lefthand) on an electronic VAS (sampling frequency: 12 Hz) for 5 min.The electronic VAS (Aalborg University, Aalborg, Denmark) is adevice composed of 20 light emitting diodes controlled by a slidingpotentiometer: 0 corresponding to no pain and 10 to intolerablepain.

The pain pro®les were averaged and the mean pain pro®le wasused to quantify the intensity of the experimentally induced pain. Asegmentation technique was used to divide the mean pain pro®lesinto four parts (Fig. 3). Changes in the slopes were used to detectvariations in pain intensity and to segment the pro®les into periodst1 (period with increasing pain intensity) and t2 (period with a highstable pain intensity). The slopes of the periods t3 (period with highpain intensity, pain intensity > 4) and t4 (period with moderate

Fig. 1 The subject, standing on a force platform (kinetic recording)with surface electrodes (electromyogram recordings from fourshoulder muscles) and re¯ective markers (kinematics recordings ofthe arm and the trunk) while performing low load, repetitive work(knife in the right hand) (a) and full bridge strain gauge knife (b)

Fig. 2 Cutting direction of CUT1 and CUT2 seen from above (a) anda typical work cycle with the di�erent events LH1, CUT1, CUT2,LH2 (with their respective durations tLH1, tCUT1, tCUT2 and tLH2, LHleft hand), inter-events durations (tLH1CUT1, tCUT1CUT2, tCUT2LH2 andtLH2LH1) and total duration tCyc (b)

Fig. 3 Mean pain intensity pro®les (n � 10,Ð), interpolated painpro®les (- - -) and weighting coe�cients in parentheses for periods t1,t2, t3 and t4 (experimental study: right trapezius muscle hypertonicsaline injection)

129

pain intensity, pain intensity < 4) were almost identical, but thetime interval was divided into two equal parts to take into accountthe decrease in pain intensity. Weighting coe�cients were com-puted as the mean pain intensity of each period divided by themean pain intensity of period t2. The weighting coe�cients (Wc) ofthe periods t2 and t3 (Wc;t2 and Wc;t3 ) were used to adjust all themeasured motor variables (MMV) during experimental musclepain, as follows:

MMV �during pain� � Wc;t2MMVt2 � Wc;t3MMVt3

Wc;t2 � Wc;t3; �1�

The MMV were: events timing, EMG, kinetics and kinematicsdata.

Motor recordings and analysis (chronic and experimentalneck-shoulder pain)

LH1 and LH2 event recordings

Force sensing resistor devices (Toptronic, Echternach, Luxem-bourg) were used to record the onset and o�set of LH1 and LH2events. These were sampled at 0.5 kHz. A light touch on the sensorresulted in clearly detected signals (Fig. 2b). The duration of theevents LH1 (tLH1), CUT1 (tCUT1), CUT2 (tCUT2), LH2 (tLH2) andthe duration of the inter-events LH1-CUT1 (tLH1CUT1), CUT1-CUT2 (tCUT1CUT2), CUT2-LH2 (tCUT2LH2) and LH2-LH1(tLH2LH1) were computed for each cycle. The total duration of eachcycle (tcyc) was also calculated. The time of onset and of o�set (tonand to�) of each event were obtained using a level detection tech-nique. To average the acquired data, a re-sampling processingtechnique was used and de®ned as:

1. A period of 3-minutes recordings (clinical study and beforepain part of the experimental study): theoretically the 3-min re-cording led to 36 cycles. Mean values of the duration of each eventand inter-event (tLH1, tLH1CUT1, tCUT1, tCUT1CUT2, tCUT2, tCUT2LH2,tLH2, tLH2LH1) were computed for the ®rst 12 cycles. The durationof the mean event and the duration of the mean inter-event werethen used to re-sample each event or inter-event to the duration ofthe mean event or inter-event. This meant that the ®rst 12 cycleswere of the same length and could be averaged with a minimal lossof information. The same procedure was used for the 12 followingcycles and for the last 12 cycles.

2. A period of 5-min recordings (during pain part of the ex-perimental study): the 5-min recordings were divided into fourparts corresponding to the periods t1, t2, t3, and t4 (Fig. 3). Theduration of the mean event and the duration of the mean inter-event were calculated for these periods and used to re-sample eachevent or inter-event to the duration of the mean event or inter-event. The weighting coe�cients of periods t2 and t3 were used toadjust the motor activity to the muscle pain intensity [see Methods,Sensory recordings and analysis, Eq. (1)].

Cutting force recordings

A full bridge strain-gauge knife was constructed and calibrated(Fig. 1b) to estimate the forces applied (FCUT1 and FCUT2) duringthe two cutting phases as well as ton and to� of CUT1 and CUT2(Fig. 2b). The sampling frequency was 0.5 kHz. Detection of themaximal force was computed for CUT1 and CUT2 and used tonormalise the EMG activity.

EMG recordings

Bipolar EMG surface electrodes (Medicotest N-10-E, élstykke,Denmark) were aligned vertically (2-cm apart) on abraded ethanol-cleaned skin along the right deltoideus anterior (halfway between thelateral one-third of clavicula and the insertion of the deltoideusmuscle) and medius muscles (halfway between the acromion and theinsertion of the deltoideus medius muscle), right infraspinatus

muscle (infraspinous fossa, two ®nger breadths below the medialportion of the spine of the scapula) and right trapezius muscle (2-cmlaterally from the mid-distance between cervical vertebra C7 and theacromion). Pre-ampli®ers with a gain of 100 were attached to theelectrodes. In total the EMG signals were ampli®ed 2000 times,band-pass ®ltered at 10±400 Hz and sampled at 1 kHz. Analysis inboth time and frequency domains were carried out as follows:

1. Time domain analysisRoot mean square (rms) values were calculated for each cuttingevent.Force root mean square (Frms) corresponding to maximal cut-ting force normalised EMG activity was computed for eachcutting event [see Appendix, EMG part, Eq. (2)]Background rms (Brms) corresponding to the static backgroundEMG activity was calculated in a 300-ms window for each cy-cle. This 300-ms window was de®ned as the EMG activity from500 ms to 200 ms before CUT1 [see Appendix, EMG part,Eq. (3)]Resting events were calculated and de®ned as a period of EMGactivity longer than 0.2 s and below 0.5 of the recti®ed averagedEMG minima and maxima detected from the end of tCUT2 tothe beginning of tCUT1.

2. Frequency domain analysisMean power frequency (MPF) ± MPF values were calculatedfor each cutting event signal (see Kwatny et al. 1970). Thepower spectrum density of the EMG signal was estimated byusing Welch's averaged periodogram method (Welch 1967).The EMG signal was divided into overlapping sections, each ofwhich was detrended, then windowed (512 or 1024 samples),then zero-padded to length (1024 or 2048 samples).

Force platform recordings

A force platform (AMTI model OR6-5-2000, Watertown, USA)measured the ground reaction forces (in the lateral-medial, ante-rior-posterior and inferior-superior directions) and reaction mo-ments (around the sagittal and frontal axes of the force platform)applied by the standing subject (Fig. 1a). The force platform sig-nals were ampli®ed 2000 times, low-pass ®ltered at 10.5 Hz (2ndorder) and sampled at 0.5 kHz. The displacement in the frontal (x)and sagittal (y) planes of the centre of pressure (CoP) were com-puted (Winter 1990). The maximal amplitude of the CoP dis-placement in both planes was calculated as the di�erence betweenthe maximum and the minimum of the CoP displacement. After-wards, the CoP surface of displacement was computed as theproduct of the CoP amplitudes in the frontal and sagittal planes.

Kinematic recordings

A 3D motion analysis system (McRe¯ex, Qualysis A/S, Partille,Sweden) with a resolution of 1/30 000 of the ®eld of view was used.Recordings of the upper right arm and of the body movement werecarried out at a sampling frequency of 30 Hz. Three cameras wereused, and they acted as an infrared illuminator and detector ofpassive and re¯ective markers. The markers were tracked in 3D bydirect linear transformation. After the tracking procedure, the 3Dco-ordinates of each marker were low-pass ®ltered with a Butter-worth ®lter (4th order, cut-o� frequency 3 Hz). It has been shownthat relative motion between two body parts can be obtained byde®ning anatomical co-ordinate systems (AnCS) and markers co-ordinate systems (MaCS), (Lafortune et al. 1992). For this purpose,two complexes of three passive markers triplet were constructedand attached with Velcro on the back and around the upper rightarm, de®ning the trunk and arm MaCS (MaCStrunk and MaCSarm),(Figs. 1a, 4).

Preliminary recordings were performed for each experiment toobtain transformations between the MaCS, the trunk and the armanatomical co-ordinate systems (AnCStrunk and AnCSarm). For thetrunk measurement, a marker was attached to the skin at lumbar

130

vertebra L2 (origin for AnCStrunk). The subject was then placed inan upright position in the laboratory co-ordinate system (LaCS)with the axes of AnCStrunk aligned with the LaCS axes and wasasked to stand still for 1 s. For the arm, a marker was placed on theskin at the olecranon (origin for AnCSarm). Similarly, the subjectstood upright and immobile with the elbow ¯exed at 90° (axes ofthe AnCSarm aligned with the LaCS axes) for 1 s. Afterwards, theL2 and olecranon markers were removed. Therefore, only the twotriplets remained on the subject's body during work task simulationrecordings.

On each video frame, overall rotation transformation matricestransforming (1) co-ordinates in the AnCSarm to co-ordinates in theAnCStrunk and (2) co-ordinates in the AnCStrunk to co-ordinates inthe LaCS were obtained (see Appendix, Kinematics part). Therotation matrices of the arm in relation to the trunk (AnCStrunk)and trunk in relation to the LaCS were then calculated frame-to-frame and expressed as Euler angles (Kabada et al. 1990). In thisway, the movements of the upper arm were expressed as anatomical¯exion-extension, abduction-adduction and internal-external rota-tion. The movements of the trunk in the three ¯exion-extension,right-left lateral ¯exion and internal-external rotation directionswere described in relation to the LaCS. The right arm and trunkangle pro®les were re-sampled according to the procedure previ-ously explained. The position in which the subject started was ex-tracted in 3D for the right arm and the trunk. The total area underthe curve as well as the area under CUT1 and CUT2 were com-puted in 3D for both right arm and trunk movement.

Statistics

Mann-Whitney (test of the two groups, patient/control) and Wi-lcoxon signed rank (test of the before/during pain condition) tests,and two-way repeated measures ANOVA (for EMG recordings)with the Student-Newman-Keuls method for multiple comparisonswere used. A value of P < 0.05 was considered signi®cant.

Results

Sensory part (experimental neck-shoulder pain)

Pain pro®le

The mean pain pro®le was divided into four segments (0±30 s), (30±78 s), (78±186 s) and (186±300 s). The weight-ing coe�cients for t1, t2, t3 and t4 were 0.51 cm á min)1,1.00 cm á min)1, 0.79 cm á min)1 and 0.39 cm á min)1,respectively (Fig. 3).

Motor part (chronic and experimentalneck-shoulder pain)

Events timing

Chronic or experimental neck-shoulder pain caused adecrease of the general working rhythm. For the clinicalstudy, the duration of tLH1, tCUT1, tCUT2, tLH2 and tcycwas signi®cantly longer for the patient group than forthe control group. In the experiment study, signi®cantlylonger duration of tCUT1 and tcyc was observed duringpain (Table 2).

EMG recordings

Time and frequency domains analysis (Table 3, Fig. 5a,b, c, d, e, Fig. 6a, b) showed for the clinical study thatthere was a signi®cant di�erence among the MPFvalues of the muscles in the measurements of thepatient and the control group, and for the experimentalstudy, that there were signi®cant di�erences among therms, Frms, Brms, and MPF values of the muscles in themeasurements of the before pain and during pain. Therewere signi®cant interactions between the rms, Brms

Fig. 4 From arm and trunk triplet, determination of the arm andtrunk markers co-ordinate system (MaCSarm, MaCStrunk), of origin-arm and origin-trunk, of constant transformation matrices TMaT/AnT

(from MaCStrunk to trunk anatomical co-ordinate system, AnCStrunk)and TAnA/MaA (from arm anatomical co-ordinate system, AnCSarm, toMaCSarm), of frame-dependent transformation matrices TMaA/Lab

(fromMaCSarm to laboratory co-ordinate system LaCS) andTLab/MaT

(from LaCS to MaCStrunk). See Appendix, Kinematics section

Table 2 E�ects of chronic and experimental neck-shoulder pain on events duration, tLH1, tCUT1, tCUT2, tLH2, tCyc (LH left hand, Cyccycle) for the clinical study (patients n = 12, controls n = 6) and for the experimental study (n = 10), P level of signi®cance

Eventduration

Clinical study Experimental study

Controls Patients P Before pain During pain P

mean SEM mean SEM mean SEM mean SEM

tLH1 (s) 0.396 0.012 0.442 0.009 0.009 0.401 0.017 0.422 0.022 0.065tCUT1 (s) 0.725 0.016 0.815 0.031 0.025 0.720 0.028 0.812 0.048 0.006tCUT2 (s) 0.679 0.020 0.786 0.029 0.010 0.626 0.020 0.638 0.029 0.695tLH2 (s) 0.443 0.011 0.474 0.008 0.036 0.434 0.021 0.448 0.024 0.131tcyc (s) 4.927 0.046 5.205 0.077 0.001 5.030 0.006 5.040 0.009 0.014

131

Table 3 EMG results. Two-way ANOVA for the clinical study(patients n = 12, controls n = 6) and for the experimental study(n = 10). E�ects of group (patient/control), muscles ´ group,condition (before/during pain) and muscles ´ condition on the rootmean square (rms), force root mean square (Frms), background root

mean square (Brms), resting events and mean power frequency(MPF) data. F Variance ratio with degree of freedom, P level ofsigni®cance. See Fig. 5a, b, c, d, e for the results of pairwise com-parison with Student-Newman-Keuls compensation

Clinical study Experimental study

Group(patient/control)

Muscles ´ group Condition(before/during pain)

Muscles ´ condition

rms F1,16 = 0.31, P = 0.590 F3,48 = 0.42, P = 0.740 F1,9 = 9.10, P = 0.015 F3,27 = 3.40, P = 0.032Frms F1,16 = 0.01, P = 0.920 F3,48 = 0.40, P = 0.760 F1,9 = 5.40, P = 0.040 F3,27 = 2.40, P = 0.090Brms F1,16 = 0.51, P = 0.490 F3,48 = 1.14, P = 0.340 F1,9 = 9.60, P = 0.001 F3,27 = 2.40, P = 0.001Resting events F1,16 = 0.60, P = 0.450 F3,48 = 1.70, P = 0.180 F1,9 = 0.60, P = 0.470 F3,27 = 2.24, P = 0.110MPF F1,16 = 11.5, P = 0.004 F3,48 = 1.73, P = 0.170 F1,9 = 16.5, P = 0.003 F3,27 = 1.30, P = 0.300

Fig. 5 Root mean square (rms) amplitude (a) force rms (Frms)amplitude (b) background rms (Brms) amplitude (c) resting events (d)and mean power frequency (MPF) (e) of trapezius (Trap.),infraspinatus (Infr.), deltoideus anterior and deltoideus medius (Delt.Ant., Delt. Med.) muscles electromyogram activity (mean and SEM)

for the clinical study (h controls n � 6), j patients n � 12) andexperimental study (h before injection n � 10, j after injectionn � 10). *Signi®cant from either patient/control or before/afterinjection group values (pairwise comparison with Student-Newman-Keuls compensation, P < 0.05)

132

values of the muscles and of the before pain and duringpain measurements.

CoP displacement

For the clinical study, patients had a larger CoP surfaceof displacement [9623 (SEM 2029) mm2] compared tocontrols [5461 (SEM 1081) mm2, P � 0.046]. For theexperimental study, only a tendency towards an in-creased CoP surface of displacement during pain wasobserved.

Kinematics

Arm movement (Table 4) ± for the clinical study, pa-tients had signi®cantly larger starting position o�set,total area and areas under the cutting events CUT1 andCUT2 in the ¯exion-extension, abduction-adduction,

internal-external rotation directions (except the startingposition o�set in the internal-external rotation direction)when comparing the patient and control groups(Fig. 7a). For the experimental study, there was a sig-ni®cant di�erence between the areas under the cuttingperiods CUT1 and CUT2 (internal-external rotationdirection) measured before pain and during pain. Theother measured parameters were not in¯uenced signi®-cantly (Fig. 8a).

Trunk movement (Table 5) ± for the clinical study, themeasured parameters, signi®cantly in¯uenced by either be-longing to the patient or the control group, were the startingposition o�set, total area, and area under the cutting eventCUT1 in the ¯exion-extension direction. A signi®cant in-crease for the patient group compared to control group wasobserved for these parameters (Fig. 7b). For the experi-mental study, the starting position o�set and areas underthe cutting periods CUT1 and CUT2 increased signi®cantlyduring pain compared to before pain (¯exion-extension;Fig. 8b).

Fig. 6 Electromyogram pro-®les of the trapezius (Trap.),infraspinatus (Infr.), deltoideusanterior and deltoideus medius(Delt. Ant, Delt. Med.) muscles(mean and SEM) for patients(n � 12, ±) and controls(n � 6, ±) (a) and for subjects(n � 10) before (±) and after(±) hypertonic saline injection inright trapezius muscle (b). Av-eraged LH1, CUT1, CUT2 andLH2 (LH left hand) time events(patients and during pain d,controls and before pain r)

133

Discussion

The aim of this clinical and experimental study was tomeasure and compare the e�ects of chronic and acuteexperimental neck-shoulder pain on the motor perfor-mance while performing standardised low load, repeti-tive work. The modulation of controlled low load,repetitive work by neck-shoulder pain was manifested as

1. A decreased working rhythm2. Reorganisation of muscle activation patterns, that is

decreased EMG activity in the injected muscle3. Shift towards higher frequencies of the EMG spec-

trum contents4. A tendency to increased amplitude of arm movements.

Furthermore, an enhanced postural activity was ob-served. The ®ndings underlined interaction betweenneck-shoulder pain and motor control.

E�ects of chronic and acute experimentalneck-shoulder pain on the work task rhythm

Numerous studies of low load, repetitive work in labo-ratory settings and studies based on method time mea-surement have been performed before (Giroux andLamontagne 1992; Sundelin and Hagberg 1992; Math-iassen and Winkel 1996; Westgaard and Winkel 1997).However, to our knowledge, no previous studies haveinvestigated the e�ect of acute experimental and chronicmuscle pain on motor control in an occupational settingwhere events and cycle duration were analysed. Thesimulated work process could be de®ned as highly re-petitive (24 movements were performed with the rightarm every minute), the duration of exertion (proportionof work in the cycle) was rather high, 4 s out of 5. Fi-nally, a 1-s micro-pause was included in the cycle inbetween tLH1 and tCUT1.

Table 4 E�ects of chronic and experimental neck-shoulder pain on arm movements, starting position, total area, area under CUT1 andarea under CUT2 in 3D for the clinical study (patients n = 12, controls n = 6) and for the experimental study (n = 10). P Level ofsigni®cance

Clinical study Experimental study

Controls Patients P Before pain During pain P

mean SEM mean SEM mean SEM mean SEM

Starting position (o)Flexion-extensiondirection

)2.16 3.5 12.55 4.8 0.0001 4.71 5.9 2.63 4.9 0.1300

Abduction-adductiondirection

20.26 4.1 28.35 4.4 0.0270 11.27 7.1 11.46 5.9 0.2300

Rotationdirection

) 22.04 3.4 )25.89 4.1 0.3470 )12.57 7.6 )13.49 7.3 0.4300

Total area (os)

Flexion-extensiondirection

50.95 10.4 138.88 14.9 0.0001 57.21 24.2 49.55 18.5 0.0650

Abduction-adductiondirection

146.01 4.4 207.88 13.8 0.0061 91.58 31.1 86.01 25.8 0.7700

Rotationdirection

136.04 15.2 191.65 11.5 0.0058 124.98 20.1 120.78 25.9 0.3750

Area CUT1 (os)

Flexion-extensiondirection

8.91 2.0 24.29 2.4 0.0001 10.37 3.6 10.22 3.7 0.4320

Abduction-adductiondirection

18.80 1.0 29.94 2.5 0.0166 11.74 5.1 12.24 4.8 0.3750

Rotationdirection

24.74 2.0 36.80 2.3 0.0001 22.42 2.4 24.50 3.1 0.0270

Area CUT2 (os)

Flexion-extensiondirection

10.47 1.8 25.10 2.7 0.0001 7.01 2.8 5.68 1.8 0.1600

Abduction-adductiondirection

28.74 1.9 40.60 2.6 0.0036 22.00 3.0 21.48 3.0 1.0000

Rotationdirection

29.06 2.4 36.78 2.2 0.0740 24.33 2.8 22.54 2.7 0.0040

134

Repetitiveness has been found to be a physical riskfactor of MSD (Sommerich et al. 1993; Sjùgaard et al.1995). Under chronic and experimental neck-shoulderpain, event duration pointed in the same direction ± adecrease in the working rhythm (Table 2). Furthermore,another important observation was the prolonged du-ration for the patients' tLH1 and tLH2 events. This indi-cated that muscle pain not only a�ected the right side(side of the dominant hand), but also a�ected the con-tralateral side. Supraspinal and/or spinal mechanismsmay mediate this spread of action through, for examplere¯ex mediated pathways.

E�ects of chronic and acute experimentalneck-shoulder pain on dynamic EMG activity

Surface EMG is often used in various occupationalhealth studies. The analysis of the EMG signals rmsvalues, in the time domain did not show signi®cant re-sults when comparing the patients and control subjects

(Table 3, Figs. 5a, 6a). However, there was a signi®cante�ect of the experimental neck-shoulder pain on thelevel of activity of the muscles, that is the activity of thetrapezius muscle decreased during experimentally in-duced muscle pain (Table 3, Figs. 5a, 6b). The trapeziusmuscle activity has been shown to be not only related toarm position and work load (Mathiassen and Winkel1990) but analysing EMG activity without taking theworkload or the force into account has some drawbacks.Indeed, force is a well-known risk factor. A decreasedforce performance has been reported for patients suf-fering from either work-related myalgia or ®bromyalgia(Elert et al. 1991). Furthermore, a reduced musclestrength after experimental muscle pain has been ob-served (Graven-Nielsen et al. 1997).

In the present study, the subjects were asked to per-form a 30-N cutting force (low muscle activation level)in all the recording sessions during the cutting phases.This normalisation of the rms values was done to enablefurther EMG comparison between the patients andcontrol subjects' measurements as well as between

Fig. 7 Arm (a) and trunk (b)movement angles in 3D (meanand SEM) for patients (n � 12,± ) and controls (n � 6, ±).Averaged LH1, CUT1, CUT2and LH2 (LH left hand) events(patients d, controls r)

135

before and during experimental neck-shoulder painmeasurements (Table 3, Figs. 5b, 6a, b). No di�erenceswere observed between the data of the patients and ofthe control subjects. On the other hand, a decrease oc-curred in the normalised EMG activity in the trapeziusmuscle. At the same time, the normalised EMG activityin the infraspinatus muscle tended to increase. Thenormalised EMG activity in the deltoideus anterior andmedius muscles tended to decrease during experimentalneck-shoulder pain. These results suggested dynamicreorganisation of EMG activity during pain; and, moreprecisely, the development of a new synergy aiming atminimising the use of the painful muscle, at least duringthe experimental neck-shoulder pain.

E�ects of chronic and acute experimentalneck-shoulder pain on the background EMG activity

The computed background rms values (Table 3,Figs. 5c, 6a, b) did not correspond to the activity at rest,but to the static background EMG activity during a

non-active part of the work cycle. For the clinical studyno di�erences were observed between the patients andthe control subjects. But in the case where hypertonicsaline was injected in the trapezius muscle, the back-ground activity of the trapezius muscle decreased duringthe experimentally induced neck-shoulder pain. Again,this underlined the functional importance of the trap-ezius muscle, and a decreased EMG activity can also beseen as a protective adaptation. Opposite results havebeen reported for the EMG activity at rest. Most likely,this can be explained by the di�erence in the loadshandled and work task pace. No di�erence has beenfound when neck-shoulder pain patients were comparedto controls (Takala and Viikari-Juntura 1991). In con-trast, EMG activity has been found to increase in be-tween contractions in ®bromyalgia patients (Elert et al.1991). Furthermore, workers with previous neck-shoul-der pain episodes have a higher static EMG level com-pared to workers without complaints (Veiersted et al.1990). Similar results have been reported in patients withlow-back pain (Arendt-Nielsen et al. 1995), and it hasbeen suggested this inability to relax may lead to MSD.

Fig. 8 Arm (a) and trunk (b)movement angles in 3D (meanand SEM) for subjects(n � 10) before (±) and after(±) hypertonic saline injection inright trapezius muscle. Aver-aged LH1, CUT1, CUT2 andLH2 (LH left hand) events(before pain r, during pain d)

136

E�ects of chronic and acute experimentalneck-shoulder pain on the EMG resting event

Resting event analysis (Table 3, 5d) did not show anydi�erence between the patients and the control subjects'measurements as well as between the before and duringexperimental neck-shoulder pain measurements. Thismay have been due to the relatively high cadence of thestandardised work cycle that did not enable the subjectto relax inbetween LH1 and CUT1. The EMG mea-surements have been used to predict trapezius musclemyalgia, and the results have shown that patients havelower EMG gaps (period of at least 0.2 s of EMG ac-tivity below 0.5% of maximal EMG activity) than non-patients (Veiersted et al. 1990). However, Takala andViikari-Juntura (1991) and Jensen et al. (1993) havefound opposite results regarding short-rest period dif-ferences. Again, one possible explanation for these dif-

ferent results is the di�erence of the work load andrhythm. However, the validity and the use of gap anal-ysis as an indicator or predictor of MSD, when per-forming a low load task, are thus questionable.

E�ects of chronic and acute experimentalneck-shoulder pain on the EMG frequency contents

Chronic and acute experimental neck-shoulder painwere characterised by higher MPF values (Table 3,Fig. 5e). The factors in¯uencing EMG frequency con-tents are activation level, velocity of contraction, type ofcontraction, muscle length, muscle temperature, musclefatigue, action potential conduction velocity, ®ring rate,and synchronisation of active motor units (MU). Usu-ally, decreased MPF values have been used to indicatemuscle fatigue (Christensen 1986). However, at a low

Table 5 E�ects of chronic and experimental neck-shoulder pain on trunk movements, starting position, total area, area under CUT1 andarea under CUT2 in 3D for the clinical study (patients n = 12, controls n = 6) and for experimental study (n = 10). P level ofsigni®cance

Clinical study Experimental study

Controls Patients P Before pain During pain P

mean SEM mean SEM mean SEM mean SEM

Starting position (o)Flexion-extensiondirection

)21.21 3.2 )16.62 1.3 0.251 )19.12 0.6 )21.64 0.8 0.002

Lateral¯exiondirection

) 3.88 4.0 8.03 2.6 0.015 1.05 0.8 1.07 0.9 0.920

Rotationdirection

)16.95 2.1 )16.23 1.5 0.166 )19.85 0.9 )18.98 1.3 0.420

Total area (os)

Flexion-extensiondirection

48.48 6.1 52.50 4.0 0.480 52.84 2.9 62.25 5.1 0.065

Lateral¯exiondirection

37.68 20.1 48.07 11.9 0.034 17.98 2.4 17.63 2.8 0.375

Rotationdirection

55.80 15.1 45.98 3.5 0.920 43.84 5.0 39.47 3.0 0.080

Area CUT1 (os)

Flexion-extensiondirection

5.92 0.8 7.39 0.7 0.215 7.35 0.8 9.24 1.4 0.037

Lateral¯exiondirection

7.56 3.3 7.90 1.1 0.021 3.98 0.6 4.20 1.0 0.770

Rotationdirection

7.41 3.1 4.08 0.6 0.190 4.65 1.4 4.60 1.3 0.850

Area CUT2 (os)

Flexion-extensiondirection

5.93 0.8 9.09 1.0 0.160 6.94 0.7 8.36 1.1 0.037

Lateral¯exiondirection

7.30 3.3 7.12 1.3 0.776 3.51 0.6 3.40 0.7 0.630

Rotationdirection

7.24 3.3 6.60 0.7 0.101 3.53 1.0 3.91 0.8 0.630

137

level of contraction an increase in MPF has been seenduring isometric submaximal long-lasting contraction(Arendt-Nielsen et al. 1989) and for the upper part of thetrapezius muscle, this increase has been interpreted asindicating fatigue (Hagberg 1981). Braakhekke et al.(1989) have also reported increased MPF of EMG inpathological fatigue and explained higher MPF by therecruitment of type II MU.

In the present study, MPF has not been used as anindex of muscle fatigue because the muscle length andthe type of contraction were not really controlled.However, greater MPF were observed with muscle pain.It has been suggested that a shift towards high fre-quencies of the power density spectrum is most likelydue to additional recruitment of MU with higher muscle®bre conduction velocities (Arendt-Nielsen and al.1989). However, a lack of harmonious recruitment be-tween MU and of relaxation of the muscles might play arole in the development of MSD (see Edwards 1988).HaÈ gg (1991) has proposed a model explaining occupa-tional myalgia based on the fact that low threshold MU(type I) are initially recruited and remain active duringthe entire work load. At low level load this mechanismcan be very potent. Henriksson et al. (1993) have sug-gested that a reduction in the micro-circulation and ex-tended static work might be risk factors for theappearance of moth-eaten or ragged red type I ®bres. Inrecent neurophysiological studies, Westgaard (1996) hasobserved that some MU may be active during the entirecontraction period but may become silent after a breakand then active again after a while; Kadefors et al.(1996) have identi®ed the same active MU over a widerange of shoulder movements. Those results are of im-portance because the overuse of some MU could lead tosome degenerative processes and ®nally to MSD.

E�ects of chronic and acute experimentalneck-shoulder pain on the postural activityand arm movements

Repetitive arm movements and a static posture haveusually been associated with the development of neck-shoulder disorders (Sommerich et al. 1993; Sjùgaardet al. 1995). In the present study, the CoP surface ofdisplacement increased in workers su�ering fromchronic neck-shoulder pain and tended to increase underexperimentally induced muscle pain. Similar results havebeen reported in a study in which an investigation wascarried out on subjective, physiological and biome-chanical responses to prolonged standing on di�erentsurfaces while performing light, repetitive, manual work,that is increased postural activity (Madeleine et al.1998).

Regarding posture angles, measurements of the upperarm postural angle have been carried out to investigatepostural load in various occupational situations (AaraÊ set al. 1988; Giroux and Lamontagne 1992). However,the present study was the ®rst one to correlate recording

of 3D arm and trunk movements with chronic and ex-perimental neck-shoulder pain. During chronic neck-shoulder pain, a signi®cant increase in almost all thearm-measured factors was found (Table 4, Fig. 7a),whereas only the amplitude of the arm movement tendedto increase after hypertonic saline injection (Table 4,Fig. 8a). Considering the trunk movements (Table 5,Figs. 7b, 8b), tendencies towards increased amplitudesof the parameters analysed were observed during eitherchronic or experimental neck-shoulder pain. Increasedpostural activity and arm movements during chronicand acute experimental neck-shoulder pain may be im-portant phenomena giving positive feedback, that is in-creased motor activity that could lead to MSD viadegenerative processes.

Neurophysiological models

Lund et al. (1991) have proposed the pain adaptationmodel to explain the interaction between muscle painand motor control. During muscle pain the model pre-dicts decreased EMG activity of the agonist muscle,increased activity of the antagonist muscles as well asless powerful and slower movements. Several studieshave reported co-ordination changes in the EMG pat-tern, that is a decreased EMG activity of the agonistmuscle and increased EMG activity of the antagonistmuscle (Arendt-Nielsen et al. 1995; Svensson et al. 1995;Graven-Nielsen et al. 1997) and thereby supporting thepain adaptation model. The ®ndings of the presentstudy, namely, decreased working rhythm and protectivereorganisation of the EMG pattern (experimental study)were in accordance with that model.

The results of the CoP surface of displacement, armand trunk movements correlated. However, when welooked at the EMG activities of the four muscles mea-sured, the hyperactivity theories were not supported. So,how can the increased arm movements and posturalactivity be explained? According to Edwards (1988),increased muscle load could result in a higher accumu-lation of metabolites in the muscles. In animal studies,DjupsjoÈ backa et al. (1994) and Pedersen et al. (1997)have observed an activation of the nociceptive a�erentsby the metabolites, and this activity may increase thestatic fusimotor drive via re¯exes. According to Peder-sen et al. (1997), increased activity in the fusimotor re¯exloop would result in an increased activity of the a-mo-toneuron a�erents during contractions.

Our results regarding EMG recordings (experimentalstudy) contradict the hypothesis of Johansson and Sojka(1991). But EMG recordings were performed on onlyfour muscles, and there is no doubt that all the rotatorcu� region muscles were involved when various armmovements were performed. Thus, muscle synergy inrepetitive work is much more complicated than the onedescribed here. Arguments in favour of the model de-veloped by Johansson and Sojka (1991) are the highspindle density of neck-shoulder muscles that has been

138

reported by Voss (1956) as well as the increased move-ment amplitude and posture activity under muscle painindicating that proprioception seems to be a�ected bymuscle pain. Furthermore, the role of neck-shouldermuscles is of importance as long as they take an activepart in maintaining posture. Finally, metabolite releasedue to low-load repetitive work may sensitise the no-ciceptive system and lead to the above-mentioned ef-fects. This might be an important factor for thedevelopment of MSD as static posture is a physical riskfactor.

In summary, the results indicated that the experi-mental method can be used as a clinically relevant modelin occupational settings for studying the interactionbetween muscle pain and motor control. Furthermore,the present comprehensive, clinical and experimentalhuman laboratory study suggested substantial modula-tion of motor co-ordination during standardised low-load repetitive work. Information on such modulatoryprocesses can enhance our knowledge of the underlyingwork-related pain mechanisms and may prove to behelpful in the design of working procedures that canminimise development of MSD.

Appendix

1. Electromyogram

The Frms was de®ned as:

Frms�x� �

���������������������������������1

N

XN

k�1jx�k�j2

!vuutFCUTmax

; �2�

where N was the number of samples corresponding tothe length of the discrete cutting event, x(k) was theEMG discrete signal in the time domain of lengthN: x�1�; x�2�; . . . ; x�k�; . . . ; x�N� and FCUT max was themaximal force of the cutting event.The Brms was de®ned as:

Brms�x� ���������������������������������1

P

XP

l�1jx�l�j2

!vuut ; �3�

where P was the number of samples corresponding tothe duration of the window (300 ms) and x(l) was theEMG discrete signal in the time domain of length P(from 500 ms to 200 ms before the cutting event CUT1:x�1�; x�2�; . . . ; x�l�; . . . ; x�P �.

2. Kinematics

Rigid body transformations matrix T(R, d) (SoÈ derkvistand Wedin 1993) can be written as1:

�y� � �Tx=y ��x� ; �4�where [x] and [y] are the 3D matrix markers co-ordinatesrespectively in position 1 and 2 and [Tx/y] is the 4 ´ 4transformation matrix.

Equation (4) can then be expressed as:

�y� � �Rx=y ��x� � �d� ; �5�where �Rx=y � is a 3 ´ 3 rotation matrix and [d] is a 3 ´ 1translation vector.

Based on the procedure explained above and thanksto the preliminary recordings (see Methods, Kinematicsrecordings section), constant rigid body transformationmatrices were computed (Fig. 4):

A. From the trunk-triplet (MaT) in the trunk markersco-ordinate system (MaCStrunk) to the trunk-triplet(AnT) in the trunk anatomical co-ordinate system(AnCStrunk): TMaT=AnT�RMaT=AnT; dMaT=AnT�, (Fig. 4,I).

B. From the arm-triplet (AnA) in the anatomicalco-ordinate system (AnCSarm) to arm-triplet (MaA)in the markers co-ordinate system (MaCSarm):TAnA=MaA �RAnA=MaA; dAnA=MaA�, (Fig. 4, II).During the working task, rigid body transformations

were then computed for each frame, from the arm(MaA) in the MaCSarm to the LaCS, (Lab) and from theLaCS (Lab) to the trunk (MaT) in the MaCStrunk. Thisprovided the transformation matrices:

TMaA=Lab�RMaA=Lab; dMaA=Lab�, (Fig. 4, III) andTLab=MaT �RLab=MaT; dLab=MaT� (Fig. 4, IV).

The overall rotation matrix obtained for each frameand transforming co-ordinates in the AnCSarm toco-ordinates in the AnCStrunk were de®ned as theproduct:

Rarm � RAnA=MaARMaA=LabRLab=MaTRMaT=AnT �6�While the overall rotation matrix obtained for eachframe and transforming co-ordinates in the AnCStrunk toco-ordinates in the LaCS were:

Rtrunk � RAnT=MaTRMaT=Lab �7�

where the rotation matrices RAnT=MaT and RMaT=Lab werethe inverse rotation matrices of RMaT=AnT and RLab=MaT.

Acknowledgements The authors would like to thank P. Frost(Clinic of Occupational Medicine, AÊ rhus University Hospital,Denmark) for making contact with the patients and the controlsubjects. This work was supported by Copernicus (grant no.ERBCIPACT930099), Arbejdsmiljùfondet, Sygekassernes Helse-fond, Det Obelske Familiefond, Ap Mùllers og Hustrus Fond andthe Danish National Research Foundation.

References

AaraÊ s A, Westgaard RH, Stranden E (1988) Postural angles as anindicator of postural load and muscular injury in occupationalwork situations. Ergonomics 31:915±933

Arendt-Nielsen L, Mills KR, Forster A (1989) Changes in muscle®bre conduction velocity, mean power frequency, and mean1For details see http://www.kin.ucalgary.ca/isb/software/kinemat/

139

EMG voltage during prolonged submaximal contractions.Muscle Nerve 12:493±497

Arendt-Nielsen L, Graven-Nielsen T, Svarrer H, Svensson P (1995)The in¯uence of low back pain on muscle activity and coordi-nation during gait: a clinical and experimental study. Pain64:231±240

Braakhekke JP, Stegeman DF, Joosten EMG (1989) Increase inmedian power frequency of the myoelectric signal in patho-logical fatigue. Electroencephalogr Clin Neurophysiol 73:151±156

Christensen H (1986) Muscle activity and fatigue in the shouldermuscles during repetitive work. Eur J Appl Physiol 54:596±601

DjupsjoÈ backa M, Johansson H, Bergenheim M (1994) In¯uenceson the gamma-muscle-spindle system from muscles a�erentsstimulated by increased intramuscular concentrations of ara-chidonic acid. Brain Res 663:293±302

Edwards RHT (1988) Hypothesis of peripheral and central mech-anisms underlying occupational muscle pain and injury. Eur JAppl Physiol 57:275±281

Elert JE, RantapaÈ aÈ -Dahlqvist SB, Henriksson-Larse n K, Loren-tzon R, Gerdle BUC (1991) Muscle performance, electromyo-graphy and ®bre type composition in ®bromyalgia and work-related myalgia. Scand J Rheumatol 21:28±34

First European survey on the work environment 1991±1992 (1992)European Foundation for the Improvement of the Living andWorking Conditions, Dublin, Ireland

Giroux B, Lamontagne M (1992) Net shoulder joint moment andmuscular activity during left weight-handling at di�erent dis-placements and frequencies. Ergonomics 35:385±403

Graven-Nielsen T, Arendt-Nielsen L, Svensson P, Jensen TS (1997)E�ects of experimental muscle pain on muscle activity andcoordination during static and dynamic motor function. Elec-troencephalogr Clin Neurophysiol 105:156±164

Hagberg M (1981) Electromyographic signs of shoulder muscularfatigue in two elevated arm positions. Am J Phys Med 60:111±121

HaÈ gg GM (1991) Static work loads and occupational myalgia ± anew explanation. In: Anderson PA, Hobart DJ, Dano� JV (eds)Electromyographical kinesiology. Elsevier, Amsterdam, pp141±144

Henriksson KG, Bengtsson A, Lindman R, Thornell L-E (1993)Morphological changes in muscle in ®bromyalgia and chronicshoulder myalgia. In: Vñrùy H, Merskey H (eds) Progress in®bromyalgia and myofascial pain. Elsevier, Amsterdam, pp 61±73

Jensen C, Nilsen K, Hansen K, Westgaard RH (1993) Trapeziusmuscle load as a risk indicator for occupational shoulder-neckcomplaints. Int Arch Occup Environ Health 64:415±423

Johansson H, Sojka P (1991) Pathophysiological mechanisms in-volved in genesis and spread of muscular tension in occupa-tional muscle pain and in chronic musculoskeletal painsyndromes: a hypothesis. Med Hypotheses 35:196±203

Kabada MP, Ramakrishnan HK, Wootten ME (1990) Measure-ment of lower extremity kinematics during level walking. JOrthop Res 8:383±392

Kadefors R, Forsman M, Zoe ga B, Herberts P (1996) Lowthreshold C-units and the development of shoulder myalgia: aneurophysiological study. Proceedings of XXVth World Con-gress on Occupational Health, ICOH'96 Publication, Interna-tional Congress on Occupational Health, Stockholm, pp 103

Kwatny E, Thomas DH, Kwatny HG (1970) An application ofsignal processing techniques to the study of myoelectric signals.IEEE Trans Bio Eng 17:303±313

Lafortune MA, Cavanagh PR, Sommer HJ, Kalenak A (1992)Three-dimensional kinematics of the human knee duringwalking. J Biomech 25:347±357

Lund JP, Donga R, Widmer CG, Stohler CS (1991) The pain-adaptation model: a discussion of the relationship between

chronic musculoskeletal pain and motor activity. Can J PhysiolPharmacol 69:683±694

Madeleine P, Voigt M, Arendt-Nielsen L (1998) Subjective, phys-iological and biomechanical responses to prolonged manualwork performed standing on hard and soft surfaces. Eur J ApplPhysiol 77:1±9

Mathiassen SE, Winkel J (1990) Electromyographic activity in theshoulder-neck region according to arm position and glenohu-meral torque. Eur J Appl Physiol 61:370±379

Mathiassen SE, Winkel J (1996) Physiological comparison of threeinterventions in light assembly work: reduced work pace, in-crease work allowance and shortened working days. Int ArchOccup Environ Health 68:94±108

Mense S (1993) Nociception from skeletal muscle in relation toclinical muscle pain. Pain 54:241±289

Mense S, Skeppar P (1991) Discharge behaviour of feline gamma-motoneurons following induction of an arti®cial myositis. Pain46:201±210

Pedersen J, SjoÈ lander P, Wenngren B-I, Johansson H (1997) In-creased intramuscular concentration of bradykinin increases thestatic fusimotor drive to muscle spindles in neck muscles of thecat. Pain 70:83±91

Sjùgaard G, Sejersted OM, Winkel J, Smolander J, Jùrgensen K,Westgaard RH (1995) Exposure assessment and mechanisms ofpathogenesis in work-related musculoskeletal disorders: signi-®cant aspects in the documentation of risk factors. In: Svane O,Johansen C (eds) Work and health scienti®c basis of progress inthe working environment. European Commission DG V, Lux-embourg, pp 75±87

SoÈ derkvist I, Wedin P (1993) Determining the movements of theskeleton using well-con®gurated markers. J Biomech 26:1473±1477

Sommerich CM, Mc Glothin J, Marras WS (1993) Occupationalrisk factors associated with soft tissue disorders of the shoulder:a review of recent investigations in the literature. Ergonomics36:697±717

Sundelin G, Hagberg M (1992) Electromyographic signs ofshoulder muscle fatigue in repetitive arm work paced bymethods-time measurement system. Scand J Environ Health18:262±268

Svensson P, Arendt-Nielsen L, Houe L (1995) Sensory-motor in-teractions of human experimental unilateral jaw muscle pain: aquantitative analysis. Pain 64:241±249

Takala E-P, Viikari-Juntura E (1991) Muscular activity in simu-lated light work among subjects with frequent neck-shoulderpain. Int J Ind Ergon 8:157±164

Travell JG, Rinzler S, Herman M (1942) Pain and disability of theshoulder and arm. J Am Med Assoc (Chicago) 120:417±422

Veiersted KB, Westgaard RH, Andersen P (1990) Pattern of muscleactivity during stereotyped work and its relation to muscle pain.Int Arch Occup Environ Health 62:31±41

Voss H (1956) Tabelle der Muskelgewiche des Mannes, berechnetund zuzammengestellt nach den Untersuchungen von W. Thi-elle (1884). Anat Anz 103:356±360

Welch PD (1967) The used of fast Fourier transform for the esti-mation of power spectra: a method based on time averagingover short modi®ed periodograms. IEEE Trans AudioElectroacoust Vol AU-15:70±73

Westgaard RH (1996) Motor unit ®ring behaviour in the trapeziusmuscle. In: Sjùgaard G, Fallentin N (eds) Risk assessment formusculoskeletal disorders ± Nordic satellite symposium underthe auspices of the International Congress on OccupationalHealth, 1996. National Institute of Occupational Health, Co-penhagen pp 93±94

Westgaard RH, Winkel J (1997) Ergonomic intervention researchfor improved musculoskeletal health: a critical review. Int J IndErgon 20:463±500

Winter DA (1990) Biomechanics and motor control of humanmovement, 2nd edn. Wiley Interscience Publications, New York

140