Normalization of surface EMG amplitude from the upper trapezius muscle in ergonomic studies — A review

J. Electromyogr.

Copyright

Kinesiol.

Vol. 5. No. 4, pp. 197-226,1995

0 1996 Elsevier

1050-6411(94)00014-X

Science Ltd. All rights reserved Printed in Great Britain 105k641119.5 $10.00 + 0.00

Normalization of Surface EMG Amplitude from the Upper Trapezius Muscle in Ergonomic Studies - A Review S. E. Mathiassen’, J. Winke11,2 and G. M. H%gg’ ‘National Institute of Occupational Health, Division of Applied Work Physiology, S-171 84 Solna ‘University Hospital of Lund, Department of Occupational and Environmental Medicine, Lund, Sweden

Surface electromyographic (EMG) amplitude from the upper trapezius muscle is widely used as a measure of shoulder-neck load in ergonomic studies. A variety of methods for normalizing EMG ampiitude from the upper trapezius (EMGamp,,) have been presentedin the literature. This impedesmeta-analysesof, for instance, upper trapezius load in relation to development of shoulder-neck disorders. The review offers a thorough discussionof different normalization proceduresfor EMGamp,,. The following main issuesare focused: output variable, location of electrodes, posture and attempted movement during normalization, load and duration of reference contractions, signal processingand test-retest repeatability. It is concluded that translations of EMGamp,, into biomechanical variables, for example relative force development in the shoulder or in the upper trapezius itself, suffer from low validity, especiallyif usedin work tasks involving large and/ or fast arm movements.The review proposesa standard terminology relating to normalization of EMGamp,, and concludesin a concrete suggestionfor a normalization procedure generating bioelectrical variables which reflect upper trapezius activation.

Summary:

Key Words: Electromyography-Shoulder-neck-Occupational-Biomechanics-Validity-Repeatability-Terminology.

J. Electromyogr. Kinesiol., Vol. 5, 197-226.December

direct information on muscle involvement during occupational work. Recordings of surface EMG amplitude have therefore been extensively used in ergonomic studies to assess the internal biomechanical exposure of the shoulder region”“~2’h. The major part of these studies have included recordings from the upper trapezius muscle, partly because the upper trapezius region is a prevalent site of work-related pain4.‘“x.‘9y and partly because the muscle is easily accessible with surface electrodes. Several investigators have used the amplitude of the raw EMG signal (i.e. in terms of volts) as a measure of upper trapezius activation. Examples

INTRODUCTION

It is widely accepted that unfavourable shoulderneck muscle loads in the work place constitute a major risk factor for developing musculoskeletal disordersx,67,21x. Surface electromyography (EMG) is at present the only non-invasive method offering ReceivedJuly 5, 1994. RevisedOctober28, 1994. AcceptedNovember16, 1994. Correspondence and reprint requeststo S. E. Mathiassen, NationalInstituteof OccupationalHealth, Divisionof Applied Work Physiology,S-17184Solna,Sweden.

197

198

S. E. MATHIASSEN

ET AL.

may be found in studies of work station design 43,55,117,130,173,231, external factors in the work environment183,209, work organization55T181, or the functional anatomy of the shoulder76,104, 112,120,121,155,180,192,206,227~

The

use

of

raw

E,~G

amplitude may be justified if the aim is to study only the relative effects of short-term interventions which may be monitored without changing the EMG electrode set-up. The raw surface EMG amplitude is, however, highly sensitive to factors determined by the specific electrode configuration, such as the electrode/skin impedance and the exact location and spacing of electrodes 16,93,193.Furthermore, the surface EMG from a specific muscle is influenced by conditions differing systematically between individuals, such as muscle fibre composition and tissue filter properties r6. Therefore studies using raw EMG amplitude from the upper trapezius in comparisons between groups and/or between days are of limited validity37,'45,'73,'~8. Such comparisons require the EMG amplitude from the upper trapezius (EMGamp,,) to be normalized, i.e. expressed in terms of a signal obtained during standardized and reproducible conditions. Evidently, normalization is also a prerequisite for an EMG study to be included in a meta-analysis of occupational shoulderneck exposure and its musculoskeletal effects. The scientific literature contains a number of fundamentally different methods for the normalization of surface EMG amplitude from the upper trapezius muscle. This lack of consensus is a serious obstacle to meta-analyses of exposureeffect relationships, and thus to the assessment of guidelines for proper shoulder-neck exposure. The present review offers a critical examination of proposed normalization methods, concentrating on essential elements in their execution while giving less attention to EMG detection techniques and basic signal processing. The latter issues have been reviewed by Basmajian and DeLucar6, Merletti and DeLuca140 and Winter et al.**O. Primarily, the review is intended to support the development of proper normalization procedures for ergonomic studies quantifying biomechanical exposure by means of upper trapezius EMG amplitude. ELEMENTS

OF THE NORMALIZATION PROCEDURE

Table 1 presents six issues which are essential in the description of any EMGamp,, normalization method. The key issue is the selection of outcome Journal

Elecrromyograpl~y

& Kitwsiology

Vol.

5. No. 4, 19%

0

RVC

MVC

Force/torque

FIG. 1. Terminology in the normalization of EMGamp,,. RVC and MVC refer to the force or torque performance during a reference and maximal voluntary contraction of the whole shoulder or (conceptually) the isolated upper trapezius muscle. RVE and MVE are the associated values of EMGamp,,, i.e. reference or maximal voluntary electrical activation. RVC and MVC pertain to a biomechanical domain, while RVE and MVE are bioelectrical variables.

variable, which to a large extent guides the concrete normalization approach. We would suggest that descriptions of normalization procedures in scientific papers should, as a minimum, respond to each of the steps in Table 1. Table 2 comprises an overview of papers evaluating exposure in occupational tasks on the basis of normalized upper trapezius surface EMG amplitude. Table 2 is arranged according to the issues in Table 1. Tables 1 and 2 provide the framework for the present review. Each issue in Table 1 is discussed in a separate section and the review concludes by suggesting minimal requirements for EMGamp,, normalization in future studies. OUTCOME

VARIABLE

Terminology

The upper trapezius surface EMG is basically a summation of motor unit action potentials detectable by the electrodes. Two fundamentally different concepts have been presented regarding the information conveyed by the amplitude of the upper trapezius EMG. One concept acknowledges the basic nature of the signal by interpreting EMGamp,, only in a ‘bioelectrical’ activation domain, normahzing in relation to the EMGamp,, obtained during a standardized contraction. The other concept assumes an association between EMG amplitude and muscle force output and operates in a ‘biomechanical’ domain by translating EMGamp,, into a force or torque variable, for example percentage of capacity the maximal voluntary contraction

NORMALIZATION

OF UPPER

TRAPEZIUS

199

EMG AMPLITUDE

b MVE

MVE

A

0-k 0

50

100

0

50

4

Synergy effort (% MVC)

100

Synergy effort (% MVC)

b

0

SO

100

Upper trapezius force (% MVC)

FIG. 2. The translation of EMGamp,, to biomechanical variables. a, An example of the relative effort in a contraction of synergic shoulder muscles and the corresponding EMGamp,,. b, Three examples of possible relationships between synergy and upper trapezius performance. c, Related examples of the resulting relationships between relative upper trapezius effort and EMGamp,,. The relationship a has been determined during the normalization procedure. The full curves in b and c illustrate the common assumption that a one-to-one relationship exists between the involvement of the upper trapezius and the synergy, implying that EMGamp,, may be translated into relative force in the upper trapezius using the function determined in a. The dashed curves illustrate two conceivable exceptions to this assumption: 1. a non-linear involvement of the upper trapezius in the synergy; and 2. a non-maximal involvement of the upper trapezius at maximal synergy effort. Both of these examples result in a relationship c between upper trapezius force and EMGamp,, which is not identical to the one assumed a. TABLE

in normalization 1. Key elements upper trapezius EMG amplitude

of

Outcome variable Electrode location Posture and attempted movement Load and duration of reference contraction(s) Signal processing Number of repetitions of the normalization

(%MVC) of the upper trapezius. The main objective of biomechanical normalizations have been to generate an estimate of the physical load on the upper trapezius which is interpretable in terms of shortterm fatigue or long-term disorders. Forty-five of the 77 studies reviewed in Table 2 have endorsed the bioelectrical concept while 29 normalized EMGamp,, in biomechanical terms. Three studies did not clearly state the output variable. Table 2 discloses a lack of consensus regarding the name of normalized output variable, in particular in the bioelectrical domain. We would therefore suggest the standard terminology presented in Figure 1. EMG amplitude normalized in the bioelectrical domain may be expressed either in percentage of a reference voluntary electrical activation (%RVE), i.e. percentage of the electrical activity obtained during a submaximal reference voluntary contraction (RVC), or in percentage of the maximal voluntary electrical activation (%MVE), referring to the electrical activity during an attempted maximal voluntary contraction (MVC). It should be noted Journal

Electromyography

& Kinesiology

Vol.

5, No.

4, 1995

that the terms ‘submaximal’ and ‘maximal’ refer to the force action in the contraction during which EMG is sampled. It may be argued that a maximal contraction is just a special case among reference contractions, but we find it important to make a clear distinction between maximal and submaximal efforts. It is shown in the present review that normalization according to these two approaches requires basically different considerations, in particular regarding validity issues. The terms %RVC and %MVC may be used in normalizations translating EMGamp,, into a biomechanical force/torque variable. %RVC and %MVC may refer either to the force/torque performance of the shoulder muscles or to force exertions of the isolated upper trapezius muscle. It is important to distinguish between these two variables as demonstrated below. Several criteria may be relevant when choosing an output variable and the appropriate procedure to obtain it. One criterion which seems statistically attractive is to aim at the variable which most effectively reduces the variance in exposure between individuals in a study group. This approach may, however, lead to erroneous conclusions. A group of individuals with similar anthropometry but different strength would appear homogeneous if all performed a work task mainly consisting of moving the unloaded arm and the EMGamp,, was normalized in terms of %RVE. The group would appear less uniform if the EMG was normalized in terms of %MVC. If, however, the individuals lifted burdens

2

f

3

Ih

$.

B 3 Y 3

$

3 e

Bjorksten

Bjelle

et al. 1987

et al. 1981

et al. 1985

Bendix

et al. 1992

Attebrant

& Hagberg

et al. 1991

Attebrant

Bendix 1984

& 1993

et al.

et al. 1988

& Westgaard

2.

Asikainen Harstela

Arborelius 1986

Aaras

Aaras 1987

Authors

TABLE

23

22

20

19

13

12

10

7

2

1

Ref.

using

t’%MVC’)

%MVE

? ?

?

%MVC

%MVC

%MVC

?

?

?

sitting

and posture

Standing

Upright sitting

?

Standing

?

Fixed

?

7

Head trunk

the upper

?

Bilateral elevation

Bilateral elevation

Above medial scapula

%MVC of

? upper angle

90”

from

Upper arms 90” abducted, lower arms horizontal, elbow angle 90”

7

Flexion

7

?

?(Drawing)

90”

?

?

Antero-lateral margin, midway occiput-acromion

Elevation; abduction

?

%MVC

%RVE

(‘TAMP-

Arm posture and attempted movement

EMG amplitude

Electrode location

normalized

%MVE R’)

%MVC

Outcome variable

Studies

Ramp %MVC

?

Ramp %MVC,

Ramp

30

2 s 3s

load

O-70

O-50 10 s

0, 15 and %MVC

1 kg

Maximal

Maximal; increase, hold

?

‘Varying’ levels

Load and duration of reference contraction(s)

trapezius

exposure

Sling

?

Handheld

FWRA

?

FWRA

rms

Sling

ms) (50 ms)

rms (100

(0.5-l

?

?

?

? Weight hanging over first digit phalanges

FWRA

?

FWRI

EMG signal smoothing

?

?

Power

Power

Linear

2

Repetitions of the procedure

or real occupational Force-EMG regression model

s) Linear

in simulated

?

?

Sling proximal to elbow/sling over acromion

Application of load

to assess

Reference to Jonsson 197898 and Jonsson 1982=

‘Techniques described by Ericson & Hagberg 1978’. Their papeP concerns elbow flexion

Regression ‘based on 50-150 data pairs’, sampling frequency stated incorrectly as 100 Hz

‘As described by Westgaard 1987’ (not included in the reference list Aaras et al.)

Vocational EMG accepted only if MVEtabd) and MVEtelev) differ by less than 50%. EMG/forcerelationship adjusted according to the larger MVE

Notes

tasks

Ekholm

& Sundelin

et al. 1986

Hagberg 1986

Hagner

Harms-Ringdahl al. 1986

&

et

et al. 1991,

et al. 1987

Hagberg

Hammarskjold 1989, 1990, 1992; Hammarskjold Harms-Ringdahl 1992

et al. 1983

Hagberg

et al. 1989

1992

Giroux & Lamontagne

Grieco

1990

et al.

et al.

et al. 1987

Lamontagne

< 0 cn Fernstrom 1994 : p Fussier-Pfohl -+ 1984

s

1987

1986

Christensen

De Groot

1986

& Norman

Christensen

Bobet 1982

1. 2s-

3

8

B % d

,$

2

B L 5a

6

79

70-74

69

66

65

63

56

53

(‘%RVC’)

(‘%MCV’)

(‘%MVC’)

%MVE (‘TAMP%‘)

%MVE (‘%EMG’ or ‘%EMGmax’)

%MVC?

%MVC

%MVC

%MVC

%MVE

%MVE

(‘%MVC’)

%MVE

52

(‘%RVC’)

(‘TAMP-

(‘%MVC’)

(‘%NC’)

RVE-fraction

%MVE

%MVE %‘)

%MVE

%MVC

%MVC

%RVE

47

46

40

31

29

28

25

the

belly

I. Antero-lateral margin; 2. C2-C6 level, covering cervical erector spinae (drawing)

Anterolateral margin

7

point

point

edge, origin(photo)

motor

motor

?fDrawing)

?

Upper midway insertion

?

Over

Over

C4-level, 2 cm lateral to col. vert.

?

1. Antero-lateral margin, midway occiputacromion; 2. Covering cervical erector spinae

2 cm below upper border, one third of distance C7acromion

?

?(Drawing)

Over

Flexion

90”

of neck

Neck extension; elevation; 90”flexion; 90” abduction

‘Standardized positions’

90”

Unilateral elevation

Unilateral elevation

Unilateral flexion

?

?

Elevation

Extension horizontal

?

?

Unilateral elevation

Bilateral elevation

Bilateral elevation

?

?

Sitting

Standing

Standing

Standing

Standing

7

Sitting

?

Sitting

Sitting

Supine

?

O-100

%MVC

Maximal

Maximal

0, 1 and

1s

3 s

O-70 10s

2 kg

Ramp O-30 %MVC, 10s

Ramp %MVC

30-40

Maximal

Maximal

2 kg

Maximal

Maximal,

Ramp %MVC,

50 %MVC,

?

?

handheld weights

Sling over acromion

Sling over acromion

Handheld

weight

Weight ‘at the level of the forehead’

?

7

Sling on the shoulder

Sling over acromion

Sling over acromion

?

(100

(100

(100

ms)

ms)

ms)

msj

s)

FWR, LPf, A (0.1 s)

FWR, LPf, time-aver (0.1 s)

rms

rms

rms

rms

RI

FWR, LPf (3 Hz)

FWR, LPf (8 Hz)

RA (30 s)

rms (100

s)

(0.12

A (0.25

rms

rms

rms

FWRA

-

-

Linear

Power

EMG vs. %MVC assumed linear

Power

?

-

2

unit

in

study as et al.

to et al

Continued

Reference Schijldt 1986’-

Reference to Schijldt & Harms-Ringdahl 1988””

Unclear if vocational EMG is normalized using the regression equation, or in terms of an RVC

Wrong figures

Same Weber

‘as described by Ekhom et al. 1979’. This paper3¶ concerns abdominal muscles

ltani

et al. 1979

1982

1988

1988

et al. 1985

et al. 1981

et al.

1978

Jonsson

Jonsson

Jonsson

Jonsson

Jonsson

Jorgensen 1989

et al. 1993

Jonsson

.?’ Jensen

b 3 ltani et al. 1987 5 2. g B Jensen et al. 1993 s

d $

105

103

102

101

?

Sitting

?

Unilateral elevation

?

%MVC

?

appr. ?

?

7

?

%MVC

?

FWR,

Sling over shoulder

? ?

Weight

rms

?

Maximal ?

30 %MVC

‘Standardized’

?

rms

?

Maximal

7

(‘%RVC’)

?

‘Standardized’

7

?

?

FWRI

FWR, ‘smoothed (time constant 0.1 s)’

rms

?

LPf

(0.2 s)

EMG signal smoothing

7

%MVC

%MVE?

indicated

Not

100

7

?

?

%MVC

99

?

?

?

90”

?

?

?

%MVC

98

Sling

Sling over shoulders

O-70

7

Application of load

Maximal

Ramp %MVC

Including maximal

Load and duration of reference contraction(s)

?

Elevation; abduction

Halfway on line C7-acromion

%MVE f’%MEMG’)

and posture

Continued

7

flexion

?

?

Head trunk

2.

Maximal

92

91

Shoulder

%MVC

90

7

?

Arm posture and attempted movement

%MVE (‘%EMGmax’)

?

Electrode location

Elevation

%MVE

Outcome variable

?

89

Ref.

TABLE

?

?

?

?

?

Linear

?

Force-EMG regression model

2

2

Repetitions of the procedure

to 198658

to 198658

Reference to Hagberg 197g6”. This paper concerns elbow flexion

Reference Hagberg

Reference Hagberg

The paper discusses a number of possible normalization procedures, but does not specify which one is used in the presented vocational recordings

Reference to a number of papers. None of these contain specific procedures for normalization of EMGamp,,

MVE: maximal EMG obtained in any of the test positions

Probably the same study as Sjpgaard et al. 1987’74

Reference to Jonsson 197898 and Jonsson 1982=9

Notes

& Ericson

et al. 1991

et al. 1992,

et al.

et al. 1988

Milerad

Nakata 1993

Nieminen 1993

Odenrick

1993

Mathiassen

Milerad 1994

et al.

et al.

Louhevaara 1990

Malmkvist 1992

et al. 1993

Lindberg

&

et al. 1989

Lannersten Harms-Ringdahl 1990

Kilbom

151

150

148 149’

143

142

133

132

128

125

118

114

(‘%RVC’)

%MVE (‘normalized amplitude

%‘)

(‘%RVC’)

(a) %MVE (‘%MVCe’). fb) %MVC’

%MVE

(b) %RVE (‘%submaxRVC’)

(a) %MVE f’%max-RVC’)

(a) %MVE (‘%max-RVC’)

RVE-fraction (‘norm-EMG amp’)

%RVE

%MVC

%MVE

%MVE (‘TAMP%‘)

%MVC

the

belly

at

?

?

Flexion

Bilateral elevation; unilateral elevation; flexion

fb) abduction, elbow angle

90”

90”

(a) abduction in scapular plane, arms vertical

(b) Abduction, elbow angle 90”. palms horizontal

(a) Abduction in scapular plane, arms vertical

Flexion, horizontal straight arms 60” to sagittal plane

90°

third

lifting’

Reference to Hagg et al. (1987)

‘Shoulder raising’

‘Arm

Neck extension; neck lateral flexion; elevation; 90” flexion; 90” abduction; external rotation

Unilateral elevation

Upper margin, midway C7acromion

?

Antero-lateral border, one of distance occiput-acromion

?(Two positions, one of which ‘trapezius transversus/supraspinatus’)

On line C7acromion, medial electrode 60% of total distance from C7

?

Over

?

1. Antero-lateral margin, midthird occiputacromion; 2. C2C6 level, covering cervical erector spinae (drawing)

7

?

Standing

Standing

Stabilized sitting

Stabilized sitting

Sitting

?

Upright sitting,

?

?

Sitting

fixed

30

2 kg

Maximal

(a) Maximal; (b) ramp 0-70-O %MVC

Maximal

(b) Weight

(a) Maximal

(b) 2 kg

(a) Maximal

Weight giving 15 Nm glenohumeral torque, arms included

?

10, 20, 35, 50 and 100 %MVC, 5-7 s each

Maximal

Maximal

10 and %MVC

?

Straps wrists

Straps wrists

around

around

fb) Handheld

(a) Manual resistance

(b) Handheld weight

(a) Manual resistance

Handheld

Pad above shoulder

?

?

Pad above shoulder

(100

rms

(a) rms (50 ms). median filtering; (b) rms (50 ms)

rms

rms (100

rms (100

ms)

ms)

ms)

ms)

median (0.3 s)

rms (100

?

rms

rms, filter

FWR, LPf, A (0.1 s)

?

?

(a)-;(b) 2nd order polynomium

-

-

-

-

?

linear

-

-

?

3 (Max) (Submax)

2

or 2

on data

(b) is

in

Continued

EMG is obtained in several test positions, but only 90” flexion is used for the investigated work task

MVE: largest EMG obtained in any of the test position

Alternative not used vocational

MVE: largest EMG obtained any of the test position. Reference to Schijldt and Harms-Ringdahl 1988””

& Hagberg

et al. 1986,

Schuldt 1987

Sundelin 1989

et al. 1987

Schuldt

et al. 1987

et al. 1987

Schiildt

Sjegaard

et al. 1989

Rohmert

182

174

168, 169

167

166

164

159

153

et al. 1990

et al. 1982

Onishi

Philipson

Ref.

Authors

(‘%RVC’)

%MVC

max-

(‘%

%MVE EMG’)

%MVE (‘TAMP%‘)

%MVE (‘TAMP%‘)

%MVE (‘TAMP%‘)

RVE-fraction (‘EMG index’)

%MVE (‘NAREMG’)

%MVC

Outcome variable

1. 2 cm above angulus superior scapulae; 2. C2C3 region (drawing)

Unilateral elevation

Bilateral elevation

(a) ‘elevation of arm and shoulder’; (b) ‘elevation of arm and shoulder’; fc) 45” abduction in scapular plane; fd) elevation; (e) medial-cranial scapular sliding; (f) neck extension; fg) neck extension

1. Antero-lateral margin, midpoint origin-insertion; 2. C2-C6 level, covering cervical erector spinae; 3. laterally, covering supraspmatus (drawing)

? (Photo)

?

1. Midpoint of antero-lateral margin; 2. C2-C6 level, covering cervical erector spinae (drawing)

arms

(a) ‘Elevation of arm and shoulder’; (b) ‘elevation of arm and shoulder’; tc) 45” abduction in scapular plane; (d) elevation; fe) neck extension; (f) neck extension

Vertical

90”

1. Antero-lateral margin, between lateral and intermediate third of occiput; acromion distance; 2. C2C6 level, covering cervical erector spinae; 3. laterally, covering supraspinatus (drawing)

?

Bilateral abduction

? part

? Middle

Arm posture and attempted movement

Electrode location

Standing

Standing

Ramp %MVC

Maximal

Maximal

?

?

Stabilized sitting

Maximal

Stabilized sitting

Rest

Maximal

Standing

?

O-30

Load and duration of reference contraction(s)

Continued

Standing

and posture

2.

?

Head trunk

TABLE

(1 s)

7

FWR, LPf,A (0.1 s)

R.LP1.A

RA

?

EMG signal smoothing

Sling over acromion

Slings over shoulders

rms (100

rms ms)

Resistance at: FWR, LPf A (a) distal (0.1 s) forearm; (b) distal upper arm; (c) distal upper arm; (d) ?; (el scapula; (f) occiput; (g) upper cervical spine

Resistance at: (a) distal forearm; (b) distal upper arm; (c) distal upper arm; fd) ?; fe) occiput; (f) upper cervical spine

Manual resistance, elbow level

?

Application of load

Power

of

MVE: largest the two trials

-

Reference to Ekholm et al. 197939. This paper concerns abdominal muscles

‘Elevation’: presumably 90” flexion or 90” abduction, scapular plane”O. MVE: largest EMG obtained in any of the test positions

Notes

‘Elevation’: presumably 90” flexion or 90” abduction, scapular plane’70. MVE: largest EMG obtained in any of the test positions

Repetitions of the procedure

-

-

-

7

Force-EMG regression model

z

s

2

$

5

h

*

Veiersted

VI

et al.

Westgaard 1986

et al. 1983

et al.

Westgaard 1993

Winkel

& Aaras

et al. 1980

et al. 1993

et al. 1990

& Viikari1991

1994

& Hagberg

Westgaard 1985

Weber

Veiersted

$

Takala Juntura

Segaard

3 d 8 %

k r+ 3 z 2. %

Sundelin 1992

’ p

? 3

212

205

204

202

200

195

194

186, 187

185

184

(‘%RVC’)

%MVC

%MVC

%MVE (‘%EMGmax’)

%MVE

RVE-fraction

%MVE (‘%MEMG’)

%MVC

%NVE (‘TAMP%‘)

%MVE (‘%MVC’, ‘%EMGmax’)

%MVC

cervical

? (Photo)

7

Halfway acromion

?

C4 level, splenius

Halfway processus prominensacromion

Halfway processus prominensacromion

C7-

over capitis

Upper margin, midpoint C7acromion (drawing)

?

Lateral;

Elevation, retraction, abduction

(a) Bilateral elevation; (bj bilateral 90” abduction

Extension with neck horizontal

?

Sitting

?

?

Sitting

Sitting

(b) 90”

?

?

(a) Bilateral elevation; (b) bilateral 90” abduction

(a) Bilateral elevation; unilateral abduction

Elevation

Ramp

7

Maximal

?

(a) Slings over shoulders; (b) slings just proximal to elbows

?

Weight

2 kg Maximal

over (b) lateral

(a) Slings over acromioclavicular joint; (b) slings 5 cm proximal to elbow fold

(a) Slings acromion; sling over epicondyle

?

and

2-5 s

?

?

Maximal

(a) Maximal ramp O-50 %MVC; (b) maximal

Maximal,

Maximal

7

(0.2 s)

(0.2 s)

(0.2 s)

rms

FWRA

FWRI

(50 ms)

(0.2 s)

RI (2 s)

RA (30 sj

FWRI

FWRI

rms

FWR

rms

Power

7

-

-

‘Approximated by a straight line’ from 0 to 30 %MVC’

?

as et

to et al.

Continued

Signal processing: reference to Ericson & Hagberg 197844

‘Largely the procedure of Jonsson 1978’98 Vocational recordings accepted only if the two calibration values differ less than 20% from their common mean

Same study Fussier-Pfohl al. 198247

Reference Veiersted 1990-4

Recordings rejected according to criteria in Westgaard 1988=”

MVE: largest 1 EMG amplitude (0.5 s average) obtained in any of the test positions (a) 3 maximal, 2 ramps; (b) 1

2-3 Elevations, abduction

Reference to Hagberg & Sundelin 1986@ and Sundelin & Hagberg 1989’82

N z

s

Winkel

Authors

procedures

et al.

an explanation

The

Ortengren 1991

1990

& Gard

1988

et al. 1983

Winkel & Oxenburgh

R= ?? 9’ sm Y. $ Winkel P

62 ;I 5

$

9 2

3 f t

have been specified has been given about

to

the extent allowed symbols, abbreviations,

by

the cited etc.

‘?‘: The electrode ‘Elevation’ denotes lifting position over upper trapezius is the shoulder with the arm not described in vertical along any detail the side of the body. If not stated otherwise, the attempted movements refer to antigravitational efforts at the glenohumeral joint with stretched arms

Terminology according to the suggestions in the present paper. If discordant, the term used in the cited study appears in brackets

?

?

? ?

7

?

Head trunk

7

Elevation

Arm posture and attempted movement

?

?

?

?

Electrode location

%MVE

%MVC

217

234

%MVC

%MVC

Outcome variable

214

213

Ref.

papers.

and posture

TABLE

2.

Literal

quotations

are

if

indicated

force transducer, not stated otherwise

The list does not include the maximal contractions required to determine MVC in biomechanical normalizations

by

citation

Fw: Fullwave; R: rectified; I: integrated; Cpf: low-pass filtered; A: averaged; rms: rootmean-square converted. Numbers in brackets denote time constant frmsconversion), time resolution (averaging, integration) or cut-off frequency (low-pass filtering)

rms

?

? ?

?

rms

EMG signal smoothing

?

?

Application of load

Maximal

Ramp

Ramp

Ramp

Load and duration of reference contraction(s)

Continued

marks

end

Stated only if explicitly more than one

Repetitions of the procedure

(‘... 7. At the

‘-‘: Regression irrelevant; ‘?‘: regression relevant, procedure not indicated

?

Power

Force-EMG regression model

of each

column

Reference to Winkel et al. 1983=‘. Normalization accepted only if regression gives an r better than 0.95. Signal processing: several references

Signal processing: reference to Ericson & Hagberg 197844

Notes

NORMALIZATION

OF UPPER

at 50% of their maximal strength, the %MVC procedure would seem the more effective in reducing intragroup variance. Thus the two output variables %RVE and %MVC operate on different scales, of which one or the other may appear more favourable depending on the occupational task and the selection of subjects. Moreover, a variance-reducing normalization may conceal interesting biological differences between individuals3. More relevant criteria for the selection of normalization procedure include the physiological information conveyed by the output variable, the scientific value of the method, its testretest reliability, its feasibility in vocational field studies and its costs in terms of equipment and operator education. Biomechanical

Variables - Basic Relations

The major part of studies normalizing EMGamp,, according to a ‘biomechanical’ concept report data in terms of %MVC of the upper trapezius muscle (%MVC,,). Figure 2 illustrates that this translation assumes a constant relative contribution of upper trapezius to the total force development of the synergy, irrespective of its force exertion, including a concert maximal activation of the synergy and the upper trapezius. The validity of translating vocational EMGamp,, into %MVC,,,,,,, depends on the robustness of the originally determined calibration curve (Figure 2a), while the relationship between EMGamp,, and %MVC,, (Figure 2c) is dislocated if the compound expression of the calibration curve (Figure 2a) and the relationship %MVC,,,,,,, vs. %MVC,, (Figure 2b) changes. The absolute force development in the upper trapezius muscle (i.e. in terms of N) cannot be measured directly with any available method and estimation of individual muscle forces in the shoulder region by means of biomechanical modelling is so far quite uncertain”0J11~190J91. Consequently, the true quantitative relationship between %MVC,,,,,,, and %MVC,, (Figure 2b) cannot be determined, but theoretical models indicate that the force partition between and within muscles in a synergy may well be a non-linear function of the synergy’s total force exertion35~‘10J1’J91. Hagberg has suggested a one-to-one relationship on a group level between %MVC,, and %MVC in glenohumeral flexioP. However, the ‘%MVC,,’ was determined from a recording of upper trapezius EMG during shoulder elevation (ad modum Figure 2a) and the data only imply that the transfer functions between Journal

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EMGamp,, and %MVC,,,,,%, are similar in shoulder elevation and glenohumeral flexion. Westgaard*“l reported an illustrative example of a decreasing accompanying an increasing synergy EMGamp,, effort. This finding is not compatible with a oneto-one relationship between %MVC,,,,,,, and %MVC,,. Any translation of EMGamp,, into a biomechanical variable faces the question of whether it is valid in the occupational situations where it is intended to be used. The validity may be influenced by a number of confounders and modifying factors. Some of these, discussed in the section below, have an effect which is largely independent of the normalization method. Other modifying factors relate closely to how the normalization is carried out and will be addressed later in the review. Biomechanical

Translations - Confounders Effect-modifiers

and

Temperature A number of studies have addressed the influence of temperature on the amplitude of EMG during supposedly constant-force contractions. With one exception 158, the amplitude has been reported to decrease with increasing skin temperature17~82~215. The amplitude may change about l-2%.“C-’ in a range of air temperatures between 1.5 and 40” C, as shown in studies of leg muscles with a low metabolic turnover17*215. It is not clear if the changes in amplitude reflect events at the muscle fibre level or if it is related to the signal detection technique. The upper trapezius has not been studied specifically, but temperature may be a factor of concern in occupations associated with large temperature fluctuations, considering that the muscle is large, flat and superficial. However, most indoor work is presumably performed in an environmental temperature range giving only minor effects on the EMG. Changes in intramuscular temperature due to metabolism may possibly modify the EMGamp,, in tasks requiring forceful and long-lasting exertions of the muscle. This has not been evaluated in the literature. Fatigue A number of studies have demonstrated that the EMGamp,, increases, in a linear or exponential pattern, during continuous isometric shoulder exercise at constant force, in healthy subjects61,75J33 as

208

S. E. MATHIASSEN

well as in subjects with disordersmJ33. The amplitude increase rate depends on the load leve1233. It is in the order of 5-lO%.min-l when the stretched arms are held horizontally without additional weight; an effort corresponding to 15-20 %MVCsynergy75J33,233. Significant increases in EMGamp,, have also been reported in intermittent isometric arm holding (2-10 % emin -1)133, flexion-extension movements of the unloaded arm (=1%.min-1)62 and repeated maximal isokinetic shoulder flexions at 60”.s-r (20%.min -l)‘O. These data are consistent with previous studies on other muscles showing that the ratio between surface EMG amplitude and force development increases during prolonged contractions as a sign of decreased muscular performance capacity, i.e. fatigue38. A fatiguing work bout may affect the EMGamp,, during subsequent tasks. Hammarskjold and Harms-Ringdah17i reported a 3040% increase in the EMGamp,, during carpenter’s work immediately after 45 min of arm cranking at about 80% of maximal aerobic capacity. Mathiassen133 showed that the EMGamp,, during an isometric test contraction was increased by up to 40% 4 h after exhaustive isometric arm holding. Thus fatigue effects on the EMGamp,, vs. %MVC,, nergyand, presumably, the EMGamp,, vs. %MVC,, relationships may be perceived as a serious obstacle to the use of EMGamp,, as a load estimate in occupational tasks. Several studies have, however, failed to show any major increase in EMGamp,, during hours of simulated or real occupational work. Amplitude increases of only about 10%-h-’ were reported during strictly controlled, simulated assembly work with light components in the laboratory146,‘81,184 and similar results were obtained during microscope work in a fixed working posture124. Light assembly work monitored either in the laboratory134 or in the field29 resulted in a 0-2%-h-’ increase in consistent with pillar drilling in the EMGamp,,, laboratory28, sewing machine operation in the fieldgl, but slightly less than results obtained during cashier’s work (&15%.h-1)131. The magnitude of these increases is low compared to the effects of other factors contaminating the validity of biomechanical translations of EMGamp,,. In some occupational tasks, however, fatigue-related amplitude changes may be a factor of more concern, for example when hand tools are used above shoulder height (amplitude increase about 1%*min-‘)208. Some of the increase in EMGamp,, during prolonged work may be due to altered motor Journal

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Vol. 5, No. 4, 1995

ET AL. patterns in the shoulder region, including increased levels of co-contraction. More co-contraction defined as simultaneous activity in agonist and antagonist muscles - may be part of the temporal adaptation to exercise, as shown in studies on kneela and elbow48 muscles. Recoordinations without apparent excessive activation may also appear within a muscle synergy during long-term exercise, as indicated in studies of elbow flexors177 and knee extensors175. Changes over time in coordination pattern may therefore in a sense reflect ‘fatigue’ in the central nervous system49. The influence of prolonged work on the coordination pattern of the shoulder-neck muscles is an important issue which has so far not been investigated. Co-contraction and recoordination due to factors other than fatigue will be considered further in a subsequent section. It has been suggested that an increase in EMGamput should be accepted as a sign of peripheral muscle fatigue only if it is accompanied by a decrease in the frequency content of the signa161,108,1s1,233. A number of studies of fatiguing constant-force contractions have, however, reported that significant increases in amplitude may occur without a concomitant decrease in signal frequency parameter61,75,233,234. This observation, amongst others, has raised doubts as to the use of EMG frequency changes as an unerring indicator of muscle fatigue, in particular at low load levels84*234. Therefore it may not be straightforward to separate EMGamp,, changes due to central factors (recoordination) from those reflecting peripheral muscle fatigue. In conclusion, the available evidence suggests that the validity of translating EMGamp,, into biomechanical variables will be seriously affected by fatigue-related events only in few occupational tasks. It may be difficult to ascertain the part of an amplitude increase explained by muscle fatigue, even with the aid of EMG frequency analysis. Recoordination

and Co-contraction

The upper trapezius is part of a complex arrangement of individual muscles in the shoulder region which may be agonists in one force action (e.g. rotation of the scapula) while at the same time antagonists in others (e.g. scapular elevation)16s833 88J36. Due to the muscular multiplicity of the region, a certain work task may be accomplished through a huge number of different combinations of individual muscle actions. The flexibility of the shoulder muscles in solving a specific work task has been

NORMALIZATION

OF UPPER

demonstrated through surface93 as well as intramuscular154 EMG recordings. Jensen et al.93 reported could be voluntarily reduced that the EMGamp,, by 40-60% if it was visually fed back to the subject during an arm elevation task at about 50% MVC. However, the spontaneous motor pattern of an individual performing a certain task seems to be much more stable than would be suggested by these numbers. The test-retest standard deviation of the EMGamp,, during repeated constant-force contraction is only of the order of 10% of the average amplitude’92~‘93. Some of the motor control solutions may imply a generally increased level of muscle activity in the whole shoulder region, and are thus analogous to agonist/antagonist co-contractions around joints with a more simple arrangement of muscles. Co-contractions may be important in stabilizing joints, although they represent an ‘excessive’ effort from a kinematic point of View32,48,176.179

A number of situations relevant to occupational work have been shown to modify the activation of the upper trapezius. Tasks demanding manual precision may imply an increased EMGamp,,, as demonstrated in a study on dentists142 and during a standardized target hitting task203. In general, however, the upper trapezius responded only slightly to precision demands, although some individuals were exceptionally sensitive. Whole-body vibration while driving a truck may more than double the as compared to the value obtained EMGamp,,, when the machine is standing stil1209. Hand and arm vibrations may induce an up to 100% increase in upper trapezius activation during hand-tool operation, depending on body posture164. An increase of about 20% in upper trapezius activation may occur during work performed after vibration exposure, as demonstrated in carpentry tasks74. Acute cold exposure to the hands may increase the upper trapezius activation by about 25% in sawing and screwing tasks73, and strong draughts have been reported to increase EMGamp,, during word processing by 10-15% in some subjects’83. The draught effect were explained by an additional recruitment of motor units in the upper trapezius and temperature effects on the EMG. A number of studies have demonstrated that a considerable upper trapezius activation may appear at the ‘relaxed’ body side during unilateral tasks. This contralateral coactivation may, depending on arm position, comprise 20-50% of the EMGamp,, at the ‘active’ body side both during submaxiJournal

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ma1136,1s6,204 and maximal17” shoulder-neck exertions. The contralateral shoulder was not reported to move in these studies, suggesting that the contralateral upper trapezius was engaged in cocontraction13”. A contralateral ‘spill-over’, however weaker, may even be observed during bilateral submaximal136 and maximall tasks. Excess activation of the upper trapezius may appear as a result of psychological stress. Psychogenic tension in the upper trapezius has been reported during a simple reaction test224, a computerized choice-reaction task203,224, a complex discrimination task47,200 and an arithmetic testlog. The EMGamp,, has also been shown to increase during competitive shooting81, stressed teaching80 and when reward is offered for good performance225. None of these provocations increased EMGamp,, by more than a few per cent of the MVE on a group level, but large individual differences were seen. Psychogenic activation may therefore affect the validity of a biomechanical normalization of EMGamp,, in some individuals, while being of negligible importance in others. Individuals developing shoulder pain during work may react by changing their motor pattern129. The pain-elicited activation of an otherwise relaxed muscle may, however, be of little quantitative importance, as shown in studies on pain provoked by connective tissue stretch77 or chemical agents9. The effects of acute pain on the EMG amplitude of a working upper trapezius muscle has not been investigated so far. In conclusion, co-contractions and recoordinations occurring after the normalization of EMGamp,, may seriously interfere with the determined relationship between EMGamp,, and %MVC,,,,,,, (Figure 2a). It may be hypothesized that recoordinations preferentially affect the relative involvement of the upper trapezius in the synergy, i.e. the %MVC,,,,,,, vs. %MVC,, relationship (Figure 2b). If so, this source of error may be of minor importance to the translation of EMGamp,, into %MVC,, (Figure 2~). Very little is known about the role of the upper trapezius in the concert motor control of the shoulder region. Fast and Forceful Movements In all the studies presented in Table 2, EMGamp,, has been normalized using isometric contractions with the arms in a fixed position. Many occupational tasks, however, require fast and forceful movements

210

S. E. MATHIASSEN

of the arms2,11s,185. The relationship between EMGamp,, and %MVC,,,,,,, was investigated by Elert and Gerdle 41 during maximal isokinetic shoulder flexions with different movement velocities. They found that the EMGamp,, during a single flexion decreased up to 22% when velocity increased. Repeated flexions resulted in a marked gradual increase in the EMGamp,,. These results contrast with similar experiments on the knee extensors, where the maxima1 EMG amplitude has been found to increase according to contraction velocity l’*. An explanation may be that the upper trapezius is not maximally activated during an (unfatigued) isokinetic shoulder flexion with maximal effort, and that the relative involvement of the upper trapezius changes according to glenohumeral flexion velocity. The latter suggestion is consistent with recent findings of recoordinations between active synergists during dynamic elbow flexions148. Movements of the arms may imply shortening or lengthening of the upper trapezius muscle, at velocities depending on both posture and velocity of the armslgO. Velocity changes may have a fundamental effect on the EMG amplitude corresponding to a certain force exertion of an individual muscler6. Furthermore, it has been suggested that velocity changes cause recoordinations between motor unit ‘task groups’ within a musc1e57,127. Consistently, a number of investigations show a weak association between EMGamp,, and force/ torque exertion in highly constrained dynamic tasks involving the arms 53,76. Similar findings have also been obtained in vocational studies under less strict conditions2v’34. The kinematics of the upper trapezius during occupational work has never been assessed, although a biomechanical framework has been developed86~s7~190~‘91. It may be suggested that the relationship EMG(Figure 2a) as determined amput vs. %MKyner,, during isometric contractions, and the relationship %MVCs,ner,, vs. %MVC,, (Figure 2b) changes in tasks involving fast and forceful movements of the arms. The effect of dynamic tasks on the EMGamp,, vs. %MVC,, relationship (Figure 2c) is therefore difficult to predict. ELECTRODE

LOCATION

Of the 77 papers reviewed in Table 2, only 21 describe the exact location of the electrode pair(s) anatomical specifications. reproducible using Eighteen papers give imprecise descriptions, such fo~nai

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ET AL.

as ‘over the belly’, ‘antero-lateral margin’ or a photo without further comments and 38 studies only report that electrodes have been placed above the upper trapezius. Of the 21 studies defining the electrode position, 13 recorded from a location midway between the processus prominens (C7) and the acromion, i.e. the ‘central lead position’ as recommended by Zipp229. Longitudinal

and Transverse Electrode Location

The significance of the location of electrodes over the upper trapezius has been demonstrated in a number of recent studies. A longitudinal displacement of the electrode pair along the muscle fibres was shown by Veiersted ly3 to imply an up to sixfold increase in the EMGamp,, associated with holding the stretched arms close to 90” pure abduction. The lowest EMG activity was observed in a ‘dip’ region slightly proximal to the midpoint of the line between the acromion and C7, while the amplitude stabilized at a higher level with the electrodes placed more than about 10 mm from the ‘dip’ in either direction. Veiersted’s experiments were confirmed and expanded by Jensen et al.93*94, detecting with a multipolar electrode during maximal contractions and standardized movements. An EMG ‘plateau region producing relatively large and stable EMG recordings was shown to appear 2-4 cm lateral to the ‘dip’. The authors suggested that the ‘dip’ region corresponds to the innervation zone of the upper trapezius. Experiments by Haggg5 on action potential propagation along the upper trapezius also indicated an innervation zone in this area. It was concluded that optima1 detection of upper trapezius EMG is achieved outside the central region of the muscle in spite of previous recommendations229. Studies on other muscles have led to similar conclusions and suggest that multipolar electrode configurations may be used to detect (and avoid) innervation zones ‘61,1L)h.The studies of Jensen et a1.93,94,have shown that the longitudinal electrode location influences the amplitude (in volts) of the maxima1 obtainable upper trapezius EMG, and that MVE will be obtained in different ‘movements’ (shoulder elevation, flexion or abduction) depending on detection site. Furthermore, the relationship between shoulder elevation force and EMGamp,, (in terms of MVE from the same electrode pair) seems to depend on electrode location. A transverse displacement of the electrode pair (i.e. in an anterior/posterior direction perpendicular

NORMALIZATION

OF UPPER

to the C7-acromion line) has likewise been shown Veiersted’93 found up to a to affect EMGamp,,. 50% increase in the EMGamp,, during a submaximal isometric contraction when moving the electrodes 15 mm in the anterior/posterior direction within the ‘dip’ zone midway between C7 and the acromion. The experiments of Mathiassen and Winkeli3” and Mathiassen133,‘34 compared EMG from sites 2-3 cm apart on a transverse line lateral to the midpoint of the C7-acromion line. The EMGamp,, from the two locations differed significantly, for example in its relationship to glenohumeral torque and in its temporal pattern during occupational work. Furthermore, transverse movement of the electrode pair has been shown to interact with the influence of arm position on EMGamp,, during submaximal glenohumeral torque exertions136 and handling of light weights’“. In line with this, Jensen and Westgaard9” present data in the present issue indicating that the activation of the upper trapezius in terms of its maximal EMG amplitude varies by up to 10% in a submaximal shoulder elevation, depending on the transverse location of electrodes. An important question is to what extent the dissimilarities between detection sites are reflected in ergonomic analyses of occupational tasks, using the amplitude probability distribution function or the exposure variation (APDF)98 analysis (EVA)137. Jensen et al. 93 studied a movement test with a median load of 3 %MVE according to electrodes in the ‘plateau’ region, and showed that longitudinal displacements of the electrode pairs in either direction increased the estimated median load to at the most 8 %MVE. Mathiassen134 reported differences in the upper trapezius activation level and repetitiveness during light assembly work according to EMG recordings from two electrode pairs transversally spaced by 2-3 cm. The EMGamput was normalized in terms of a submaximal reference EMG (an RVE). The quantitative significance of electrode location may depend on the EMG normalization procedure and the investigated work task, as indicated by Bao et al. in the present issue’“. The evidence given above indicates that the location of upper trapezius electrodes should be carefully ascertained and reported in ergonomic studies, preferably in terms of anatomical landmarks. A large proportion of previous studies are insufficient in this respect (see Table 2). Even careful placement of electrodes may result in a ‘random’ inaccuracy sufficiently large to influence the properJournal

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ties of the EMG amplitude. Veiersted’“” used a meticulous procedure based on individual tracing sheets referring to anatomical landmarks, but was able to replace electrodes on the upper trapezius with a precision of only +2 mm from the target (90% confidence interval). Evidently, less careful procedures may be expected to be less reliable. The precision of feasible field procedures for placing electrodes, for instance using standardized anatomical landmarks, has not so far been evaluated. Functional

Subdivision

of the Upper Trapezius

The significance of electrode location above the upper trapezius has been interpreted as a sign of the upper trapezius being ‘compartmentalized’95,134,136, meaning that the motor unit recruitment pattern within the upper trapezius changes according to the instantaneous character of its action in terms of forcefulness, muscle length and movement velocity. has been Compartmentalism (or ‘multipartism’) demonstrated in several other human muscles27~57,107,138,163,222and has been suggested as a general principle in motor contro142,127,210. Compartmentalism would be particularly convenient in a muscle with highly diversified biomechanical functions, such as the upper trapezius16~s3~96~‘91. Long-term isometric contractions have been suggested as a cause of oscillating recoordinations within the biceps brachii45, supposedly in order to attenuate the development of fatigue. Temporal recoordination has not so far been studied systematically in the upper trapezius, but Winkel et a1.212 suggested alternating recruitment as an explanation of their upper trapezius EMG findings during floorcleaning. If the distribution of active motor units within the upper trapezius changes according to the kinetics and/or duration of a work task, several EMG detection sites will be needed to satisfactorily describe the compound action of the upper trapezius. However, in ergonomic exposure assessment especially in the field - it may be more feasible to represent the upper trapezius by only one electrode set-up. The consequences of this approach in terms of information loss and restricted validity should be further investigated. Cross-talk All of the papers summarized detected upper trapezius EMG

in Table 2 have by means of a

212

S. E. MATHIASSEN

bipolar, single differential technique with the common reference electrode placed over an electrically neutral tissue16. This electrode configuration provides stable detection of surface EMG, but it is sensitive to cross-talk from adjacent muscles. Recordings from the bulky belly of the upper trapezius have been commonly assumed to be uncontaminated, while cross-talk is a recognized problem when detecting at locations where the upper trapezius is thin and covers other muscles, such as close to vertebral colthe umn47,79,118,136,166,168,169,182,184,200

Or

c]ose

to

the

acromio-clavicular joint93,136,142,166,*68,169. Crosstalk may be partly eliminated by using an annular electrode configuration230, by reducing the surface area and spacing of the electrodeP9, or by double differential processing of signals from multiple electrode arrays34,‘39. The latter technique has recently been applied to the upper trapeziuF and in a paper in the present issue, Jensen and Westgaard95 demonstrate that double differentiation does provide a modified EMG signal from the lateral part of the upper trapezius. Their results indicate that cross-talk may be a factor of concern, even at detection sites commonly assumed to reflect only the upper trapezius. However, the quantitative significance of cross-talk at different electrode locations above the upper trapezius remains to be investigated. Multiple electrode arrays may, in addition to offering a comprehensive and specific signal, provide an approach for assessing muscle activation on a motor unit level’62.226. So far the technique has not been tested on the upper trapezius, but it might give further insight into the representativeness of conventional surface EMG. The rest of the present review will concentrate on upper trapezius EMG as detected in most studies (presumably including those which do not specify the electrode location), i.e. with an electrode pair in line with the muscle fibres in the vicinity of the midpoint between C7 and the acromion. POSTURE

AND ATTEMPTED

MOVEMENT

Arm Posture and ‘Movement’ A large variety of arm positions and attempted movements have been used for EMGamp,, normalization in the literature (see Table 2). The most prevalent are vertical shoulder elevation with the arms along the body (28 out of 77 studies) and anti-gravity ‘movements’ of the horizontal arms in journal

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either pure abduction (nine studies) or pure flexion (six studies). Twenty-six studies do not report the arm position used for the normalization. A normalization in terms of MVE must aim at finding the ‘movement’ by which the largest possible EMGamp,, can be obtained. In a paper containing guidelines for EMGamp,, normalization99, Jonsson stated - without experimental evidence - that the MVE should be obtained with the arms in 90” flexion. Schtildt and Harms-Ringdah1170 compared shoulder elevation, glenohumeral abduction in the scapular plane and 90” glenohumeral flexion, the two latter positions with horizontal arms. They found that ‘5.5’ out of 10 subjects produced the maximum EMGamp,, in abduction, ‘2.5’ in flexion and one in elevation. The last subject achieved the MVE in a retraction/elevation movement of the isolated scapula. Westgaard201 reported that the highest upper trapezius activation may appear both in abduction and elevation; in some subjects the two estimates of maximal EMG differed up to fourfold in magnitude. This result was confirmed by Veiersted et al.194,1y5and Jensen et al.92 using similar normalization procedures. In contrast, Nieminen et al.150 support Jonsson’s recommendation by reporting that the maximum EMGamp,, was obtained most often in 90” glenohumeral flexion, and never in shoulder elevation or glenohumeral abduction with the arms along the body. Substantial differences in maximal EMG amplitude due to the position of the involved joint has also been reported in other muscle synergies, for example the lumbar back muscles144. The maximal EMG amplitude of a muscle seems to be independent of its length19’ and differences related to joint position may be explained in part by the electrodes sampling from different volumes of the underlying muscle, and in part by recoordinations between muscles in the synergy and within the muscle itself. Twenty-nine studies in Table 2 translate EMGamput into a biomechanical variable. All of these studies carry out the normalization in only one arm position. A number of recent studies have discussed whether such a relationship will be valid during occupational work where the arms are moved within a range of postures. Takala and Viikari-Junturals6 reported up to 10% differences in the relationship between EMGamp,, and glenohumeral torque during slow arm movements within 510” from a 60” glenohumeral flexion in the sagittal plane. Veiersted193 found a systematic standard deviation of 23% in values of EMGamp,, obtained when the

NORMALIZATION

OF UPPER

straight and unloaded arms were held in postures deviating less than 20” from a reference position close to 90” glenohumeral abduction. Moving the arms within 25” from a position of 90” horizontal flexion changed the relationship between EMGamtorque by up to 30%, P and glenohumeral aiiording to Nieminen et a1.150. Mathiassen and Winke1136 showed that the EMGamp,, associated with a glenohumeral torque of 15 Nm (-30 %MVC) changed by up to 50% when the arms were moved over 90” in the horizontal and 45” in the vertical plane from an initial position of horizontal 90 abduction. The relationship between EMGamp,, (Figure 2a) has consistently been and %MVC,,,,,,, shown to depend on arm position18n~‘92. Changes in arm position influence the length of the upper trapezius fibresr9”, but studies on other muscles indicate that muscle length may only influence the relationship between EMG amplitude and relative muscle force to a minor degree15’%“‘. All of the cited studies therefore confirm the notion that arm position profoundly influences the relative engagement of the upper trapezius during a glenohumeral torque or force exertion (Figure 2b). However, the studies do not object to the use of EMGamp,, as a measure of %MVC,, (Figure 2~). Even a work station of good ergonomic design may allow arm movements within a wide range of angles in front of the body, particularly in the horizontal planes4. Studies by Aards et al.* and Kilbom and Persson’ Is have demonstrated that occupational work in ‘constrained’ postures may be performed with the upper arms deviating more than 20” from their median flexion or abduction position for considerable parts of the total working time. As shown above, this deviation may imply changes in the EMGamp,, of the order of 30% with only minor changes in the force/torque exertion of the synergy. It has been suggested that EMGamp,,, should be normalized in arm positions resembling the occupational work in which the biomechanical estimate is to be used. Conceptually, this may be an attractive approach in occupational tasks with restricted arm movements, but it seems less feasible in tasks involving large and frequent movements. First, it would require normalizations to be made in a selection of representative arm positions, and second, a continuous monitoring of arm position during work would be needed as a basis for choosing the adequate normalization at each instant. If posture is not measured, normalization of EMGGourd

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amput in several arm positions does not offer any advantages compared to normalization in one arm position in the middle of the relevant range of movement. Procedures operating in the bioelectrical domain do not have to consider whether the normalization posture is representative of occupational work. Instead, the principal objective of, for instance, an MVE procedure would be to find the position and effort providing the largest EMGamp,,. In summary, arm posture has a large effect on the relationship between EMGamp,, and %MVC,,,,,,, (Figure 2a). The effect may be explained mainly by changes in the relative engagement of the upper trapezius in the synergy (Figure 2b). Translation of EMGamp,, to %MVC,, (Figure 2c) may therefore be less sensitive to changes in arm position. Head and Trunk

Posture

More than half of the papers listed in Table 2 (41 out of 77) do not indicate the body position of the subject during the normalization procedure. In 25 studies the subject was sitting, in 10 standing, and one study used a supine position. The role of the upper trapezius as an anatomical link between the shoulder girdle and the vertebral column suggests that neck position or back curvature may influence the involvement of the upper trapezius in a specific work task. The influence of head/ neck position on upper trapezius activity has been extensively studied. Movements or maintained postures of the head in the flexion/extension plane do not seem to involve the lateral parts of the upper trapezius to any particular extent2~4n~78~79J69.Even a maximal isometric exertion in neck extension or flexion causes only a slight lateral upper trapezius activity 112~‘61~171.Lateral bending”* or twisting209 of the neck may, however, provoke the upper trapezius to a considerable extent. In a laboratory study of office work, Bendix et al.*’ found higher lo- and 50-percentiles in the cumulated distribution (the APDF) of EMGamp,, during standing than during sitting, although the work task was performed in a more upright posture with the arms closer to the body at the standing work station. This finding suggests that the relative engagement of the upper trapezius during manual tasks may depend on the body position, but the issue has not been specifically investigated so far. A carefully fixed sitting posture during normalization may be preferable to standing. Sitting offers

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better conditions for obtaining a pure force/torque action of the shoulder muscles, avoiding undesirable actions in other body parts. In summary, the exact posture of the upper back and head seems to be of minor importance to the result of an EMGamp,, normalization, while the significance of standing/sitting/supine postures is not known. The validity of translating EMGamp,, into synergy force/torque exertion may be low in vocational tasks involving twisted or laterally bent head postures. LOAD

AND DURATION OF THE REFERENCE CONTRACTION(S) Load

Normalization procedures in terms of MVE and MVC inevitably include a determination of the individual’s maximal strength. This applies to 70 of the 77 studies in Table 2. Maximal force exertions are conceptually well-defined and offer a physiological anchor point which is normalized between individuals. However, the assessment of ‘maximal’ performance may have a questionable validity if the subject is unaccustomed, unmotivated or suffers from pain, in particular if measurements are made in the field143,193,201. This may present a serious source of differential misclassification165 in epidemiological studies expressing workload in diseased and non-diseased groups in terms of the individual’s ‘maximal’ capacity. As discussed in a previous section, the MVE of the upper trapezius differed between glenohumeral abduction and flexion. The maximal glenohumeral torque is approximately the same in the two positions 61J19J36. Together, these results point out that the upper trapezius is not necessarily maximally activated during a maximal exertion of the shoulder. A similar conclusion follows from the results of Bao et al. presented in this issuei that the upper trapezius is activated more during a maximal bilateral arm abduction than during a corresponding unilateral exertion. It is at present not known whether it is possible to provoke the upper trapezius to its true maximum by any voluntary effort of the whole shoulder complex. Studies of the elbow musclesls9 indicate that it may not be possible to activate muscles involved in a co-contraction maximally, due to inhibition in the central nervous system. This may apply to the upper trapezius according to its function as a stabilizer of the scapula Journal

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rather than a prime mover16,83,96. In addition, it seems reasonable to assume that the upper trapezius is seldom the limiting link in a maximal force exertion of the whole shoulder. The validity of ‘maximal’ upper trapezius activations should therefore be treated with caution. Non-maximal upper trapezius activation during a maximal synergy effort is a violation of the one-toone relationship between %MVC,,,,,,, and %MVC,, (Figure 2b). The considerations above are therefore relevant when discussing the validity of translating EMGamp,, into %MVC,, (Figure 2~). On the other hand, non-maximal upper trapezius activation does not as such interfere with the use of a EMGamp,t vs. %MVC,,,,,,, relationship (Figure 2a). Maximal contractions may cause tissue trauma, discomfort and delayed soreness193,201. This should also be considered when discussing the appropriateness of normalization procedures including maximal force exertions. The relationship between EMGamp,, and %MVC synergyhas most often been determined using a ramp procedure, i.e. a simultaneous monitoring of EMGamp,, and %MVC,,,,,,, during a gradually increasing force/torque exertion in the shoulder. This technique has been used in 1.5 studies in Table 2. Ramp contractions require concentrated cooperation by the subject and may therefore be less feasible in field conditions”J5. Constant-force contractions at stepwise increasing levels of exertion (‘. . repeated reference voluntary contractions (KC), Figure 1) have been suggested as an alternative to ramps 9g, Only a few of the studies referred to in Table 2 have used the stepwise RVC approach 13,69J28. In this issue, Attebrant et al.” present data indicating that load estimates obtained by a stepwise RVC approach may be about 20% less than estimates obtained by a power regression ramp. The result was, however, not statistically significant due to large differences between individuals. Consistently, Mathiassen and Winke1’36 reported that the EMGamp,, corresponding to a glenohumeral torque of 15 Nm (-30 %MVC) was about 25% less if the torque was obtained by handheld weights than if it was exerted during a ramp contraction. The authors suggested that the motor control patterns differed between the two tasks. Studies on other muscles have confirmed that this hypothesis may be valid2*. Mathematical modelling of relationships between EMGamp,, and

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%MVG,n,r,, will be discussed in the subsequent section on signal processing. The relative force development of the upper trapezius has been widely accepted as a meaningful expression of upper trapezius load during OCCUpational work. However, several physiological variables relevant to occupational discomfort and risk may be more closely associated with the absolute workload than with relative strength performance. This may apply to intramuscular pressure15,‘“5 and development of discomfort and fatigue24,30,2”7. Epidemiological evidence consistently indicates a weak association between shoulder muscle strength and disorders in occupational work with mostly low force requirements .‘13,*18. We therefore suggest that more attention is paid to developing expressions of upper trapezius EMG which refer to absolute load. Examples may be to normalize EMGamp,, in terms of the amplitude obtained while holding a standardized weight”.12,14,47.75,134*143 or while exerting a standardized submaximal glenohumeral torque (in Nm) 136. These procedures also have the advantage of avoiding maximal force exertions. In the bioelectrical domain they offer a reference EMGamp,, obtained during a standardized submaximal contraction (i.e. an RVE). The cited examples, however, vary widely in the load and arm posture of the reference contraction. The resulting RVEs therefore differ in their relationship to previous normalization outcomes11,14,‘43. The development of standardized RVE procedures is an important research issue. A biomechanical normalization of EMGamp,, by means of isometric reference contractions may, as discussed in a previous section, be of questionable validity in vocational tasks comprising fast and forceful arm movements. It may therefore seem an attractive idea to normalize EMGamp,, during dynamic reference contractions. Normalization of EMG amplitude by values obtained during a ‘test task’ has been applied in gait analysis15* and to dynamic movements in the elbow3 and the anklerIb. So far it has not been tested in shoulder-neck activities. However, the approach requires an elaborate biomechanical calculation of the force/torque exertions during the test task. Furthermore, it may be difficult to construct a standardized task template which is of general relevance to many occupational situations. Therefore the feasibility of a dynamic normalization of EMGamp,, may be questioned.

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Duration The optimal duration of a single reference contraction must be viewed as a compromise between obtaining sufficient and valid data and avoiding development of fatigue. Only a few of the studies in Table 2 state specifically the duration of the normalization reference contractions. Ramps are reported to have lasted for 10 s with a force increase rate of 3-7 %MVC.s-’ 2”,29,h6, or for about 5 s with 10 %MVCs-’ increase in force194. Westgaard*” presents an illustration of a O-40 %MVC ramp about 30 s long. In contrast, Jensen et a1.93 have used a ramp procedure comprising a force increase from O-100 %MVC within 3-4 s. The relationship between EMG and force may be distorted if the force is increased too fast”“, but at force increase rates below 20 %MVC.s-’ only mild effects may be expected 123. The majority of the ramp procedures mentioned lie well within this limit. If, however, the force exertion continues for too long, fatiguerelated changes in the EMG amplitude may appear. Within 15 s of holding the stretched arms horizontal without additional weight (~15 %MVC) the EMGamp,, may increase by 2-3%75,133,233. If a 2 kg hand-held weight is added, the amplitude increase may amount to 45% within the 15 s233. Criteria for selecting the duration of a submaximal constant-force RVC have not been discussed in the literature, and examples may be found in the whole Research range between 1 and 15 s 11.25,128,136,201~ is needed to ascertain the optimal duration of a RVC according to level of exertion, considering variance in the corresponding EMGamp,, and fatigue effects. Maximal exertions constitute a special case because the definition of ‘maximal’ most often includes the time aspect. The subject is typically required to attempt an MVC for a few seconds during which the EMG is recorded7~31~128~18”~1s7. Afterwards, the MVE may be defined as the maximal EMG amplitude obtained at a single sampling point or during a well-defined averaging period. The outcome value obviously depends on this definition94, but most studies fail to report the exact procedure. Questions specifically related to the repetition of reference contractions will be dealt with in a subsequent section.

S. E. MATHIASSENETAL.

216 SIGNAL Signal Amplitude

PROCESSING Detection and Smoothing

Amplitude detection of upper trapezius EMG has been performed either by a full-wave linear rectification of the raw EMG or by applying a root mean square (rms) technique, i.e. squaring the raw EMG, smoothing it (see below) and taking the square root of the resulting signalz20. In the linear rectification, smoothing is performed in a subsequent, separate step. Basically, three different classes of smoothing techniques have been used on EMGamp,,: analogue low-pass filtering, digital moving-window averaging and digital averaging in consecutive windows. All three approaches can be applied in connection with linear rectification or as a part of the rms conversion, and the cut-off characteristics may be varied using different filtering time constants, window lengths or averaging techniques. It should be noted that analogue filtering with a certain time constant is not equivalent to averaging with a moving window of the same length220. Twenty-eight of the studies in Table 2 used an rms conversion and in 26 the linearly rectified EMG was averaged. Only a minority of these papers report the procedure in exact and reproducible terms and many fail to indicate the low-pass properties of the smoothing filter used. More sophisticated filtering techniques have been suggested to offer a better representation of important components of a vocational signal, such as its ‘static’ and ‘dynamic’ properties149. These techniques, including median filtering, have been used in only a few studies125,150. The smoothing procedure must be adjusted according to the type of information aimed at in the signal. Different techniques may well be optimal for EMGamp,, interpreted in a bioelectrical domain and EMGamp,, used as an estimator of force. The significance of the low-pass filter setting has been investigated by Zuniga and Simons232, concluding from visual inspection that a rms time constant of 200 ms produced the ‘desired amount of smoothing’ of biceps brachii EMG during an elbow flexion ramp. In another study on the biceps muscle, Hagbergm aimed at establishing the rms time constant resulting in the best possible agreement between processed EMG amplitude and elbow flexion torque. On the basis of cumulated distribution functions (APDF) of processed EMG amplitude and force, he recommended a time constant of 50-100 ms. Later EMG studies on gait have Journal

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confirmed that muscle force and EMG amplitude correlate best if the linearly rectified EMG is lowpass filtered with cut-off frequencies between 1 and 4 Hz152,221, corresponding to time constants of 160-40 ms. Winkel and Bendix21* accepted rmsconverted EMG as an expression of muscle force only if it attained a stable value for more than 0.3 s. A number of vocational studies have applied smoothing procedures which are even coarser, such as signal averaging within consecutive windows of l-2 s1;164,202or even 30 Sag,*@‘. In the latter cases, the ambition of translating EMGamp,, into a continuous force equivalent has obviously been abandoned. In this issue, Jensen et al.94 present MVE data from the upper trapezius showing that the signal amplitude may be decreased by about 15% if the length of an averaging window is increased from 0.2-2.0 s. A rectified and smoothed EMG signal may still exhibit a substantial variance, which is probably not due to force changes in the underlying muscle but is rather an expression of the inherent stochastic character of surface EMG33. Several papers present illustrative examples of variance in rms-converted EMG which cannot be explained by force variations29,149,232. In isometric constant-force arm flexions at about 30% of maximal glenohumeral torque, the standard deviation in rms-converted (time constant 100 ms) EMGamp,, corresponds to 1520% of the signal’s mean amplitude (Mathiassen, unpublished data). The value is consistent with Figure 3 in Jensen et al.91, showing the distribution of EMGamp,, during a constant-force contraction at a similar force level. A standard deviation of about 15% may theoretically be expected in a stationary, stochastic signal with EMG frequency characteristics which is rms-converted in consecutive windows of 100 ms18. A point-to-point translation of EMGamp,, into a biomechanical variable may therefore be subject to substantial inaccuracy. In a study of ramp normalization reported in the present issue, MathiassenL35 shows that an EMGamp,, assumed to correspond to 25 %MVC,,,,,,, may in fact in 5% of all cases originate from torques less than 15 %MVC or larger than 35 %MVC. This estimation error is particularly unfortunate when EMG is analysed in a real-time domain. During recent years, a number of ergonomic methods have been presented which aim at quantifying repetitiveness, i.e. the real-time frequency of shifts between amplitude levels2r6. Examples are the ‘contraction frequency analysis’*ll, different ‘gap’

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analysis techniques 92.187,194,‘temporal pattern analysis’L26 and ‘exposure variation analysis’137. Analysis methods such as the APDF procedureY8 which operate only in a relative time domain of force or torque will be much less sensitive to residual variance in the EMGamp,,, if they are only carried out on sufficiently long sequences of EMG. The above-mentioned study by HagbergW, evaluated EMG smoothing only on the basis of APDF distributions, and little is known of how to optimize signal processing with reference to ergonomic realtime analysis of EMGamp,,. In summary, the signal smoothing technique may have profound effects on the validity of EMGamp,, normalization. The optimal smoothing procedure may depend on whether the outcome variable belongs to the bioelectrical or the biomechanical domain. Models of the Relationship between EMGamp,, and Biomechanical Load The relationship between EMGamp,, and was originally suggested by Jonsson %MKynergy to be linear up to about 30 %MVC99. The suggestion was based on averaged data from intermittent isometric elbow flexions lasting several minutes5*“. The linearity of the instantaneous relationship between EMGamp,, and %MVC,,,,,,, was not supported by experimental evidence. Nevertheless, Jonsson’s statement has been cited and used in a number of studies13,63,69,*94 and has been assumed also to apply in a wider force range90.i2”. In addition, a number of studies may be found in Table 2 in which upper trapezius EMG is recorded only during a maximal effort, while vocational EMG is stated in terms of %MVC,,“‘,31,52,53.56. These studies assume a linear relationship between EMGamp,, and %MVC,, in the entire O-100 %MVC force range and we suspect that some of them confuse electrical activation (%MVE) and force exertion (%MVC). A large number of papers have reported positively accelerating (concave) relationships between EMGamp,, and %MVCynerg,, (Figure 2a) when the force range is extended beyond 30 %MVC29,“,59.h8.136,150,*3~. At an EMGamp,, level of about 25 %MVE, the corresponding exertion in the synergy may be of the order of 30 %MVC4 or even 50 %MVC’so~23s, presumably depending on the reference contraction used during normalization and the subsequent mathematical modelling. Data presented by Jensen et al.93,95 and Attebrant et Journal

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a1.l’ suggest that the EMGamp,, vs. %MVCSynergy relationship may be far from linear even at low force levels. A linear or concave relationship between EMGamp,, and %MVC,, (Figure 2c) is consistent with the extensive literature dealing with forceEMG relationships123’156,223. A number of studies11,59,93,95,201have, however, reported that negatively accelerating (convex) relationships between EMGamp,, and %MVC,,,,,,, may occur. A convex relationship between EMGamp,, and %MVC,, is incompatible with basic electrophysiology and the cited data strongly suggest that the relative involvement of the upper trapezius has changed according to the force exertion of the synergy (Figure 2b). A number of mathematical models have been suggested to describe the relationship between EMGamp,, and %MVC,,,,,,,. All have been based on a least-square regression approach. According to Table 2, the one most frequently used is a power model of the form: EMGamp,, = a.%MVCb. Other models used in the literature on EMGamp,, have been rectilinear (as mentioned above), piecewise rectilinear51.59, polynomial of the second150, thirdg5 or fourth93 order, or exponential’3h. The best-fit model according to an r2 criterion (see below) may differ according to the health status of the subject68 and between right and left upper trapezius in the same subject59. The idea of mathematical modelling was discussed on a common-sense level early in the history of vocational EMG44,97, but very little quantitative data has been presented so far on the physiological and statistical qualities of different approaches. Specifically, the scientific precedence of power regression remains to be documented. The power model forces the EMGamp,, vs. %MVC,,,,,,, relationship through the origo. This constraint is reasonable from a physiological point of view, but may not always be justified in the modelling of real data. Christensen2” showed an obvious example of data inappropriately regressed by a power model. A thorough discussion is also lacking as to whether EMG vs. force data from ramps or stepwise RVC procedures fulfil the statistical prerequisites for straightforward least-square regression2h. Consecutive samples of EMG vs. force from a ramp contraction are required to be statistically independent and the variance in EMG amplitude must be constant, irrespective of the force level. Examples29~1s0 exist to show that the latter condition may be severely violated if the EMGamp,, data are used without a variance-stabilizing transformation26.

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Furthermore, the conventional choice of EMGamp,, as the dependent variable, changing in response to the independent, controlled variable %MVC,,,,,,, may be questioned. From a physiological viewpoint, the surface EMG may rather be seen as an expression of bioelectrical events causing force to develop. Accordingly, it has been suggested that the regression should be carried out using EMG amplitude as an independent variable122z14i. Obviously this may result in a different quantitative relationship between EMG amplitude and force than the conventional reversed regression approach. The quality of the regression model has conventionally been assessed by stating the relative variance in EMGamp,, explained by regression, i.e. the value of 13 19,93,136;150*214.A larger value of r2 may be obtained by increasing the range of the independent variable (in this case %MVC,,,,,,,). This may have led some authors to extend the force range of the ramp far beyond the load levels occurring in the studied occupational tasks. Evaluating the quality of the regression model only on the basis of r2 may, however, be misleading. First, the inclusion of extreme values may incline the regression equation to be less correct at lower load levels. Second, the quality of the model in terms of providing a confident estimate of the dependent variable depends on the absolute residual variance after regression 26. Mathiassen demonstrates, in this issue’35, that a narrow ramp force range may indeed imply a smaller r2 in the EMGamp,, vs. force regression than a wide range, but that force may be estimated from EMGamp,, with similar confidence in the two cases. Quality measures based on absolute residual variance have been approached in only a few other studies of upper trapezius EMG59,150 and it is suggested that a higher priority should be given to this issue in future studies using regression. NUMBER

OF REPETITIONS NORMALIZATION

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A variety of measures are available to quantify normalization repeatability116. Below, most attention will be given to the coefficient of variation (CV), i.e. the standard deviation as a percentage of the average. This variable has been extensively used in the literature as a measure of repeatability and provides a basis for estimating the number of repetitions required to achieve a certain statistical confidence. The random error associated with normalization of EMGamp,, will appear as a component journal

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ET AL. of the total random error on determinations of upper trapezius activation in ergonomic studies. Test-retest Repeatability,

Same Electrodes

Repeated determinations of upper trapezius MVE using the same electrode application has been reported to be associated with CVs from 614% 14.93.192. Recalculations of the data presented by Jensen et al. in this issue94 gives a CV of about 12% on MVE values from repetitions within a few minutes of a maximal shoulder elevation and about 8% on MVEs obtained in the course of 2 h during either shoulder elevation or glenohumeral abduction. A CV of about 9% has been reported in MVE investigations on the triceps brachii muscle228. A submaximal RVE may likewise be repeated with a CV of 613% under carefully controlled circumstances in the laboratory14~‘23~1Y2,1~3, while semi-field conditions may increase the CV slightly”. This emphasizes the importance of a meticulous standardization of the conditions during normalization. A normalization procedure based on one reference (MVE or RVE) will impose a random error equal to its CV on normalized occupational irrespective of the level of muscle EMGamp,,, activation. In contrast, the test-retest repeatability of a ramp normalization (power-regressed volts vs. %MVC synergy)depends on the load level, showing a CV of 46% at 5 %MVCsynergy, but only 11% at 30 % MVCsynergy l l. Ramp normalization may therefore offer a better confidence than single-point procedures in heavy occupational work, while the opposite seems to apply to work with a low upper trapezius load. A CV of 10% implies that if the reference contraction is performed only once - which is the case in most vocational studies - the probability of getting an EMGamp,, value within *lo% of the ‘true’ normalization reference is only 68%. If four repetitions of the normalization procedure are carried out instead, their average EMG will lie within ?lO% from the ‘true’ value with 95% probability. More repetitions increase the exactness of the estimated reference EMGamp,, in proportion to the square root of their number, but may result in the development of fatigue-related changes in amplitude95z193. As shown, MVE and RVE procedures may be of quite similar test-retest repeatability when performed on healthy young subjects in the laboratory. Important information is lacking about the

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repeatability of different EMGamp,, normalization procedures in unaccustomed subjects entering an ergonomic field study. Test-retest Repeatability,

Electrodes Replaced

Replacing the electrodes introduces additional sources of test-retest variance. A major source may be changes in the location of electrodes, even if these are intended to be identically repositioned’93. Other conceivable sources, especially in field studies, may originate in the measurement situation, for example climatic changes or disturbances from the surroundings. This compound variance has a direct bearing on the power of comparing any two situations in which upper trapezius EMG have not been sampled with the same electrodes. Thus the calculations below apply to comparisons of different studies within a meta-analysis. Veiersted193 estimated the between-days CV in a carefully controlled RVE determination to be 23%. Mathiassen and Winkel136 reported a CV of 25% on the EMGamp,, corresponding to a 15 Nm glenohumeral torque obtained during a ramp contraction and normalized by an RVE determined during weight-holding in 90” abduction. In a study of assembly work 134 Mathiassen found a betweendays within-subject’ CV of 20% in normalized median EMGamp,, during work (unpublished data). This CV includes variance contributions related both to modifications in motor performance during the work task and to errors in the normalization RVE, which were obtained during a 15 Nm glenohumeral torque. Assuming these two sources to be of equal magnitude, the compound CV could have been reduced to 16% by performing the normalization procedure four times instead of one, as outlined above. A between-days CV of 20-25% is somewhat larger than the value obtained on elbow extensors228. The difference may show that normalization of EMGamp,, is particularly sensitive to small variations in the normalization procedure. The between-days within-subject CV on normalized EMGamp,, in a given occupational task performed in the field may well be larger than the 2@-25% obtained in the laboratory. A CV of 25% on the normalized EMGamp,, corresponding to a certain work task implies that the probability is only 30% that two observations on different days of ‘the same’ EMGamp,, will differ less than 10%. A CV of 25% also implies that 14 subjects are required in a paired experimental design to detect

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a true intervention change of 20% in normalized EMGamp,, at a 0.05 significance level with a power of 0.803(j. Twenty-six subjects would be necessary to detect the same difference between two independent groups (i.e. in an unpaired design). Study power could theoretically be improved if the normalization variance was reduced, by using a refined and carefully standardized normalization procedure. Table 2 illustrates that previous studies may have been insufficient in this respect and that considerate normalization may indeed improve the statistical quality of future ergonomic studies. RECOMMENDATIONS FOR THE NORMALIZATION OF EMGamp,, Most of the recommendations below are our practical conclusions to the discussion in the present review. In case of lacking scientific evidence, we have suggested temporary solutions which may be changed according to future research. The procedure is suggested to be incorporated in future studies as a mandatory minimum, in addition to which the researcher may optionally add other normalizations according to his/her own preferences. The guidelines apply to bipolar surface EMG. Outcome Variable Normalization should be made only in terms of the electrical activity of the upper trapezius (i.e. restricted to the bioelectrical domain). The procedure should always include the EMGamp,, during a submaximal effort (i.e. an RVE) and, if not contraindicated, the electrical activity during a maximal effort (MVE). All vocational EMG results should be reported in terms of RVE, and the most important findings also in terms of MVE, if possible. Electrode

Location

A thorough estimation of the involvement of the upper trapezius may only be obtained by several electrode pairs in the upper trapezius region. All studies should include recordings from electrodes placed 2 cm apart on the line between C7 and the acromion with the electrodes centred 2 cm lateral to the mid-point of the line. Posture and Attempted

Movement

The RVE should be obtained while holding both arms straight and horizontal in 90” abduction (i.e.

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in the frontal plane of the body). The hands should be held relaxed, with the palm pointing downwards. The subject should be dressed in a light shirt adding negligible weight to the arms. If the normalization includes a MVE, this should be determined with the arms in the same position as used for the RVE. The subject should be seated and fixed during the normalization, with the head in an untwisted and upright position. Load and Duration

of Reference Contraction(s)

The RVE should be obtained while holding the arms as described above without additional load. Each determination should last 15 s and the average EMG amplitude should be calculated for the middle 10 s of that contraction. During the determination of MVE, resistance should be given proximal to the elbow. The subject should attempt to hold a maximal contraction for 5 s and the MVE should be defined as the highest EMG value obtained within a digital moving average window of 1 s length. Signal Processing The EMG signal should be smoothed using a rms procedure with an averaging moving window of 100 ms length. Number

of Repetitions

of the Normalization

Four consecutive determinations should be made of the RVE, with at least 1 min in between each. The average of these four RVE values constitutes the RVE to be used as normalization reference. Three determinations should be made of MVE, separated by at least 2 min. The highest value among the three MVEs should be chosen as the normalization reference. CONCLUDING

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thorough description of proper normalization procedures for upper trapezius EMG amplitude. Specifically, we draw attention to the suggested guidelines. In addition, we wish to encourage further research on the issues pointed out by the review. REFERENCES 1. Aarls A, Westgaard RH: Further studies of postural load and musculo-skeletal injuries of workers at an electromechanical assembly plant. Appl Ergon l&211-219, 1987. 2. Aaris A. Westeaard RH. Stranden E: Postural angles as an indicator of postural load and muscular injury in occupational work situations. Ergonomics 31:915-933, 1988. 3. Allison GT, Marshall RN, Singer KP: EMG signal amplitude normalization technique in stretch-shortening cycle movements. J Electromyogr Kinesiol 3:236-244, 1993. 4. Anderson JAD: Shoulder pain and tension neck and their relation to work. Scand J Work Environ Health 10:435-442, 1984. 5. Antti C-J, Bjiirksttn M, Ericson B-E, Jonsson B: Relationship between work and integrated myoelectric activity in long-term vocational studies. In: Riomechanics V-A, ed by Komi PV, University Park Press, Baltimore, pp 515-519, 1976. 6. Antti CJ: Relationship between time means of external load and EMG amplitude in long term myoelectric studies. Electromyogr Clin Neurophysiol 17:45-53, 1977. 7. Arborelius UP, Ekholm J, NCmeth G, Svensson 0, Nisell R: Shoulder joint load and muscular activity during lifting. Stand J Rehabil Med 18:71-82, 1986. 8. Armstrong TJ, Buckle P, Fine LJ, Hagberg M, Jonsson B, Kilbom A et al.: A conceptual model for work-related neck and upper-limb musculoskeletal disorders. Stand J Work Environ Health 19:73-84, 1993. 9. Ashton-Miller JA, McGlashen KM, Herzenberg JE, Stohler CS: Cervical muscle myoelectric response to acute experimental sternocleidomastoid pain. Spine 15:1006-1012, 1990. 10. Asikainen A, Harstela P: Influence of small control levers of grapple loader on muscle strain, productivity and control errors. J Forest Engng 5:2>28, 1993. 11. Attebrant M, Mathiassen SE, Winkel J: Normalizing upper trapezius EMG amplitude: comparison of ramp and constant force procedures. J Electromyogr Kinesiol 5:245-250, 1995. 12. Attebrant M, Winkel J, Hansson J-E: Hand control operation and arm rest design evaluated by muscle load measurements. In: Designing for Everyone, ed by Qutinnec Y and Daniellou F, Taylor &-Francis, London, pp 15%155, 1991. 13. Attehrant M. Winkel J. Petersson NF. Mathiassen SE, Hansson J-E: Evaluation of two hand controls regarding upper trapezius load. In: International Scientific Conference on Prevention (PREMUS),

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Musculoskeletal

Disorders

ed by Hagberg M and Kilhom A, National of Occupational Health, Stockholm, pp 31-33,

REMARKS

The present review demonstrates a lack of consensus between previous ergonomic studies regarding normalization of upper trapezius EMG amplitude. Furthermore, the normalization procedure has most often been insufficiently reported. This obstructs comparison and compilation of ergonomic data from different sources. We hope that dissemination of the contents of this review will stimulate the considerate use and Journal

ET AL.

14. 15. 16. 17.

Institute 1992. Bao S, Mathiassen SE, Winkel J, Bjoring G: Normalizing upper trapezius EMG amplitude: comparison of different procedures. J Electromyogr Kinesiol 5:251-257, 1995. Barnes WS: The relationship between maximum isometric strength and intramuscular circulatory occlusion. Ergonomics 23:351-357, 1980. Basmaiian JV, DeLuca C: Muscles Alive, Williams & Wilkins, Baltimore, 1985. Bell DG: The influence of air temperature on the EMG/ force relationship of the quadriceps. Eur J Appl Physiol 67:256260, 1993.

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