Eur J Appl Physiol (1993) 67:266-273 ,uro,..n A p p l i e d Journal of

P h y s i o l o g y and Occupational Physiology © Springer-Verlag 1993

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle Chris Jensen, Ottar Vasseljen, and Rolf H. Westgaard

Division of Organization and Work Science, The Norwegian Institute of Technology, N-7034 Trondheim, Norway

Accepted April 28, 1993

Summary. The effect of electrode position on the up- per trapezius muscle on the myo-electric signal ampli- tude was investigated with special reference to arm po- sition and estimate of force output. Previously, a de- pression of the electromyogram (EMG) signal has been reported midway between the seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62: 91-98) although this electrode posi- tion has been recommended (Zipp 1982, Eur J Appl Physiol 50:41-54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical abducted and flexed positions and they performed a dynamic movement test. The myo-electric signal was recorded along the length of the right upper trapezius muscle by a 16-channel bipolar array electrode and was integrated with a 0.2-s time resolution. A region just lateral to the midpoint between C7 and the lateral edge of acromion was found with high and stable am- plitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flex- ion probably due to sliding of the skin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral and the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an interelectrode distance of 2 cm a centre position 2 cm lateral to the midpoint between C7 and acromion provided good re- peatability and high signal yield.

Key words: Trapezius muscle - Electromyography - Electrode position - Force - Arm position

Introduction

Shoulder muscle load is commonly estimated from bi- polar surface electromyographic (EMG) recordings of the upper trapezius muscle. The EMG signal has been

Correspondence to: C. Jensen

either related to the EMG response obtained at maxi- mal voluntary contraction (MVC) or a regression anal- ysis has been performed to determine the relationship between EMG and force in shoulder elevations of in- creasing force (Bjelle et al. 1981; Christensen 1986; Jensen et al. 1993; Schtildt et al. 1987; Veiersted et al. 1990; Westgaard 1988). The recommended position of the bipolar surface electrode has been a symmetrical placement about the midpoint between the seventh cervical vertebra and acromion to achieve the stated requirements of 1. Good repeatability, 2. Consideration of individual body dimensions, and 3. High signal yield of the EMG recording (Zipp 1982).

These requirements may not be fulfilled satisfactori- ly with the above location of the recording electrodes. Veiersted (1991) has performed EMG recordings along the length of the muscle and found a depression of the EMG amplitude above the midpoint relative to amplitudes obtained at a more lateral position. Fur- thermore, threefold differences in the EMG amplitude have been observed with the above electrode location when comparing shoulder MVC with the upper arms in different positions (Westgaard 1988). Thus, highly vari- able signal yield may have contributed to the reported differences in activation patterns for the trapezius mus- cle when the arms are abducted at different angles (Mathiassen and Winkel 1990). It may also affect the requirement of good repeatability if arm movements cause a displacement of the skin relative to the muscle in an area with large variations in the recorded signal amplitude. Finally, the EMG amplitude-force relation- ship obtained under a specified recording condition (i.e. vertical arm position) may not be valid for other recordings performed with elevated arms even when the electrode location is unchanged. This would also impose difficulties in establishing EMG-force relation- ships for recordings of occupational activities with vari- able arm position.

The main purpose of the present study was to inves- tigate the relationship between the central depression

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in t h e E M G s igna l o f t h e u p p e r t r a p e z i u s m u s c l e a n d t h e o b s e r v e d v a r i a t i o n in E M G a m p l i t u d e s b y s i m u l t a - n e o u s m e a s u r e m e n t o f E M G a c t i v i t y w i t h 16 b i p o l a r s u r f a c e e l e c t r o d e s p l a c e d p a r a l l e l to t h e m u s c l e f ib res . T h i s was c a r r i e d o u t by s t u d y i n g t h e e f f e c t o f d i f f e r e n t a r m p o s i t i o n s d u r i n g i s o m e t r i c c o n t r a c t i o n s a n d dy- n a m i c m u s c l e ac t iv i ty . T h e r e c o r d i n g s a lso a l l o w e d us to e x a m i n e t h e E M G - f o r c e r e l a t i o n s h i p a l o n g t h e l e n g t h o f t h e m u s c l e f ib res .

Methods

Subjects. Five women and five men participated in the study. All were healthy volunteers without pain or diseases in the muscles of the upper body region.

EMG recordings. A 16-channel bipolar array electrode was used for the recordings of the muscle activity of the right upper trape- zius muscle (trapezius pars descendens). The individual (Ag) electrodes were circular with 3-ram diameter. The interelectrode distance of each of the 16 bipolar electrodes was 20 mm and the distance between adjacent electrode centres 5 mm. The array electrode had a total recording length of 7.5 cm. It was placed on a straight line from the spine of the seventh cervical vertebra (C7) to the lateral edge of acromion (acromion) spanning the midpoint (central lead point) between the two "landmarks" as shown in Fig. 1. The signal was amplified 4000 x , band-pass fil- tered (bandwidth 10-1250 Hz) and stored on a digitizing recorder (Earth Data 128). The E M G signal was reconverted to an ana- logue form and quantified by A-D-convertion at a sample fre- quency of 2 kHz on a PDP 11/83 computer. The digitized signals were full-wave rectified and integrated with a time resolution of 0.2 s. Artefacts were detected and removed by an artefact rejec- tion procedure, sensitive to sharp transients and slow deviations from the signal base-line. The system noise level corresponded to approximately 1.5 ixV rms. This was subtracted from the E M G signal and a 3-point moving average procedure was performed to smooth the signals. The median activity level in tests was deter- mined by using the amplitude probability distribution function of Jonsson (1982).

Procedure. Three MVC of approximately 5-s duration were per- formed with simultaneous recordings of E M G and force pro- duced during isometric shoulder elevation. Adjustable straps were placed over the shoulders and connected to two strain- gauge force transducers from which the signals were transferred

Fig. 1. The position of the 16-channel array electrode on the lead- line between the seventh cervical vertebra (C7) and acromion (acr)

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Fig. 2. Force versus time curves for one subject performing shoulder elevations with the straps placed over the shoulders

to the recorder. During these contractions the subject was seated on a chair with the back and the arms in a vertical position (0 ° abduction). Thereafter, maximal contractions were performed with 90 ° arm abduction (2 x ), 90 ° arm flexion (2 x ) and 30 ° and 60 ° arm abduction (1 x , only seven subjects with 30 ° and 60 ° arm abduction). These contractions were carried out with the straps placed just proximal to the elbows. The contraction series were performed in the order indicated above for all subjects.

In shoulder elevation care was taken to minimize the E M G activity before the contraction started, using E M G biofeedback if necessary. The tension of the shoulder straps was adjusted to in- dicate about 10 N of force, ensuring that any attempted shoulder elevation would immediately be measured by the force transduc- er. The first part of the contraction took place at a slow rate, about 10%-30% MVC.s -1, increasing to about 30%-50% M V C . s - 1 at high force levels (Fig. 2). This procedure minimized the time lag between the E M G and force signals at low force lev- els and avoided fatigue effects at high force levels. Variation in force rates within the indicated limits we assumed to have only weak effects on the EMG-force relationship (Lawrence and De Luca 1983). The subjects performed sub-maximal trial contrac- tions until they were able to carry out a continuous contraction sequence. For the further analyses the "best" contraction, i.e. the most continuous contraction during the entire force range was chosen, based on visual inspection of the EMG signals.

Dynamic movements were performed in a standardized test situation for seven subjects (Westgaard and BjCrklund 1987). The right arm and hand was required to move between three target areas placed in front of the subject on a table. The upper right arm moved continuously from a vertical position to approximate- ly 45 ° flexion and further to approximately 45 ° abduction before it reached a vertical position again. Each round was performed at a speed of 80. rain - 1 for 2 rain.

Visual feedback tests were carried out by three subjects in an attempt to reduce the E M G activity in the trapezius muscle: 3-kg loads were held in the hand for 15 s with the arm abducted or flexed 45 ° and the elbow joint held at a straight angle without visual feedback. The subjects were then asked to repeat the pro- cedure and make an effort to reduce the E M G signal which was displayed on an oscilloscope while the arm position was main- tained.

Statistics. Regression lines were generated using the method of least squares and Pearsons" correlation coefficient (r) was calcu- lated. Student's t-test was carried out to test for equal means with the significance level set at P equal to or less than 0.05. The coef- ficient of variation (CV) is expressed as the standard deviation as a percentage of the mean value. The NCSS statistical package (Dr. Jerry L. Hintze, Kaysville, Utah) was used in these calcula- tions.

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Table 1. Personal data, distance from C7 to acromion (C7-acr) and the mean force of three maximal contractions produced by the right shoulder with the arms at 0 ° abduction are shown for all subjects

Sex Age Body Body C 7 - a c r Mean (years) mass height (cm) force

(kg) (cm) (N)

Female 33 55 165 20.0 505 Female 27 65 172 22.9 646 Female 24 56 161 18.0 528 ~, Female 34 52 161 20.0 552 ~" Female 26 57 168 20.0 470

r~ Male 33 80 178 22.5 833 Male 24 83 176 24.0 730 Male 28 68 182 23.5 636 Male 33 68 174 23.0 660 Male 47 77 178 22.5 790

Results

The distances between C7 and acromion and the mean forces of the contractions produced by the right shoulder with the arms at 0 ° abduction are shown in Table 1.

The EMG amplitude profile

Consistent individual amplitude profiles recorded dur- ing maximal contraction with the arm at 0 ° abduction were found along the length of the trapezius muscle (Fig. 3A). The mean profile of the subjects is shown in Fig. 3B. Moving in a medial direction from the most lateral electrode positions the maximal amplitude in- creased and reached the highest values at a region lo- cated 10%-30% of the distance from the midpoint (be- tween acromion and C7) to the acromion. This "pla- teau" was followed by a decrease towards the midpoint and a subsequent increase medial to the midpoint visu- alised as a dip in the amplitude profile. The mean posi- tion of the dip was at 52% of the distance from acrom- ion to C7.

The plateau was 2-2.5 cm wide and spanned by at least five electrode centres of the array electrode. The CV of the maximal amplitudes was calculated for the plateau region which showed the lowest interelectrode variation in the E M G recordings (Table 2). The mean CV was 5.6%. The exact location of the region varied slightly between the subjects, but it covered a position from 2 cm to 2.5 cm lateral to the midpoint between C7 and acromion for all of the subjects (Table 2).

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Fig. 3. A Individual electromyogram (EMG) amplitude profiles recorded during maximal voluntary contractions with the arms in a vertical position. The position along the length of the right up- per trapezius muscle is indicated as the distance from the dip po- sition to C7 (positive values) or to acromion (negative values). A signal amplitude of 1 mV root mean square corresponds to a val- ue of 18850 in the figure. B Mean curve of the data in A. Bars indicate standard deviation

Table 2. Location of the "plateau" region which is given as the distance from the centre of this plateau to the midpoint of the line between acromion and C7, The coefficient of variation (CV) of the electromyogram amplitudes at maximal voluntary force (0 ° arms abduction) is indicated for the five electrodes spanning the plateau

Subject no. Plateau location CV (cm) (%)

1 3.0 4.7 2 2.0 4.6 3 2.0 3.2 4 2.0 3.1 5 2.5 5.9 6 2.0 6.2 7 1.5 3.7 8 2.5 2.5 9 2.0 9.8

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Effect of arm position on maximal amplitudes

Figure 4A shows amplitude profiles of one subject re- corded during three maximal contractions with the arm in different positions. The dip was slightly displaced to a more lateral position for the profiles with 90 ° arm abduction or flexion compared to the recording with

the arm at 0 ° abduction. The ratio between the ampli- tudes at maximal voluntary force for 90 ° and 0 ° arm abduction was calculated for all electrode positions. If the amplitude recorded with 0 ° arm abduction was larger than the amplitude with 90 ° abduction the in- verse negative ratio was calculated. The ratios varied considerably about the midpoint whereas they were close to unity at the plateau (Fig. 4B). The same calcu-

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Fig. 4. A Electromyogram (EMG) amplitude profiles recorded during maximal voluntary contractions with the arms in a vertical position (0 ° abduction), at 90 ° abduction or 90 ° flexion for one subject (1 mV root mean square = 18850). The position along the length of the right upper trapezius muscle is indicated as the relative distance from the midpoint to C7 (positive values) or to acromion (negative values). B The ratio between maximal ampli- tudes at 90 ° and 0 ° abduction or the negative ratio between max- imal amplitudes at 0 ° and 90 ° abduction if the former was less than 1

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lations were made for the other subjects who all showed similar patterns. The mean ampli tude ratio for the recordings located within the pla teau region was 1.03 [(SD 0.30) n =46]. The majori ty of the ratios in this region (94%) were in the range f rom - 1.50 to 1.50 (Fig. 5) which has previously been used as a criteria for

Table 3. Position of the "dip" in recordings of maximal voluntary contractions with the upper arms at 30 ° , 60 ° and 90 ° abduction for seven subjects. The position is indicated as the distance in centi- metres from the electrode with the lowest maximal electromyo- gram amplitude with the arms in a vertical position (0 °) and the direction is indicated in parenthesis

0 o 30 ° 60 ° 90 °

0 0 0.5 (lateral) 0.5 (lateral) 0 0 0 0 0 0 0.5 (medial) 0.5 (medial) 0 0 0 1.0 (lateral) 0 1.5 (lateral) 1.0 (lateral) 1.5 (lateral) 0 0.5 (lateral) 0.5 (lateral) 0.5 (lateral) 0 1.0 (lateral) 1.0 (lateral) 1.0 (lateral)

accepting calibration recordings (Westgaard 1988). A t the dip position with 0 ° a rm abduction the mean ratio was 2.93 (SD 1.61). Two subjects reached values higher than 4 at this position. The mean inverse ratio for the electrode position at the dip recorded with the arm ab- ducted 90 ° was -1 .93 (SD 0.43).

Maximal contractions with the arm abducted at 0 °, 30 °, 60 ° and 90 ° were compared to search for systematic displacements of the dip. The position of the dip at dif- ferent a rm positions is shown in Table 3 for seven sub- jects. The dip was recorded by a more lateral electrode in five cases when a displacement was caused by ab- duction of the arm. One subject showed the opposite pattern.

EMG-force relationship

Relative E M G ampli tude-force curves were generated for all electrode positions recorded during contractions with the arms at 0 ° abduction. A 4th order polynomial regression was fitted to each curve. The variat ion in E M G ampli tude explained by the variation of the re- gression models ( r2x 100%) ranged f rom 98% to 99%. For simplicity only three curves for each of three sub- jects are shown in Fig. 6. The three curves represent the dip position, a position within the plateau region and a more lateral position, all recorded simultaneous- ly. The E M G ampli tude at the dip position and the lat- eral position increased faster at low contraction levels than the ampli tude recorded at the plateau position. For some subjects the ampli tude at the dip position reached the maximal E M G ampli tude (EMGmax) be- fore M V C was attained.

The mean relative E M G ampli tudes of all subjects are shown as a function of relative force level in Fig. 7A and the curves represent the same electrode posi- tions as in Fig. 6. The E M G levels at the dip position and at the most lateral positions were clearly elevated compared to the plateau region. This elevation was most p ronounced at low force levels, where 10% M V C corresponded to 22% EMGmax for the most lateral electrode and 15% for the dip electrode, significantly different f rom the mean E M G value for the plateau

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Fig. 6. Relative electromyogram amplitude-force curves for three electrode positions for each of three subjects (A, B and C). The symbols denote the electrode location: (O) 6 cm (5 cm for the wo- men) medial to acromion ("lateral position"), and (O) 2 cm lat- eral to the midpoint between C7 and acromion ("plateau posi- tion"), and (V) the electrode position with the lowest recorded EMG amplitude ("dip position"). The 4th order regression lines are fitted to the data. MVC, Maximal voluntary contraction. EMGm,×, maximal electromyogram

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Fig. 7. A Mean relative electromyogram amplitude-force curves for the same electrode positions as in Fig. 6. The curves are based on the mean values of ten subjects. The linear regression line is based on the mean values at the plateau position. B Mean rela- tive EMG amplitudes at 10% and 50% MVC related to the EMGmax amplitude obtained with the arms at 0 ° abduction for ten subjects. The electrode position refers to the distance from the electrode where the dip in the maximal amplitude profile was found. Negative values indicate the lateral direction and bars show the standard deviation. For definitions see Fig. 6

parent for some of the subjects. The mean relative E M G amplitudes at 10% and 50% M V C for 13 elec- t rode positions shared by all subjects when related to the position of the dip are shown in Fig. 7B. This de- monstra tes both the lower relative E M G levels and the smaller variability in the plateau region in comparison to the lateral and the "dip" regions.

electrodes (10% E M G . . . . P.~ 0.01). At 50% M V C the mean level of 47% EMGmax for the plateau electrode differed significantly f rom the mean of the E M G am- plitude at the lateral position (56% E M G . . . . P < 0.01) and it tended to differ f rom the mean ampli tude at the dip position (57% E M G . . . . P=0.069) . The mean EMG-force curve at the plateau region appeared lin- ear; in fact, a linear regression model explained 99% of the variat ion in the data. However , not all of the indi- vidual plateau curves were linear and a curvilinear shape with a s teeper slope at high force levels was ap-

The m o v e m e n t test

The profile of the median E M G activities for the movemen t test showed a dip about the midpoint be- tween C7 and acromion similar to the dip in the maxi- mal ampli tude profile, but slightly displaced. In this test the E M G levels at the most lateral electrodes were not lower than the E M G levels at the plateau region which was also the case for the E M G ampli tude re- corded at 1% M V C with the arms in a vertical position. Thus, in both cases the relative E M G levels at the most

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Fig. 8. A Relative median EMG activity during the movement test and the relative EMG amplitude recorded at 1% MVC with 0 ° arm abduction along the length of trapezius muscle for one subject. The electrode location is indicated as the relative dis- tance from the midpoint to C7 (positive values) or to acromion (negative values). B Relative median EMG activity during the movement test for the subject in A. The activity was related to the EMG amplitude obtained during MVC with the arms at 0 ° abduction and 90 ° abduction. For definitions see Fig. 6

l a t e r a l e l e c t r o d e s w e r e high, s imi la r to the r e l a t ive E M G level in the d ip p o s i t i o n (Fig. 8A) .

T h e effect o f ca l ib ra t i ng the E M G act iv i ty of the m o v e m e n t tes t wi th the E M G a m p l i t u d e r e c o r d e d dur - ing M V C wi th the a rms at 0 ° o r 90 ° a b d u c t i o n is shown in Fig. 8B. T h e r e l a t ive ac t iv i ty in the m o s t l a t e r a l p a r t of the musc le was s imi la r wi th the two ca l i b r a t i on con- t rac t ions , and h ighe r t han at the p l a t e a u reg ion . T h e r e l a t i ve E M G levels n e a r the m i d p o i n t we re h igh ly de - p e n d e n t on the choice of c a l i b r a t i o n c o n t r a c t i o n due to the d i s p l a c e m e n t of the d ip in the m a x i m a l a m p l i t u d e prof i les .

T h e m e a n r e l a t i ve E M G levels of seven subjec t s we re ca l cu l a t ed for b o t h ca l i b r a t i on p r o c e d u r e s at e lec- t r o d e pos i t i ons s e p a r a t e d by 1 cm. T w o - t a i l e d p a i r e d S t u d e n t ' s t - tests we re p e r f o r m e d to iden t i fy the pos i - t ions wi th the m o s t s t ab le E M G levels . T h e resul t s a r e p r e s e n t e d in T a b l e 4 showing tha t the m e d i a n levels w e r e s imi la r for the pos i t i ons 2 cm to 3 cm l a t e r a l to the m i d p o i n t . This c o n f i r m e d tha t the r e c o r d e d E M G act iv i ty was leas t sens i t ive to the a r m p o s i t i o n in this r eg ion .

Feedback mediated reduction o f the E M G signal

Table 4. Median activity levels for the movement test in percent- age of the maximal electromyogram (EMGm,~) amplitude re- corded during maximal shoulder contractions with the upper arms in a vertical position and with 90 ° arm abduction

Electrode position Arms vertical Arms abducted P (cm) (% EMOmax) (% gMGmax)

- 5.0 8.2 5.8 0.008 -4 .0 5.9 4.1 0.180 - 3.0 3.7 3.3 0.233 -2 .0 3.0 3.2 0.410 - 1.0 2.4 3.6 0.132

0.0 3.4 4.4 0.347 1.0 6.1 3.0 0.022

Values are means for seven subjects and the probability value (P) for equal means at each electrode position is given. The electrode position refers to the distance from the midpoint between C7 and acromion with negative values lateral to the midpoint

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Fig. 9. A and B Median activity during a 3-kg load carrying task with and without visual feedback for one subject. The arm was abducted or flexed 45 °. C The ratio between the median activities obtained with and without feedback. EMG . . . . Maximal electro- myogram

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tion was larger at arm flexion than abduction as exem- plified by the recordings in Fig. 9A, B. The recordings from all three subjects showed a larger decrease in the EMG activity in the plateau region than at a more lat- eral position. One of the subjects reduced the ampli- tude of the signal in the plateau region to 40% of the amplitude recorded without feedback. However, the signal at the most lateral electrode was only reduced to 90%, indicating the influence of another signal source than the trapezius muscle at the lateral position (Fig. 9C).

Discussion

The present series of experiments has demonstrated that the estimate of upper trapezius muscle force by surface EMG recording is highly dependent on the lo- cation of the electrode along the length of the muscle fibres. In particular, the electrode location recom- mended in the literature results in unstable recording conditions. Broadly, three regions were identified along the length of the trapezius muscle fibres and are considered below.

The "plateau" region

At the region 1 cm to 3 cm lateral to the midpoint be- tween acromion and C7, the highest and most stable EMG amplitudes were recorded during maximal shoulder contractions. The maximal EMG signals were similar in this region for shoulder elevation with the arm in 0 ° abduction and with the arm in 90 ° abduction. Thus, a change in arm position or a displacement of the skin relative to the muscle caused only a minimal change in the EMG response. The smallest variation between the individual EMG-force curves were pro- duced for the position 2 cm lateral to the midpoint and the mean curve was on average near linear at this posi- tion. The commonly used calibration procedure, where the relative muscle load is estimated from the EMG amplitude as a fraction of the EMG amplitude at max- imal force level, would therefore seem valid with the electrode located in this region. Some error in the esti- mation of the load will remain, however, due to the individual variation in the EMG-force relationship.

The "dip" region

This study supports the observations of Veiersted (1991) who has shown a dip in the EMG amplitude lo- cated 56% of the distance from acromion to C7, using a bipolar surface electrode in consecutive measure- ments at different electrode positions. This was similar to the dip location found in the present study with EMG signals recorded simultaneously at different po- sitions along the length of the muscle fibres.

In the dip region there are large variations in the relative EMG levels when performing a standardised

movement test. The variation is most likely due to the sliding of the skin relative to the underlying muscle. Near the muscle end plate region the position of the electrodes is likely to be critical, due to common mode rejection of symmetrical waveforms spreading from the end plates in medial and lateral directions. A study of autopsy material has shown that the innervation zone of the upper trapezius muscle is about 1 cm wide and located midway between the acromion and the C7 ver- tebra (O. Wa~rhaug and K. B. Veiersted, personal com- munication, 1993) which would support the idea that the dip position corresponds to the endplate region with the monopolar electrodes located on opposite sides of the endplates. Outside the muscle endplate re- gion the change in the pick-up area of the electrode due to the skin-muscle displacement is of minor impor- tance as the displacement is small. The importance of the electrode configuration is further indicated by two studies where Vigreux et al. (1979) have found a de- pression in the EMG amplitude in the middle region of the muscle with bipolar surface electrodes, whereas Zuniga et al. (1970), using monopolar recordings, have observed a bell-shaped amplitude profile over the mus- cle belly.

The EMG-force relationship seemed somewhat dif- ferent in this region compared to the plateau region. On average the relative EMG activity was more ele- vated at low force levels in the dip region, a finding we cannot explain unless the cancellation of symmetrical signals reaching the two electrodes was more pro- nounced at higher force levels.

The lateral region

In the lateral direction the relative EMG levels were more elevated at low force-levels, producing highly nonlinear curves. Several factors may be responsible for this pattern. The upper trapezius muscle has been shown to be a relatively flat muscle with a thickness of about 0.5 cm in the middle region (Jensen 1992). When moving towards acromion the trapezius muscle be- comes thinner, inserting in the tendon 4 cm-5 cm me- dial to acromion. The surface EMG signal from the trapezius muscle would be expected to decline in a lat- eral position, as seen by the amplitude profile at maxi- mal shoulder elevation. However, the supraspinatus muscle may contribute to the observed EMG signal at this position. The experiments with a voluntary reduc- tion in the trapezius muscle EMG signal have provided indirect evidence for another contribution to the EMG signal which is not related to the trapezius muscle as the signal was not reduced for the lateral electrodes to the same extent as for the more medially placed elec- trodes (Fig. 9C). The elevated EMG response for the lateral electrodes in the movement test may also be in- terpreted in support of this hypothesis, as the supraspi- natus muscle would be active during this movement with abducted and flexed arm. However, the similar re- sponse during shoulder elevation with the arm in 0 ° ab- duction is difficult to explain as the supraspinatus mus-

273

cle is not supposed to be active in this movement . The contr ibution to the E M G response f rom the supraspi- natus muscle thus remains undetermined, but it seems clear that the lateral region should be avoided when aiming to record trapezius muscle E M G activity. Con- sequently, it would seem that studies of trapezius mus- cle activity with this electrode position should be t reated with caution (Mathiassen and Winkel 1990).

The force-EMG calibration curve as an indicator of fibre type distribution

The shape of the f o r c e - E M G calibration curve has been interpreted to indicate the propor t ion of type I and type I I fibres in the muscle (Chaffin et al. 1980; Lawrence and De Luca 1983; Hagberg and Hagberg 1989). This interpretat ion is based on the assumption that the smaller type I fibres are recruited first and produce smaller ampli tude increments than the larger type I I fibres which are recruited at higher force levels. In our experience, the reliability of an est imate of fibre propor t ion by this me thod is low (see also Lawrence and De Luca 1983). If the present data had been ap- plied to obtain an est imate of fibre proport ions, this est imation would have produced variable results along the length of the muscle fibres, especially near the end- plate region, even though the underlying fibre type dis- tr ibution can be assumed to be constant.

Some E M G activity before the start of the contrac- tion or exclusion of the very low points on the force- E M G calibration curve considerably influence the slope of the regression line at low force levels. In fact, if the zero point is left out, the calibration curve for the pla teau region in our study can be approximated by two linear lines in the region 5-40% M V C and 60- 100% M V C with a slope ratio of 0.7, al though the var- iation explained was 99% with a linear regression f rom 0 to 100% MVC.

Conclusion

In summary, when using a bipolar surface electrode with an interelectrode distance of 2 cm to obtain an es- t imate of upper trapezius muscle force, a centre posi- tion 2 cm lateral to the midpoint be tween C7 and acromion would seem to provide good repeatabi l i ty and a high signal yield. This is in agreement with the general r ecommenda t ion of Basmajian and De Luca (1985) of recording be tween the moto r point and the distal tendon of a muscle. The data should not be inter- pre ted as if the E M G activity recorded in this region was representat ive of the whole upper trapezius mus- cle. However , they do present us with a relatively large and well-defined region of the upper trapezius muscle where reproducible results may be obtained in most of

the potential laboratory or occupational settings where the measuremen t of shoulder muscle activity is of in- terest.

Acknowledgements. This project was supported by grants from the Norwegian Research Council for Science and the Humani- ties.

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