Eur J Appl Physiol (1992) 65:251-257
European
oum=o, A p p l i e d Physiology and Occupational Physiology © Springer-Verlag1992
Electromyographic changes in work-related myalgia of the trapezius muscle Tommy Oberg 1'2, Leif Sandsj6 3'4, Roland Kadefors 3'4, and Sven-Erik Larsson 2 1 Department of Biomechanics, UniversityCollege of Health Sciences, P.O. Box 1038, S-55111 JOnk6ping, Sweden 2 Department of Orthopaedics, Universityof LinkOping, Sweden 3 Department of Applied Electronics, Chalmers Universityof Technology, G6teborg, Sweden 4 Lindholmen DevelopmentCenter, GOteborg, Sweden Accepted March 12, 1992
Summary. In 11 patients, all women, 21-55 years of age, with unilateral work-related myalgia of the trapezius muscle, the right and left trapezius muscles were examined simultaneously for electromyogram (EMG) signs of localized muscle fatigue. All patients were tested with 0-kg hand load for 5 min, holding the arms straight at 90 ° of elevation in the scapular plane. Only 4 of the patients tolerated exposure to higher load levels. They were tested with 1 kg hand load for 3 rain and 2 kg hand load for 2 min, with a period of rest of 30 min between the trials. The EMG mean power frequency (MPF) and root mean square (rms) were calculated. Data were normalized with the initial value as a reference and regression analyses were performed. On both sides a decrease of MPF and an increase of rms were found with increasing time and load, i.e. classical EMG signs of localized muscle fatigue. Compared with the nonaffected side smaller changes were found on the affected side, possibly due to pain inhibition, impaired microcirculation and biochemical changes along the muscle fibres. At 0-kg hand load we found no change of MPF on either side despite subjective feelings of fatigue and pain. We interpreted these findings as an indication of reduced capacity of the affected trapezius muscle to sustain static load with early development of pain-associated local fatigue. Key words: Electromyogram - Amplitude - Frequency Fatigue - Shoulder pain - Trapezius muscle
Introduction In the modern industrialized world, many occupations such as assembly work in electronics factories involve long-lasting static and repetitive loads to the neck and shoulders, in particular the trapezius muscle. This may cause absence from work due to pain syndromes in the neck-shoulder region (Kvarnstr6m 1983; Hagberg and Correspondence to: T. Oberg
Wegman 1987; Larsson et al. 1988). Localized muscle fatigue has been studied by electromyography (EMG), but usually at high load levels and on healthy subjects. Only a few EMG studies have been concerned with frequency analysis of fatigue in diseased muscles (e.g. Braakhekke et al. 1989; Hagberg and Hagberg 1989b; Kadefors et al, 1969; Linssen et al. 1990; Ronager et al. 1989). Chronic, work-related myalgia of the trapezius muscle with reduced capability may have unfavourable results with regard to working ability (KvarnstrOm 1983). Recently, morphological evidence of disturbed mitochondrial function (Larsson et al. 1988) and reduced microcirculation (Larsson et al. 1990) has been reported but the condition requires further EMG studies. In healthy individuals localized muscle fatigue appears in the EMG as an increase of the amplitude in the time domain and as a shift of the spectrum towards lower frequencies in the frequency domain. This spectral shift in sustained muscle contraction was described by Kogi and Hakamada in 1962 and has been interpreted as a sign of localized muscle fatigue (Chaffin 1973; Kuorinka 1976; Lindstr0m et al. 1970, 1974). Spectral parameters such as the EMG mean power frequency (MPF) and the median frequency (e.g. DeLuca 1984; Basmajian and DeLuca 1985; DeLuca and Knaflitz 1990) have been generally used to describe a spectral shift relative to an initial reference value. The EMG parameters, when used as estimators of localized muscle fatigue, have been shown to be subjected to different kinds of variation, systematic as well as random (Oberg et al. 1990, 1991, 1992). Such variations can disturb the interpretation of EMG signals and must be considered in the EMG analysis. If EMG methods are to be used for evaluation of muscle fatigue in ergonomics and in clinical practice, it is necessary to know whether muscles affected by work-related pain react in the same manner as healthy muscles. The aim of this study was: 1. To expose patients with unilateral myalgia of the trapezius muscle to a fatiguing, continous load and to analyse by EMG the development of localized muscle fatigue on the affected and nonaffected side,
252
Table 1. Occupation, age, duration of complaints, handedness and side of complaints Subject no.
Occupation
Age (years)
Duration (months)
Handedness
Complaints
1 2 3 4 5 6 7 8 9 10 11
Cashier Cleaner Secretary Nurse's assistant Shop assistant Cashier Cleaner Nurse's assistant Dentist School meal assistant (formerly seamstress) Physiotherapist
30 46 26 36 44 21 43 25 46 55 42
26 60 7 24 7 4 24 40 6 many years many years
R R R R R R R R R R L
R R L R L L R R R R R
R, Right; L, left
2. To relate the findings to basic facts concerning the generation of work-related myalgia. Methods Patients. Patients with strictly unilateral complaints from longstanding myalgia of the trapezius muscle were examined. Criteria for inclusion in the study were muscular pain localized to the upper part of the trapezius muscle, no other clinical signs of disease and a history of long-standing static or repetitive arm work. Patients with generalized muscle pain, joint related pain or neuralgic pain were excluded. All the patients were women, with a mean age of 37.6 (21-55) years. The occupations, the duration of symptoms, the age and handedness of the patients are listed in Table 1. The patients had been referred to our research laboratory by physiotherapists, general practitioners and from the county hospital of J6nk6ping during the course of one year. They were clinically examined before admission. Most patients were right-handed (10/ 11), most of them had their complaints on the right side (8/11) and most of them were affected on the dominant side (7/11). All patients had pain at rest as well as at work. The trapezius muscle was examined by palpation prior to the E M G recordings. All patients had at least one tender point in the upper part of the trapezius muscle. No attempt was made to quantify the tenderness. Two of the patients were on full-time sick leave and 5 on halftime.
~
0 kg 1 kg 2 kg
Exposure to static load. The seated subject kept her arms straight and elevated to 90 ° of abduction in the scapular plane, half-way between the frontal and sagittal planes (Johnston 1937; see Fig. la, b) for 5 min with a hand load of 0 kg. Because of pain related to the sustained muscle contraction most of the patients did not want to go on to higher load. However, 4 patients managed 1-kg hand load for 3 min and 2-kg hand load for 2 min. All had a rest period of about 30 min between the trials.
EMG recording and signal processing. Bipolar surface electrodes (pre-gelled child electrocardiogram electrodes, Medicotest, ¢31stycke, Denmark) were placed over the upper part of the trapezius muscle, half-way between the spinous process of C-7 and the acromion (Basmajian and Blumenstein 1983; Fig. 2). The reference electrode was placed over the spinous process of C-7. The E M G was recorded simultaneously from the right and left sides. The signal was preamplified close to the electrodes (preamplifiers from Johne and Reilhofer, Munich, FRG) and amplified in a sixchannel amplifier, built in our own laboratory. The signal was then stored on a frequency modulated tape recorder (TEAC-R71, Tokyo, Japan). It was later replayed through a fourth order Butterworth anti-aliasing filter with a cutoff frequency of 800 Hz, AD-converted with a sampling frequency of 2 kHz and fast Fourier transform was performed on a minicomputer (Digital Equipment
Fig. 1. a) Loads for the hands and b) position of the arms in 90 ° abduction in the scapular plane P D P 11/44, Maynard, Massachusets, USA). A quality control routine was applied to the signal (Arvidsson 1982; Kadefors et al. 1983). The signal was analysed in sequences of 0.5 s. The MPF and root mean square (rms) values were calculated and 10-s means of the E M G parameters were used for further calculations. A detailed description of the E M G recording and processing procedures has been given in a previous paper (Oberg et al. 1990).
253
Regression
coefficients:
(N.S.)
0 -2 x 10 .5 •
6 x 10-5 (N.S.) o
o o
o o o
o ° 8 ° ° °
80
o
°°°
o
o
o
o " ' ' l
..':':
-,
: ~.°.o;:.o:o,..o.O..~ ~3 LJJ
"o
0
[\1
III
European
oum=o, A p p l i e d Physiology and Occupational Physiology © Springer-Verlag1992
Electromyographic changes in work-related myalgia of the trapezius muscle Tommy Oberg 1'2, Leif Sandsj6 3'4, Roland Kadefors 3'4, and Sven-Erik Larsson 2 1 Department of Biomechanics, UniversityCollege of Health Sciences, P.O. Box 1038, S-55111 JOnk6ping, Sweden 2 Department of Orthopaedics, Universityof LinkOping, Sweden 3 Department of Applied Electronics, Chalmers Universityof Technology, G6teborg, Sweden 4 Lindholmen DevelopmentCenter, GOteborg, Sweden Accepted March 12, 1992
Summary. In 11 patients, all women, 21-55 years of age, with unilateral work-related myalgia of the trapezius muscle, the right and left trapezius muscles were examined simultaneously for electromyogram (EMG) signs of localized muscle fatigue. All patients were tested with 0-kg hand load for 5 min, holding the arms straight at 90 ° of elevation in the scapular plane. Only 4 of the patients tolerated exposure to higher load levels. They were tested with 1 kg hand load for 3 rain and 2 kg hand load for 2 min, with a period of rest of 30 min between the trials. The EMG mean power frequency (MPF) and root mean square (rms) were calculated. Data were normalized with the initial value as a reference and regression analyses were performed. On both sides a decrease of MPF and an increase of rms were found with increasing time and load, i.e. classical EMG signs of localized muscle fatigue. Compared with the nonaffected side smaller changes were found on the affected side, possibly due to pain inhibition, impaired microcirculation and biochemical changes along the muscle fibres. At 0-kg hand load we found no change of MPF on either side despite subjective feelings of fatigue and pain. We interpreted these findings as an indication of reduced capacity of the affected trapezius muscle to sustain static load with early development of pain-associated local fatigue. Key words: Electromyogram - Amplitude - Frequency Fatigue - Shoulder pain - Trapezius muscle
Introduction In the modern industrialized world, many occupations such as assembly work in electronics factories involve long-lasting static and repetitive loads to the neck and shoulders, in particular the trapezius muscle. This may cause absence from work due to pain syndromes in the neck-shoulder region (Kvarnstr6m 1983; Hagberg and Correspondence to: T. Oberg
Wegman 1987; Larsson et al. 1988). Localized muscle fatigue has been studied by electromyography (EMG), but usually at high load levels and on healthy subjects. Only a few EMG studies have been concerned with frequency analysis of fatigue in diseased muscles (e.g. Braakhekke et al. 1989; Hagberg and Hagberg 1989b; Kadefors et al, 1969; Linssen et al. 1990; Ronager et al. 1989). Chronic, work-related myalgia of the trapezius muscle with reduced capability may have unfavourable results with regard to working ability (KvarnstrOm 1983). Recently, morphological evidence of disturbed mitochondrial function (Larsson et al. 1988) and reduced microcirculation (Larsson et al. 1990) has been reported but the condition requires further EMG studies. In healthy individuals localized muscle fatigue appears in the EMG as an increase of the amplitude in the time domain and as a shift of the spectrum towards lower frequencies in the frequency domain. This spectral shift in sustained muscle contraction was described by Kogi and Hakamada in 1962 and has been interpreted as a sign of localized muscle fatigue (Chaffin 1973; Kuorinka 1976; Lindstr0m et al. 1970, 1974). Spectral parameters such as the EMG mean power frequency (MPF) and the median frequency (e.g. DeLuca 1984; Basmajian and DeLuca 1985; DeLuca and Knaflitz 1990) have been generally used to describe a spectral shift relative to an initial reference value. The EMG parameters, when used as estimators of localized muscle fatigue, have been shown to be subjected to different kinds of variation, systematic as well as random (Oberg et al. 1990, 1991, 1992). Such variations can disturb the interpretation of EMG signals and must be considered in the EMG analysis. If EMG methods are to be used for evaluation of muscle fatigue in ergonomics and in clinical practice, it is necessary to know whether muscles affected by work-related pain react in the same manner as healthy muscles. The aim of this study was: 1. To expose patients with unilateral myalgia of the trapezius muscle to a fatiguing, continous load and to analyse by EMG the development of localized muscle fatigue on the affected and nonaffected side,
252
Table 1. Occupation, age, duration of complaints, handedness and side of complaints Subject no.
Occupation
Age (years)
Duration (months)
Handedness
Complaints
1 2 3 4 5 6 7 8 9 10 11
Cashier Cleaner Secretary Nurse's assistant Shop assistant Cashier Cleaner Nurse's assistant Dentist School meal assistant (formerly seamstress) Physiotherapist
30 46 26 36 44 21 43 25 46 55 42
26 60 7 24 7 4 24 40 6 many years many years
R R R R R R R R R R L
R R L R L L R R R R R
R, Right; L, left
2. To relate the findings to basic facts concerning the generation of work-related myalgia. Methods Patients. Patients with strictly unilateral complaints from longstanding myalgia of the trapezius muscle were examined. Criteria for inclusion in the study were muscular pain localized to the upper part of the trapezius muscle, no other clinical signs of disease and a history of long-standing static or repetitive arm work. Patients with generalized muscle pain, joint related pain or neuralgic pain were excluded. All the patients were women, with a mean age of 37.6 (21-55) years. The occupations, the duration of symptoms, the age and handedness of the patients are listed in Table 1. The patients had been referred to our research laboratory by physiotherapists, general practitioners and from the county hospital of J6nk6ping during the course of one year. They were clinically examined before admission. Most patients were right-handed (10/ 11), most of them had their complaints on the right side (8/11) and most of them were affected on the dominant side (7/11). All patients had pain at rest as well as at work. The trapezius muscle was examined by palpation prior to the E M G recordings. All patients had at least one tender point in the upper part of the trapezius muscle. No attempt was made to quantify the tenderness. Two of the patients were on full-time sick leave and 5 on halftime.
~
0 kg 1 kg 2 kg
Exposure to static load. The seated subject kept her arms straight and elevated to 90 ° of abduction in the scapular plane, half-way between the frontal and sagittal planes (Johnston 1937; see Fig. la, b) for 5 min with a hand load of 0 kg. Because of pain related to the sustained muscle contraction most of the patients did not want to go on to higher load. However, 4 patients managed 1-kg hand load for 3 min and 2-kg hand load for 2 min. All had a rest period of about 30 min between the trials.
EMG recording and signal processing. Bipolar surface electrodes (pre-gelled child electrocardiogram electrodes, Medicotest, ¢31stycke, Denmark) were placed over the upper part of the trapezius muscle, half-way between the spinous process of C-7 and the acromion (Basmajian and Blumenstein 1983; Fig. 2). The reference electrode was placed over the spinous process of C-7. The E M G was recorded simultaneously from the right and left sides. The signal was preamplified close to the electrodes (preamplifiers from Johne and Reilhofer, Munich, FRG) and amplified in a sixchannel amplifier, built in our own laboratory. The signal was then stored on a frequency modulated tape recorder (TEAC-R71, Tokyo, Japan). It was later replayed through a fourth order Butterworth anti-aliasing filter with a cutoff frequency of 800 Hz, AD-converted with a sampling frequency of 2 kHz and fast Fourier transform was performed on a minicomputer (Digital Equipment
Fig. 1. a) Loads for the hands and b) position of the arms in 90 ° abduction in the scapular plane P D P 11/44, Maynard, Massachusets, USA). A quality control routine was applied to the signal (Arvidsson 1982; Kadefors et al. 1983). The signal was analysed in sequences of 0.5 s. The MPF and root mean square (rms) values were calculated and 10-s means of the E M G parameters were used for further calculations. A detailed description of the E M G recording and processing procedures has been given in a previous paper (Oberg et al. 1990).
253
Regression
coefficients:
(N.S.)
0 -2 x 10 .5 •
6 x 10-5 (N.S.) o
o o
o o o
o ° 8 ° ° °
80
o
°°°
o
o
o
o " ' ' l
..':':
-,
: ~.°.o;:.o:o,..o.O..~ ~3 LJJ
"o
0
[\1
III
