Ottar Vasseljen

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle

SummaryThe effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression 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 position 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 amplitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion 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 repeatability and high signal yield.

Trapezius muscle activity as a risk indicator for shoulder and neck pain in female service workers with low biomechanical exposure.

Electromyographic activity of the upper trapezius muscles was recorded over the workday for two groups of service workers, shopping centre (n = 22) and healthcare workers (n = 44), both with low observed biomechanical exposure. Static and median EMG activity level, number of EMG gaps and gap time were determined. The variability of these variables over the workday was examined by calculating the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) of 1-h consecutive recording periods. All variables except gap time showed acceptable reliability (ICC = 0.69-0.78), i.e. the largest fraction of variance in the data set was due to intersubject variance, despite relatively large hour-to-hour variation (CV = 0.21-0.62). The EMG activity level in the trapezius muscles was low (static activity level < 1% EMGmax), despite the high prevalence of shoulder and neck pain for both groups of workers. In addition to the work recordings, tests were performed to determine intersubject variation in muscle activity when adopting a standardized resting posture, and in a dynamic muscle activity pattern during paced arm movement. Neither the EMG variables from the work recordings nor the tests with EMG recording indicated higher trapezius EMG activity levels for workers with pain in the shoulders and neck in this study. The low EMG levels are interpreted to indicate a low risk of developing shoulder and neck complaints due to biomechanical exposure for both groups of workers. The possibility of pain-initiating mechanisms, associated with stress and not mediated through muscle activity, is considered in the discussion.

A case-control study of trapezius muscle activity in office and manual workers with shoulder and neck pain and symptom-free controls

A case-control study with matched pairs was initiated to investigate the relationship between shoulder-neck complaints and activity in the upper trapezius muscle. The matching was done so that the physical demands from work (external exposure) were equal for both the case and the control. Each pair was also matched for gender, age, working hours, and employment time. Male (n = 18) and female workers (n = 78) employed in both manual and office work were included. Muscle activation levels and pause patterns during work and muscle activity during tests of attention, coordination, and rest were recorded by surface electromyography. The results showed consistent associations between pain and signs of increased activation of the upper trapezius for the cases in the manual group. No such associations were observed in the office group. The results are consistent with the hypothesis that muscle activation patterns may in some instances, but not in all, explain why some workers develop pain while others do not in work situations where the physical demands are similar.

Can stress-related shoulder and neck pain develop independently of muscle activity?

&NA; A case‐control designed was used to investigate associations and interactions between muscle activity measured by surface electromyography (EMG) in the upper trapezius muscle and subjectively reported risk factors in workers with and without shoulder and neck pain. EMG data were collected both in the workplace (indicating vocational muscle activity) and in a laboratory setting (indicating non‐specific muscle activity). Women in manual (15 pairs) and office (24 pairs) work were included. The pairs were matched on age, gender and on current and historical work load, such as working hours, type and length of employment. Previous reports of this study have indicated that shoulder and neck myalgia was associated with increased muscle activity for the manual workers, and with psychological and psychosocial factors for the office workers. These risk factors were in the present report used as the basis for studying associations and interactions between muscle activity (1), psychological and psychosocial factors (2), and shoulder and neck pain (3). Subjectively reported or perceived general tension, a stress symptom presumed related to psychosocial and psychological factors, was previously found to be the strongest and only variable separating cases and controls in both work groups. In this paper, no relationship was found between perceived general tension and EMG variables for the office workers. For the manual workers a strong interaction was found; perceived general tension correlated positively with EMG variables for the controls, and negatively with EMG variables for the cases. It is hypothesised that the feeling of geneal tension represents a physiological activation response that may or may not include muscle fibre activation. This implies that pain provoked by psychosocial stress factors may not be mediated through increased muscle activity.

Low-level Laser versus Traditional Physiotherapy in the Treatment of Tennis Elbow

Summary The effect of low-level laser (GaAs) versus a combination of pulsed ultrasound and deep friction massage was investigated in a randomised trial. The laser therapy was given at a dose of 3.5 J/cm 2 and the ultrasound at a space and time average intensity of 0.32 W/cm 2 , and the deep friction massage was applied for ten minutes continually. Thirty patients with lateral epicondylalgia were assigned equally to the two treatment groups. All patients received eight treatments and were evaluated subjectively and objectively before and at the end of treatment, and four weeks after treatment. From selection to the last assessment, a significant decrease in pain (p 1 1 = 0.01) and the laser group (p 1 = 0.02). Comparing the two groups, a significant decrease in pain (p 2

Shoulder and neck complaints in customer relations: individual risk factors and perceived exposures at work

Perceived psychosocial and biomechanical exposures, individual factors and pain in the shoulder and neck were recorded in two groups of female service workers (healthcare and shopping centre workers). The jobs investigated were characterized by ‘much’ direct human relations, ‘little’ sitting and ‘much’ standing, and were light work by physiological or biomechanical criteria but potentially psychosocially demanding. A screening survey (n = 400 females) was the basis for the selected sample (n = 66 females), which was the object of the main investigation of this study. Reliability of the questionnaires was tested in a separate group of female healthcare workers (n = 29). Heart rate recordings through the work day estimated workload. There was a high prevalence of shoulder and neck pain (>50%) for both work groups. In the two populations it proved difficult to explain shoulder and neck pain by reported physical and psychosocial exposures or individual factors, except by the variable ‘perceived general tension’, which clearly differentiated workers with and without pain. The findings in this study indicated, first, that perceived general tension might be an independent risk factor for muscle pain and, second, that this might be related to personality factors. However, this putative relationship must be verified in a longitudinal study. As no variable describing exposures in the working environment was associated with shoulder and neck pain, the question is posed whether such complaints can be considered work-related. Alternatively, the variables used to describe mechanical and psychosocial exposures in this study may have low specificity in characterizing work-related risk factors for service workers with customer relations.

A case-control study of psycholotical and psychosocial risk factors for shoulder and neck pain at the workplace

Risk factors associated with work-related shoulder and neck myalgia were investigated in a case-control study with pairs matched for age, gender, and physical exposure. Guided interviews with standardized and self-constructed questionnaires were performed among manual (n = 15 pairs) and office (n = 24 pairs) workers. Perceived general tension was the variable with the strongest association with shoulder and neck pain in both work groups. Otherwise, the results in the two groups were very different, indicating that different risk factors and mechanisms were associated with shoulder and neck pain in the two work groups. The study provides background information for future attempts to establish causal relationships between physical and psychosocial exposure and shoulder and neck pain, which can be more accurately investigated in a longitudinal rather than a cross-sectional experimental design.

Arm and trunk posture during work in relation to shoulder and neck pain and trapezius activity

OBJECTIVE: To investigate work technique in relation to work-related shoulder and neck pain (SNP) and upper trapezius muscle activity. DESIGN: A matched pair, case-control field study of female employees with and without SNP. BACKGROUND: It has proved difficult to distinguish subjects with SNP from those without by vocational electromyographic recordings from the upper trapezius muscle. Other potential risk indicators include psychosocial factors and work technique. This study focuses on the latter. METHODS: Manual (14 pairs) and office workers (24 pairs) were recorded during a 30-min work period. Simultaneous recordings of upper trapezius activity by surface electromyography and arm and upper back postures by inclinometers were analysed. RESULTS: Cases and controls were not differentiated on the basis of arm elevation or of trunk posture in the sagittal plane. No significant correlations were found between variables averaging the muscle activity and the arm elevation over the recording period. Statistically significant correlations were, however, found between these variables when analysing recordings at high time resolution (0.2 s) and adjusting for the delay in arm elevation relative to the upper trapezius muscle activity (r = 0.43, manual group; r = 0.32, office group). CONCLUSIONS: Factors other than arm elevation probably contribute more significantly to the load in the upper trapezius muscle, and to the development of work-related SNP in work situations with moderate arm elevation. RELEVANCE: The study suggests that arm and trunk posture recordings are not a sensitive indicator of risk of shoulder and neck complaints in work with low to moderate biomechanical demands.

Estimating maximal EMG amplitude for the trapezius muscle: On the optimization of experimental procedure and electrode placement for improved reliability and increased signal amplitude.

The amplitude of electromyographic (EMG) surface recordings on the upper trapezius muscle and its reproducibility during maximal voluntary contractions were examined. At different electrode positions 1.5-3.0 cm lateral to the midpoint between the seventh cervical vertebra (C7) and the acromion the mean maximal EMG amplitude estimates were within 90% of the highest EMG amplitude that was recorded, when the estimate was based on the highest value of two shoulder elevations and two arm abductions. Near the midpoint between C7 and the acromion the mean maximal EMG amplitude was only 42% of the highest value. The repeatability coefficient based on two estimates of the maximal EMG amplitude obtained with 2 h interval indicated that 95% of the differences in the recorded amplitudes should be less than 22% of the mean maximal amplitude. Increasing the averaging period during signal processing from 0.6-2.0 s decreased the maximal amplitude estimate by 8%. It is concluded that electrode positioning is an important factor to consider and that more than one shoulder elevation and arm abduction should be performed when estimating maximal amplitudes.

For patients with tennis elbow, physiotherapy is superior to corticosteroid injections in the long term

Summary of Smidt N, van der Windt AWM, Assendelft WJJ, Deville WLJM, Korthals-de Bos IBC and Bouter LM (2002): Corticosteroid injections, physiotherapy, or a wait-and-see policy for lateral epicondylitis: a randomised controlled trial.