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The aim of this randomized controlled trial was to assess changes in myalgic trapezius activation, muscle oxygenation, and pain intensity during repetitive and stressful work tasks in response to 10 weeks of training. In total, 39 women with a clinical diagnosis of trapezius myalgia were randomly assigned to: (1) general fitness training performed as leg-bicycling (GFT); (2) specific strength training of the neck/shoulder muscles (SST) or (3) reference intervention without physical exercise. Electromyographic activity (EMG), tissue oxygenation (near infrared spectroscopy), and pain intensity were measured in trapezius during pegboard and stress tasks before and after the intervention period. During the pegboard task, GFT improved trapezius oxygenation from a relative decrease of -0.83 ± 1.48 μM to an increase of 0.05 ± 1.32 μM, and decreased pain development by 43%, but did not affect resting levels of pain. SST lowered the relative EMG amplitude by 36%, and decreased pain during resting and working conditions by 52 and 38%, respectively, without affecting trapezius oxygenation. In conclusion, GFT performed as leg-bicycling decreased pain development during repetitive work tasks, possibly due to improved oxygenation of the painful muscles. SST lowered the overall level of pain both during rest and work, possibly due to a lowered relative exposure as evidenced by a lowered relative EMG. The results demonstrate differential adaptive mechanisms of contrasting physical exercise interventions on chronic muscle pain at rest and during repetitive work tasks. [ABSTRACT FROM AUTHOR]

The purpose of this laboratory study was to investigate the effects of surface electromyography (EMG)- and mechanomyography (MMG)-based audio and visual biofeedback during computer work. Standardized computer work was performed for 3 min with/without time constraint and biofeedback in a randomized order. Biofeedback was given on the basis of an individual preset threshold value for the right trapezius EMG and MMG signal and a time factor (repetition of events above the threshold). The duration of muscle activity above the preset threshold, the right trapezius EMG and MMG root mean square (RMS) values as well as the work performance in terms of number of completed graph/mouse clicks/errors, the rating of perceived exertion (RPE) and the usefulness of the biofeedback were assessed. The duration of muscle activity above the threshold was significantly lower with MMG compared with EMG as source of biofeedback (p < 0.05). Biofeedback led to a significant decrease in the right trapezius EMG RMS, lower RPE and decreased number of errors and mouse clicks, but also decreased number of completed graphs (p < 0.05). Audio and visual biofeedbacks were as effective. MMG-based biofeedback is a potential reliable alternative to EMG in ergonomics. A lowering of the trapezius muscle activity may contribute to diminish the risk of work related musculoskeletal disorders development. [ABSTRACT FROM AUTHOR]

Background: Computer work is associated with low level sustained activity in the trapezius muscle that may cause development of trapezius myalgia. Such a low level activity may be attention related or alternatively, be part of a general multi joint motor program providing stabilization of the shoulder joint as a biomechanical prerequisite for precise finger manipulation. This study examines single motor unit (MU) firing pattern in the right trapezius muscle during fast movements of ipsilateral or contralateral index finger. A modulation of the MU firing rate would support the existence of a general multi joint motor program, while a generally increased and continuous firing rate would support the attention related muscle activation., Method: Twelve healthy female subjects were seated at a computer work place with elbows and forearms supported. Ten double clicks (DC) were performed with right and left index finger on a computer mouse instrumented with a trigger. Surface electromyographic signals (EMG) was recorded from right and left trapezius muscle. Intramuscular EMG was recorded with a quadripolar wire electrode inserted into the right trapezius. Surface EMG was analyzed as RMS and presented as %MVE. The intramuscular EMG signals were decomposed into individual MU action potential trains using a computer algorithm based on signal shape recognition and manual editing. Instantaneous firing rate (IFR) was calculated as the inverse of each inter-spike interval (ISI). All ISI shorter than 20 ms were defined as doublets. For all MU IFR was spike triggered averaged across the 10 DC to show the modulation during DC as well as for calculation of the cross correlation coefficient (CCC)., Results: All subjects showed surface EMG activity in both right and left trapezius ranging from 1.8 %MVE to 2.5 %MVE. Regarding intramuscular EMG during right hand DC a total of 32 MUs were identified. Four subjects showed no MU activity. Four showed MU activity with low mean firing rate (MFR) with weak or no variations related to the timing of DC. Four subjects showed firing patterns with large modulation in IFR with a clear temporal relation to the DC. During left hand DC 15 MUs were identified in four subjects, for two of the subjects with IFR modulations clearly related to DC. During both ipsi- and contralateral DC, doublets occurred sporadically as well as related to DC Conclusion: In conclusion, DC with ipsi- and contralateral fast movements of the index finger was found to evoke biomechanically as well as attention related activity pattern in the trapezius muscle. Doublets were for three of the subjects found as an integrated part of MU activation in the trapezius muscle and for one subject temporarily related to DC.

Unlabelled: Continuous electromyographic (EMG) activity level is considered a risk factor in developing muscle disorders. EMG biofeedback is known to be useful in reducing EMG activity in working muscles during computer work. The purpose was to test the following hypotheses: (1) unilateral biofeedback from trapezius (TRA) can reduce bilateral TRA activity but not extensor digitorum communis (EDC) activity; (2) biofeedback from EDC can reduce activity in EDC but not in TRA; (3) biofeedback is more effective in no time constraint than in the time constraint working condition. Eleven healthy women performed computer work during two different working conditions (time constraint/no time constraint) while receiving biofeedback. Biofeedback was given from right TRA or EDC through two modes (visual/auditory) by the use of EMG or mechanomyography as biofeedback source. During control sessions (no biofeedback), EMG activity was (mean ± SD): 2.4 ± 1.1, 2.5 ± 2.1, and 9.1 ± 3.1%max-EMGrms for right and left TRA and EDC, respectively. During biofeedback from TRA, activity was reduced in right TRA (1.7 ± 1.6%max-EMGrms) and left TRA (1.2 ± 2.0%max-EMGrms) compared to control. During biofeedback from EDC, activity in EDC was reduced (8.3 ± 3.3%max-EMGrms) compared with control. During time constraint, activity was reduced in right TRA (1.9 ± 1.3%max-EMGrms), left TRA (1.5 ± 1.5%max-EMGrms), and EDC (8.4 ± 3.2%max-EMGrms), during biofeedback compared to control., Conclusion: biofeedback reduced muscle activity in TRA by ∼ 30-50% and in EDC by ∼ 10% when given from the homologous or bilateral muscle but not from the remote muscle, and was significant in the time constraint condition; while feedback source and presentation mode showed only minor differences in the effect on reducing homologous muscle activity. This implies that biofeedback should be given from the most affected muscle in the occupational setting for targeting relief and prevention of muscle pain most effectively., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Surface electromyograms(EMG) during test contractions (TCs) were studied to assess the muscle strain in simulated mushroom picking. Additionally, the duration of the TC for the effective assessment was investigated. Nine female subjects performed standardized shoulder abduction and a stooped posture for one minute as TCs. Each experiment consisted of a 60-min rest, three work periods (W1-W3), a 30-min rest, and two work periods (W4 and W5) separated by a 30-min rest period. The duration of each work period was about 20 min. A total of 18 TCs was performed between the work periods and every 10 minutes in the rest periods. EMGs were recorded from the trapezius, infraspinatus, deltoid, and erector spinae muscles. The amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated. Each TC was divided equally into three parts. Ratings of perceived exertion (RPE) in the neck, shoulder and low-back were reported during TCs. The work increased RPE of all the parts. AEMG and RPE were increased and MPF was decreased by W1, W2 and W3 in the neck and shoulder muscles. MPF of the erector spinae was increased by the work. The results were not affected by the duration of TCs and the parts during the TCs. AEMG and MPF fluctuated before W1 although the changes of RPE were small. Averaging several TCs was recommended to get stable results from TCs. EMG changes and appropriate TC conditions were discussed in relation to the adaptation in fatiguing contractions.

Work-related neck/shoulder muscle pain has been associated with increased anaerobic muscle metabolism. Thus, interventions to enhance oxygenation of painful muscles seem relevant. While cycling with relaxed shoulders has been shown to result in acute neck/shoulder muscle pain reduction, the effect on tissue oxygenation remains unknown. The aim of the present study was to investigate tissue oxygenation of the passive trapezius muscle during and after cycling in female workers with (MYA) and without (CON) trapezius myalgia. Eligible participants (n = 17 MYA, n = 8 CON) performed 20 min sub-maximal cycling in an upright position with relaxed shoulders. Near-infrared spectroscopy was used to measure trapezius muscle oxygenation during and 2 min after the cycling period. For both MYA and CON, oxygenation of the passive trapezius increased in a linear fashion over time, to values approximately 5 microM above baseline at the end of the cycling period, with no significant group difference (CON 5.2, MYA 4.9 microM). Two min after termination of exercise, oxygenation was increased further in both groups, but significantly more in CON (8.8 microM) than in MYA (7.0 microM) (P = 0.05). In conclusion, cycling increases oxygenation of resting neck/shoulder muscles in women with and without trapezius myalgia, indicating acute positive effects of either neural or humoral factors on vascular beds of distant relaxed muscles. Although this beneficial response was observed in both groups, the post-exercise response was lower in women with trapezius myalgia.

The aim of this investigation was to study female workers active in the labour market for differences between those with trapezius myalgia (MYA) and without (CON) during repetitive pegboard (PEG) and stress (STR) tasks regarding (1) relative muscle load, (2) trapezius muscle blood flow, (3) metabolite accumulation, (4) oxygenation, and (5) pain development. Among 812 female employees (age 30-60 years) at 7 companies with high prevalence of neck/shoulder complaints, clinical examination identified 43 MYA and 19 CON. At rest, during PEG, and STR the trapezius muscle was measured using (1) EMG and MMG, (2) microdialysis, and (3) NIRS. Further, subjective pain ratings were scored (VAS). EMGrms in %MVE (Maximal Voluntary EMG-activity), was significantly higher among MYA than CON during PEG (11.74 +/- 9.09 vs. 7.42 +/- 5.56%MVE) and STR (5.47 +/- 5.00 vs. 3.28 +/- 1.94%MVE). MANOVA showed a group and time effect regarding data from the microdialysis: for MYA versus CON group differences demonstrated lower muscle blood flow and higher lactate and pyruvate concentrations. Potassium and glucose only showed time effects. NIRS showed similar initial decreases in oxygenation with PEG in both groups, but only in CON a significant increase back to baseline during PEG. VAS score at rest was highest among MYA and increased during PEG, but not for CON. The results showed significant differences between CON and MYA regarding muscle metabolism at rest and with PEG and STR. Higher relative muscle load during PEG and STR, insufficient muscle blood flow and oxygenation may account for the higher lactate, pyruvate and pain responses among MYA versus CON.

Purpose: The aim of the present study is to examine the association between physical activity and perceived job demand, job control, perceived stress and energy, and physiological arousal reflected by morning and evening concentrations of cortisol in saliva among white-collar workers., Methods: Physical activity during the last week was assessed during work and leisure time by a Danish version of the International Physical Activity Questionnaire and saliva samples were collected. The study group comprised 389 white-collar workers, aged 25-67 years, and of which 257 were women., Results: We found that physical activity during leisure time was associated with higher perceived energy, and for men also with lower perceived stress. Further, we found that physical activity at leisure time affected the association between salivary cortisol and perceived stress and energy so that respondents being physically active at leisure time and perceiving higher energy showed higher evening saliva cortisol., Conclusion: Physically active employees perceive less stress and more energy. The association between stress-energy and salivary cortisol was affected by vigorous physical activity. No association between job control-demand and the degree of physical activity was found. Based on the present data, we recommend office workers-exposed to high job strain and inactivity at the job-to perform physical activity, preferably of high intensity, in order to reduce stress and increase energy.

Objectives: The aim of this case-control study was to investigate characteristics of cleaners with good musculoskeletal health after years with physically heavy work., Methods: One hundred and 41 female seniority cleaners participated. Twenty-five reported no musculoskeletal symptoms, whereas 83 reported severe symptoms in the low back, neck shoulders or upper limbs. The groups were of matching age, height, body weight and seniority (19 years). Muscular strength was recorded by isometric maximal voluntary contractions on a day without pain. Exposure to physical risk factors at work, psychosocial work factors, and leisure time physical activity were assessed by a postal questionnaire., Results: Cleaners with good musculoskeletal health were not reporting different exposure to physical risk factors at work or leisure time physical activity, but had higher muscular strength and reported higher influence at work than cleaners with severe symptoms., Conclusions: These findings suggest that muscular strength and influence at work are of relevance for sustaining good musculoskeletal health in workers with physically heavy work.

Objective: To investigate the effect of two contrasting physical activity worksite interventions versus a reference intervention (REF) on various health outcomes., Methods: A 1-year randomized controlled trial was conducted with specific resistance training (SRT), all-round physical exercise (APE), and REF., Results: SRT and APE compared with REF showed significant reductions in systolic blood pressure (approximately 6 mm Hg), body fat percentage (approximately 2.2 body fat%), as well as shoulder and back pain (approximately 30% reduction in duration). Muscle strength (APE and SRT) and maximal oxygen uptake (APE) increased approximately 10%., Conclusions: Worksite intervention with both SRT as well as APE is recommended, since these activities compared with REF resulted in clinically relevant reductions of cardiovascular and metabolic syndrome-related risk factors as well as musculoskeletal pain symptoms, in combination with minor increases in physical capacity.

The aim of this study was to investigate effects of biofeedback training on trapezius activity and rest (gaps) during occupational computer work. A randomized controlled trial with 164 computer workers was performed. Two groups working with computer mouse more than 50% (n = 64) and less than 25% (n = 49) of their work time performed five sessions with unilateral electromyographical (EMG) biofeedback from the dominant trapezius during computer work. A third group working with computer mouse more than 50% of their work time (n = 51) served as controls. Bipolar EMG from the bilateral upper trapezius muscles during normal computer work was recorded. Changes in discomfort/pain were not recorded. The biofeedback training reduced activity (P < 0.05), and increased the frequency of short (P < 0.05) and long (P < 0.05) gaps, and the relative rest time (P < 0.05) of the trapezius during computer work. By improving trapezius inactivity during computer work, biofeedback training may have the potential to prevent trapezius myalgia in computer workers.

Muscle strain was assessed with surface EMG during simulated mushroom picking. Nine female subjects performed five periods of work (W1-W5). The duration of each period was about 20 min. W1, W2 and W3 were separated by a short break of several minutes. W3, W4 and W5 were separated by a rest period of about 43 min. EMGs were recorded from the muscles of trapezius, infraspinatus, deltoid and erector spinae muscles. Amplitude of EMG (AEMG) and mean power frequency (MPF) of EMG were calculated every minute. Ratings of perceived exertion (RPE) were recorded for every 5 min. RPE increased during each work period and with the advance of W1-3. AEMG increased during some of the work periods in the trapezius and infraspinatus. AEMG decreased in a few of the work periods in the other muscles. MPF decreased during some of the work periods in the infraspinatus, deltoid and erector spinae. The increase of AEMG in the trapezius was interpreted as spare capacity for the work. The infraspinatus and deltoid were assessed to be more fatigued than the trapezius. The fatigue level of the erector spinae was low.

Objectives: This study evaluates the effect of two different worksite physical-activity interventions on neck-shoulder symptoms, together with perceived work ability and sick leave among office workers., Methods: An examiner-blinded randomized controlled trial was conducted with 549 office workers allocated to one of three intervention groups: one with specific resistance training (SRT) of the neck-shoulder region (N=180), one with all-round physical exercise (APE) (N=187), and one which acted as a reference group, which was informed about general health-promoting activities but did not include a physical activity program (N=182). Questionnaires were filled out at baseline and after 1 year of training., Results: The duration and intensity of neck and shoulder symptoms was lower after the specified worksite physical-activity interventions than in the reference group. On an intervention group level, SRT was not more effective than APE in reducing the duration and intensity of neck and shoulder symptoms. However, those asymptomatic at baseline had a significant lower prevalence of neck-shoulder symptoms at follow-up when allocated to the SRT group than placed in the APE group or reference group. At baseline the work ability index (WAI) was close to 90% of the maximum score, and the mean sick leave was 5 days per year, both being unaffected by the interventions., Conclusions: Different physical-activity interventions were successful in reducing neck-shoulder symptoms, and SRT was superior to APE in the primary prevention of such symptoms. The initially relatively high WAI was the most probable reason for no further increase in WAI. Likewise the mean sick leave the year before the intervention was very low, and it was probably not possible to reduce it further.

Dynamic muscle contractions have been shown to cause greater energy turnover and fatigue than static contractions performed at a corresponding force level. Therefore, we hypothesized that: (1) electro- (EMG) and mechanomyography (MMG), intramuscular pressure (IMP), and reduction in muscle oxygen tension (rTO(2)) would be larger during dynamic (DYN) than intermittent static (IST) low force contractions; and that (2) oxygen tension would remain lower in the resting periods subsequent to DYN as compared to those following IST. Eight subjects performed elbow flexions with identical time-tension products: (1) DYN as a 20 degrees elbow movement of 2 s concentric and 2 s eccentric followed by a 4 s rest; and (2) IST with a 4 s contraction followed by a 4 s rest. Each session was performed for 1 min at 10 and 20% of the maximal voluntary contraction (MVC). The force, bipolar surface EMG, MMG, IMP, rTO(2) were measured simultaneously from the biceps brachii, and the data presented as the mean values together with the standard error of the means. Comparison of the corresponding time periods showed the EMG(rms) and MMG(rms) values to be larger during DYN than IST (concentric phase: DYN vs IST were 14.2 vs 9.4, and 22.0 vs 15.9%(max)-EMG(rms); eccentric phase: in DYN, the MMG was approximately 1.5 and approximately 2.0-fold IST at 10 and 20%MVC, respectively). In contrast, the IMP of the concentric phase in DYN was lower than in IST (2.3 vs 29.5 and 10.9 vs 42.0 mmHg at 10 and 20%MVC, respectively), and a similar picture was seen for the eccentric phase. However, no differences were seen in rTO(2) in either the contraction or the rest periods. In a prolonged rest period (8 s) after the sessions, DYN but not IST showed rTO(2) above baseline level. In conclusion, rTO(2) in DYN and IST were similar in spite of major differences in the MMG and EMG responses of the muscle during contraction periods. This may relate to the surprisingly lower IMP in DYN than IST.

Aim: To test the hypothesis that time-wise increase in intramuscular pressure (IMP) and subsequent decrease in muscle tissue oxygenation (TO(2)) results in muscle fatigue development during a non-exhaustive, low-force contraction evidenced by changes in electromyogram (EMG) and particular mechanomyogram (MMG)., Methods: Seven subjects performed static elbow flexion at 10% maximal voluntary contraction (MVC) for 10 min (10% MVC(10 min)). Surface EMG, MMG, IMP and TO(2) measured by near-infrared spectroscopy was recorded from m. biceps brachii during 10% MVC(10 min) and during 5% MVC test contractions of 1 min duration performed before 10% MVC(10 min), 10 and 30 min post-exercise. EMG and MMG were analysed for root mean square (rms) and mean power frequency (mpf)., Results: During 10% MVC(10 min) MMGrms increased from initial level of 0.04 +/- 0.01 to 0.11 +/- 0.07 m s(-2) in the last minute and MMGmpf and EMGmpf decreased from 34.9 +/- 8.2 to 21.3 +/- 3.8 Hz and from 71.7 +/- 10.9 to 61.7 +/- 10.0 Hz respectively. Similar changes were present in 5% MVC test contractions 30 min post-exercise. Initially, TO(2) decreased by 6.9 +/- 6.5% of resting level but returned to rest within 1 min. IMP remained constant during the contraction after an initial fourfold increase from resting level of 12.2 +/- 10.4 mmHg., Conclusions: IMP was anticipated to increase with time of contraction due to e.g. increased muscle water content; but this was not confirmed. Consequently, muscle blood flow was unlikely to be impeded with contraction time, which may account for the maintenance of TO(2). Thus, decreased TO(2) did not underlie either acute or long-term muscle fatigue development evidenced by changes in EMG and particular MMG variables.

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC10 min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC10 min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC10 min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067+/-0.028 mV before 10%MVC10 min to 0.107+/-0.049 and 0.087+/-0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054+/-0.039 ms(-2) to 0.133+/-0.104 and 0.127+/-0.099 ms(-2), respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.

Aim: Local muscle metabolic responses to repetitive low-force contractions and to intense static contractions were studied by microdialysis in humans., Methods: Microdialysate and electromyography (EMG) were sampled from the trapezius muscle, mixed venous blood samples were taken and perceived exertion was rated (0-9) before and during 20 min of standardized repetitive arm movement (REP), 60 min recovery (R1), and 10 min 90 degrees sustained arm position (SUS) at 20% maximum voluntary contraction, followed by 60 min recovery (R2) in six healthy male participants (28-33 years)., Results: Average muscle activity was 8 +/- 2% of EMGmax-RMS (mean +/-SEM) during REP and 22 +/- 5% of EMGmax-RMS during SUS. Perceived exertion increased from 0 to 3.2 +/- 0.5 during REP and from 0 to 8.5 +/- 0.3 during SUS. During REP interstitial muscle lactate increased from 2.1 +/- 0.2 to 2.9 +/- 0.2 mmol L(-1) (P < 0.001) and returned to the baseline level during R1, while dialysate [K+] increased from 3.8 +/- 0.2 to 4.7 +/- 0.2 mmol L(-1) (P < 0.002) and returned to 3.8 +/- 0.2 mmol L(-1) during R1. In contrast, plasma lactate and [K+] remained unchanged. During SUS interstitial muscle lactate increased from 2.3 +/- 0.2 to 3.3 +/- 0.3 mmol L(-1) (P < 0.003), increased further to 6.5 +/- 1.3 mmol L(-1) post-exercise (P < 0.001) and returned to baseline levels during R2. Dialysate [K+] increased from 3.9 +/- 0.2 to 4.6 +/- 0.2 mmol L(-1) (P < 0.05) and returned to baseline level during R2. Plasma lactate increased significantly during SUS whereas plasma [K+] was unchanged. During REP and SUS interstitial pyruvate was unchanged but increased in the post-exercise period proportional to the exercise intensity., Conclusions: The microdialysis technique was effective in revealing muscle metabolic events that were not found systemically. Furthermore, the trapezius muscle showed an anaerobic metabolism during low-force contraction, which could indicate inhomogeneous muscle activation.

The aim was to evaluate the cardiovascular and subjective stress response to a combined physical and mental workload, and the effect of rest. Twelve females who had no prior experience of laboratory experiments participated in the study. Computer-work-related mental stressors were either added to or removed from a standardized computer work session in the laboratory. Beat-to-beat blood pressure and electrocardiogram (ECG) were recorded continuously during the experiment. The participants reported subjective experiences of stress in six categories using an 11-point scale before and at the end of the work. Heart rate variability (HRV) variables were calculated from the ECG recordings, and a reduction in the high-frequency component of HRV and an increase in the low- to high-frequency ratio were observed in the stress situation compared to the control session. No changes were seen in the low-frequency component of HRV. The stressors induced an increase in blood pressure compared to baseline that persisted, and for the diastolic pressure it even increased in the subsequent control session. No differences were observed for subjective experience of stress with the exception of a time trend in the exhaustion scale, i.e. a progression in reported exhaustion with time. The results-and the dissociation between HRV and blood pressure variables-indicate that HRV is a more sensitive and selective measure of mental stress. It could be speculated that heart rate-derived variables reflect a central pathway in cardiovascular control mechanisms ("central command"), while the blood pressure response is more influenced by local conditions in the working muscles that partly mask the effect of changes in mental workloads. In the rest period after each work session, HRV and blood pressure variables were partly normalized as expected. However, an 8-min period of rest was insufficient to restore blood pressure to resting values.

Unlabelled: The overall aim was to investigate the effect of psychosocial loads on trapezius muscle activity during computer keying work and during short and long breaks. In 12 female subjects, surface electromyography (EMG) was recorded bilaterally from the upper trapezius muscle during a standardized one hand keying task-interspaced with short (30 s) and long (4 min) breaks-in sessions with and without a combination of cognitive and emotional stressors. Adding psychosocial loads to the same physical work did not increase the activity of the trapezius muscle on either the keying or the control side, both of which remained at median and static EMG activity levels of around 5% and 2.5% of the maximal voluntary electrical activity (EMG(max)), respectively. The difference between the keying and the control side was significant; and further the control side activity was significantly increased above resting level. During both short and long breaks, exposure to psychosocial loads also did not increase the activity of the trapezius muscle either on the side of the keying or the control hand. Of note is that during long breaks the muscle activity of the keying side as well as that of the control side remained at the same level as during the short breaks, which was increased above resting level. This was to be seen from the static and the median EMG activity levels as well as gap times, the overall mean values being: 0.4%EMG(max), 1.1%EMG(max), and 50% in gap time, respectively., In Conclusion: psychosocial loads are not solely responsible for increased non-postural muscle activity; and increasing the duration of breaks does not per se cause muscle relaxation.

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.

The aim was to investigate whether quantitative job demands influence muscle activity among women, and whether there are gender differences in duration of computer, mouse, and keyboard use and muscle activity of shoulder and forearm muscles during work. The study was carried out in an occupational setting, and 24 women and 11 men from a municipal administration participated. The duration of computer, mouse, and keyboard use was measured by a commercial software package. Quantitative job demands were registered by questionnaire. Electromyography (EMG) was measured bilaterally from the upper trapezius and the extensor digitorum communis muscles. No association was found between self-reported quantitative job demands and muscle activity among the women ( n=24). The women used the keyboard more frequently ( p=0.020) and tended to perform fewer mouse clicks compared to men ( p=0.057), but no difference was seen in EMG activity between men ( n=11) and women ( n=11) from the same department. However, office assistants (six women) showed significantly higher static EMG activity levels ( p=0.042) and almost significantly shorter EMG gap times ( p=0.060) than the rest of the subjects (5 women and 11 men). This indicated that shorter muscular resting periods among female office assistants as compared to the other subjects were due to differences in job content rather than gender differences.