Karen Søgaard

The effect of mental stress on heart rate variability and blood pressure during computer work

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.

Increase in muscle nociceptive substances and anaerobic metabolism in patients with trapezius myalgia: microdialysis in rest and during exercise.

&NA; Local metabolic changes are suggested to be involved in muscle pain development in humans. Nineteen women with chronic work‐related trapezius myalgia (TM) and 20 healthy female controls (CON) were studied during baseline rest, 20 min repetitive low‐force exercise, and 120 min recovery. Interstitial serotonin (5‐HT), glutamate, lactate, pyruvate, and blood flow were determined by microdialysis in the trapezius muscle. Baseline pressure pain threshold (PPT) was lower (143±18 (TM) vs. 269±17 (CON) kPa) (mean±SEM), pain intensity (visual analogue scale, VAS) higher (33±5 vs. 2±1 mm), muscle 5‐HT higher (22.9±6.7 vs. 3.8±1.3 nmol/l), and glutamate higher (47±3 vs. 36±4 μmol/l) in TM than in CON (all P<0.05), whereas muscle blood flow was similar in groups. Furthermore, muscle pyruvate was higher (180±15 vs. 135±12 μmol/l) and lactate higher (4.4±0.3 vs. 3.1±0.3 mmol/l) in TM than in CON (P<0.001). In response to exercise, VAS and glutamate increased in both TM and CON (all P<0.05). In TM only, lactate and pyruvate increased significantly (P<0.02), whereas blood flow increased to similar levels in both groups. During the initial 20 min recovery period, blood flow remained increased in TM (P<0.005) whereas it decreased to baseline levels in CON. In conclusion, patients with chronic work‐related TM have increased levels of muscle 5‐HT and glutamate that were correlated to pain intensity (r=0.55, P<0.001) and PPT (r=−0.47, P<0.001), respectively. In addition, TM was associated with increased anaerobic metabolism, whereas a normal rise in blood flow was seen with exercise. These findings indicate that peripheral nociceptive processes are active in work‐related TM.

Effect of two contrasting types of physical exercise on chronic neck muscle pain

OBJECTIVE The prevalence of neck muscle pain has steadily increased and especially pain from the descending part of the trapezius muscle has been associated with monotonous work tasks such as computer work. Physical exercise is generally recommended as treatment, but it is unclear which type of training is most effective. Our objective was to determine the effectiveness of specific strength training of the painful muscle versus general fitness training without direct involvement of the painful muscle (leg bicycling) on work-related neck muscle pain. METHODS We conducted a randomized controlled trial and recruited subjects from 7 workplaces characterized by monotonous jobs (e.g., computer-intensive work). Forty-eight employed women with chronic neck muscle pain (defined as a clinical diagnosis of trapezius myalgia) were randomly assigned to 10 weeks of specific strength training locally for the affected muscle, general fitness training performed as leg bicycling with relaxed shoulders, or a reference intervention without physical activity. The main outcome measure was an acute and prolonged change in intensity of neck muscle pain (100-mm visual analog scale [VAS]). RESULTS A decrease of 35 mm (approximately 79%; P<0.001) in the worst VAS pain score over a 10-week period was seen with specific strength training, whereas an acute and transient decrease in pain (5 mm; P<0.05) was found with general fitness training. CONCLUSION Specific strength training had high clinical relevance and led to marked prolonged relief in neck muscle pain. General fitness training showed only a small yet statistically significant acute pain reduction.

Musculoskeletal symptoms and duration of computer and mouse use

Abstract The primary aim was to study associations between duration of computer and mouse use and musculoskeletal symptoms among computer users. A questionnaire was delivered to 5033 employees in 11 Danish companies and institutions and 3475 subjects responded (69%). Logistic regression analyses on 2579 full-time working employees showed that working almost the whole working day with a computer was associated with neck symptoms (OR=1.92, CI: 1.21–3.02) and shoulder symptoms (OR=1.83, CI: 1.13–2.95) among women and hand symptoms (OR=2.76, CI: 1.51–5.06) among men. These odds ratios were adjusted for psychosocial factors. Among respondents working almost all of their work time with a computer the gender and age-adjusted odds ratio for mouse use more than half of the work time was 1.68 (CI: 1.22–2.31) for hand/wrist symptoms. Call center and data entry workers experienced the lowest possibilities for development at work. All work tasks involving computer use, except computer maintenance, were characterised by a higher frequency of movements than desk work without using a computer. Call center work was characterised by the highest level of repetitiveness as both work tasks and movements were perceived as repetitive. Relevance to industry Computer users with a long daily duration of computer use and mouse use experienced more musculoskeletal symptoms than those with a short duration of computer use. Computer work in general seemed characterised by repetitive movements, which may be a risk factor for musculoskeletal symptoms.

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.

The health paradox of occupational and leisure-time physical activity

Background Occupational and leisure-time physical activity are considered to provide similar health benefits. The authors tested this hypothesis. Methods A representative sample of Danish employees (n=7144, 52% females) reported levels of occupational and leisure-time physical activity in 2005. Long-term sickness absence (LTSA) spells of ≥3 consecutive weeks were retrieved from a social-transfer payment register from 2005 to 2007. Results 341 men and 620 females experienced a spell of LTSA during the period. Cox analyses adjusted for age, gender, smoking, alcohol, body mass index, chronic disease, social support from immediate superior, emotional demands, social class and occupational or leisure-time physical activity showed a decreased risk for LTSA among workers with moderate (HR 0.85, CI 0.72 to 1.01) and high (HR 0.77, CI 0.62 to 0.95) leisure-time physical activity in reference to those with low leisure-time physical activity. In contrast, an increased risk for LTSA was shown among workers with moderate (HR 1.59, CI 1.35 to 1.88) and high (HR 1.84, CI 1.55 to 2.18) occupational physical activity referencing those with low occupational physical activity. Conclusion The hypothesis was rejected. In a dose–response manner, occupational physical activity increased the risk for LTSA, while leisure-time physical activity decreased the risk for LTSA. The findings indicate opposing effects of occupational and leisure-time physical activity on global health.

Worksite interventions for preventing physical deterioration among employees in job-groups with high physical work demands: Background, design and conceptual model of FINALE

BackgroundA mismatch between individual physical capacities and physical work demands enhance the risk for musculoskeletal disorders, poor work ability and sickness absence, termed physical deterioration. However, effective intervention strategies for preventing physical deterioration in job groups with high physical demands remains to be established. This paper describes the background, design and conceptual model of the FINALE programme, a framework for health promoting interventions at 4 Danish job groups (i.e. cleaners, health-care workers, construction workers and industrial workers) characterized by high physical work demands, musculoskeletal disorders, poor work ability and sickness absence.Methods/DesignA novel approach of the FINALE programme is that the interventions, i.e. 3 randomized controlled trials (RCT) and 1 exploratory case-control study are tailored to the physical work demands, physical capacities and health profile of workers in each job-group. The RCT among cleaners, characterized by repetitive work tasks and musculoskeletal disorders, aims at making the cleaners less susceptible to musculoskeletal disorders by physical coordination training or cognitive behavioral theory based training (CBTr). Because health-care workers are reported to have high prevalence of overweight and heavy lifts, the aim of the RCT is long-term weight-loss by combined physical exercise training, CBTr and diet. Construction work, characterized by heavy lifting, pushing and pulling, the RCT aims at improving physical capacity and promoting musculoskeletal and cardiovascular health. At the industrial work-place characterized by repetitive work tasks, the intervention aims at reducing physical exertion and musculoskeletal disorders by combined physical exercise training, CBTr and participatory ergonomics. The overall aim of the FINALE programme is to improve the safety margin between individual resources (i.e. physical capacities, and cognitive and behavioral skills) and physical work demands, and thereby reduce the physical deterioration in a long term perspective by interventions tailored for each respective job-group.DiscussionThe FINALE programme has the potential to provide evidence-based knowledge of significant importance for public health policy and health promotion strategies for employees at high risk for physical deterioration.Trial registrationsISRCTN96241850, NCT01015716 and NCT01007669

Motor control and kinetics during low level concentric and eccentric contractions in man

Motor unit (MU) recruitment patterns were studied in 6 female subjects during dynamic contractions at relative workloads corresponding to 10% maximum voluntary contraction. The contractions consisted of a 20 degree elbow flexion (concentric contraction) and extension (eccentric contraction) and MU action potential trains were recorded from the brachial biceps muscle. The mean angular velocity of the dynamic contractions was 10 degrees/s, during which a total of 119 MUs were identified. Additionally, a few contractions were studied at 20 degrees/s during which 30 MUs were identified, and 9 MUs during the 40 degrees/s contraction. About 60% of the identified MUs were active during the concentric as well as the eccentric phase for each of the velocities. Mean firing rate decreased significantly when the contraction changed from concentric to eccentric, whereas the number and properties of identified active MUs were similar. This emphasizes firing rate modulation as important during low level dynamic contractions rather than selective recruitment of different types of MUs in the concentric versus the eccentric phase. Similar kinetic demands occur frequently in occupational tasks, especially during monotonous work. The present data indicate that only a limited pool of MUs are being recruited during such tasks. Extensive recruitment of these MUs may cause fatigue and start a potentially vicious circle leading to work-related muscle disorders.

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms

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.

Shoulder muscle load and muscle fatigue among industrial sewing-machine operators

SummaryPhysiological responses to physical work were assessed for 29 female industrial sewing-machine operators during an 8-h working day under ordinary working conditions. During sewing-machine work, the average (left and right) static load in the trapezius muscle was 9% of the maximal electromyogram (EMG) amplitude (% EMGmax), while the average mean load was 15% EMGmax, and the average peak load was 23% EMGmax. The static load level was unrelated to the muscle strength of the sewing-machine operators, which for the group as a whole was within the normal range. The load levels remained unchanged during the working day, while changes in the EMG mean power frequency and zero crossing frequency rate occurred, both indicating the development of muscle fatigue in left and right trapezius muscle during the working day. In line with this, the rating of perceived exertion in the shoulder and neck region increased during. the working day. Dividing the group of sewing-machine operators into two groups, those with the highest frequency and those with the lowest frequency of shoulder/neck troubles showed that the former group had significantly lower muscle strength, despite the fact that no differences in the surface EMG during sewing were found between the two groups. It was concluded that industrial sewing-machine work involves a pattern of shoulder muscle activity which induces fatiguing processes in the shoulder and neck regions. Furthermore, since the static shoulder muscle load was independent of muscle strength, factors other than working posture may be of significance for the static shoulder muscle load.