Nils Fallentin

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.

Risk factors in the onset of neck/shoulder pain in a prospective study of workers in industrial and service companies

Aims: To quantify the relative contribution of work related physical factors, psychosocial workplace factors, and individual factors and aspects of somatisation to the onset of neck/shoulder pain. Methods: Four year prospective cohort study of workers from industrial and service companies in Denmark. Participants were 3123 workers, previously enrolled in a cross sectional study, where objective measurement of physical workplace factors was used. Eligible participants were followed on three subsequent occasions with approximately one year intervals. Outcomes of interest were: new onset of neck/shoulder pain (symptom cases); and neck/shoulder pain with pressure tenderness in the muscles of the neck/shoulder region (clinical cases). Results: During follow up, 636 (14.1%) participants reported neck/shoulder pain of new onset; among these, 82 (1.7%) also had clinical signs of substantial muscle tenderness. High shoulder repetition was related to being a future symptom case, and a future clinical case. Repetition was strongly intercorrelated with other physical measures. High job demands were associated with future status as a symptom case, and as a clinical case. A high level of distress predicted subsequent neck/shoulder pain, and neck/shoulder pain with pressure tenderness. Conclusions: High levels of distress, and physical and psychosocial workplace factors are predictors of onset of pain in the neck and/or shoulders, particularly pain with pressure tenderness in the muscles.

Motor unit recruitment during prolonged isometric contractions

SummaryMotor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P<0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed “motor unit rotation” and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.

Physical, psychosocial, and individual risk factors for neck/shoulder pain with pressure tenderness in the muscles among workers performing monotonous, repetitive work.

STUDY DESIGN Cross-sectional study. OBJECTIVES To evaluate the effect of individual characteristics and physical and psychosocial workplace factors on neck/shoulder pain with pressure tenderness in the muscles. SUMMARY OF BACKGROUND DATA Controversy prevails about the importance of workplace factors versus individual factors in the etiology of pain in the neck and/or shoulders. METHODS Study participants were 3123 workers from 19 plants. Physical risk factors were evaluated via video observations, and psychosocial risk factors were assessed with the job content questionnaire. Other procedures included symptom survey, clinical examination, and assessment of health-related quality of life (SF-36). The main outcome variable, neck/shoulder pain with pressure tenderness, was defined on the basis of subjective pain score and pressure tenderness in muscles of the neck/shoulder region. RESULTS The prevalence of neck/shoulder pain with pressure tenderness was 7.0% among participants performing repetitive work and 3.8% among the referents. We found an association with high repetitiveness (prevalence ratio 1.8, 95% confidence interval 1.1-2.9), high force (2.0, 1.2-3.3), and high repetitiveness and high force (2.3, 1.4-4.0). The strongest work-related psychosocial risk was high job demands (1.8, 1.2-2.7). Increased risk was also associated with neck/shoulder injury (2.6, 1.6-4.1), female gender (1.8, 1.2-2.8), and low pressure pain threshold (1.6, 1.1-2.3). Neck/shoulder pain was strongly associated with reduced health-related quality of life. CONCLUSIONS Work-related physical and psychosocial factors, as well as several individual risk factors, are important in the understanding of neck/shoulder pain. The findings suggest that neck/shoulder pain has a multifactorial nature. Reduced health-related quality of life is associated with subjective pain and clinical signs from the neck and shoulders. The physical workplace factors were highly intercorrelated, and so the effect of individual physical exposures could only be disentangled to a minor degree.

Assessment of work postures and movements using a video-based observation method and direct technical measurements.

The aim was to study postures and movements during repetitive work using video-based observations and direct technical measurements (inclinometers and goniometers). A total of 21 healthy women from a poultry processing plant volunteered. Neck flexion > 20 degrees was registered during 92% of the recorded time with the observation method, while the corresponding value measured with the inclinometer was 65%. Different reference positions and different measured variables apparently contributed to the differences between the methods. Mean wrist position was measured to be 0 degrees in flexion-extension and 19 degrees in ulnar deviation. Differences between the methods in the registered hand positions were small. The number of repetitive movements/minute and mean power frequency (MPF) of the electrogoniometer data was significantly related, showing both variables to be relevant measures of repetitiveness. In conclusion, the observation method and the technical measurements supplemented each other well. A reduction in class categories was suggested for future observation methods.

Risk Factors for Neck and Upper Extremity Disorders among Computers Users and the Effect of Interventions: An Overview of Systematic Reviews

Background To summarize systematic reviews that 1) assessed the evidence for causal relationships between computer work and the occurrence of carpal tunnel syndrome (CTS) or upper extremity musculoskeletal disorders (UEMSDs), or 2) reported on intervention studies among computer users/or office workers. Methodology/Principal Findings PubMed, Embase, CINAHL and Web of Science were searched for reviews published between 1999 and 2010. Additional publications were provided by content area experts. The primary author extracted all data using a purpose-built form, while two of the authors evaluated the quality of the reviews using recommended standard criteria from AMSTAR; disagreements were resolved by discussion. The quality of evidence syntheses in the included reviews was assessed qualitatively for each outcome and for the interventions. Altogether, 1,349 review titles were identified, 47 reviews were retrieved for full text relevance assessment, and 17 reviews were finally included as being relevant and of sufficient quality. The degrees of focus and rigorousness of these 17 reviews were highly variable. Three reviews on risk factors for carpal tunnel syndrome were rated moderate to high quality, 8 reviews on risk factors for UEMSDs ranged from low to moderate/high quality, and 6 reviews on intervention studies were of moderate to high quality. The quality of the evidence for computer use as a risk factor for CTS was insufficient, while the evidence for computer use and UEMSDs was moderate regarding pain complaints and limited for specific musculoskeletal disorders. From the reviews on intervention studies no strong evidence based recommendations could be given. Conclusions/Significance Computer use is associated with pain complaints, but it is still not very clear if this association is causal. The evidence for specific disorders or diseases is limited. No effective interventions have yet been documented.

Criteria for classification of posture in repetitive work by observation methods: A review

Abstract Observation methods have been widely used to assess mechanical exposure in ergonomic epidemiology. In this review study, eight published and still applied observation methods are reviewed and classified with special emphasis on criteria used for classification of body angles and movement intervals. An obvious lack of standardized body angles hampers comparison between the methods. A more general question is raised concerning the internal and external validity of observation methods in epidemiologic studies. The internal validity has shown to be acceptable in certain work situations. The external validity of observation methods seems more questionable. Only frequency and duration of movements and posture are included in the methods while other possibly more important risk factors of cumulative trauma disorders i.e. force, angular velocity, and acceleration are not included. It is suggested that observation methods should be standardized. Furthermore, they should be expanded to include some of the other known risk factors or alternatively be supplemented by more direct measurements at the workplace. Relevance to industry Data from observation methods concerning classification of posture allows a more precise exposure definition than job title. Valid observation methods are in many cases a prerequisite in epidemiologic studies and constitute a practical tool in the evaluation of ergonomic intervention.

Work load during floor cleaning. The effect of cleaning methods and work technique

Cardiovascular and muscle load levels were evaluated during floor cleaning. A group of 12 experienced female cleaners participated in the study. Of the subjects 6 used a mopping method and 6 a traditional scrub and cloth method. Heart rate, oxygen consumption, rating of perceived exertion, electromyography from the trapezius muscle and video recordings were obtained during floor cleaning. With respect to relative heart rate and oxygen consumption the two groups did not differ. The results revealed a high cardiovascular load corresponding on average to 53% of the individual maximal oxygen uptake. In addition 65% of the observed period was spent with the back in a position forward flexed more than 20°. The time spent in extreme forward back flexion was shorter for the mopping group. Both groups exhibited high static, median and peak shoulder muscle load levels of 10%, 25% and 54% maximal voluntary contraction, respectively. At the same time, however, the mopping group tended to have a higher shoulder load than the group using the scrub and cloth method. Furthermore, electromyographic signs of fatigue in the trapezius muscle indicated a more stereotype activation of the shoulder muscles during mopping than during scrubbing. Based on these results, it was concluded that mopping cannot be recommended as less strenuous than scrubbing.

Mechanisms contributing to different joint moments observed during human walking

The present study investigated factors that contribute to the formation of a previously reported knee joint flexor moment during the stance phase of walking. Contradictory results have been reported on this flexor moment, which some but not all individuals exhibit. Seven healthy male subjects were high speed filmed while walking across a force platform, and EMG recordings were obtained from five leg muscles. To investigate segment interactions, net joint moments about the ankle, knee and hip joint were calculated by inverse dynamics and each term in the equation used for the moment calculation was evaluated during the timeecourse of the step cycle. To test the hypothesis that net joint moments are balanced by an external moment formed by the resulting ground reaction vector multiplied by the perpendicular distance to the actual joint, external moment arms were calculated by the floor reaction force vector approach (FRFV). Contrasting two subjects with different net joint moments about the knee and ankle joint revealed that the knee joint flexor moment could not be explained by an opposite external moment. The external moments were calculated by a simplified method (FRFV) in which the point of force application is incorrect for joints above the ankle joint. However, at the ankle joint the net joint moment was always opposed by an external moment of opposite polarity. A detailed examination of the equation used for the net joint moment calculation showed that a knee joint flexor moment can be caused directly by a large plantar flexor moment about the ankle joint. For example, the soleus muscle can pull the tibia and generate an extensor moment about the knee joint, which in turn has to be opposed by a knee flexor moment from the hamstring muscles. Otherwise the desired joint angles cannot be obtained during human walking. It is therefore suggested that the kinematics regarding how the foot is placed on the ground may influence the net ankle joint moment, while the moment patterns about the knee and hip joint are determined by segment interaction and the requirements for controlling the direction of the resulting ground reaction vector. In vertical jumping it is advantageous to generate extensor moments about the knee and hip joint simultaneously, while in horizontal locomotion this would result in inefficient vertical movements.

Prognosis of shoulder tendonitis in repetitive work: a follow up study in a cohort of Danish industrial and service workers.

Background: The physical and psychosocial work environment is expected to modify recovery from shoulder disorders, but knowledge is limited. Methods: In a follow up study of musculoskeletal disorders in industrial and service workers, 113 employees were identified with a history of shoulder pain combined with clinical signs of shoulder tendonitis. The workers had yearly reexaminations up to three times. Quantitative estimates of duration, repetitiveness, and forcefulness of current tasks were obtained from video recordings. Perception of job demands, decision latitude, and social support was recorded by a job content questionnaire. Recovery of shoulder tendonitis was analysed by Kaplan-Meier survival technique and by logistic regression on exposure variables and individual characteristics in models, allowing for time varying exposures. Results: Some 50% of workers recovered within 10 months (95% CI 6 to 14 months). Higher age was strongly related to slow recovery, while physical job exposures were not. Perception of demands, control, and social support at the time when the shoulder disorder was diagnosed, were associated with delayed recovery, but these psychosocial factors did not predict slow recovery in incident cases identified during follow up. Conclusion: The median duration of shoulder tendonitis in a cross sectional sample of industrial and service workers was in the order of 10 months. This estimate is most likely biased towards too high a value. Recovery was strongly reduced in higher age. Physical workplace exposures and perceived psychosocial job characteristics during the period preceding diagnosis seem not to be important prognostic factors.