Thomas Läubli

Continuous, intermitted and sporadic motor unit activity in the trapezius muscle during prolonged computer work

The Cinderella hypothesis postulates the continuous activity of specific motor units (MUs) during low-level muscle contraction. The MUs may become metabolically overloaded, with the subject developing muscle pain and strain. The hypothesis requires MUs that are active for a time long enough to actually damage muscle fibers. The aim of this study was to determine if there are continuously active MUs in the right trapezius muscle during normal computer work using a computer mouse. Fourteen healthy subjects executed an interactive computer-learning program (ErgoLight) for 30 min. Six-channel intramuscular EMG and two-channel surface EMG signals were recorded from two positions of the trapezius muscle. Decomposition was achieved with automated, multi-channel, long-term decomposition software (EMG-LODEC). In two out of the 14 subjects, three MUs were continuously active throughout the 30 min. Although the majority of the MUs were active during only part of the experimental session, an ordered on-off behavior (e.g. substitution) pattern was not observed. As long-lasting activity was verified in some subjects, the results support the Cinderella hypothesis. However, it cannot be concluded here how long the MUs could stay active. If continuous activity overloads low threshold MUs, the potential exists for selective fibre injuries in low threshold MUs of the trapezius muscle in subjects exposed to long-term computer work.

A software package for the decomposition of long-term multichannel EMG signals using wavelet coefficients

This paper presents a method to decompose multichannel long-term intramuscular electromyogram (EMG) signals. In contrast to existing decomposition methods which only support short registration periods or single-channel recordings of signals of constant muscle effort, the decomposition software EMG-LODEC (ElectroMyoGram LOng-term DEComposition) is especially designed for multichannel long-term recordings of signals of slight muscle movements. A wavelet-based, hierarchical cluster analysis algorithm estimates the number of classes [motor units (MUs)], distinguishes single MUAPs from superpositions, and sets up the shape of the template for each class. Using three channels and a weighted averaging method to track action potential (AP) shape changes improve the analysis. In the last step, nonclassified segments, i.e., segments containing superimposed APs, are decomposed into their units using class-mean signals. Based on experiments on simulated and long-term recorded EMG signals, our software is capable of providing reliable decompositions with satisfying accuracy. EMG-LODEC is suitable for the study of MU discharge patterns and recruitment order in healthy subjects and patients during long-term measurements.

Work-life conflict and musculoskeletal disorders: a cross-sectional study of an unexplored association

BackgroundThe health consequences of work-family or rather work-life conflict (WLC) have been studied by numerous researchers. The work-related causes of musculoskeletal disorders (MSD) are also well explored. And stress (at work) has been found to be a consequence of WLC as well as a cause of MSD. But very little is known about a potential association between WLC and MSD and the possible mediating role of stress in this relationship.MethodsSurvey data collected in 2007 among the workforces of four large companies in Switzerland were used for this study. The study population covered 6091 employees. As the exposure variable and hypothesized risk factor for MSD, WLC was measured by using a 10-item scale based on an established 18-item scale on work-family conflict. The outcome variables used as indicators of MSD were (low) back pain and neck/shoulder pain. Stress as the assumed intervening variable was assessed by a validated single-item measure of general stress perception. Correlation coefficients (r), standardized regression coefficients (β) and multiple adjusted odds ratios (OR) were calculated as measures of association.ResultsWLC was found to be quite strongly associated with MSD (β = .21). This association turned out to be substantially confounded by physical strain at work, workload and job autonomy and was considerably reduced but far from being completely eliminated after adjusting for general stress as another identified risk factor of MSD and a proven strong correlate of WLC (r = .44). A significant and relevant association still remained (β = .10) after having controlled for all considered covariates. This association could be fully attributed to only one direction of WLC, namely the work-to-life conflict. In subsequent analyses, a clear gradient between this WLC direction and both types of MSD was found, and proved to be consistent for both men and women. Employees who were most exposed to such work-to-life conflict were also most at risk and showed a fivefold higher prevalence rate (19%-42%) and also an up to sixfold increased relative risk (OR = 3.8-6.3) of suffering greatly from these types of MSD compared with the least exposed reference group showing very low WLC in this direction. Including stress in the regression models again reduced the strength of the association significantly (OR = 1.9-4.1), giving an indication for a possible indirect effect of WLC on MSD mediated by stress.ConclusionFuture research and workplace interventions for the prevention of MSD need to consider WLC as an important stressor, and the MSD risk factor identified in this study.

Magnetic resonance imaging of the lumbar spine: findings in female subjects from administrative and nursing professions.

Study Design. Cross-sectional magnetic resonance imaging (MRI) study. Objective. To find out whether different patterns of lumbar disc degeneration related to different physical work types (static vs. dynamic) can be found. Summary of Background Data. The association of lumbar disc degeneration with physical workload as well as the association of disc degeneration with low back pain (LBP) has been debated controversially. Nevertheless, many of the mainly invasive treatment concepts are based on disc degeneration models. Methods. An MRI study was performed in female subjects aged 45 to 62 years with persistent LBP and in age-matched controls. Subjects (n = 109) were selected from nursing and administrative professions. A questionnaire was used to assess the subjects’ exposure to physical workplace factors. Disc degeneration, disc herniation, nerve root compromise, high intensity zones, endplate changes, and facet joint osteoarthritis were evaluated based on sagittal T1- and T2-weighted as well as axial T2-weighted images. Results. The two occupational groups could clearly be distinguished by the physical workplace factors: administrative work was associated with predominantly sitting postures whereas nursing work was associated with frequent walking, pushing, pulling, and lifting. Except for endplate (Modic) changes at L5–S1, MRI findings did not differ between the four groups, namely, nurses and secretaries with and without LBP. When analyzing the MRI findings within the two vocational groups, nerve root compromise and endplate changes in the lower lumbar spine were found to be significant (P < 0.05) risk factors for LBP. Disc degeneration, disc herniation, nerve root compromise, and facet joint osteoarthritis were present in more than 50% of the intervertebral spaces studied. Conclusions. These findings give evidence that in subjects performing nonheavy work, patterns of lumbar disc degeneration are not associated with the job type and characteristic physical loadings.

The role of back muscle endurance, maximum force, balance and trunk rotation control regarding lifting capacity

Evaluation of lifting capacity is widely used as a reliable instrument in order to evaluate maximal and safe lifting capacity. This is of importance in regard to planning rehabilitation programs and determining working ability. The aim of this study was to investigate the influence of basic functions on the lifting capacity measured by the progressive isoinertial lifting evaluation (PILE) and the functional capacity evaluation (FCE) tests in a lower (floor to waist) and an upper (waist to shoulder) setting and compare the two test constructs. Seventy-four female subjects without acute low back pain underwent an examination of their lifting capacities and the following basic functions: (1) strength and endurance of trunk muscles, (2) cardiovascular endurance, (3) trunk mobility and (4) coordination ability. A linear regression model was used to predict lifting capacity by means of the above-mentioned basic functions, where the F statistics of the variables had to be significant at the 0.05 level to remain in the model. Maximal force in flexion showed significant influence on the lifting capacity in both the PILE and the FCE in the lower, as well as in the upper, lifting task. Furthermore, there was a significant influence of cardiovascular endurance on the lower PILE and also of endurance in trunk flexion on the lower FCE. Additional inclusion of individual factors (age, height, weight, body mass index) into the regression model showed a highly significant association between body height and all lifting tasks. The r2 of the original model used was 0.19/0.18 in the lower/upper FCE and 0.35/0.26 in the lower/upper PILE. The model r2 increased after inclusion of these individual factors to between 0.3 and 0.4. The fact that only a limited part of the variance in the lifting capacities can be explained by the basic functions analyzed in this study confirms the assumption that factors not related to the basic functions studied, such as lifting technique and motor control, may have a strong influence on lifting capacity. These results give evidence to suggest the inclusion of an evaluation of lifting capacity in clinical practice. Furthermore, they raise questions about the predictive value of strength and endurance tests in regard to lifting capacity and work ability.

Trapezius Muscle Motor Unit Activity in Symptomatic Participants during Finger Tapping Using Properly and Improperly Adjusted Desks

Work-related musculoskeletal disorders in the neck-shoulder area and upper extremities are common among computer users, especially women. We comparedtemporal changes of motor unit (MU) activation in the trapezius muscle during finger tapping using both appropriate and inappropriate ergonomic desk adjustments. Sixteen intensive and nonintensive computer users with either moderate or severe musculoskeletal disorders participated in the study. Six-channel intramuscular electromyographic (EMG) signals and 2-channel surface EMG were recorded from 2 positions of the trapezius muscle. A statistically significant increase in activity was observed with a desk adjusted 5 cm higher than appropriate and was attributable mainly to increased duration of MU activity. Participants with severe symptoms activated more MUs, and these were also active longer. In women, on average, MUs were active nearly twice as long as in men during the same tapping task. This study demonstrates that it is possible to evaluate ergonomic topics on the MU level and that incorrectly adjusted office equipment, in addition to motor demands imposed by the work task, results in prolonged activity of MUs. A potential application of this research is an increased awareness that certain individuals who work with incorrectly adjusted office equipment may be at greater risk of developing work-related musculoskeletal disorders.

Co-activity of the trapezius and upper arm muscles with finger tapping at different rates and trunk postures.

Abstract In the context of finding a model that describes the pathophysiological mechanisms leading to muscle pain at low-intensity repetitive work, in this study we investigated whether a simplified finger motor task that requires little mental demand can cause increased muscle activity in the upper arms and neck, and examined the impact of the variation of two parameters, finger tapping rate and body posture. Using the 5th and 95th percentiles from the surface electromyogram of six muscles of the fingers, upper arm and neck, we determined the static and dynamic components of the muscle activity. Correlation methods were used to find a component in the muscle activity that originated from the rhythm of the finger tapping. Further investigations included tapping steadiness and finger force. It was found that in many, but not all subjects, low or even high activity was constantly present in the upper arm and trapezius muscles, sometimes even during relaxation. Fast tapping and a forward-leaning body posture caused considerable increases, while a slightly reclined posture helped to reduce co-activity. However, motor control patterns varied strongly between individuals. Since certain subjects showed no co-activity at all we can assume that trapezius and upper-arm activation is not necessarily required for the completion of a task similar to ours. This may explain why some VDU users develop work-related musculoskeletal disorders while others remain healthy.

Effects of a low alcohol dose on static balance, fine motor activity, and mental performance.

The effects of a single low alcohol dose (men 0.54 g and women 0.44 g alcohol per kg body weight) were measured by static balance, fine motor activity, and mental performance. In 10 healthy volunteers balance was registered by a temporally and spatially high resolution platform measuring the center of foot pressure and a three-dimensional coordination measurement system. Fine motor activity and mental performance were tested with selected experiments from the NES2 (Neurobehavioral Evaluation System) neuropsychological test battery. Changes of bipedal and monopedal balance could be detected after the alcohol consumption. Neither the fine motor activity nor the mental performance test demonstrated significant effects. Thus, the static balance test proved to be a sensitive, fast, and atraumatic method to identify slight neurotoxic disturbances.

Course of back pain across 5 years: a retrospective cohort study in the general population of Switzerland.

Study Design. A retrospective cohort study in the general population of Switzerland. Objective. To investigate the course of back pain (BP) across 5 years and the impact of BP history on its incidence and recurrence. Summary of Background Data. Longitudinal studies on BP performed in the general population have reported varying prevalence and incidence rates. Most studies compared two points in time with varying time periods. This study adds information about the course of BP exploring five points in time with annual intervals. Methods. The Swiss Household Panel is a representative population-based cohort study (N = 7799). The question analyzed in the present study asked about “bad back or lower back problems at least once a month in the last 12 months (BP).” Among 7791 persons who answered this question during the baseline survey in 1999, 3881 persons (49.8%) completed all annual follow-up surveys through 2003 and represent the study sample. In each year, the 1-year prevalence, incidence, and recurrence of BP were calculated. The course of BP was analyzed according to the number of years with BP, the constancy of BP status, and the trend of BP. For each analysis, the observed frequency was compared with expected frequencies on the basis of two theoretical models. Results. In the study sample (age 44.0 ± 15.6 years, 57.7% women), BP prevalence was 33.2% at baseline. In the follow-up surveys, mean prevalence was 37.7%, mean incidence 19.6%, and mean recurrence 69.0%. The most frequently observed courses across 5 years were those with a constant status: BP always absent (n = 1346, 34.7%) or BP always present (n = 538, 13.9%). BP recurrences increased with increasing numbers of previous consecutive years with BP from 46.9% (1 year of previous BP) to 88.1% (at least 4 years of previous BP). Conclusion. BP history is highly predictive for future BP episodes.

Touch&Type: a novel pointing device for notebook computers

The widespread use of the mouse as an input device for notebook computers indicates that many users are reluctant to use alternative built-in pointing devices. We present a novel input method called Touch&Type™™ which is meant to overcome some of the drawbacks encountered with conventional built-it pointing devices. Touch&Type™™ combines a conventional keyboard with an extended touch pad whereby the touch pads sensitive area is formed by the surface of the keys themselves and thus can be made as large as the whole key area. A comparative study of pointing operation is presented with a Touch&Type™™ prototype in comparison with the mouse and the conventional touch pad. While the mouse outperformed its two counterparts, Touch&Type™™ was found to be superior to the conventional touch pad (after a short learning period) with a confidence level of 73%. The study investigated pointing operation only, not taking homing time into account.