Background: Introduction of sit-stand tables has been proposed as an initiative to decrease sedentary behavior among office workers and thus reduce risks of negative cardiometabolic health effects. However, ensuring proper and sustainable use of such tables has remained a challenge for successful implementation. Objective: We developed a new system to promote and sustain the use of sit-stand tables. Methods: The system was programmed to change the position of the table between “sit” and “stand” positions according to a regular pre-set pattern, if the user agreed to the system-generated prompts prior to each change. The user could respond to the system-generated prompts by agreeing, refusing or delaying the changes by 2 minutes. We obtained user compliance data when this system was programmed to a schedule of 10 minutes of standing after every 50 minutes of sitting. Compliance was investigated in nine office workers who were offered the semi-automated sit-stand table for two months. Results: On average, the system issued 12-14 alerts per day throughout the period. Average acceptance rate ranged from 75.0-82.4%, and refusal rate ranged from 11.8-10.1% between the first and eighth weeks of intervention (difference not statistically significant). During the first week after introduction, the table was in a standing position for 75.2 min on average, increasing slightly to 77.5 min in the eighth week. Conclusion: Since the workers were essentially sitting down before the table was introduced, these results suggest that the system was accepted well, and led to an effective reduction of sitting during working hours. Users also reported that the system contributed positively to their health and wellbeing, without interrupt their regular work, and that they would like to continue using the sit-stand table even beyond the two-month period, as part of their regular work. Compliance beyond two months of use, however, needs to be verified.

Background. There is increasing evidence that sedentary behaviour during the workday is associated with negative health effects. In this context, interventions to reduce total sedentary time and breaking up periods of continuous sitting during computerized office work are urgently needed. Several reviews conclude that introducing sit-stand stations may lead to positive effects, but they also state that long-term interventions in real occu-pational settings are still rare. Therefore, the aim of this study was to investigate usage of sit-stand tables among Brazilian office workers during an intervention lasting two months.

Methods.Nine office workers (6 females, 3 males; age 42 [SD 12] years) participated. The workers received traditional sit-stand tables and ergonomics information. They then used the workstation for two months. The tables were furnished with a system that recorded and kept track of table use during the intervention period. Table use early and late in the intervention period was compared using the Wilcoxon signed-rank test for repeated measurements.

Results. In the beginning of the eight-week intervention period, workers, in median, changed table position 2.4 (1.9 – 4.7) times per day, decreasing to 2.3 (1.0 – 3.3) times at the end (P=0.09). Moreover, we also found a non-significant decrease in total time stand-ing per day, from 88.6 (67.4 – 94.3) minutes to 58.8 (33.1 – 95.7) minutes (P=0.31).

Discussion. Two months after introducing sit-stand tables, some decrease in usage could be seen, if not statistically significant. Based on this, we emphasize that introduction of sit-stand tables should be accompanied by continued encouragement of the workers, preferably informed by a personalized follow up of actual use.

Postures and muscle activity in the upper body were recorded from 50 academics office workers during 2 hours of normal work, categorised by observation into computer work (CW) and three non-computer (NC) tasks (NC seated work, NC standing/walking work and breaks). NC tasks differed significantly in exposures from CW, with standing/walking NC tasks representing the largest contrasts for most of the exposure variables. For the majority of workers, exposure variability was larger in their present job than in CW alone, as measured by the job variability ratio (JVR), i.e. the ratio between min–min variabilities in the job and in CW. Calculations of JVRs for simulated jobs containing different proportions of CW showed that variability could, indeed, be increased by redistributing available tasks, but that substantial increases could only be achieved by introducing more vigorous tasks in the job, in casu illustrated by cleaning.

Sedentary behaviors in office workers has become a major public health concern and several initiatives have been proposed to break up sedentary behavior patterns during the performance of computer-intensive office work. Among such initiatives, the use of sit-stand workstations has been suggested to be one of the most promising by recent reviews. However, there still is only limited scientific evidence showing how effective sit-stand workstations are, in reducing sedentary behaviors and also documentation of their sustainability of use in studies of regular office work (i.e. as the “newness” of the system wears off, with time since introduction). This study aimed to document user behaviors and compare the use of two sit-stand workstation based interventions among two groups of administrative office workers: an “autonomous” group in which these workstations were introduced following some general ergonomic guidelines, and another “feedback-system” group in which the sit-stand tables were furnished with a semi-automatic reminder system, programmed to raise the table to a high (i.e. standing) position for 10 minutes after every accumulated 50 minutes of the table being in a low (i.e. sitting) position, i.e. to result in about 83% sitting per day. In addition, the sustainability of the use of these two kinds of sit-stand workstation interventions over two continuous months since their introduction was also studied. The results averaged over two months of usage of the two interventions showed that the percentage (%) sitting time was 87.4 (84.9-89.2) on average in the autonomous group and 84.0 (83.5-85.4) on average in the feedback-system group (P=0.001), and the frequency of switches between sitting and standing was 0.3 (0.2-0.3) per hour in the autonomous group and 0.7 (0.6-0.7) per hour in the feedback-system group (P=0.001). Thus, the sit-stand table system integrated with the automatic reminder system led to more reduction in sitting time and more switches in posture between sitting and standing as compared to the traditional sit-stand table, and behaviors of both groups were seen to be sustained over the 2-month intervention period (no difference across time for any of the variables tested for any group).

The aim of this study was to determine the extent of upper arm, neck and trunk posture variation that can be obtained by combining seated and standing computer work, compared to performing only seated computer work. Posture data were recorded for two hours during each of three days of ordinary work from 24 office workers that had been using a sit-stand station for two months. Periods with sitting and standing computer work were identified using on-site observations, and posture means and minute-to-minute variance were determined for both. Expected minute-to-minute posture variability in different temporal combinations of sitting and standing computer work were determined by simulation, and expressed in terms of a Job Variance Ratio, i.e. the relative increase in variability compared to sitting-only work. For all three postures, mean values differed between sitting and standing computer work, and both showed a notable minute-to-minute variability. For most workers, posture variability was larger when combining sitting and standing than when sitting only, and simulations suggested to introduce more standing than what the worker currently practiced. The results indicate that introducing a sit-stand table could, for most office workers, have a positive effect on upper arm, neck and trunk posture variability.

Objectives: Many current jobs are characterized by sedentary behaviour (SB) and lack of physical activity (PA). Interventions addressing SB and PA at the workplace may benefit workers’ health. The present review is the first to focus on the effectiveness of interventions implemented during productive work with the intention to change workers’ SB and/or PA while working.

Methods: Scopus was searched for articles published from 1992 until March 12, 2015. Relevant studies were evaluated using the Quality Assessment Tool for Quantitative Studies and summarized in a best-evidence synthesis.

Results: 40 studies describing 41 interventions were included and organized into three categories: alternative workstations (20), interventions promoting stair use (11) and personalized behavioural interventions (10). Strong evidence was found for alternative workstations leading to positive effects on overall SB, while evidence was conflicting for effects on SB and PA at work, overall PA, and work performance. Evidence was moderate for alternative workstations to have no effect on hemodynamics and cardiorespiratory fitness. Stair use promotion interventions were found to increase PA at work, while personalized behavioural interventions increased overall PA; both with moderate evidence. Personalized behavioural interventions were found to have no effect on anthropometric measures (moderate evidence). Regarding work performance and lipid and metabolic profiles, evidence was either conflicting or insufficient.

Conclusions: Current evidence supports that introduction of alternative workstations may have positive effects on overall PA and SB, likely without reducing work performance, while the long-term health effects of all three reviewed categories of interventions remain to be established.

Purpose Heart rate variability (HRV) is often monitored in occupational studies as a measure of cardiac autonomic activation, but the reliability of commonly used HRV indices is poorly understood. In the present study, we determined the variability between and within subjects of common HRV indices during a repetitive low-force occupational task, i.e. pipetting, and interpreted the results in terms of necessary sample sizes in studies comparing HRV between conditions or groups.

Methods Fourteen healthy female subjects performed a standardized pipetting task in the laboratory on three separate days within a short time-span (<2 weeks), and on one additional occasion six months later. A number of standard HRV indices were calculated in both time and frequency domains. For each HRV index, variance components were estimated between subjects, within subjects between occasions far apart in time, and within subjects between days within a two-week period.

Results We found that the time interval between repeated measurements did not influence the extent of HRV variability, and that the reliability of most HRV indices was sufficient for even small study samples (30 subjects or less) to be able to detect, with satisfying power (>0.80), a significant 10% to 20% difference in HRV between groups, and between conditions within individuals.

Conclusions We conclude that HRV can be used as a reliable and feasible marker of autonomic activity in occupational studies of repetitive low-force work.

INTRODUCTION: Sit-stand workstations are a popular intervention to reduce sedentary behavior (SB) in office settings. However, the extent and distribution of SB in office workers long-term accustomed to using sit-stand workstations as a natural part of their work environment are largely unknown. In the present study, we aimed to describe patterns of SB in office workers with long-term access to sit-stand workstations and to determine the extent to which these patterns vary between days and workers. METHODS: SB was objectively monitored using thigh-worn accelerometers for a full week in 24 office workers who had been equipped with a sit-stand workstation for at least 10 months. A comprehensive set of variables describing SB was calculated for each workday and worker, and distributions of these variables between days and workers were examined. RESULTS: On average, workers spent 68% work time sitting [standard deviation (SD) between workers and between days (within worker): 10.4 and 18.2%]; workers changed from sitting to standing/walking 3.2 times per hour (SDs 0.6 and 1.2 h-1); with bouts of sitting being 14.9 min long (SDs 4.2 and 8.5 min). About one-third of the workers spent >75% of their workday sitting. Between-workers variability was significantly different from zero only for percent work time sitting, while between-days (within-worker) variability was substantial for all SB variables. CONCLUSIONS: Office workers accustomed to using sit-stand workstations showed homogeneous patterns of SB when averaged across several days, except for percent work time seated. However, SB differed substantially between days for any individual worker. The finding that many workers were extensively sedentary suggests that just access to sit-stand workstations may not be a sufficient remedy against SB; additional personalized interventions reinforcing use may be needed. To this end, differences in SB between days should be acknowledged as a potentially valuable source of variation.

Background. In a systematic literature review, we investigated the effect on work performance and metabolic and physiological outcomes of interventions aimed at reducing sedentary behaviour (SB) and/or increasing physical activity (PA) during productive work time.

Methods. Scopus was searched for articles published from 1992 until March 12, 2015. We included studies: (1) addressing interventions aimed at reducing SB and/or increasing PA at the workplace, during productive work; (2) using a design including a control group or control condition; (3) being published as a full-length paper in a peer-reviewed journal in English; (4) reporting on work performance outcomes and metabolic and physiological outcomes (i.e. lipid and metabolic profiles, hemodynamic and cardiorespiratory measures and anthropometric measures). Relevant studies were evaluated using the Quality Assess-ment Tool for Quantitative Studies and summarized in a best evidence synthesis.

Results. 18 interventions were included and organized into two categories: (1) alternative workstation interventions (n=15), i.e. sit-stand workstations or “active” workstations; and (2) personalized behavioural interventions (n=3), i.e. interventions involving personalized goals and/or giving behavioural feedback using prompts or messages.There was moderate evidence for alternative workstations not influencing hemodynamics and cardiorespiratory fitness as well as personalized behavioural interventions notinfluencing anthropometric measures. Evidence was insufficient (alternative workstations) or conflicting (personalized behavioral interventions) on lipid and metabolic profiles. For work performance, there was insufficient evidence for personalized behavioural interven-tions and conflicting evidence for alternative workstations. But for the latter, only one out of 11 studies showed a negative effect.

Discussion. Current evidence suggests that work performance is not negatively affected by alternative workstations. Furthermore, there is no strong case for introducing interven-tions aimed at reducing SB and/or increasing PA during productive work time in the hope of getting a positive effect on metabolic and physiological outcomes. However, large-scale, high quality studies with long-term follow-ups are needed before more definite conclusions on this topic can be drawn.

Background.Increased motor variability while performing repetitive tasks has been suggested to decrease the risk of developing musculoskeletal disorders. However, support for this positive effect is lacking outside of short, simple, highly controlled tasks. It is also currently unknown whether or not existing motor variability metrics are viable for characterising occupational tasks. The purpose of this study was to assess motor variability during a long-cycle simulated occupational task. Using metrics previously validated for short-cycle tasks, this study aimed to determine the extent to which: (1) individuals dif-fered in motor variability with respect to kinematics and/or EMG activation; (2) individual motor variability was consistent across days; and (3) kinematics and EMG motor variability were correlated.

Methods.Following a stringent, three-day training regime, 15 females proved sufficiently proficient to participate. On two occasions, participants performed 36 cycles of an assembly task (combining gross and fine motor skills) at 110 MTM pacing (51 s per cycle). For each cycle, multiple upper arm kinematic and trapezius EMG summary mean and SD variables were calculated; for each variable, the variability across the 36 cycles was assessed. The relative size of variability across individuals, and the consistency of each individual’s motor behaviour across days were assessed using kinematic and EMG vari-ables. The correlation between kinematic and EMG variables was also assessed.

Results.Distinct individual behaviours were observed across days: some participants were clearly more consistent in their motor behaviour than others. Further, a high correlation was found between some kinematic and muscle activation variables.

Discussion. Using previously validated upper arm assessment metrics, we were able to differentiate between individuals performing a long-cycle assembly task based on their degree of motor variability. Given the nature of our study task, we believe the metrics that we found to be successful at identifying individual behaviours could be used for assessing field tasks.

This study aimed at determining the extent to which individuals with neck-shoulder pain and non-symptomatic individuals differ in muscle activation patterns, when performing computer work, as quantified by exposure variation analysis (EVA). As a secondary aim, we also aimed to quantify the day-to-day reliability of EVA variables describing trapezius muscle activation in a non-symptomatic control group. Thirteen touch-typing computer users (pain: n=5, non-symptomatic: n=8) completed three pre-selected computer tasks in the laboratory.

Upper trapezius muscle activity was recorded using electromyography and analyzed using EVA with five amplitude and five duration categories. Individuals with neck-shoulder pain spent less time at low amplitudes and exhibited longer uninterrupted periods of muscle activation compared to their non-symptomatic counterparts. Thus, non-symptomatic workers tended to switch between exposure levels more often than individuals with pain. For a majority of EVA variables, ICCs ranged from 0.6 to 0.9, and between-days coefficients of variation were between 0.4 and 2.2.

The purpose of this study was to quantify upper-trapezius muscle activation patterns using exposure variation analysis (EVA) in healthy computer workers and those with chronic neck-shoulder pain. Eight healthy and five chronic pain participants were asked to complete three computer-based tasks (TYPE, CLICK, and FORM) in two pacing conditions (self-paced and control-paced). EVA was used to quantify variation using five amplitude classes and five duration classes. Performance in each task was also quantified. Healthy workers and those with chronic pain did not differ in performance, and they both exhibited similarly low levels of muscle activation amplitude. Pain participants, however, were found to spend less time in lower duration classes across tasks and conditions. These results indicate that individuals with chronic neck-shoulder pain utilize movement strategies involving sustained durations of continuous muscle activation. This may be suggestive of decreased temporal variation in muscle activation patterns in those with chronic pain.

Aim: This study investigated the extent to which controlled changes in work pace in a cyclic pick-and-place task influence upper extremity kinematics and muscle activity, and whether an effect depends on working height. Methods: Thirteen participants performed the task four minutes at each of five work paces ranging from 8 to 12 cycles·min^-1 in each of two experimental conditions where the hand was moved horizontally with an average upper arm elevation of 30° and 50°, respectively. For each work cycle, we calculated the average and standard deviation of the upper arm elevation angle and the activity of the trapezius and deltoid muscles, as well as the angular peak velocity. We summarized these seven variables by calculating averages across cycles and cycle-to-cycle variabilities. Results: At 30° arm elevation, pace significantly influenced within-cycle angle variation, cycle-to-cycle variability of the average angle, angular peak velocity, and cycle-to-cycle variability of peak velocity. However, only angular peak velocity increased monotonically across all paces from 8 to 12 cycles·min^-1). Average activity in the trapezius and the deltoid were the only muscle activity variables to increase consistently with pace. These effects of work pace did not change with working height. Conclusion: The present study did not find any consistent work pace effect on upper extremity kinematics and muscle activity, in spite of a comprehensive empirical basis compared to previous literature. While our results suggest that work pace may not be of critical concern in an occupational health context, we encourage further studies verifying or disproving this notion.

Background. Sedentary behaviour studies often describe the extent of sitting by a time proportion; typically per cent time spent sitting. Proportions are examples of so-called “compositional data,” since they add up to a constrained sum (i.e. 100%). Compositional data differ from non-compositional data in aspects of essential importance to their analy-sis and interpretation, including how to address variability. Compositional data analysis (CDA) acts in a space of logarithmically transformed ratios of proportions, rather than on the proportions per se. We compared the statistical properties of confidence intervals (CI) of group mean values of sitting-time proportions obtained using standard procedures and CDA, exemplified by sample sizes required to obtain a specified precision.

Methods. Sitting and non-sitting time proportions calculated from whole-day accelerom-eter recordings in 25 office workers were used as a heuristic example. Variability between subjects was assessed using standard statistics and CDA. In both cases, the size and shape of a 95% CI on the estimated mean sitting-time proportion of n subjects was assessed for different sizes of the mean and values of n.

Results. While standard CIs at a specific n are independent of the mean value and sym-metric, CDA-derived CIs are asymmetric, except at a mean of 50%, and wider at “medium” than at “extreme” mean values. In the example, a 95% CI of ±5% around the mean was ob-tained using n=26 subjects according to standard procedures. However, using CDA, upper 95% CI limits of +5% were obtained with n=5 for a mean value of 90%, but required n=58 when the mean value was 60%. Similar-sized lower 95% CI limits of -5% were obtained with n=13 and n=63 at 90% and 60% means, respectively.

Discussion. CDA-based estimates of sample sizes differed markedly from estimates based on standard statistics. Properties and implications of CDA in sedentary behaviour research deserve further consideration.

Background: Cervical range of motion (ROM) is commonly assessed in clinical practice and research. In a previous study we decomposed active cervical sagittal ROM into contributions from lower and upper levels of the cervical spine and found level- and direction-specific impairments in women with chronic non-specific neck pain. The present study aimed to validate these results and investigate if the specific impairments can be explained by the neutral posture (defining zero flexion/extension) or a movement strategy to avoid large gravitationally induced torques on the cervical spine.

Methods: Kinematics of the head and thorax was assessed in sitting during maximal sagittal cervical flexion/extension (high torque condition) and maximal protraction (low torque condition) in 120 women with chronic non-specific neck pain and 40 controls. We derived the lower and upper cervical angles, and the head centre of mass (HCM), from a 3-segment kinematic model. Neutral head posture was assessed using a standardized procedure.

Findings: Previous findings of level- and direction-specific impairments in neck pain were confirmed. Neutral head posture was equal between groups and did not explain the direction-specific impairments. The relative magnitude of group difference in HCM migration did not differ between high and low torques conditions, lending no support for our hypothesis that impairments in sagittal ROM are due to torque avoidance behaviour.

Interpretation: The direction- and level-specific impairments in cervical sagittal ROM can be generalised to the population of women with non-specific neck pain. Further research is necessary to clarify if torque avoidance behaviour can explain the impairments.

The aim of this study was to estimate the between days test-retest reliability of nonlinear metrics used to quantify motor variability in a repetitive precision task. On three separate days, 14 healthy subjects performed pipetting as a general model of repetitive precision tasks. The task consisted of transferring liquid 20 times with a cycle time of 2.8 s from a pickup tube to eight target tubes placed on a table in front of the subjects. The motion of hand, arm and the pipet tip was tracked in 3D and the shoulder elevation and elbow flexion angle were obtained. Motor variability was assessed using nonlinear metrics based on information theory and recurrence quantification analysis. Between- and within- (between-days) subject variance components were computed using a one-way random effect model, and intra-class correlation coefficients (ICC) were calculated from the variance components as standardized measures of reliability. Most of the metrics displayed a considerable between-days variance component and therefore the ICC showed a slight to moderate reliability. The reported data on between- and within-subject variability can be used to design future studies using non-linear motor variability metrics on kinematics data.

Background. Muscle fatigue develops at markedly different rates among individuals. The pattern of variation in muscle activity has been suggested as a determinant of the rate of fatigue development. This variation can occur between muscles or between compart-ments of a muscle. Thus, we investigated the pattern of muscular activity in shoulder and arm regions during a fatiguing repetitive work task.

Methods. 21 healthy young women performed a repetitive pipetting task at 2.8 seconds/cycle. The session continued until the subject rate of perceived exertion reached 8 on Borg CR-10 scale. High density (HD) surface electromyogram (EMG) over upper trapezius, and bipolar EMG from extensor carpi radialis, flexor carpi radialis, biceps, triceps, deltoi-deus anterior, serratus anterior, upper and lower trapezius were collected to investigate intra- and inter-muscle variation patterns. EMG amplitude and mean power frequency (MPF) were obtained for all the recorded EMG signals. The barycenter of activity over the HD-EMG grid was also determined. Normalized mutual information (NMI) was determined for each pair of muscles. The extent of variability of the outcomes was also assessed.

Results. As fatigue developed, EMG amplitude increased and the MPF decreased for all muscles except the MPF for upper trapezius and deltoideus. The activity of trapezius was higher on the lateral side of HD-EMG grid than on the medial side and the barycenter showed a lateral shift across time. NMI between the muscle pairs also increased with fatigue. The variability of the investigated outcomes was not associated with the time to the task ending.

Discussion. Myo-electrical manifestations of muscle fatigue were observed but none of the outcomes had an association with the rate of fatigue development. Using multivariate approaches, synergistic pattern of muscular activity is yet to be investigated.

The spatio-temporal distribution of muscle activity has been suggested to be a determinant of fatigue development. Pursuing this hypothesis, we investigated the pattern of muscular activity in the shoulder and arm during a repetitive dynamic task performed until participants' rating of perceived exertion reached 8 on Borg's CR-10 scale. We collected high density surface electromyogram (HD-EMG) over the upper trapezius, as well as bipolar EMG from biceps brachii, triceps brachii, deltoideus anterior, serratus anterior, upper and lower trapezius from 21 healthy women. Root mean square (RMS) and mean power frequency (MNF) were calculated for all EMG signals. The barycenter of RMS values over the HD-EMG grid was also determined, as well as normalized mutual information (NMI) for each pair of muscles. Cycle-to-cycle variability of these metrics was also assessed. With time, EMG RMS increased for most of the muscles, and MNF decreased. Trapezius activity became higher on the lateral side than on the medial side of the HD-EMG grid and the barycenter moved in a lateral direction. NMI between muscle pairs increased with time while its variability decreased. The variability of the metrics during the initial 10% of task performance was not associated with the time to task termination. Our results suggest that the considerable variability in force and posture contained in the dynamic task per se masks any possible effects of differences between subjects in initial motor variability on the rate of fatigue development.

Motor variability (MV) has been suggested to be a determinant of the risk for developing musculoskeletal disorders in repetitive work. In this study we examined whether individuals consistently differed in the extent of motor variability when performing a standardized short-cycle manual task. On three separate days, arm kinematics was recorded in 14 healthy subjects performing a pipetting task, transferring liquid from a pick-up tube to eight target tubes with a cycle time of 2.8 s. Cycle-to-cycle standard deviations (SD) of a large selection of shoulder and elbow kinematic variables, were processed using principal component analysis (PCA). Thereafter, between-subjects and between-days (within-subject) variance components were calculated using a random effects model for each of four extracted principal components. The results showed that MV differed consistently between subjects (95% confidence intervals of the between-subjects variances did not include zero) and that subjects differed consistently in MV between days. Thus, our results support the notion that MV may be a consistent personal trait, even though further research is needed to verify whether individuals rank consistently in MV even across tasks. If so, MV may be a candidate determinant of the risk of developing fatigue and musculoskeletal disorders in repetitive occupational work.

Background:

A longstanding hypothesis in physical therapy and occupational research is that workers repeating a task very stereotypically will be more prone to develop musculoskeletal disorders than workers that manage to vary postures and loads. Increased movement variability (MV), presumably, modify tissue loads, distribute stresses more equally, and thus reduce the cumulative load on any particular tissue. A handful of studies of MV have indicated less overuse injuries and faster recovery from musculoskeletal pain disorders. Even when repeating strictly controlled tasks individuals may differ in motor consistency, some showing higher levels of MV than others. However, whether the extent of MV is indeed a consistent individual trait across different tasks and different days is not known.

Purpose:

To investigate whether individual profiles of MV is stabile between days, the consistency of MV patterns from kinematic recordings, repeated across three days, was studied when performing repetitive upper-extremity precision work.

Methods:

A laboratory-based simulation of precision work; a 'pipetting' task paradigm, was developed in which liquid was repeatedly transferred from one tube to another, with a cycle time of 2.8s. Fourteen healthy female subjects, aged 20-45 years, right-handed and with experience in pipetting participated on 3 different days under identical conditions. Kinematic data were obtained using an electromagnetic motion capture system (FASTRAK). MV in shoulder elevation, elbow flexion and shoulder-elbow coordination were operationalized using cycle-to-cycle standard deviations across 20 pipetting cycles of kinematics parameters including joint range of motion, average and peak velocities, time to peak velocities, average angle and phase. Multivariate analysis was conducted using principal component analysis (PCA) (SIMCA+P, 12.0) to analyze relationships among variables and individual patterns in the data matrix of the recordings from day1. Thereafter, in order to confirm the observed structure of inter-individual MV patterns, classification of the data from day2 and day3 was performed using the parameters of the model from day1.

Results:

Four PCA components (Eigenvalues>1) accounted for 80 percent of the total variance in the model for day1. In the subsequent prediction model where data from day2 and 3 were projected into the model of day1, all subject observations except one could be predicted with 95% confidence (Hotelling T2). And individual data scores from all three days were clustered in relative proximity to each other, indicating consistency in MV between days.

Conclusion(s):

The findings indicate, even in this small and homogenous sample of young healthy females, that there may indeed be consistent individual traits in motor variability. A next step would be to answer whether these traits remain consistent if work factors such as work pace or precision are altered, and whether individual profiles of MV are associated with physiological responses related to risk for developing musculoskeletal disorders.

Implications:

Consistency of individual MV patterns substantiate previous notions that some people appear prone to repeat themselves while others tend to vary their motor behavior when performing the same task. Assessment of MV by physical therapists in research and practice could be valuable to further explore and address the relation of MV and musculoskeletal health.

Neuromuscular strategies employed in multijoint movements during repetitive motion-induced fatigue are still unclear, and movement variability may present a novel way to understand the compensatory control mechanisms that occur during fatigue. The aim of this study was to assess changes in shoulder and elbow joint kinematic variabilities and shoulder-elbow coordination variability associated with neck-shoulder fatigue, and whether these changes affected the spatio-temporal aspects of task performance. Nineteen healthy young adults continuously performed a repetitive pointing task between two targets placed in front of them at shoulder height at 1 Hz until fatigue (Borg CR10 rating of 8). Shoulder and elbow kinematics were recorded and used to compute shoulder abduction-adduction and elbow flexion-extension joint angles, and fingertip trajectories were used to compute the movement time and 3D spatial coordinates of the endpoint in each repetitive pointing movement cycle. Cycle-to-cycle movement variability of the shoulder and elbow joint angles from 15 consecutive forward pointing movements and cycle-to-cycle variability of continuous relative phase between the shoulder and elbow joint angles were compared between the first (baseline) and last (fatigue-terminal) minutes of performance. Shoulder kinematic variability and shoulder-elbow coordination variability were found to significantly increase with fatigue (by 60% and 30% of their respective baseline values). However, movement timing errors and spatial variability of the endpoint were found to be unchanged with fatigue. Results suggest that fatigue-related increase in shoulder variability may have been compensated by changes in shoulder-elbow coordination with an overall task performance objective and associated hierarchical control mechanisms.

The correlation between gaze strategy and kinematics of bimanual movements isassessed using repetitive bimanual object transfers as an experimental paradigm. The hypothesis isthat visual demand in such tasks may be a critical bottleneck determining bimanual coordination.Kinematics and eye-movements were compared before and after practice of this repetitive task.New eye-hand coordination strategies emerged with practice. Also, with practice, a systematicprioritization of the left hand movement to be „primary‟ and the right hand movement to be„secondary‟ emerged. This choice implied that the left hand movement kinematics was similar tounimanual left hand movements, whereas the performance of the right hand task was contingent onsuccessful completion of the primary task. This was revealed by „anticipatory adjustments‟ of theright hand kinematics (Right-hand peak velocity ranged from 100%-70% of the left-hand, and thescaling was dependent on task conditions and the corresponding eye-hand coordination strategiesused). We use this evidence to argue that the CNS, aware of an inherent asymmetry between thetwo hand systems, learns to anticipate the need and availability of visual feedback for successfultask completion, and uses this knowledge to optimize movement coordination - specifically suchthat the right-hand control was modulated to take visual constraints into account.

Fitts’ law cannot be used to predict movement times (MTs) of bimanual tasks since no empirical relationships associating task difficulty and bimanual MT have been demonstrated yet. Development of a ‘bimanual task difficulty index’ has been challenged by the complex patterns of coordination involved in simultaneously performing two tasks, one with each hand, under a control system with limited visual and attentional resources. To address this fundamental issue in human motor performance, bimanual object transfers with the left and right hands to targets of various precision requirements and separated by different distances were studied in six healthy subjects. Visual resource allocation during task performance was used to identify ‘primary’ and ‘secondary’ hand movements in bimanual tasks. While the primary movement was similar to a unimanual movement, the secondary MT varied with its own, as well as the contralateral hand’s task constraints. These results, which were stable and consistent across six subjects, provide preliminary evidence that visual behaviour, indicating closed-loop control, can be used to systematically derive bimanual MTs.

Several recent reviews have reported that 'repetitive movements' is a risk factor for occupational musculoskeletal disorders (MSD) in the neck, shoulder and arm regions. More variation in biomechanical exposure is often suggested as an effective intervention in such settings. Since increasing variation using extrinsic methods like job rotation may not always be possible in an industrial context, the intrinsic variability of the motor system may offer an alternative opportunity to increase variation. Motor variability (MV) refers to the natural variation in postures, movements and muscle activity observed to different extents in all tasks. The current review discusses relevant MV research appearing in motor control, sports sciences and occupational biomechanics literature to answer whether MV is important to consider in an occupational context, and if yes, whether it can be manipulated by training the worker or changing the working conditions so as to increase biomechanical variation without jeopardizing production

Several recent reviews have reported that ‘repetitive movements’ is a risk factor for occupational musculoskeletal disorders (MSD) in the neck, shoulder and arm regions. More variation in biomechanical exposure is often suggested as an effective intervention in such settings. While increasing variation using extrinsic methods like job rotation may not always be possible in an industrial context, the intrinsic variability of the motor system may offer an alternative opportunity to increase variation. Motor variability (MV) refers to the natural variation in postures, movements and muscle activity observed to different extents in all tasks. The current review explores the state of the art in MV research from motor control, sports and occupational biomechanics literature to answer whether MV is important to consider in an occupational context, and if yes, whether this literature stimulates further studies to test if MV can be manipulated as a deliberate intervention for increasing biomechanical variation without jeopardizing production.

Purpose Most previous studies of concurrent physical and cognitive demands have addressed tasks of limited relevance to occupational work, and with dissociated physical and cognitive task components. This study investigated effects on muscle activity and heart rate variability of executing a repetitive occupational task with an added cognitive demand integral to correct task performance.

Methods Thirty-five healthy females performed 7.5 min of standardized repetitive pipetting work in a baseline condition and a concurrent cognitive condition involving a complex instruction for correct performance. Average levels and variabilities of electromyographic activities in the upper trapezius and extensor carpi radialis (ECR) muscles were compared between these two conditions. Heart rate and heart rate variability were also assessed to measure autonomic nervous system activation. Subjects also rated perceived fatigue in the neck–shoulder region, as well as exertion.

Results Concurrent cognitive demands increased trapezius muscle activity from 8.2 % of maximum voluntary exertion (MVE) in baseline to 9.0 % MVE (p = 0.0005), but did not significantly affect ECR muscle activity, heart rate, heart rate variability, perceived fatigue or exertion.

Conclusion Trapezius muscle activity increased by about 10 %, without any accompanying cardiovascular response to indicate increased sympathetic activation. We suggest this slight increase in trapezius muscle activity to be due to changed muscle activation patterns within or among shoulder muscles. The results suggest that it may be possible to introduce modest cognitive demands necessary for correct performance in repetitive precision work without any major physiological effects, at least in the short term.

The effect of concurrent physical and cognitive demands on arm motor control is poorly understood. This exploratory study compared movement kinematics in a repetitive high-precision pipetting task with and without additional concurrent cognitive demands in the form of instructions necessary to locate the correct target tube. Thirty-five healthy female subjects performed a standardized pipetting task, transferring liquid repeatedly from one pick-up tube to different target tubes. In the reference condition, lights indicated the target tube in each movement cycle, while the target tube had to be deciphered from a row and column number on a computer screen in the condition with additional cognitive demands. Kinematics of the dominant arm was assessed using the central tendency and variability of the pipette-tip end-point trajectory and joint kinematics properties of the shoulder and elbow. Movements slowed down (lower velocities and higher area under the movement curves) and end point trajectory variability increased in the condition with additional cognitive demands, but there were no changes in the kinematics properties such as joint range of motion, times of acceleration and deceleration (as indicated by the time to peak velocity), average angles, or phase relationships between angle and angular velocity of shoulder or elbow movements between the two conditions. Further, there were also no differences in the size or structure of variability of the shoulder and elbow joint angles, suggesting that subjects could maintain the motor repertoire unaltered in the presence of these specific additional cognitive demands. Further studies should address motor control at other levels of concurrent cognitive demands, and with motor tasks that are less automated than the pipetting task used in the present study, so as to gain an increased understanding of the effect of concurrent cognitive demands for other activities of relevance to daily life.

Thirty-five healthy women, experienced in pipetting, each performed four pipetting sessions at different pace and accuracy levels relevant to occupational tasks. The size and structure of motor variability of shoulder and elbow joint angles were quantified using cycle-to-cycle standard deviations of several kinematics properties, and indices based on sample entropy and recurrence quantification analysis. Decreasing accuracy demands increased both the size and structure of motor variability. However, when simultaneously lowering the accuracy demand and increasing pace, motor variability decreased to values comparable to those found when pace alone was increased without changing accuracy. Thus, motor variability showed some speed-accuracy trade-off, but the pace effect dominated the accuracy effect. Hence, this trade-off was different from that described for end-point performance by Fitts’ law. The combined effect of accuracy and pace and the resultant decrease in motor variability are important to consider when designing sustainable work systems comprising repetitive precision tasks.

Kinematic motor variability is extensively studied in occupational, clinical and sports biomechanics, but the consistency of most motor variability metrics have never been reported. In this study, fourteen subjects performed a repetitive pipetting task on three separate days. Movements of hand, arm and pipette tip were recorded in 3D and used to compute shoulder elevation, elbow flexion and shoulder-arm coordination angles, as well as pipette-tip endpoint precision. Cycle-to-cycle motor variability was quantified using linear dispersion measures of standard kinematics properties such as peak velocity, range of motion, and inter-segmental relative phase. Between- and within-subject consistencies of these variability metrics were quantified by variance components estimated using a nested random effects model. For most metrics, the variance between subjects was larger than that between days and cycles. Entering the variance components in statistical power equations showed that for most metrics, a total of 80-100 subjects will be required to detect a 20% difference between two groups with sufficient power, while this difference can typically be detected  in repeated-measures (paired) designs using 25 subjects. The reported between- and within-subject variance components can be used as a data base to facilitate efficient designs of future studies of kinematic motor variability.

Increased movement variability has been suggested to reduce the risk of developing musculoskeletal disorders caused by repetitive work. This study investigated the effects of work pace on arm movement variability in a standardized repetitive pipetting task performed by 35 healthy women. During pipetting at slow and fast paces differing by 15%, movements of arm, hand and pipette were tracked in 3D, and used to derive shoulder and elbow joint angles. The size of cycle-to-cycle motor variability was quantified using standard deviations of several kinematics properties, while the structure of variability was quantified using indices of sample entropy and recurrence quantification analysis. When pace increased, both the size and structure of motor variability in the shoulder and elbow decreased. These results suggest that motor variability drops when repetitive movements are performed at increased paces, which may in the long run lead to undesirable outcomes such as muscle fatigue or over-use.

Motor variability (MV) refers to the intrinsic variability naturally present in the motor control system. Occurring even in the simplest movements, it is usually manifested as a difference in joint movements, joint coordination and/or muscle activities between successive repeats of a task which are identical in performance. Contrary to the traditional view that MV is detrimental to performance, it is now widely accepted that MV may actually have an important functional role in skill acquisition, and that skilled performance may, actually, be associated with increased MV. Further, MV is related to pain and fatigue, and may play a decisive role in rehabilitation (reviewed in Srinivasan & Mathiassen 2012). Hypothetically, individuals with a larger MV would be better protected against overuse injuries, and recover faster after disorders affecting motor performance. However, whether the extent of MV is, indeed, a consistent individual trait across different tasks is not known.    

The purpose of this study was to let individuals perform a laboratory-based simulation of repetitive upper-extremity precision work and determine:

(i)             Whether it is possible to systematically classify individuals according to the size of their MV in repetitive work;

(ii)            Whether classification of individuals in one working condition on one day persists even when some work-factors are slightly changed, and between different days when they perform the same work.

Repetitive pipetting with a cycle time of 2.8s was performed in the laboratory by a group of 14 healthy female subjects, aged 20-45 years, right-handed and experienced in pipetting, on 3 different days under identical protocol and experimental conditions. Work factors such as work-pace, precision and cognitive load (on top of the pipetting work) were manipulated within each day. Kinematic data were obtained using electromagnetic motion capture systems (FASTRAK).

MV in shoulder elevation, elbow flexion and shoulder-elbow coordination were operationalized using cycle-to-cycle standard deviations of motor parameters such as peak velocities, time lag of peak velocities, phase angle and inter-segmental phase angle. The resulting traits in individuals and the consistency of those traits across tasks and days will be presented.

Background. More women suffer from musculoskeletal disorders (MSDs) in the neck-shoulder and hand-arm regions than men, even when they are performing similar jobs. Fatigue is a known predictor of MSDs, and gender differences in fatigue responses could help explain the difference in MSD occurrence. This study aimed to assess the extent to which genders differ in fatigability by examining muscle-activity responses when perform-ing a low-force, repetitive arm-task leading to muscle fatigue in the neck-shoulder region.

Methods. 108 healthy individuals (55 males, 53 females) repeatedly touched two targets placed at shoulder height in front of them. The targets were placed at 30% and 100% of arm’s length and touched at a rate established by a 1Hz metronome until the subjects reported a perceived exertion of 8 on the Borg CR-10 scale for the neck-shoulder region. Bipolar surface EMG was recorded from the upper trapezius, anterior deltoid, biceps and triceps. Task duration and EMG amplitude (average and cycle-to-cycle variability) during the first minute (baseline) and last minute (fatigue-terminal) were compared between men and women.

Results. There were no gender differences in task duration, EMG variability at baseline, or change in average EMG amplitude with fatigue. Change in EMG variability from baseline to fatigue-terminal was lower for women than men in the upper trapezius (7.2% vs. 30.6% increase, p=0.02), and higher for women than men in the biceps (11.8% increase vs. 23.2% decrease, p=0.0006).

Discussion.This is the first study to report gender differences in muscle-activity responses to a fatiguing, dynamic manual task relevant to working life based on a comprehensive sample of healthy individuals. Despite no differences in task duration, there may be gen-der differences in the physiological mechanisms behind fatigue adaptations: while women may use compensatory mechanisms mainly involving the elbow, men may use more shoulder-driven strategies. These gender differences in muscle-activity responses may contribute to explaining why women suffer more from neck-shoulder MSDs than men.

Purpose: Epidemiological research has identified women to be more susceptible to developing neck-shoulder musculoskeletal disorders when performing low-force, repetitive work tasks. Whether this is attributable to gender differences in fatigability and motor control is currently unclear. This study investigated the extent to which women differ from men in fatigability and motor control while performing a short-cycle repetitive task.

Methods: 113 healthy young adults (58 women, 55 men) performed a standardized repetitive pointing task. The task was terminated when the subject's perceived exertion reached 8 on the Borg scale. Time to task termination, and changes in means and cycle-to-cycle variabilities of surface electromyography signals from start to end of the task were compared between women and men, for the upper trapezius, anterior deltoid, biceps and triceps muscles.

Results: Women and men terminated the task after 6.5 (SD 3.75) and 7 (SD 4) min on average (p>0.05). All 4 muscles showed an increase of 25-35% in average muscle activity with fatigue (no significant sex differences). However, men exhibited a higher increase than women in trapezius muscle variability with fatigue (31% vs. 7%; p<0.05), and a decrease in biceps muscle variability where women had an increase (-23% vs. 12%; p<0.05).

Conclusions: Our results suggest that women and men may not differ in the ability to perform repetitive tasks at low-to-moderate force levels. However, differences in motor control strategies employed in task performance may explain gender differences in susceptibility to developing musculoskeletal disorders when performing repetitive work for prolonged periods in occupational life.

The objectives of this study were to assess the sex-specific relationships between motor and sensory adaptations to repetitive arm motion-induced neck/shoulder fatigue, and measure how additional sensory stimulation affected these adaptations. Twenty-three participants performed two sessions of a repetitive pointing task until scoring 8 on the Borg CR10 scale for neck/shoulder exertion or for a maximum of 45min, with and without sensory stimulation (i.e. light touch) applied on the fatiguing shoulder. Just before reaching the task termination criteria, all participants showed changes in mean and variability of arm joint angles and experienced a five-fold increase in anterior deltoid (AD) sensory threshold in the stimulus-present condition. Women with the greatest increases in AD sensory thresholds demonstrated the greatest increases in shoulder variability (r= .66) whereas men with the greatest increases in upper trapezius sensory thresholds demonstrated greatest changes in shoulder angle (r= -.60) and coordination (r= .65) variability. Thus, sensory stimulation had no influence on time-to-termination but affected how men and women differently adapted, suggesting sex differences in the sensorimotor fatigue response mechanisms.