Michael J. Agnew

Influence of mental workload on muscle endurance, fatigue, and recovery during intermittent static work

Most occupational tasks involve some level of mental/cognitive processing in addition to physical work; however, the etiology of work-related musculoskeletal disorders (WMSDs) due to these demands remains unclear. The aim of this study was to quantify the interactive effects of physical and mental workload on muscle endurance, fatigue, and recovery during intermittent work. Twelve participants, balanced by gender, performed intermittent static shoulder abductions to exhaustion at 15, 35, and 55% of individual maximal voluntary contraction (MVC), in the absence (control) and presence (concurrent) of a mental arithmetic task. Changes in muscular capacity were determined using endurance time, strength decline, electromyographic (EMG) fatigue indicators, muscle oxygenation, and heart rate measures. Muscular recovery was quantified through changes in strength and physiological responses. Mental workload was associated with shorter endurance times, specifically at 35% MVC, and greater strength decline. EMG and oxygenation measures showed similar changes during fatigue manifestation during concurrent conditions compared to the control, despite shorter endurance times. Moreover, decreased heart rate variability during concurrent demand conditions indicated increased mental stress. Although strength recovery was not influenced by mental workload, a slower heart rate recovery was observed after concurrent demand conditions. The findings from this study provide fundamental evidence that physical capacity (fatigability and recovery) is adversely affected by mental workload. Thus, it is critical to determine or evaluate occupational demands based on modified muscular capacity (due to mental workload) to reduce risk of WMSD development.

Effectiveness of an on-body lifting aid at reducing low back physical demands during an automotive assembly task: assessment of EMG response and user acceptability.

The purpose of this study was to investigate the effectiveness and user acceptability of a Personal Lift-Assist Device (PLAD) at an automotive manufacturing facility, with operators who perform an on-line assembly process requiring forward bending and static holding. Surface EMG data were collected at six sites on the low back and abdomen, and an accelerometer was used to measure trunk inclination. Use of the PLAD significantly reduced the thoracic and lumbar erector spinae activity and EMG-predicted compression at the 10th, 50th, and 90th APDF percentile levels (p < or = 0.05), without significantly increasing rectus abdominus activity or trunk flexion. Similarly, ratings of perceived exertion were found to be significantly lower when wearing the PLAD (p = 0.006). Subjective opinions were positive, with 8/10 subjects indicating they would wear the device everyday. With slight changes, workers felt that the PLAD could be beneficial at reducing forces and discomfort in similar industrial or manual materials handling tasks that place excessive physical demands on the low back.

Effects of rotation frequency and task order on localised muscle fatigue and performance during repetitive static shoulder exertions

Though widely considered to reduce physical exposures and increase exposure variation, there is limited evidence that rotating between tasks is effective in reducing the risk of work-related musculoskeletal disorders (WMSDs). The purpose of this study was to assess the effects of rotation, specifically focusing on rotation frequency and task order, on muscle fatigue and performance when rotating between tasks that load the same muscle group. Twelve participants completed six experimental sessions during which repetitive static shoulder abduction tasks were performed at two exertion levels for one hour either with or without rotation. Compared to only performing a higher or lower exertion task, rotating between the two tasks decreased and increased fatigue, respectively. Increasing rotation frequency adversely affected task performance, and task order had a minor effect on muscle fatigue. These rotation parameters may be important considerations when implementing rotation in the workplace. Practitioner Summary: Rotation is widely used and assumed to reduce the risk of WMSDs, yet little research supports that it is effective in doing so. Results here show that specific aspects of a rotation scheme may influence muscle fatigue and task performance, though further research is needed under more realistic task conditions.

Muscle- and task-dependent responses to concurrent physical and mental workload during intermittent static work

Many workers experience combined physical and mental demands in their jobs, yet the contribution of these demands to the development of musculoskeletal disorders is unclear. The purpose of this study was to investigate muscle- and task-dependent responses to concurrent demands during intermittent static work. Twenty-four participants performed shoulder, wrist, and torso exertions at three levels of physical workload (PWL) in the absence (control) and presence (concurrent) of a mental arithmetic task. Compared to the control, concurrent demand conditions resulted in decreased muscle activity (4–9% decrease), increased cardiovascular load (2–4% increase), and impaired motor co-ordination (9–24% increase in force fluctuation). Furthermore, these outcomes were more prominent at higher PWL levels and within postural (shoulder and torso) muscles. Mental task performance exhibited greater interference with the physical task at low and high PWL levels. Thus, it may be important to consider these muscle- and task-specific interactions of concurrent demands during job design to address worker health and performance issues. Practitioner Summary: Occupational tasks place both physical and mental demands on workers. These demands can adversely affect physiological responses and performance, and are muscle- and task-dependent. Findings from this research may facilitate the development of ergonomics interventions, such as task redesign and tool/workstation design, that may help reduce risk of workplace injuries.

Reliability of quantitative EMG analysis of the extensor carpi radialis muscle

This study investigated the within-subject, intra- and inter-operator reliability of quantitative electromyographic (EMG) analysis using decomposition-based quantitative electromyography (DQEMG). Needle and surface-detected EMG signals were collected during low-level isometric contractions of the extensor carpi radialis (ECR) muscle. DQEMG was used to extract needle-detected motor unit potential (MUP) trains and surface-detected MUPs (SMUPs) associated with each train. Two independent experienced operators re-decomposed and processed the MUP data on two separate occasions. One-way analyses of variance (ANOVA) were performed to identify within-subject differences (test-retest, n=6), and operator and trial differences (intra- and inter-operator, n=20) for the MUP morphological variables. The within-subject reliability, as well as the intra- and inter-operator reliability were estimated using intraclass correlation coefficients (ICCs). The 95% limits of agreement were calculated to measure within-subject and between operator agreements. MUP durations were found to be significantly different between days (p<0.05). For intra- and inter-operator reliability, a significant difference was found within and between the operators for MUP duration, and between the operators for MUP number of turns (p<0.05). SMUP morphological parameters yielded higher ICC values for both intra (0.96-0.99) and inter (0.96-0.99) operator scores when compared to MUP morphological parameters (intra-operator: 0.81-0.99; and inter-operator: 0.67-0.98). Mean motor unit (MU) firing rate was found to be a highly reliable measure for both intra- and inter-operators (0.99-0.97, respectively). The between-operator agreement was above 95% for all morphological parameters. These results concerning within-subject, intra- and inter-operator reliability and levels of agreement for quantitative motor unit analysis suggest that DQEMG provides sufficiently consistent results to allow it to be effectively used for QEMG analysis. Needle-detected MUP morphology although clinically useful in the diagnosis and monitoring of neuropathies, was not as reliable as surface MUP QEMG measures.

Exertion-Dependent Effects of Physical and Mental Workload on Physiological Outcomes and Task Performance

OCCUPATIONAL APPLICATIONS The findings from the current study illustrate the adverse effects of concurrent physical and mental demands on shoulder muscle activation. Furthermore, the results also suggest that the effects of concurrent physical and mental demands are more pronounced during static, as opposed to dynamic, exertions. As such, it is important to consider the interaction of said work parameters (i.e., force levels, mental demands, and type of exertion) when evaluating/designing tasks. Occupational tasks requiring concurrent physical and mental work should be redesigned to reduce static loading (by changing postures or providing frequent breaks). With tasks requiring static exertions, other physical parameters (such as force) should be reduced to minimize interference due to additional cognitive processing. To conclude, concurrent physical and mental demands affect muscle activation and impede worker performance; ergonomists should consider this interaction during task design/redesign, evaluations of injury risk, and potential causal models of injury development. TECHNICAL ABSTRACT Background: Static work is considered an occupational risk factor in the development of injuries, thus there is an emphasis on employing dynamic exertions to work tasks. With workers experiencing concurrent physical and mental demands in their daily jobs, it is unclear whether these exertion types affect overall task demands differentially. Objective: The aim of this study was to compare exertion-dependent physiological responses due to concurrent physical and mental workload during intermittent shoulder exertion. Methods: Twelve young participants, balanced by gender, performed intermittent static and dynamic shoulder abduction for 3 minutes at three levels of physical workload (low, moderate, and high) in the absence and presence of a mental arithmetic task. Study measures included muscle activity, muscle oxygenation, motor and mental arithmetic task performance, and subjective responses (NASA-Task Load Index and Borg CR10 Scale). Results: Static exertions and higher physical demands adversely affected physiological responses (i.e., muscle activity and oxygenation) and performance measures, and they were associated with higher levels of perceived exertion and workload. Additional mental demands negatively affected muscle activity, mental task performance, and subjective workload measures. However, these results were more pronounced during static exertions at high physical demand levels. Conclusions: Results indicated that certain job parameters (static exertions and high physical demands) are more susceptible to interference with mental demands than others (dynamic exertions and low demands). When assessing overall demands placed on workers during concurrent physical and mental work, it is important to consider the interaction of work parameters, specifically physical demand levels and exertion type, with mental demands.

Occupational Health Outcomes for Workers in the Agriculture, Forestry and Fishing Sector: Implications for Immigrant Workers in the Southeastern US

BACKGROUND Workers in the Agriculture, Forestry, and Fisheries (AgFF) sector experience exposures directly related to the work itself, as well as the physical environment in which the work occurs. Health outcomes vary from immediate to delayed, and from acute to chronic. METHODS We reviewed existing literature on the health outcomes of work in the AgFF sector and identified areas where further research is needed to understand the impact of these exposures on immigrant Latino workers in the southeastern US. RESULTS Outcomes related to specific body systems (e.g., musculoskeletal, respiratory) as well as particular exposure sources (e.g., pesticides, noise) were reviewed. The most extensive evidence exists for agriculture, with a particular focus on chemical exposures. Little research in the southeastern US has examined health outcomes of exposures of immigrant workers in forestry or fisheries. CONCLUSION As the AgFF labor force includes a growing number of Latino immigrants, more research is needed to characterize a broad range of exposures and health outcomes experienced by this population, particularly in forestry and fisheries.

Functional data analysis as a means of evaluating kinematic and kinetic waveforms

Researchers use peak and mean characteristics to quantify changes in kinematics and kinetic loading patterns. The variability of human motion warrants a more complex method of analysis. This paper will demonstrate that functional data analysis (FDA) can quantify differences throughout a lifting waveform that would not be evident when applying standard statistical methods to discrete variables. A functional ANOVA was used to identify variation in kinematic and kinetic waveforms resulting from fatigue experienced over a 45-min lifting task. Only trunk angular motion and hip angular velocity were significantly different across the work task using discrete peak values. FDA was able to demonstrate that additional areas outside of the peaks were significantly different in the late phase for several waveforms. FDA was able to demonstrate significant variation across the work period for kinetic and kinematic variables when statistics performed on discrete variables reported no variation. More robust methods for follow-up procedures were recommended.

Females exhibit shorter paraspinal reflex latencies than males in response to sudden trunk flexion perturbations

BACKGROUND Females have a higher risk of experiencing low back pain or injury than males. One possible reason for this might be altered reflexes since longer paraspinal reflex latencies exist in injured patients versus healthy controls. Gender differences have been reported in paraspinal reflex latency, yet findings are inconsistent. The goal here was to investigate gender differences in paraspinal reflex latency, avoiding and accounting for potentially gender-confounding experimental factors. METHODS Ten males and ten females underwent repeated trunk flexion perturbations. Paraspinal muscle activity and trunk kinematics were recorded to calculate reflex latency and maximum trunk flexion velocity. Two-way mixed model analyses of variance were used to determine the effects of gender on reflex latency and maximum trunk flexion velocity. FINDINGS Reflex latency was 18.7% shorter in females than in males (P=0.02) when exposed to identical trunk perturbations, and did not vary by impulse (P=0.38). However, maximum trunk flexion velocity was 35.3% faster in females than males (P=0.01) when exposed to identical trunk perturbations, and increased with impulse (P<0.01). While controlling for differences in maximum trunk flexion velocity, reflex latency was 16.4% shorter in females than males (P=0.04). INTERPRETATION The higher prevalence of low back pain and injury among females does not appear to result from slower paraspinal reflexes.

Effects of physical and mental demands on shoulder muscle fatigue.

Mental demands have been associated with increased risk of injuries; however, its influence on muscle fatigability remains unclear. The aim of this study was to investigate the interaction of mental workload and physical workload on muscle fatigability during repetitive shoulder work. Twelve young participants, balanced by gender, performed shoulder abduction exercises until exhaustions at three levels of physical workload (low (5% maximum voluntary contraction (MVC)), moderate (35% MVC), and high (55% MVC)) in the absence and presence of a mental arithmetic test. Endurance time and rate of strength decline were employed as indicators of muscle fatigue. Concurrent physical and mental processing was found to adversely decrease muscle endurance by ~25% at the moderate intensity level. Furthermore, concurrent demands were associated with faster rate of strength decline compared to the control, irrespective of the physical intensity level. Findings from the current study provide evidence of the adverse effects of mental workload on muscle capacity (i.e., endurance and fatigue). It is therefore important to consider potential changes in worker capacity with concurrent physical and cognitive demands before designing work tasks/products.