A mirror in the sky: the effects of map format and user expertise on navigation performance and mental workload.

A novel map display concept named Mirror in the Sky (MitS) has been introduced to improve performance and reduce workload in navigation tasks. However, this display will be novel to most users and as such, an evaluation of MitS in comparison with more conventional map formats is warranted. This study investigated the effects of map display format (MitS vs. north-up and track-up maps) and user expertise on mental workload (MWL) and performance, using both soldiers (experts) and civilians (novices) as participants. Participants followed a prescribed route to a destination in a virtual environment (route following task) while also performing a secondary task (detection response task). Soldiers generally performed better than civilians. Soldiers reported a higher MWL with MitS than with the north-up map, whereas civilians reported a higher MWL with MitS than with the track-up map. Regardless of user expertise, there were performance and workload challenges with MitS, despite its potential. Practitioner summary: A new map display concept called Mirror in the Sky (MitS) was compared with two conventional map formats: a north-up and track-up map. The experiment tested soldier and civilian users in a route following task. Both groups got further into the route and had fewer obstacle collisions with north-up and track-up maps than they did with the MitS map. MWL measures generally indicated higher workload with MitS. Abbreviations: MitS: mirror in the sky; FFOV: forward field of view; AR: augmented reality; MWL: mental workload; VR: virtual reality; HF: human factors; HR: heart rate; HRV: heart rate variability; DRT: detection response task; DRDC: defence research and development Canada; VE: virtual environment; RT: response time; ANOVA: analysis of variance.

Subjective evaluation of physical and mental workload interactions across different muscle groups.

Both physical and mental demands, and their interactions, have been shown to increase biomechanical loading and physiological reactivity as well as impair task performance. Because these interactions have shown to be muscle-dependent, the aim of this study was to determine the sensitivity of the NASA Task Load Index (NASA TLX) and Ratings of Perceived Exertion (RPE) to evaluate physical and mental workload during muscle-specific tasks. Twenty-four participants performed upper extremity and low back exertions at three physical workload levels in the absence and presence of a mental stressor. Outcome measures included RPE and NASA TLX (six sub-scales) ratings. The findings indicate that while both RPEs and NASA TLX ratings were sensitive to muscle-specific changes in physical demand, only an additional mental stressor and its interaction with either physical demand or muscle groups influenced the effort sub-scale and overall workload scores of the NASA TLX. While additional investigations in actual work settings are warranted, the NASA TLX shows promise in evaluating perceived workload that is sensitive not only to physical and mental demands but also sensitive in determining workload for tasks that employ different muscle groups.

Rotation during lifting tasks: effects of rotation frequency and task order on localized muscle fatigue and performance.

Though widely considered to reduce the risk of work-related musculoskeletal disorders, there is limited evidence suggesting that rotating between tasks is effective in doing so. The purpose of the current study was to quantify the effects of rotation and parameters of rotation (frequency and task order) on muscle fatigue and performance. This was done using a simulated lifting task, with rotation between two levels of loading of the same muscle groups. Twelve participants completed six experimental sessions during which repetitive box lifting was performed for one hour either with or without rotation. When rotation was present, it occurred every 15 minutes or every 30 minutes and was between two load levels (box weights). Rotation reduced fatigue and cardiovascular demand compared to the heavier load without rotation, with a mean reduction of ∼33% in perceived discomfort and a ∼17% reduction in percentage of heart rate reserve. Further, rotation increased fatigue and cardiovascular demand compared to the lighter load without rotation, with a mean increase of ∼34% perceived discomfort and a ∼19% increase in percentage of heart rate reserve. Neither rotation frequency nor task order had definitive effects, though maximum discomfort ratings were nearly 20% higher when starting with the lighter load task. These parameters of rotation should be further evaluated under more realistic task conditions.

Ergonomic evaluation of a wearable assistive device for overhead work.

Overhead work is an important risk factor for upper extremity (UE) musculoskeletal disorders. We examined the potential of a mechanical arm and an exoskeletal vest as a wearable assistive device (WADE) for overhead work. Twelve participants completed 10 minutes of simulated, intermittent overhead work, using each of three payloads (1.1, 3.4 and 8.1 kg) and with/without the WADE. Ratings of perceived discomfort (RPDs) and electromyography (EMG) were obtained for the upper arms, shoulders and low back. Using the WADE, UE RPDs decreased by ∼50% with the heavier payloads, whereas smaller (∼25%) and non-significant increases in low-back RPDs were found and were relatively independent of payload. Changes in RPDs with WADE use were consistent with physical demands indicated by EMG, though EMG-based differences in fatigue were less apparent. Participants generally preferred using the WADE, particularly with heavier payloads. These results supported the potential utility of a WADE as an intervention for overhead work.

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.

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.

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.

Passive stiffness characteristics and neutral zone quality of the scoliotic lumbar torso in the principle anatomical planes of motion.

BACKGROUND: The 1-10% prevalence rate of adult scoliosis frequently requires expensive therapy and surgical treatments and demands further research into the disease, especially with an aging population. Most studies examining the mechanics of scoliosis have focused on in vitro testing or computer simulations. This study quantitatively defined the passive stiffness properties of the in vivo scoliotic spine in three principle anatomical motions and identified differences relative to healthy controls. METHODS: Adult scoliosis (n = 14) and control (n = 17) participants with no history of spondylolisthesis, spinal fracture, or spinal surgery participated in three different tests (torso lateral side bending, torso axial rotation, and torso flexion/extension) that isolated mobility to the in vivo lumbar spine. The spinal stiffnesses and spinal neutral zone width were calculated. These parameters were statistically compared between factor of population and within factor of direction. FINDINGS: Torque-rotational displacement data were fit using a double sigmoid function, resulting an in excellent overall fit (Avg. R2 = 0.95). There was a significant interaction effect between populations when comparing axial twist neutral zone width vs. lateral bend neutral zone width and axial twist stiffness vs. lateral bend stiffness. The axial twist neutral zone width magnitude was significantly larger in scoliosis patients. INTERPRETATION: The present study is the first investigation to quantify the whole trunk neutral zone of the scoliotic lumbar spine. Future research is needed to determine if lumbar spine mechanical characteristics can help explain progression of scoliosis and complement scoliosis classification systems.

Comparison of 3D dynamic virtual model to link segment model for estimation of net L4/L5 reaction moments during lifting.

The purpose of this study was to validate a 3D dynamic virtual model for lifting tasks against a validated link segment model (LSM). A face validation study was conducted by collecting x, y, z coordinate data and using them in both virtual and LSM models. An upper body virtual model was needed to calculate the 3D torques about human joints for use in simulated lifting styles and to estimate the effect of external mechanical devices on human body. Firstly, the model had to be validated to be sure it provided accurate estimates of 3D moments in comparison to a previously validated LSM. Three synchronised Fastrak units with nine sensors were used to record data from one male subject who completed dynamic box lifting under 27 different load conditions (box weights (3), lifting techniques (3) and rotations (3)). The external moments about three axes of L4/L5 were compared for both models. A pressure switch on the box was used to denote the start and end of the lift. An excellent agreement [image omitted] was found between the two models for dynamic lifting tasks, especially for larger moments in flexion and extension. This virtual model was considered valid for use in a complete simulation of the upper body skeletal system. This biomechanical virtual model of the musculoskeletal system can be used by researchers and practitioners to give a better tool to study the causes of LBP and the effect of intervention strategies, by permitting the researcher to see and control a virtual subject's motions.

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.

An on-body personal lift augmentation device (PLAD) reduces EMG amplitude of erector spinae during lifting tasks.

BACKGROUND: A new on-body personal lift augmentation device was developed to support the back muscles during the repetitive lifting task. METHODS: Nine male subjects participated in the study. Three Fastrak units were used to record positions and rotations of the segments. Trunk muscle normalized and integrated electromyography of the left and right thoracic erector spinae, lumbar erector spinae, external obliques, and rectus abdominis, as well as the kinematic variables of peak lumbar angle, peak pelvis angle, peak trunk acceleration, peak load acceleration were compared in symmetrical lifting for three different loads (5 kg, 15 kg, 25 kg) with three different styles (stooped, squat, free) under two conditions of with and without personal lift assist device. FINDINGS: The lift assist device significantly reduced the required muscular effort of the lumbar (p = 0.001) and thoracic erector spinae with no significant differences in the level of abdominal muscle activity. The amount of integrated electromyography reduction ranged from 14.4% to 27.6% for the lumbar and thoracic erector spinae respectively. Simple measures of trunk posture and accelerations confirmed that there were no differences in lifting technique that would cause the integrated electromyography activity to be reduced. No major kinematic differences were found when the lift assist device was worn indicating that it did not alter these specific technique variables. INTERPRETATION: The lift assist device did reduce the required muscular effort of the lumbar and thoracic erector spinae without adversely affecting the level of abdominal muscle activity. This reduction may help reduce the risk of recurring back injuries or assist in the return to work phase, especially in repetitive tasks.

Development of a sliding mode control model for quiet upright stance.

Human upright stance appears maintained or controlled intermittently, through some combination of passive and active ankle torques, respectively representing intrinsic and contractile contributions of the ankle musculature. Several intermittent postural control models have been proposed, though it has been challenging to accurately represent actual kinematics and kinetics and to separately estimate passive and active ankle torque components. Here, a simplified single-segment, 2D (sagittal plane) sliding mode control model was developed for application to track kinematics and kinetics during upright stance. The model was implemented and evaluated using previous experimental data consisting of whole body angular kinematics and ankle torques. Tracking errors for the whole-body center-of-mass (COM) angle and angular velocity, as well as ankle torque, were all within ∼10% of experimental values, though tracking performance for COM angular acceleration was substantially poorer. The model also enabled separate estimates of the contributions of passive and active ankle torques, with overall contributions estimated here to be 96% and 4% of the total ankle torque, respectively. Such a model may have future utility in understanding human postural control, though additional work is needed, such as expanding the model to multiple segments and to three dimensions.

A new method to assess passive and active ankle stiffness during quiet upright stance.

Both passive and active ankle torque contribute to postural stability during quiet upright stance, yet directly measuring their relative contributions is difficult. Here, a new method was developed to estimate passive and active ankle stiffness (ST) and damping (DA). In contrast to earlier approaches, the proposed method does not require external mechanical or sensory perturbations. Instead, the method is based on the assumption that upright stance is intermittently controlled, and that active ankle torque is in-phase coherent with ankle angular acceleration. Thus, identifying the local maxima of ankle angular accelerations facilitates the identification of time windows that include substantial active ankle torque. After identifying these local maxima and associated windows, estimates of passive and active ankle ST and DA were obtained using linear regression analyses. Consistent with earlier work, passive ankle torque was estimated to account for 94-97% of the total ankle torque, and to have linear relationships with ankle angle and angular velocity. Predicted values of passive and active ankle stiffness were also consistent with earlier reports. This new approach may be a useful tool to efficiently investigate passive and active joint stiffness during quiet upright stance.

A time-frequency approach to estimate critical time intervals in postural control.

The critical time interval (CTI) is a parameter that has been used to distinguish open-loop from closed-loop control during upright stance. The aim of this study was to develop a new method to determine CTIs. The new approach, termed the intermittent critical time interval (ICTI) method, was motivated from evidence that upright standing is an intermittent rather than an asymptotic stability control process. For this ICTI method, center-of-pressure time series are first transformed to the time-frequency domain with a wavelet method. Subsequently, the CTI is assumed equal to the time span between two local maxima in the time-frequency domain within a distinct frequency band (i.e., 0.5-1.1 Hz). This new method may help facilitate better estimates of the transition time interval between open and closed-loop control during upright stance and can also be applied in future work such as in simulating postural control. In addition, this method can be used in future work to assess temporal changes in CTIs.

Use of wavelet coherence to assess two-joint coordination during quiet upright stance.

Joint coordination plays a critical role in maintaining postural stability, yet there is limited existing work describing joint coordination patterns in the time-frequency domain. Here, two-joint coordination was examined during quiet upright stance. A wavelet coherence method was applied to quantify the coherence between ankle-trunk and ankle-head angles in the sagittal and frontal planes. Wavelet coherence results indicated intermittent joint coordination particularly for frequencies of 2.5-4.0Hz. Coherence results were further processed to estimate mean time intervals between coherence instances, coherence burst frequency, and the ratio of in-phase versus anti-phase behaviors. Time intervals between intermittent coherence were 1.3-1.5sec, coherence burst frequency was ~0.4Hz, and phase ratios were ~1.0. Intermittent "bursting" of postural muscles may account for the finding of intermittent coherence in the noted frequency band. Some age and/or gender differences in coherence were found, and may be related to comparable differences in postural control ability or strategies. Results from application of this new method support earlier evidence that kinematic coordination is achieved intermittently rather than continuously during quiet upright stance. This method may provide richer information regarding such coordination, and could be a useful approach in future studies.

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.

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.

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.

The effect of an on-body personal lift assist device (PLAD) on fatigue during a repetitive lifting task.

Occupations demanding frequent and heavy lifting are associated with an increased risk of injury. A personal lift assist device (PLAD) was designed to assist human muscles through the use of elastic elements. This study was designed to determine if the PLAD could reduce the level of general and local back muscle fatigue during a cyclical lifting task. Electromyography of two erector spinae sites (T9 and L3) was recorded during a 45-min lifting session at six lifts/lowers per minute in which male participants (n=10) lifted a box scaled to represent 20% of their maximum back extensor strength. The PLAD device reduced the severity of muscular fatigue at both muscle sites. RMS amplitude increased minimally (22% and 26%) compared to the no-PLAD condition (104% and 88%). Minimal median frequency decreases (0.33% and 0.41%) were observed in the PLAD condition compared to drops of 12% and 20% in the no-PLAD condition. The PLAD had an additional benefit of minimizing pre-post changes in muscular strength and endurance. The PLAD also resulted in a significantly lower rate of perceived exertion across the lifting session. It was concluded that the PLAD was effective at decreasing the level of back muscular fatigue.

Effects of two hospital bed design features on physical demands and usability during brake engagement and patient transportation: a repeated measures experimental study.

BACKGROUND: Work-related musculoskeletal disorders (WMSDs) are prevalent among healthcare workers worldwide. While existing research has focused on patient-handling techniques during activities which require direct patient contact (e.g., patient transfer), nursing tasks also involve other patient-handling activities, such as engaging bed brakes and transporting patients in beds, which could render healthcare workers at risk of developing WMSDs. OBJECTIVES: Effectiveness of hospital bed design features (brake pedal location and steering-assistance) was evaluated in terms of physical demands and usability during brake engagement and patient transportation tasks. DESIGN: Two laboratory-based studies were conducted. In simulated brake engagement tasks, three brake pedal locations (head-end vs. foot-end vs. side of a bed) and two hands conditions (hands-free vs. hands-occupied) were manipulated. Additionally, both in-room and corridor patient transportation tasks were simulated, in which activation of steering-assistance features (5th wheel and/or front wheel caster lock) and two patient masses were manipulated. PARTICIPANTS: Nine novice participants were recruited from the local student population and community for each study. METHODS: During brake engagement, trunk flexion angle, task completion time, and questionnaires were used to quantify postural comfort and usability. For patient transportation, dependent measures were hand forces and questionnaire responses. RESULTS: Brake pedal locations and steering-assistance features in hospital beds had significant effects on physical demands and usability during brake engagement and patient transportation tasks. Specifically, a brake pedal at the head-end of a bed increased trunk flexion by 74-224% and completion time by 53-74%, compared to other pedal locations. Participants reported greater overall perceived difficulty and less postural comfort with the brake pedal at the head-end. During in-room transportation, participants generally reported "Neither Low nor High" physical demands with the 5th wheel activated, compared to "Moderately High" physical demands when the 5th wheel was deactivated. Corridor transportation was similarly reported to be easier when a steering-assistance feature (the 5th wheel or front caster lock) was activated. CONCLUSIONS: Braking and steering-assistance features of hospital beds can have important effects on task efficiency and physical demands placed on healthcare workers. Selection of specific designs may thus be able to improve productivity and contribute to a reduction in WMSDs risk among healthcare workers.