Quantification of Exposure to Risk Postures in Truck Assembly Operators: Neck, Back, Arms and Wrists

The study assessed the proportion of time in risky postures for the main joints of the upper limbs in a truck assembly plant and explored the association with musculoskeletal symptoms. Fourteen workstations (13 individuals) of a truck assembly plant were selected, and seven sensors were placed on the body segments of the participants. The sensors included tri-axial accelerometers for the arms and back, inclinometers for the neck and electro-goniometry for quantifying flexion/extension of the right and left hands. The proportions of time in moderate awkward postures were high at all workstations. Neck and wrist excessive awkward postures were observed for most workstations. The average values of the 91st percentile for back flexion and right/left arm elevation were 25°, 62°, and 57°, respectively. The 91st and 9th percentile averages for neck flexion/extension were 35.9° and −4.7°, respectively. An insignificant relationship was found between the percentage of time spent in awkward upper limb posture and musculoskeletal symptoms. The findings provide objective and quantitative data about time exposure, variability, and potential risk factors in the real workplace. Quantitative measurements in the field provide objective data of the body postures and movements of tasks that can be helpful in the musculoskeletal disorders (MSDs) prevention program.

Using Digital and Physical Simulation to Focus on Human Factors and Ergonomics in Aviation Maintainability

OBJECTIVE

This research aimed to evaluate the differences in the assessments made by three simulation tools used in a maintainability design office to perform human factor/ergonomics (HFE) analysis: digital human modeling (DHM), virtual reality (VR), and physical mock-up (PMU).

BACKGROUND

Maintainability engineers use digital/physical simulation tools in the early design phase to analyze whether the design is well adapted for maintenance operators. Knowing the potential of these simulation tools would encourage maintainability stakeholders to integrate HFE in the design process more efficiently.

METHOD

Eleven maintenance tasks were analyzed from the participation of six maintenance operators. Various HFE indicators including physical, cognitive, and organizational indicators were assessed. Each operator repeated 11 maintenance tasks on VR and PMU. Based on the anthropometric parameters, six manikins were created to analyze 11 maintenance tasks on DHM.

RESULTS

A significant difference was found for the organizational indicators between VR and PMU, whereas the physical and cognitive indicators are similar. DHM, VR, and PMU are compared with the common HFE indicators for the physical dimension and present a significant difference for individual tasks.

CONCLUSION

To reduce the gap between simulation tools, a better physical representation is requested on the VR platform, improving the perception of work sequences in the virtual world. Concerning DHM, a new paradigm is proposed to study a few tasks per work area instead of studying each task independently.

APPLICATION

This study will help develop a new methodology and tools specifically for non-HFE experts in the maintainability department.

Within and between Individual Variability of Exposure to Work-Related Musculoskeletal Disorder Risk Factors

Industrial companies indicate a tendency to eliminate variations in operator strategies, particularly following implementation of the lean principle. Companies believe when the operators perform the same prescribed tasks, they have to execute them in the same manner (completing the same gestures and being exposed to the same risk factors). They attempt to achieve better product quality by standardizing and reducing operational leeway. However, operators adjust and modify ways of performing tasks to balance between their abilities and the requirements of the job. This study aims to investigate the variability of exposure to physical risk factors within and between operators when executing the same prescribed tasks. The Ergonomic Standard method was used to evaluate two workstations. Seven operators were observed thirty times between repeated cycle times at those workstations. The results revealed the variability of exposure to risk factors between and within operators in the repeated execution of the same tasks. Individual characteristics and operators’ strategies might generate the variability of exposure to risk factors that may be an opportunity to reduce the risks of work-related musculoskeletal disorders (WR-MSDs). However, sometimes operators’ strategies may cause overexposure to risk factors; operators most often adopt such strategies to undertake their tasks while reducing the workload.

Physical risk factors identification based on body sensor network combined to videotaping

The aim of this study was to perform an ergonomic analysis of a material handling task by combining a subtask video analysis and a RULA computation, implemented continuously through a motion capture system combining inertial sensors and electrogoniometers. Five workers participated to the experiment. Seven inertial measurement units, placed on the worker's upper body (pelvis, thorax, head, arms, forearms), were implemented through a biomechanical model of the upper body to continuously provide trunk, neck, shoulder and elbow joint angles. Wrist joint angles were derived from electrogoniometers synchronized with the inertial measurement system. Worker's activity was simultaneously recorded using video. During post-processing, joint angles were used as inputs to a computationally implemented ergonomic evaluation based on the RULA method. Consequently a RULA score was calculated at each time step to characterize the risk of exposure of the upper body (right and left sides). Local risk scores were also computed to identify the anatomical origin of the exposure. Moreover, the video-recorded work activity was time-studied in order to classify and quantify all subtasks involved into the task. Results showed that mean RULA scores were at high risk for all participants (6 and 6.2 for right and left sides respectively). A temporal analysis demonstrated that workers spent most part of the work time at a RULA score of 7 (right: 49.19 ± 35.27%; left: 55.5 ± 29.69%). Mean local scores revealed that most exposed joints during the task were elbows, lower arms, wrists and hands. Elbows and lower arms were indeed at a high level of risk during the total time of a work cycle (100% for right and left sides). Wrist and hands were also exposed to a risky level for much of the period of work (right: 82.13 ± 7.46%; left: 77.85 ± 12.46%). Concerning the subtask analysis, subtasks called 'snow thrower', 'opening the vacuum sealer', 'cleaning' and 'storing' have been identified as the most awkward for right and left sides given mean RULA scores and percentages of time spent at risky levels. Results analysis permitted to suggest ergonomic recommendations for the redesign of the workstation. Contributions of the proposed innovative system dedicated to physical ergonomic assessment are further discussed.

Sweating responses and body temperatures during nocturnal sleep in humans

The changes in the central control of sweating were investigated in five sleeping subjects under neutral and warm conditions [operative temperature (To) = 30, 33, and 34 degrees C; dew-point temperature = 10 degrees C]. Esophageal (Tes) and mean skin (Tsk) temperatures, chest sweat rate (msw,1), and concomitant electroencephalographic data were recorded. Throughout the night, msw,1 was measured under a local thermal clamp of 38 degrees C. Results showed that the thermal environment exerted a strong influence on both the levels and the time patterns of body temperatures. Moreover, local sweating rate correlated positively with Tes, and this relationship varied according to sleep stages. For a given Tes level, there was a sleep stage-related gradation in msw,1 that was higher in slow-wave sleep (SWS) than in stage 1-2 and the lowest in rapid-eye-movement (REM) sleep. This is explained by a change in the excitability or the sensitivity of the thermoregulatory system. The msw,1 differences between stage 1-2 and SWS are accounted for by a decrease in the Tes threshold (Tset) for sweating while the slope of the msw,1-Tes relation remains unchanged. The lower msw,1 in REM sleep is explained by a lesser slope for the msw,1-Tes relation without any Tset change from stage 1-2.

Temperature regulation during intermittent exercise with progressive dehydration

Effects of dehydration (3% of initial body weight) on temperature regulation were investigated in 5 men during intermittent exercise of 4 h duration at a dry air temperature of 34 degrees C. Relative mechanical work load was 50% of the subject's steady state heart rate, which was 170 beats . min-1. During rehydration from the 70th min to the end of the exercise, the subjects drank, every 10 min in equal portions, an amount of water (20 degrees C) totaling up to 80% of the body weight loss recorded during dehydration runs. Continuous measurements were made of rectal (Tre) and mean skin (Tsk) temperatures and of whole body weight loss. Chest sweating rate (msw) was measured from a capsule located under a local thermal clamp (36 degrees C). Blood samples were obtained during rest periods and after the 1st and the 4th hour of exercise. Compared to dehydration runs, water intake did not always cause an increase of msw while body temperatures always decreased. Dehydration resulted in a decrease in plasma volume and in increases of plasma osmolality, [Na+] and [K+]. Water intake induced a thermoregulatory response whose intensity largely differs from one body area to another. The change in the slope of the relation of msw to Tre features a decrease in the sensitivity of the thermoregulatory system with dehydration. The whole body water loss is significantly correlated with the change in plasma volume and body temperatures (Tre, Tsk). This suggests that the reduced sweating response observed during dehydration can be related to plasma hypovolemia.

Hydration during exercise. Effects on thermal and cardiovascular adjustments

Five young unacclimatised subjects were exposed for 4 h at 34 degrees C (10 degrees C dew-point temperature and 0.6 m X s-1 air velocity), while exercising on a bicycle ergometer: 25 min work--5 min rest cycles for 2 hours followed by 20 min work--10 min rest cycles for two further hours. 5 experimental sessions were carried out: one without rehydration (NO FLUID) resulting in 3.1% mean loss of body weight (delta Mb), and four sessions with 20 degrees C fluid ingestion of spring water (WATER), hypotonic (HYPO), isotonic (ISO) and hypertonic (HYPER) solutions to study the effects of fluid osmolarity on rehydration. Mean final rehydration (+/- SE) after fluid intake was 82.2% (+/- 1.2). Heart rate was higher in NO FLUID while no difference among conditions was found in either delta Mb or hourly sweat rates. Sweating sensitivity was lowest in the dehydration condition, and highest in the WATER one. Modifications in plasma volume and osmolarity demonstrated that NO FLUID induced hyperosmotic hypovolemia, ISO rehydration rapidly led to plasma isoosmotic hypervolemia, while WATER led to slightly hypoosmotic normovolemia. It is concluded that adequate rehydration through ingestion of isotonic electrolyte-sucrose solution, although in quantities much smaller than evaporative heat loss, rapidly restored and expanded plasma volume. While osmolarity influenced sweating sensitivity, the plasma volume changes (delta PV) within the range -6% less than or equal to delta PV + 4% had little effect on temperature adjustments in our conditions.

Contribution of skin thermal sensitivities of large body areas to sweating response

The thermal sensitivity of different parts of the body was investigated by heating large areas of the body surface while the mean skin temperature calculated from Hardy and DuBois ' formula (1938) was always kept constant. The right arm sweating responses recorded under a local thermal clamp were related to changes in segmental skin temperatures of the different parts of the body. The results show that: 1) the various local peripheral signals are projected into integrating structures in the central nervous system; 2) the thermal sensitivity is greater for the head-and-trunk area in comparison with other parts of the body. For resting nude subjects, the formula of Hardy and DuBois remains a pertinent way for evaluating the role of skin thermal signals in the central drive for sweating. The peripheral contribution to the central sweating drive depends only on the skin temperature change and on the size of the stimulated area.

Oral temperature as an index of core temperature during heat transients

Rectal (Tre), oral (Tor) and oesophageal (Tes) temperatures were measured in five exercising subjects exposed for two hours to five conditions (1) a steady condition (WR) involving a constant work load (50 W) at a constant air temperature (Ta = 36.5 degrees C); (2) air temperature variations (delta Ta) between 28 degrees C and 45 degrees C and (3) between 23 degrees C and 50 degrees C at constant work load (50 W); (4) and (5) to work load variations (delta W) between 25 W and 75 W at a constant Ta (= 36.5 degrees C). Oral temperature recordings were taken sublingually and were either continuous or discontinuous. When discontinuous, the time needed for Tor to stabilize after the mouth opening was taken into account. The respective reliability of Tor and Tre as estimates of Tes were compared in each condition. Results showed that the resting (Tor - Tes) difference (+ 0.12 degrees C) was barely modified after two hours of exposure, whereas Tre overestimated Tes by 0.2 degrees C to 0.4 degrees C depending on the condition. The Tor variations were highly correlated with Tes variations under steady condition and under air temperature variations. In these conditions, Tor represented the best estimate of Tes. Under work-load variations, Tor was less closely related to Tes than was Tre. It is suggested that the relative inertia of Tor to step changes in exercise intensity could be ascribed to work induced variations in mouth blood flow.

Thermophysiological responses to humid heat: sex differences

1. Thermophysiological responses of four men and four pre- and postovulatory women were compared in humid heat conditions. Responses of pre- and postovulatory women are similar except for body temperature levels, which were significantly higher after ovulation. 2. Pronounced sex-related differences were observed in sweating rate and in body temperature variations. For the same evaporation, the sweat rate in men was higher than in women; as a consequence of this, the dripping rate was larger in men and thus the sweat decline was more important. Body temperature increases were larger in men in function of time and therefore temperature regulation in women was considered to be more efficient.

Respiratory mechanics of Pekin ducks under four conditions: pressure breathing, anesthesia, paralysis or breathing CO2-enriched gas

Impedance magnitude (Z) of the lower respiratory system was studied in Pekin ducks, using forced oscillations of a small volume at the airways opening in the range 1.6-16 Hz. The experiments were performed on 5 awake ducks enclosed in a body plethysmograph and spontaneously breathing ambient air at a transrespiratory pressure (Prs, the pressure difference between the lung and the body surface) which was varied in steps from -10 cm H2O (compression) to +10 cm H2O (distension). In 3 anesthetized birds, the effects of CO2 breathing and muscular paralysis were also studied. Analysis of end-expiratory Z data yielded estimates of respiratory resistance (R), inertance (I) and compliance (C). During positive or negative pressure breathing in conscious ducks, minute volume (V) and end-tidal CO2 (PETCO2) remained unchanged from normal (Prs = zero) while tidal volume (VT) and ventilatory period (Ttot) decreased. The respiratory system in late expiration can be modelled well with a simple series R-I-C mechanical model at Prs values of zero, +10 and -10 cm H2O. The value of Z increased at all frequencies studied during compression of the respiratory system (Prs = -10 cm H2O) and did not change much from normal (Prs = zero) during distension (Prs = +10 cm H2O). Both resistance and inertance increased during compression. During distension contradictory changes in resistance and inertance suggest that complex changes in flow profile and/or in flow pathways occurred with positive pressure breathing. Anesthesia or paralysis did not noticeably change the oscillatory resistance or inertance, but increased oscillatory compliance. CO2-breathing did not affect the respiratory impedance in late expiration, but reduced its flow dependence along the ventilatory cycle.

Impedance of the lower respiratory system in ducks measured by forced oscillations during normal breathing

The lower respiratory system of 10 conscious Pekin ducks breathing normally was subjected to superimposed oscillations of 1.3 to 16 Hz by a small volume piston pump. Induced sinusoidal flow (VO) and pressure (PO) signals were measured in late expiration, when the gas flow ventilated by the animal was minimum. The modulus of the respiratory impedance was calculated as the ratio PO/VO. The values obtained at the various oscillatory frequencies were compared to those predicted on the basis of a series mechanical network model consisting of resistive, inertial and compliant elements (RIC) using a least squares non linear regression method. The experimental data fitted well the frequency response of a simple RIC mathematical model. After correcting for the effects of the endotracheal tube, the mean values +/- SE of the optimized parameters were: resistance 4.8 +/- 0.4 cm H2O . L-1 . sec; inertance: 0.05 +/- 0.01 cm H2O . L-1 . sec2; compliance: 7.7 +/- 0.5 ml . cm H2O-1; natural frequency: 8.0 +/- 0.4 Hz. It is concluded that: (1) the lower respiratory system in ducks can be closely modeled by a RIC mechanical series network model; (2) the forced oscillation method can be used to investigate avian mechanics of breathing, with the birds awake and breathing spontaneously.