The relationship between shoulder torques and the perception of muscular effort in loaded reaches

The objective of this study was to define the quantitative relationship between external dynamic shoulder torques and calibrated perceived muscular effort levels for load delivery tasks, for application in job analyses. Subjects performed a series of loaded reaches and, following each exertion, rated their perceived shoulder muscular effort. Motion and task physical requirements data were processed with a biomechanical upper extremity model to calculate external dynamic shoulder torques. Calculated torque values were then statistically compared to reported calibrated perceived muscular effort scores. Individual subject torque profiles were significantly positively correlated with perceived effort scores (r2 = 0.45-0.77), with good population agreement (r2 = 0.50). The accuracy of the general regression model improved (r2 = 0.72) with inclusion of factors specific to task geometry and individual subjects. This suggests two major conclusions: 1) that the perception of muscular shoulder effort integrates several factors and this interplay should be considered when evaluating tasks for their impact on the shoulder region; 2) the torque/perception relationship may be usefully leveraged in job design and analysis.     

Modelling of shoulder and torso perception of effort in manual transfer tasks

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the ‘sense of effort’ is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.     

Applying psychophysics to prevent overexposure: On the relationships between acceptable manual force,joint loading,and perception

Psychophysical methods are commonly used to establish guidelines for task acceptability or to indicate perceived task demands. Despite their widespread use and practical application, the subjectivity of psychophysical methods can limit their perceived benefit to field ergonomists and engineers. However, recent work has revealed important links between biomechanical limitations and psychophysical force acceptability and also between perceived effort and moment loading at the shoulder. These twin findings support the continued use of psychophysical methods in ergonomics, and implore future developments to improve their use in the field. Through a combination of literature review and research vignettes, these links are described, discussed, tested and applied in both experimental and simulation scenarios. Whether the goal is to design work based on psychophysically acceptable guidelines, or to gauge existing task demands using perceived effort, this paper highlights the power of psychophysics for ergonomic design.     

Predictors of perceived effort in the shoulder during load transfer tasks

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r² = 0.50) and muscle activity model-based muscle force predictions (MFPs): r² = 0.42, electromyography (EMG): r² = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r² = 0.74; MFP: r² = 0.67, EMG: r² = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.     

Investigating shoulder muscle loading and exerted forces during wall painting tasks: Influence of gender,work height and paint tool design

The task of wall painting produces considerable risk to the workers, both male and female, primarily in the development of upper extremity musculoskeletal disorders. Insufficient information is currently available regarding the potential benefits of using different paint roller designs or the possible adverse effects of painting at different work heights. The aim of this study was to investigate the influence of gender, work height, and paint tool design on shoulder muscle activity and exerted forces during wall painting. Ten young adults, five male and five female, were recruited to perform simulated wall painting at three different work heights with three different paint roller designs while upper extremity muscle activity and horizontal push force were recorded. Results demonstrated that for female participants, significantly greater total average (p = 0.007) and integrated (p = 0.047) muscle activity was present while using the conventional and curly flex paint roller designs compared to the proposed design in which the load was distributed between both hands. Additionally, for both genders, the high working height imposed greater muscular demands compared to middle and low heights. These findings suggest that, if possible, avoid painting at extreme heights (low or high) and that for female painters, consider a roller that requires the use of two hands; this will reduce fatigue onset and subsequently mitigate potential musculoskeletal shoulder injury risks.     

The effect of overhead drilling position on shoulder moment and electromyography

The effect of overhead drilling tasks on electromyographic (EMG) activity and shoulder joint moment was examined in this study. Twenty subjects simulated an overhead drilling task using a close, middle and far reach position while standing on either a lower or a higher step of a stepladder. Root mean square amplitude (AMP) of EMG activity from the dominant side anterior deltoid, biceps brachii and triceps brachii muscles was used to determine muscular load. Digital video was used to determine shoulder joint moment using 2-dimensional static link segment modelling in the sagittal plane. The results demonstrated that, compared to the far reach position, using the close reach position significantly decreased anterior deltoid AMP and biceps brachii AMP and moment, but increased triceps brachii AMP. Compared to the lower step, using the higher step significantly decreased anterior deltoid AMP and triceps AMP and moment, while increasing biceps AMP in the close position. There was no significant change noted in EMG median frequency indicating that fatigue was minimized. Moment increased monotonically with AMP. The findings indicated that workers performing overhead tasks should work close to their body in order to minimize shoulder forces. The implications of this recommendation are discussed.     

Modelling of shoulder and torso perception of effort in manual transfer tasks

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the 'sense of effort' is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.     

Problems and solutions in manual materials handling: the state of the art

For several decades manual materials handling has been a topic of interest in many fields of research. The reason for this interest is the devastating cost of and human suffering caused by injuries associated with MMH. Prevention and control of these injuries has been a concern shared by many researchers. In order to control the nature and extent of these injuries, limits must be established for MMH, especially lifting. This paper summarizes the variables affecting the ability to handle materials and the three basic approaches to determining capacities and limits for MMH: the biomechanical, the physiological, and the psychophysical approaches. The paper presents the various models developed in each of these approaches, together with a discussion of progress made and difficulties encountered in deriving capacity recommendations from each. Finally, the paper presents the latest recommendation on how these approaches can be integrated into a single comprehensive model for establishing lifting limits.     

An experimental investigation on push force and its perception during a flexible hose insertion task encountered in a truck assembly line

The push force and its perception when inserting a flexible hose laterally into a connector were investigated. Effects of hose diameter, glove, target position and obstacle condition were studied. Maximum voluntary insertion forces (MVFs) under similar working conditions were also measured. The larger the diameter, the higher the force required. The peak axial forces for the hoses of 6, 12 and 16 mm in diameter were on average respectively 94, 122 and 184 N, representing 45%, 61% and 93% of MVF. Glove condition, target position and obstacle did not significantly affect the axial insertion force and moment, but they did affect effort perception. Lower effort was perceived with gloves and high and near position. High intra- and inter-individual variability in insertion force for a given hose may suggest that feedback of successful insertion was insufficient. The recognition of a successful insertion must be ensured to avoid unnecessary extra force exertion.

Practitioner summary: The effects of glove, hose diameter, target location and obstacle on push force and its perception were studied when inserting a flexible hose. Solutions for improving the recognition of a successful insertion and the hose/connector system design must be found to reduce force exertion to safe levels.     

The effects of shoulder posture on neck and shoulder musculoskeletal loading and discomfort during smartphone usage

While using their smartphone, users tend to adopt awkward neck and shoulder postures for an extended duration. Such postures impose the risk of MSDs on those body parts. Numerous studies have been undertaken to examine neck posture; however, few studies have investigated shoulder postures. This study examined various shoulder postures during smartphone use and their effect on neck and shoulder kinematics, muscle loading, and neck/shoulder discomfort. Thirty-two asymptomatic young adult smartphone users randomly performed texting tasks for 3 min at four different shoulder flexion angles (15°, 30°, 45°, and 60°), while maintained a neck posture in the neutral position (0° neck flexion angle). Measures were taken of neck and shoulder muscle activity of the cervical erector spinae (CES), anterior deltoid (AD), upper trapezius (UT) and lower trapezius (LT), and kinematic data (angle, distance and gravitational moment). Results showed AD and LT muscle activity significantly increased when the shoulder flexion angle increased with an opposite effect on CES and UT. A recommended shoulder posture was identified as 30° flexion, as this yielded the best compromise between activation levels of the four muscles studied. This angle also induced the lowest neck/shoulder discomfort score. The findings suggest smartphone users hold their device at approximately 30° shoulder flexion angle with their neck in a neutral posture to reduce the risk of shoulder and neck musculoskeletal disorders when smartphone texting.Relevance to industrySmartphone use in the manufacturing and service industries is an integral part of work and useful means of communication tool. Awkward postures during extensive smartphone use impose an increased risk of both neck and shoulder musculoskeletal disorders. Shoulder flexion angles need consideration when making recommendations about safe work postures during smartphone use.     

Comparing the revised NIOSH lifting equation to the psychophysical,biomechanical and physiological criteria used in its development

The purpose of this study was to perform a systematic and comprehensive comparison between the recommended weights of lift (RWL) from the revised NIOSH lifting equation, and the specific biomechanical, psychophysical and physiological criteria that were stated to be used in the equation's development. A composite acceptable load (CAL) table was developed for the 216 conditions presented in the female lifting table of Snook and Ciriello (1991). Each CAL value was calculated to correspond to the most conservative load of the three design criteria. The RWLs caused lumbar compression forces close to the biomechanical limit of 3400 N for lifts below knuckle height, but resulted in compression forces that are much lower at higher lifting heights. At moderate lifting frequencies, the average RWL would be acceptable to more than 95% of female workers according to the psychophysical criterion. At the highest frequencies, the RWL was found to be an average of 63% of the CAL values based on the integration of the psychophysical and physiological criteria. It is recommended that ergonomists acknowledge the very conservative nature of the revised NIOSH lifting equation when using it to evaluate occupational lifting tasks.     

On the human perception of dissimilarities between postures of humanoids

This paper analyzes the perceived dissimilarity between postures of humanoid robots. First, the human perception of absolute distance between postures is focused. It is shown that results derived in computer graphics for human figures can be replicated with humanoids, despite the difference in body proportions and arrangement of the degrees of freedom. Successively, the perception of relative distances between postures is considered. It is shown that, paradoxically, better prediction of the distance between a pair of postures does not necessary lead to better predictions of which of multiple distances is the shortest. Finally, the paper concludes by briefly discussing possible implications of this finding to the fields of motion retrieval and motion blending.     

The perception of hazard I. Hazard analysis and the contribution of visual search to hazard perception

Rockfall accidents are an important determinant of safety performance in deep level mining. Previous research which is reviewed suggests that a lack of skill in perceiving warnings of imminent danger may be an important factor contributing to this kind of accident. The perceptual task of mine workers is described briefly and a survey of the visual characteristics of dangerous rock is reported upon. The visual search performance of mine workers in a stope simulator, designed on the basis of the survey results, was studied. The findings show not only that novice mine workers, when compared with experienced men, lack the ability to search adequately in a simulation of dangerous rock, but also that their search skills can be improved significantly by training.     

Manual material handling guidelines for the shoulder: Biomechanical support for the Liberty Mutual Tables as developed by Snook and Ciriello

Stover Snook and Vincent Ciriello laid the groundwork for psychophysical material handling guidelines in the 1970s. Since then, further research into psychophysical guidelines has been performed by numerous researchers. However, there still exists a gap between psychophysical and biomechanical guidelines. Snook and Ciriello's work eventually led to development of the Liberty Mutual Tables to reduce low-back pain episode in workers due to MMH tasks. Epidemiological evidence indicates pushing tasks may be more related to shoulder pain than low-back pain. A novel approach to protecting worker's shoulder complex by comparing the Liberty Mutual Table guidelines for pushing tasks to biomechanically derived pushing guidelines is presented. These biomechanically derived guidelines are based on muscle activation levels of the subscapularis muscle as determined using a biomechanical model of the shoulder complex. The subscapularis muscle may be a marker for subacromial impingement syndrome. In general, the psychophysical guidelines and the biomechanical guidelines achieve general agreement with respect to magnitude and shape. Differences between the two models range from 6 to 67%.     

Relations between shoulder/neck disorders and EMG zero crossing shifts in female assembly workers using the test contraction method

Pain and discomfort in the shoulder/neck region is a serious problem amongst workers in many categories of assembly work. It has been suggested that these disorders are related to local muscular fatigue. In this study 14 female assembly workers were studied. Several of these women had occupational shoulder/neck disorders of various degrees. The method (FTC), applied during work, involved short test contractions every ten minutes during which EMG from m. trapezius and m. infraspinatus bilaterally was recorded. The EMG was analysed using the zero crossing technique. The subjects filled in a standard questionnaire concerning their muscular shoulder/neck disorders. It is shown that subjects reporting disorders have more pronounced EMG alterations than those without disorders.     

Free shoulder space requirements in the design of high backrests

The objective of this study was to determine the influence of scapular support on the effects of lumbar support and to prove that a high and straight backrest is inappropriate. In literature the importance of a lumbar support is noted, although data about optimal dimensions is an under-researched topic and in earlier studies on force distribution and muscle activity the backrest had a fixed form. The lumbar support is needed to maintain the lumbar lordosis but no studies deal with the question of the precise dimensions of the backrest at shoulder level. With a specially designed apparatus, forces on shoulder and seat were measured separately, and the force on the pelvis calculated, while varying seat and backrest inclination within the range from 0° to 17°. Seat-to-backrest angle (at the level of lumbar support) was kept constant at 90°. The distance between the tangent to the lumbar support and the parallel tangent to the scapular support was varied from 0, 2, 4, 6 and 8 cm. This distance is called the free shoulder space. Electromyography was measured at the erector spinae at the levels of the L1, T8 and T5 vertebrae. For all seat angles, a free shoulder space of d=0 cm resulted in the highest back muscle activity. In agreement with the biomechanical model, EMG activity reduced with an increase of seat tilt and increase of free shoulder space. With increasing free shoulder space, a larger part of the total backrest force was carried by the lumbar support. This study shows that a high and straight backrest overrules lumbar support. Offering free shoulder space of at least 6 cm reduces back muscle activity and allows for lumbar support.     

The influence of the search complexity and the familiarity with the website on the subjective appraisal of aesthetics,mental effort and usability

The user experience is defined as ‘a person's perceptions and responses that result from the use and/or anticipated use of a product, system or service’ (ISO FDIS 9241-210, 2009 ISO FDIS 9241-210, 2009. Ergonomics of human system interaction Ergonomics of human system interaction – Part 210: human-centered design for interactive systems (formerly known as 13407). Switzerland: International Organization for Standardization. [Google Scholar]) [Ergonomics of human system interaction Ergonomics of human system interaction – Part 210: human-centered design for interactive systems (formerly known as 13407). Switzerland: International Organization for Standardization]. Accordingly, some authors have argued that an interactive system has to be evaluated not only with regard to its usability and utility levels, but also with regard to emotional, attractiveness, and aesthetic levels. These last aspects play a substantial role on the general assessment of such systems and on the satisfaction of users. Some studies focused on the immediate aesthetic subjective perception of systems, on their subjective usability and preference perceptions. However, few studies, at least to our knowledge, have been focused on the reverse, that is, on the effect of difficulties experienced by individuals in using systems on the aesthetic appraisal. The present study aimed at determining the role of familiarity level with the website and the search complexity on the search performance and post-experiment appraisals of aesthetics, usability, and mental effort. The main results revealed that the search complexity affected negatively search performance, whereas the familiarity level affected only the re-reading of the search questions. The post-use assessments of aesthetics, mental effort, and usability satisfaction were affected by search performance. In addition, these variables were correlated except the expressive aesthetics, which seemed to be independent from the search performance and other subjective appraisals. Then, we discuss these findings in line with prior studies and present future ways of research.     

Phase change materials and the perception of wetness

Phase change materials (PCMs) are increasingly incorporated in textiles in order to serve as a thermal buffer when humans change from a hot to a cold environment and the reverse. Due to the absence of wetness sensors in the skin, cooling of the skin may be perceived as a sensation of wetness instead of cold. In order to investigate if this phenomenon occurs when manipulating textiles, nine subjects were asked to touch or manipulate PCM-treated and untreated fabrics. In 75% of the cases, the subjects indicated that the treated material felt wetter than the untreated material independent of the way the textiles were manipulated. We conclude that incorporating PCMs in textiles may lead to a feeling of wetness which might be uncomfortable. Therefore, we recommend investigating a change in cooling properties to minimise this feeling. PRACTITIONER SUMMARY: This article describes a psychophysical experiment into the sensation of wetness of textiles treated with phase change materials. It was found that in 75% of the cases, subjects found the treated fabric to feel wetter than the untreated. This may affect the comfort of wearing clothes made of these textiles.     

Interfering effects of the task demands of grip force and mental processing on isometric shoulder strength and muscle activity

The purpose of this study was to examine the interfering effects of physical and mental tasks on shoulder isometric strength in different postures. Fifteen volunteers (seven women, eight men) performed a series of isometric shoulder exertions at 30°, 60° and 90° of both shoulder flexion and abduction alone and with the addition of a 30% grip force, a mental task (Stroop test) and both additional tasks simultaneously. The shoulder tasks were completed either at maximal intensity, or while maintaining a shoulder posture without any additional effort. Surface electromyography (EMG) from seven muscles of the shoulder girdle and shoulder moment were collected for each 6 s shoulder exertion. When normalized to maximum exertion, no differences were found between genders and no differences existed between conditions when subjects maintained each posture without exerted force. In the maximal shoulder exertion trials, an increase in shoulder angle (in either plane) resulted in an increase in EMG in most muscles, while shoulder moment decreased in flexion and remained constant in abduction. Shoulder moments and muscle activation were greatest in the shoulder exertion alone condition followed by adding a 30% grip and the Stroop test, with the addition of both tasks further reducing the exerted shoulder moment and EMG. However, muscle activity did not always decrease with shoulder strength and remained elevated, indicating a complex coactivation pattern produced by an interfering role of the tasks. Overall, it was found that a mental task can have the same or greater effect as a concurrent grip and should be considered when assessing muscular loading in the workplace, as typical biomechanical modelling may underestimate internal loads. The results not only provide valuable shoulder strength data but also practical strength values, depending on additional tasks.