Physical Workload in Different Body Postures,While Working Near to,or Below Ground Level

The physical load of different working postures during a light job at low working level was compared while varying the frequency of forward movement and working level. The postures compared were bending, kneeling, squatting and sitting on a low stool. Energy expenditure, heart rate and the increase in heart rate over a five minutos' period were used as parameters. The following conclusions can be drawn. Bending and kneeling require less energy and arc less strenuous, when one hand (arm) is used as a support to balance the trunk. At high forward movement frequencies (more than 4 metres per minute or 5 movements per minute) the bending posture without arm support is loss strenuous than squatting and sitting on a one-legged stool. At low movements frequencies the squatting posture is preferable. Heart rate and energy consumption were slightly higher than that when sitting, but performance and also the gradient of the heart rate were slightly more favourable. The working height, varied from ? 14 cm to + 16 cm in comparison with the floor level, influenced the workload less. A remarkable increase in the work load was observed in the bending position only when the working level was lower than the level of the feet. If work has to be carried out in the ground itself, then squatting appears to be the most favourable position.     

Electromyography of the thigh muscles during lifting tasks in kneeling and squatting postures

Underground coal miners who work in low-seam mines frequently handle materials in kneeling or squatting postures. To assess quadriceps and hamstring muscle demands in these postures, nine participants performed lateral load transfers in kneeling and squatting postures, during which electromyographic (EMG) data were collected. EMG activity was obtained at five points throughout the transfer for three quadriceps muscles and two hamstring muscles from each thigh. ANOVA results indicated that EMG data for nine of 10 thigh muscles were affected by an interaction between posture and angular position of the load lifted (p < 0.001). Muscles of the right thigh were most active during the lifting portion of the task (lifting a block from the participant's right) and activity decreased as the block was transferred to the left. Left thigh muscles showed the opposite pattern. EMG activity for the majority of thigh muscles was affected by the size of the base of support provided by different postures, with lower EMG activity observed with a larger base of support and increased activity in postures where base of support was reduced (p < 0.05). Thigh EMG activity was lowest in postures with fully flexed knees, which may explain worker preference for this posture. However, such postures are also associated with increased risk of meniscal damage. STATEMENT OF RELEVANCE: Kneeling and squatting postures are sometimes used for manual lifting activities, but are associated with increased knee injury risk. This paper examines the EMG responses of knee extensors/flexors to lifting in these postures, discusses the impact of posture and kneepads on muscle recruitment and explores the implications for work in such postures.     

Task-specific postures in low-seam underground coal mining

The objective of this study was to determine low-seam mine worker exposure to various postures as they pertain to job classifications and job tasks. Sixty-four mine workers from four low-seam coal mines participated. The mine workers reported the tasks they were required to complete and the two postures they used most frequently to perform them. They were provided with a schematic of postures from which to select. The two postures reported most frequently were identified for each task along with the job classification of the workers performing the tasks. Of the 18 tasks reported, over two thirds were performed by at least two different job classifications and over one third were performed by four or more job classifications. Across tasks, the postures used appeared to vary greatly. However, when grouped by job classification, the most frequently reported posture across all job classifications was kneeling near full flexion. Operating the continuous miner was associated with frequent squatting and was likely used because it affords great mobility, allowing operators to move quickly to avoid hazards. However, for environments with a restricted vertical height such as low-seam mining, the authors recommend squatting be avoided as data demonstrates that large amounts of femoral rollback and high muscle activity for the extensors when performing lateral lifts in this posture. Kneeling near full flexion was reported as the most frequently used posture by all job classifications and was likely due to the fact that it requires the least amount of muscle activity to maintain and has reduced pressures at the knee. However, the authors recommend this posture be avoided when performing lateral lifting tasks. Like squatting, kneeling near full flexion results in increased femoral rollback and may increase the stresses applied to the meniscus. Unlike lateral lifting, maintaining a static posture results in knee loading and muscle activity such that the mine worker should consider kneeling near full flexion and sitting on their heels. Although kneeling near full flexion is associated with injuries, there are benefits to this posture that are realized when statically kneeling (minimal muscle activity, allows worker to maintain an upright torso in low heights, and decreased loading at the knee). However, cartilage is avascular and nourished by synovial fluid. Therefore, one should frequently rotate between postures, assuming a more upright kneeling posture when possible and frequently fully flexing and extending the knee allowing nutrients to the cartilage.Relevance to industryIn 2009, over one fourth of underground coal mines that produced coal in the United States were considered low seam with an average working height of <109.2 cm (MSHA, 2009) restricting workers to their knees. Data exists regarding the biomechanical implications of kneeling postures and demonstrates the possibility of detrimental consequences to varying degrees for each posture. With each posture posing a different level of exposure to musculoskeletal disorder risk factors, it is essential to determine the postures mine workers use to perform their job tasks and how their postural options are restricted by the low-seam underground mining environment.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible—particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p< 0.001) low back compressive loads (mean±SD 1698±467 N) than those experienced by the lumbar spine during standing (1076±243 N). Subjects were equally divided into adopting one of two sitting strategies: a single ‘static’ or a ‘dynamic’ multiple posture approach. Within each individual, standing produced a distinctly diVerent spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level diVerences in the individuals who adopted diVerent sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.     

EMG analysis of sitting work postures in women

Based on EMG analysis on six middle-aged women, seven sitting work postures were compared with reference to relaxed standing position. The muscles included in the investigation are pectoralis major, levator scapulae, deltoideus, latissimus dorsi, upper fibres of the trapezius, erector spinae, vastus medialis and lateralis, rectus femoris and gastrocnemius muscles. The commoner sitting postures examined were sitting on the floor with crossed legs, sitting on the floor with right leg bent at the knee, sitting on the floor with left leg bent at the knee, squatting with both legs bent at the knee without any back support, sitting on a plank of 10 cm height with both legs bent at the knee, sitting on the floor with legs extended, and sitting upright on a stool of 40 cm height. The relative load on the muscles was highest in the case of sitting on the floor with the right leg bent at the knee. However, less muscle activity was noted when sitting on the floor with the legs extended, a posture commonly adopted by women performing various domestic and other activities.     

Effects of common working postures on balance control during the stabilisation phase of transitioning to standing

Standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to assess the magnitude of this imbalance. Forty-five male participants completed three replications of conditions created by four static postures and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Body segment location and displacement of the centre of pressure (COP) were recorded using a motion capture system and two forceplates, respectively. Balance control measures were calculated during the stabilisation phase. All balance control measures were significantly affected by static posture but not duration within posture. Bending over at waist generally caused the smallest changes in balance control measures, whereas the reclined kneeling posture resulted in the largest. Findings may lead to recommendations for redesign of tasks to reduce the use of certain working postures, particularly in high-risk environments such as construction. STATEMENT OF RELEVANCE: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that the working posture chosen affects stabilisation during a transition to a standing position. Bending at the waist or squatting seems to have less of an affect on balance control measures, whereas both types of kneeling postures evaluated resulted in greater imbalance.     

Dynamic biomechanical modelling of symmetric and asymmetric lifting tasks in restricted postures

This article describes investigations of dynamic biomechanical stresses associated with lifting in stooping and kneeling postures. Twelve subjects volunteered to participate in two lifting experiments each having two levels of posture (stooped or kneeling), two levels of lifting height (350 or 700 mm), and three levels of weight (15,20, or 25 kg). One study examined sagitally symmetric lifting, the other examined an asymmetric task. In each study, subjects lifted and lowered a box every 10 s for a period of 2 min in each treatment combination. Electromyography (EMG) of eight trunk muscles was collected during a specified lift. The EMG data, normalized to maximum extension and flexion exertions in each posture, was used to predict compression and shear forces at the L3 level of the lumbar spine. A comparison of symmetric and asymmetric lifting indicated that the average lumbar compression was greater in sagittal plane tasks; however, both anterior-posterior and lateral shear forces acting on the lumbar spine were increased with asymmetric lifts. Analysis of muscle recruitment indicated that the demands of lifting asymmetrically are shifted to ancillary muscles possessing smaller cross-sectional areas, which may be at greater risk of injury during manual materials handling (MMH) tasks. Model estimates indicated increased compression when kneeling, but increased shear forces when stooping. Increasing box weight and lifting height both significantly increased compressive and shear loading on the lumbar spine. A multivariate analysis of variance (MANOVA) indicated complex muscle recruitment schemes—each treatment combination elicited a unique pattern of muscle recruitment. The results of this investigation will help to evaluate safe loads for lifting in these restricted postures.     

Lifting in four restricted lifting conditions: psychophysical, physiological and biomechanical effects of lifting in stooped and kneeling postures

Many underground coal mines are less than 1.2 m in height, and require that manual lifting tasks be performed in restricted postures (usually stooped or kneeling). Unfortunately, little is known about how these postures affect the underground coal miner's capabilities to perform such work. A previous Bureau of Mines study indicated that lifting capacity is greater when stooped than kneeling when lifting under a 1.2 m roof height (Gallagher et al, 1988). However, many low-seam coal mines present even more restricted headroom than 1.2 m. Therefore, a study was conducted to: (1) examine the psychophysical lifting capacity of low-seam coal miners under four restricted lifting conditions, (2) investigate the associated metabolic costs, and (3) analyse the electromyography (EMG) of trunk muscles when lifting in these restricted positions. Subjects were 12 coal miners accustomed to handling materials in restricted postures (mean age = 35.9 yrs +/- 6.4 SD). Results of this study indicated that lifting capacity was greater when the subjects could assume a stooped posture than when kneeling (p < 0.01). Furthermore, the metabolic cost was greater in the kneeling posture for heart rate (p < 0.001), oxygen consumption (p < 0.001), minute ventilation (p < 0.05] ), and respiratory exchange ratio (p < 0.05), despite the fact that miners lifted less weight in this posture. Analysis of the EMG data indicated increased activity of the electores spinae when kneeling (p < 0.001), but higher latissimus dorsi activity when stopped (p < 0.001). The findings of this study indicate that the weight of supply items should be reduced approximately 14-18% when the kneeling posture must be used for lifting.     

The effects of knee angles on subjective discomfort ratings, heart rates, and muscle fatigue of lower extremities in static-sustaining tasks

The purpose of this study was to investigate the effects of knee-flexion angles on subjective discomfort ratings, heart rates, and muscle fatigue using median frequency (MDF) in a static-sustaining task. Thirty healthy participants maintained 13 postures including standing, squatting, sitting, and kneeling postures and then MDFs from the erector spinae, biceps femoris, vastus medialis, gastrocnemius, and tibialis anterior muscles, subjective discomfort, and heart rates were collected every 3 min during a sustained 15 min task. Results showed that the discomfort, heart rate and muscle fatigue were significantly influenced by the body postures. In general, standing and sitting postures showed less discomfort as well as lower heart rates, whereas squatting postures (KF120, KF90, KF60) had higher discomfort and heart rates. Three MDF change trends were reported associated with postures in this study. First, there were less changes of MDFs for standing and sitting postures; second, all patterns of MDFs for KF 150 and KF120 decreased, and lastly some MDFs had increasing trends and others showed decreasing trends for KF30, KF30T, and kneeling postures.     

The effects of knee angles on subjective discomfort ratings,heart rates,and muscle fatigue of lower extremities in static-sustaining tasks

The purpose of this study was to investigate the effects of knee-flexion angles on subjective discomfort ratings, heart rates, and muscle fatigue using median frequency (MDF) in a static-sustaining task. Thirty healthy participants maintained 13 postures including standing, squatting, sitting, and kneeling postures and then MDFs from the erector spinae, biceps femoris, vastus medialis, gastrocnemius, and tibialis anterior muscles, subjective discomfort, and heart rates were collected every 3 min during a sustained 15 min task. Results showed that the discomfort, heart rate and muscle fatigue were significantly influenced by the body postures. In general, standing and sitting postures showed less discomfort as well as lower heart rates, whereas squatting postures (KF120, KF90, KF60) had higher discomfort and heart rates. Three MDF change trends were reported associated with postures in this study. First, there were less changes of MDFs for standing and sitting postures; second, all patterns of MDFs for KF 150 and KF120 decreased, and lastly some MDFs had increasing trends and others showed decreasing trends for KF30, KF30T, and kneeling postures.     

Effects of posture on dynamic back loading during a cable lifting task

This study evaluated spinal loads associated with lifting and hanging heavy mining cable in a variety of postures. This electrical cable can weigh up to 10 kg per metre and is often lifted in restricted spaces in underground coal mines. Seven male subjects performed eight cable lifting and hanging tasks, while trunk kinematic data and trunk muscle electromyograms (EMGs) were obtained. The eight tasks were combinations of four postures (standing, stooping, kneeling on one knee, or kneeling on both knees) and two levels of cable load (0 N or 100 N load added to the existing cable weight). An EMG-assisted model was used to calculate forces and moments acting on the lumbar spine. A two-way split-plot ANOVA showed that increased load (p < 0.05) and changes in lifting posture (p < 0.05) independently affected trunk muscle recruitment and spinal loading. The increase in cable load resulted in higher EMG activity of all trunk muscles and increased axial and lateral bending moments on the spine (p < 0.05). Changes in posture caused more selective adjustments in muscle recruitment and affected the sagittal plane moment (p < 0.05). Despite the more selective nature of trunk EMG changes due to posture, the magnitude of changes in spinal loading was often quite dramatic. However, average compression values exceeded 3400 N for all cable lifting tasks.     

Safe levels of manual forces for young males (1)

This is the first of a three-part series in which diagrams show the single loads or forces which can be applied safely by fit young males, standing or squatting with the back erect. Subsequent sections will cover other standing, sitting and kneeling contours.     

Stabilisation times after transitions to standing from different working postures

Transitioning to standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to quantify the magnitude of imbalance using a stabilisation time metric. Forty-five male participants completed three replications of conditions created by one of four working postures (bent at waist, squat, forward kneel, reclined kneel) and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Stabilisation time, based on changes in centre of pressure velocity, was used to indicate the initiation of steady state while standing. Stabilisation time was significantly affected by static postures but not duration within posture. The largest stabilisation times resulted from transitions initiated from a bent at waist posture. The smallest were associated with the kneeling postures, which were not significantly different from each other. Findings may lead to recommendations for redesign of tasks, particularly in high-risk environments such as construction.

Statement of Relevance: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that working posture affects stabilisation during transition to a standing position. Bending at the waist and squatting resulted in longer stabilisation times, whereas both kneeling postures evaluated resulted in greater imbalance but for a shorter duration.     

Effects of posture on dynamic back loading during a cable lifting task

This study evaluated spinal loads associated with lifting and hanging heavy mining cable in a variety of postures. This electrical cable can weigh up to 10 kg per metre and is often lifted in restricted spaces in underground coal mines. Seven male subjects performed eight cable lifting and hanging tasks, while trunk kinematic data and trunk muscle electromyograms (EMGs) were obtained. The eight tasks were combinations of four postures (standing, stooping, kneeling on one knee, or kneeling on both knees) and two levels of cable load (0 N or 100 N load added to the existing cable weight). An EMG-assisted model was used to calculate forces and moments acting on the lumbar spine. A two-way split-plot ANOVA showed that increased load (p<0.05) and changes in lifting posture (p<0.05) independently affected trunk muscle recruitment and spinal loading. The increase in cable load resulted in higher EMG activity of all trunk muscles and increased axial and lateral bending moments on the spine (p<0.05). Changes in posture caused more selective adjustments in muscle recruitment and affected the sagittal plane moment (p<0.05). Despite the more selective nature of trunk EMG changes due to posture, the magnitude of changes in spinal loading was often quite dramatic. However, average compression values exceeded 3400 N for all cable lifting tasks.     

Where Does the Mouse Go? An Investigation into the Placement of a Body-Attached TouchPad Mouse for Wearable Computers

We investigated the effects of placement of a TouchPad input device on a user's body for the control of a wearable computer. This study involved 25 subjects performing selection tasks with a TouchPad mouse while wearing a wearable computer on their back and using a head-mounted display. Each subject performed the tasks in 27 different combinations of four postures (sitting, kneeling, standing and prone) and seven different placements of the TouchPad mouse on the subject's body (forearm, thigh by 2, torso by 2, and upper arm by 2). We measured the time and error rate to complete the selection of a circular target. The results for the effects due to posture showed that there were similar time effects for sitting, standing and kneeling. When examining the effects resulting from mouse position, the front of the thigh was shown to be the best position of the mouse. When the posturing and mouse position conditions were combined, the results indicated that the thigh front mouse position would be most appropriate for sitting, kneeling and standing postures, and the forearm mouse position would be best for the prone position.     

Static and dynamic postural loadings during computer work in females: Sitting on an office chair versus sitting on an exercise ball

Seated computer work results in prolonged static loading, which has been associated with the development of musculoskeletal disorders. A popular alternative to sitting on an office chair while performing computer work is to sit on an exercise ball. Sitting on an exercise ball might affect static and dynamic aspects of working posture. We monitored posture, muscle activation and spinal shrinkage in 10 females performing a 1-h typing task, while sitting on an office chair with armrests and while sitting on an exercise ball. Sitting on an exercise ball resulted in 33% more trunk motion and in 66% more variation in lumbar EMG. Both of these findings can be considered to be an advantage for the exercise ball. However, the fifth percentile and average lumbar EMG were also higher when sitting on an exercise ball, with 38% and 78%, respectively. In addition, more spinal shrinkage occurred when sitting on an exercise ball than when sitting on an office chair. Arm flexion was reduced, but trapezius activation was unaffected when sitting on an exercise ball. It is concluded that the advantages with respect to physical loading of sitting on an exercise ball may not outweigh the disadvantages.     

A study stressing the need for a static postural force model for work analysis

The maximum endurance time (MET) in static force exertions was used as a parameter for the assessment of five working postures. By applying the methodology of Rohmert to the construction of a general model for static muscular work and evaluating the measured MET results, the need for a new static posture model has been shown.

The aim of the present pilot study was to test MET in load situations that would indicate when the general model can be used or when a new static postural force model is needed.

Subjects exerted static postural forces at different load levels until exhaustion. In the first two postures, the strain was concentrated on the upper limbs, where active forces (muscular) play a key-role and justify the use of the model. In the remaining postures, the strain affected mainly the back/trunk, where the mechanical equilibrium of the body is brought about by active (muscles) and passive (skeleton and ligaments) structures.

During the tests electromyographic (EMG) measurements of selected muscles (objective measurements) as well as rated perceived exertion (RPE; subjective measurements on Borg's CR-10 scale) were recorded.

The results show that the maximum endurance times in upper extremity postures are predicted by the general model whilst in the back/trunk postures the measured MET was longer than predicted by the model. New models are presented for static postural force on the back.

The EMG measurements supported the conclusion that the muscles studied play a key role in the chosen upper-extremity postures but gave no clear indication in the back-oriented postures. Ratings of perceived exertion coincided with the EMG measurements in upper extremity postures and proved to be a good substitute for measurement and calculation of the load levels studied. The initial RPE can therefore be used in models for predicting maximal endurance times in complex cases.

For the range of relative postural loads tested, an exponential function for predicting MET in static posture exertions produced the best fit curve.     

Comparison of stoop versus prone postures for a simulated agricultural harvesting task

Physical and psychophysical differences between working in the stooped and prone postures were compared while performing a simulated agricultural harvesting task for 30 min. Fifteen male subjects participated. The measures used to compare the two postures included perceived discomfort, electromyography (EMG), and heart rate (HR). Average hamstrings localized discomfort (0-10 scale) was 6.17 (SD=2.9) for the stoop posture and 0.67 (SD=1.29) for the prone posture. Erector spinae and hamstring EMG RMS increased 68% and 18%, respectively, while mean power frequency for the hamstrings decreased 13% for the stoop task. Mean power frequency for the middle trapezius muscle decreased in both postures (stoop 4.13%, prone 3.79%). Average heart rate during the last work cycle was 35% greater than the resting heart rate for the stoop posture while average heart rate was 17% greater for the prone posture. Subjects worked on the prone workstation without rest during the 15 min work simulations with less discomfort, no localized fatigue in the back or leg muscles tested, and lower working heart rates than subjects working in the stoop posture.     

Work activities and musculoskeletal complaints among preschool workers

The potential for musculoskeletal trauma among preschool workers has been largely unexplored in the United States. This case report describes an investigation conducted to identify and evaluate possible causes of back and lower extremity pain among 22 workers at a Montessori day care facility. Investigators met with and distributed a questionnaire to school employees, and made measurements of workstation and furniture dimensions. Investigators also recorded the normal work activities of school employees on videotape, and performed a work sampling study to estimate the percentage of time employees spend performing various tasks and in certain postures.

Questionnaire results from 18 employees indicated that back pain/discomfort was a common musculoskeletal complaint, reported by 61% of respondents. Neck/shoulder pain, lower extremity pain and hand/wrist pain were reported by 33, 33 and 11% of respondents, respectively. Observation and analysis of work activities indicated that employees spend significant periods of time kneeling, sitting on the floor, squatting, or bending at the waist. Furthermore, staff members who work with smaller children (i.e. six weeks to 18 months of age) performed more lifts and assumed more awkward lower extremity postures than employees who work with older children (3–4 years of age). Analysis of two lifting tasks using the revised NIOSH lifting equation indicated that employees who handle small children may be at increased risk of lifting-related low back pain. Investigators concluded that day care employees at this facility are at increased risk of low back pain and lower extremity (i.e. knee) injury due to work activities that require awkward or heavy lifts, and static working postures. Recommendations for reducing or eliminating these risks by modifying the workplace and changing the organization and methods of work are presented.