The effects of posture on forearm muscle loading during gripping

The purpose of this study was to quantify the response of the forearm musculature to combinations of wrist and forearm posture and grip force. Ten healthy individuals performed five relative handgrip efforts (5%, 50%, 70% and 100% of maximum, and 50 N) for combinations of three wrist postures (flexed, neutral and extended) and three forearm postures (pronated, neutral and supinated). ‘Baseline’ extensor muscle activity (associated with holding the dynamometer without exerting grip force) was greatest with the forearm pronated and the wrist extended, while flexor activity was largest in supination when the wrist was flexed. Extensor activity was generally larger than that of flexors during low to mid-range target force levels, and was always greater when the forearm was pronated. Flexor activation only exceeded the extensor activation at the 70% and 100% target force levels in some postures. A flexed wrist reduced maximum grip force by 40-50%, but EMG amplitude remained elevated. Women produced 60-65% of the grip strength of men, and required 5-10% more of both relative force and extensor activation to produce a 50 N grip. However, this appeared to be due to strength rather than gender. Forearm rotation affected grip force generation only when the wrist was flexed, with force decreasing from supination to pronation (p<0.005). The levels of extensor activation observed, especially during baseline and low level grip exertions, suggest a possible contributing mechanism to the development of lateral forearm muscle pain in the workplace.     

Wrist and forearm postures and motions during typing

Awkward upper extremity postures and repetitive wrist motions have been identified by some studies as risk factors for upper extremity musculoskeletal disorders during keyboard work. However, accurate body postures and joint motions of typists typing on standardized workstations are not known. A laboratory study was conducted to continuously measure wrist and forearm postures and motions of 25 subjects while they typed for 10 – 15 min at a standard computer workstation adjusted to the subjects' anthropometry. Electrogoniometers continuously recorded wrist and forearm angles. Joint angular velocities and accelerations were calculated from the postural data. The results indicate that wrist and forearm postures during typing were sustained at non-neutral angles; mean wrist extension angle was 23.4 ± 10.9 degrees on the left and 19.9 ± 8.6 degrees on the right. Mean ulnar deviation was 14.7 ± 10.1 degrees on the left and 18.6 ± 5.8 degrees on the right. More than 73% of subjects typed with the left or right wrist in greater than 15 degrees extension and more than 20% typed with the left or right wrist in greater than 20 degrees ulnar deviation. Joint angles and motions while typing on an adjusted computer workstation were not predictable based on anthropometry or typing speed and varied widely between subjects. Wrist motions are rapid and are similar in magnitude to wrist motions of industrial workers performing jobs having a high risk for developing cumulative trauma disorders. The magnitude of the dynamic components suggests that wrist joint motions may need to be evaluated as a risk factor for musculoskeletal disorders during typing.     

Relation between intra-abdominal pressure and lumbar moments when lifting weights in the erect posture

Abstract

Several investigations have shown that during physical activity there is a relationship between the magnitude of trunk stresses and increases in intraabdominal pressure (IAP). This study was undertaken to quantify this relationship for moments acting at lumbar level (L4/5) during lifting activities. Fourteen young males performed a series of 60 bimanual lifts in the sagittal plane in 12 different hand positions, while standing in an erect posture. In each hand position, loads ranging from 59 to 706 N were selected in order to apply identical forces at the shoulder, whatever the hand-shoulder distance. The moment about L4/5 was determined through a biomechanical model deriving data from the subject's anthropometry and the photographically recorded posture.

Results showed that IAP was well correlated with the lumbar moment in all the hand positions but one, whose postural configuration put some limitations on the exertion of force. When data from that position were excluded, IAP (kPa) was related to the moment (Nm) at L4/5 level by y=0·079x?1127 (r=0·75). IAP measurement may thus be used as an index of spinal stress in real-life lifting tasks.     

Co-activation and maximal EMG activity of forearm muscles during key tapping

Many workers with repetitive motion tasks develop work related musculoskeletal diseases. In this study, the impact of required forces in dynamic key touch pressure and key displacements on forearm extensor and flexor muscles was investigated. The aims were to evaluate the co-activation of forearm extensor and flexor muscles during a 2 min tapping task at 4 Hz and to assess possible changes in maximal surface electromyography (sEMG) activation during tapping using different keys with variable force–displacement characteristics. 13 subjects took part in the experiment and performed ten tapping sequences, using a different key make (with different force-displacement characteristics) each time. Two channels of sEMG were used to monitor forearm muscle activation. Results showed (a) that the co-activation of forearm extensor and flexor muscles increases with rising key makes force, and (b) that coordination between extensor and flexor muscles improves over time.The findings indicate that keyboards with make-force levels over 0.59 N are ergonomically inadequate. Differences in key displacement were less important.

Relevance to industry

Chronic pain in the forearm due to repetitive tasks is common among employees working with computers. This paper presents and discusses results showing higher muscle activity and bad intramuscular coordination when using high make-force keys.     

Lower back muscle forces in pushing and pulling

Abstract

In the investigation of lower back stress, the muscle forces of the erector spinae and the rectus abdominis are often calculated using the two-dimensional biomechanical model. These muscle forces are used to estimate the compressive forces at L5/S1 disc This paper presents a study of the muscle forces predicted by a two-dimensional biomechanical model during pushing and pulling and myoelectric activity from the corresponding muscles. The goal was to investigate whether a simple two muscle torso model would reasonably estimate the muscle actions in pushing and pulling tasks. Six subjects participated in the experiment. EMG (rms) value was used as an indicator of muscle forces. The results show high correlation between the predicted muscle forces and the measured root-mean-square EMG values in trunk pushing and pulling (r²=0.93) and hand pushing and pulling (r²=0.96) in an erect posture with hips braced but low in hand pushing and pulling using a free posture (r²=0.37).     

Source document position as it affects head position and neck muscle tension

Abstract

Measurements of head position, mechanical load at C7, and level of muscle EMG activity in the muscles of the neck were compared across six document positions in both a reading task and a typing (word processing) task. Source documents, identified as a primary visual task, were placed in two positions in front of the subject, flat on the table to each side of the keyboard, and on a document stand at each side of the keyboard. Each of the 20 subjects performed both a reading task and a typing task in each of these document positions while measurements were taken. It was found that the greatest variations in head position occurred in head rotation with documents flat on the table. This document position on each side also produced the greatest level of muscle tension in both the neck extensors and, to a lesser degree, the sternocleidomastoid muscles. In addition, greater variability between document position measures was found in the typing condition.     

Muscle activity and range of motion during active trunk rotation in a sitting posture

Twisted trunk postures during tractor driving are associated with low-back pain. The purposes of this study were to quantify the muscle activity as a function of twisting angle, to quantify the range of motion (ROM) during active trunk rotation and to determine whether there were any differences between tractor drivers and office workers and between twisting direction for these variables. The subjects performed exertions in a seated position, twisting from the neutral position to the end of the ROM. The results showed that external oblique and erector spinae had significantly different activation patterns depending on twisting direction. For the contralateral external oblique and the ipsilateral erector spinae, the muscle effort required to twist the trunk was low up to about 20° twisting angle, then the muscle effort needed to twist the trunk increased progressively. No significant differences due to occupation or twisting direction were found. The result implies that work in twisted trunk postures might be a risk factor for low-back pain.     

Effects of changes in sitting work posture on static neck and shoulder muscle activity

Abstract

In order to analyse the effect of changing the sitting posture on the level of neck and shoulder muscular activity, an electromyographic (EMG) study of ten healthy experienced female workers from an electronics plant was undertaken. A standardized, simulated task was performed in eight different sitting work postures. Using surface electrodes, the level of muscular activity was recorded as normalized, full-wave rectified low-pass filtered EMG. The results showed that the whole spine flexed sitting posture gave higher levels of static activity in several neck and shoulder muscles than the posture with a straight and vertical spine, which in turn gave higher levels than the posture with slightly backward-inclined thoraco-lumbar spine.     

The influence of desk and display design on posture and muscle activity variability whilst performing information technology tasks

Desk design and computer display height can affect posture and muscle activation during computer use. Amplitudes of postural variables and muscle activity during computer use do not explain the results from epidemiological studies of musculoskeletal discomfort and disorders related to computer use. The purpose of this study was to assess variability of posture and muscle activity during work with two computer display heights and book/paper, in conjunction with a curved desk designed to provide forearm support and a traditional, straight desk.18 male and 18 female participants performed 10-min tasks involving keying, mousing, reading and writing in six desk/display conditions. 3D posture and surface emg were assessed for the final 2 min of each task.The curved desk resulted in greater postural and muscle activity variation, suggesting an advantage of this supportive surface over the straight desk. There was little difference in variability associated with the two display heights. However, greater variability of posture and muscle activity was evident with the book/paper condition. Non-touch typists had greater neck flexion variation.The design of information technology tasks and workstations can influence the short term variation in posture and muscle activity. Variation is influenced independently of mean postures and muscle amplitudes and therefore needs to be considered to adequately assess the risk of musculoskeletal disorders.     

Perceived safety and biomechanical stress to the lower limbs when stepping down from fire fighting vehicles

Injuries related to emergency vehicles represent 19% of compensated work accidents for fire fighters, 37% of which occur while stepping down from their vehicles. This study compared the impact forces, the use of upper limbs and the perception of danger of fire fighters as they step down from five different locations on fire trucks. The results show that stepping down from the crew cab facing the street produces impact forces averaging 3.2 times the subject's body weight, but is also perceived as the safest way to descend in one of the two groups of fire fighters that participated in the study. Stepping down from the same location, but facing the truck, produced significantly less impact force and a better distribution of the energy over time. This may be achieved through better control of the descending leg, ankle flexion, and the use of grab bars. A re-design of the access to emergency vehicles should take into account both the safety needs and reduction in biomechanical stress of fire fighters.     

Effects of a thermal-insulating mouse pad on temperature of forearm and hand during computer tasks

This laboratory experiment studied the effects of a thermal-insulating mouse pad on arm temperature and comfort during computer work. Fourteen subjects performed two 20-min computer tasks (a mouse task and a combined task alternating keyboard and mouse use), under three conditions, namely with: 1) a thermal-insulating pad; 2) a placebo pad; 3) no pad (desktop). The temperatures of the forearm, wrist, hand and fingers were measured with four thermocouples. Comfort and discomfort were determined by two visual analogue scales. No arm temperature differences were found between the experimental conditions after performing the combination task in any location. After the mouse task, however, arm temperature decreased significantly less with the thermal-insulating mouse pad than with the placebo pad. The thermal-insulating pad was rated as more comfortable and less uncomfortable than a regular desktop during mouse tasks. A large size is recommended for the thermal-insulating pad.     

Analysis of the musculoskeletal system of the hand and forearm during a cylinder grasping task

Musculoskeletal disorders of the hand are mostly due to repeated or awkward manual tasks in the work environment and are considered a public health issue. To prevent their development, it is necessary to understand and investigate the biomechanical behavior of the musculoskeletal system during the movement. In this study a biomechanical analysis of the upper extremity during a cylinder grasping task is conducted by using a parameterized musculoskeletal model of the hand and forearm. The proposed model is composed of 21 segments, 28 musculotendon units, and 20 joints providing 24 degrees of freedom. Boundary conditions of the model are defined by the three-dimensional coordinates of 43 external markers fixed to bony landmarks of the hand and forearm and tracked with an optoelectronic motion capture system. External marker positions from five healthy participants were used to test the model. A task consisting of closing and opening fingers around a cylinder 25 mm in diameter was investigated. Based on experimental kinematic data, an inverse dynamics process was performed to calculate output data of the model (joint angles, musculotendon unit shortening and lengthening patterns). Finally, based on an optimization procedure, joint loads and musculotendon forces were computed in a forward dynamics simulation. Results of this study assessed reproducibility and consistency of the biomechanical behavior of the musculoskeletal hand system.     

Evaluation of document location during computer use in terms of neck muscle activity and neck movement

This study evaluated the impact on neck movement and muscle activity of placing documents in three commonly used locations: in-line, flat desktop left of the keyboard and laterally placed level with the computer screen. Neck excursion during three standard head movements between the computer monitor and each document location and neck extensor and upper trapezius muscle activity during a 5 min typing task for each of the document locations was measured in 20 healthy participants. Results indicated that muscle activity and neck flexion were least when documents were placed laterally suggesting it may be the optimal location. The desktop option produced both the greatest neck movement and muscle activity in all muscle groups. The in-line document location required significantly more neck flexion but less lateral flexion and rotation than the laterally placed document. Evaluation of other holders is needed to guide decision making for this commonly used office equipment.     

Shoulder and neck muscle activities during typing with articulating forearm support at different heights

Use of forearm support is known to reduce physical stress of computer users, but research about how to properly position the forearm support is insufficient. This study was aimed to determine whether the height of forearm support influences muscular loads during typing. Twenty four subjects performed a typing task with a pair of articulating forearm support at three different heights as well as without any support, while shoulder, neck and forearm muscle activities and posture data were recorded. Typing with the support at resting elbow height produced significantly (p < 0.05) lower shoulder and neck muscle activities than that of no support condition. Typing with the support at heights higher than the resting elbow height produced significantly greater shoulder and neck muscle activities compared to the no support condition. Results suggest that forearm support can help computer users lessen physical stress in typing, but only when the supports are positioned at resting elbow height.

Practitioner Summary: Use of forearm support is known to alleviate physical stress of PC users in computer works such as typing. This experimental study addressed the importance of proper positioning of forearm support by comparing neck and upper extremity muscle activities between conditions with varying heights of forearm support in keyboard typing.     

The effects of lumbar massage on muscle fatigue,muscle oxygenation,low back discomfort,and driver performance during prolonged driving

This paper reviews results of comparisons between novice and expert operators in the supervision of dynamic environments. Firstly, the characteristics of expertise and its operational definitions are discussed. Secondly, the main dimensions of the complexity of these environments are described and the difficulties operators have in coping with complexity are examined. Research work carried out in field studies is then overviewed in order to assess the role of expertise in the three components of process control: monitoring, diagnosis and decision-making and executive control. The discussion stresses the convergent results of various studies. In spite of the diversity of task domains and of novice-expert definitions, results converge on three points: experts are better at producing inference, in anticipating, and have a more functional view of the process. Divergent results on directionality of reasoning, depth of knowledge in relation to performance and synchronization of actions are also discussed. Finally, further research trends are outlined.     

Modeling and simulation of an artificial muscle and its application to biomimetic robot posture control

The shape memory effect exhibited by Nitinol wire can be utilized to construct an artificial muscle. The muscle is activated by an electric current, which produces heat and initiates a phase transformation. The Nitinol artificial muscle stress–strain–power relationship was determined by experiments, and a mathematical model was developed. The artificial muscle model was utilized for the posture control of a biomimetic underwater robot. The optimal activation patterns for height, pitch, and roll postures were determined. Simulation results for the height postures are in agreement with the experiments. The separation between the center of gravity and the centroid of the robot has a stabilizing effect on pitch and roll postures.     

The effect of handle design on upper extremity posture and muscle activity during a pouring task

In this study, the effect of container handle parameters on shoulder and upper limb muscle activity and joint posture during a pouring task is investigated. Results indicated that a low handle position and a vertical handle slope minimised the loading of the shoulder muscles. A high and sloped handle minimised the muscle activity and wrist deviation of the lower arm. The effects of diameter were not significant for most dependent variables during the lifting phase of the task; however, beneficial effects were seen with the smallest handle diameter during the pouring phase. A trade-off existed between the shoulder and the hand/wrist posture with the different handles. The findings of significance with relatively small effect size suggest a high sensitivity of the system to any changes. In the real world, speed, space and work conditions are important factors that influence how a task is performed. This emphasises the importance of proper handle design.

Practitioner Summary: In this study, the effect of container handle design on the muscle activity and postures of the upper extremity during a pouring task were analyzed using the experimental data collected from electromyography and motion tracking systems. The low handle height and vertical handle slope design yielded the lowest shoulder muscle activity.     

Effect of a desk attachment board on posture and muscle activity in women during computer work

Working at a computer is part of a large number of jobs and has been associated with upper extremity musculoskeletal disorders and back pain. The study evaluated the effects of a board attachment on upper extremity and back. The findings are mixed in that the board may have a positive effect in preventing back pain, but may be detrimental to upper extremities. Effect of a desk attachment board on upper extremity and trunk posture, and muscle activity was assessed in women video display terminal users. Participants completed a standard 20-min computer task under two conditions: 1) using a standard desk; 2) using a desk attachment board designed to support the forearms. Bilateral electromyography of the trapezius, multifidus and longissimus muscles and the right anterior deltoid and forearm extensor muscles was recorded. 3-D trunk and upper extremity posture was monitored. Participants were tested before and after 2 weeks of familiarisation with the board in their workplace. Perceived tension and discomfort were recorded before and after use of the board. Use of the board tended to increase muscle activity in the right trapezius and forearm extensor and to decrease muscle activity in the back. Perceived tension in the low back decreased slightly with the board. The board may be useful in reducing tension in the low back during computer work, but may adversely affect the upper extremities.     

The effect of upper extremity support on upper extremity posture and muscle activity during keyboard use

Forearm support during keyboard use has been reported to reduce neck and shoulder muscle activity and discomfort. However, the effect of forearm support on wrist posture has not been examined. The aim of this study was to examine the effect of 3 different postures during keyboard use: forearm support, wrist support and "floating". The floating posture (no support) was used as the reference condition. A wrist rest was present in all test conditions. Thirteen participants completed 20 min wordprocessing tasks in each of the test conditions. Electromyography was used to monitor neck, shoulder and forearm muscle activity. Bilateral and overhead video cameras recorded left and right wrist extension, shoulder and elbow flexion and radial and ulnar deviation. The forearm support condition resulted in significantly less ulnar deviation (p < or = 0.007), less time spent in extreme ulnar deviation (p = 0.002) and less reports of discomfort than the "floating" condition (p = 0.002). The wrist support but not the forearm support condition resulted in less trapezius and anterior deltoid muscular activity (p < 0.007). These findings indicate that typing with upper extremity support in conjunction with a wrist rest may be preferable to the "floating" posture implicit in current guidelines.