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.     

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.     

The effect of typing posture on wrist extensor muscle loading

High static loading of the forearm extensor musculature has been observed during keying tasks. To reduce the level of loading, one must first understand the contributing factors. A simulation of the human finger was used to determine muscle force contributions during a static index finger key press at several wrist postures. The planar model included active and passive muscle forces of the intrinsic and extrinsic finger muscles. The model was expanded to include the passive forces from the other fingers as well as the weight of the hand to determine the exertion required of the wrist extensor muscles to maintain the given wrist and finger postures. Model results indicated that greater than 25% of maximal exertion is required of the wrist extensors when the wrist is extended to 30. The increased moment contribution from passive forces of the extrinsic finger flexor muscles was responsible for the majority of the increased wrist extensor contribution as the wrist was extended. These findings are in relative agreement with previous electromyographic studies and may indicate a mechanism for forearm extensor pain in office workers. Potential applications of this research include ergonomic modeling of the upper limb to determine internal loads that may lead to work-related disorders.     

Skill-level based examination of forearm muscle activation associated with efficient wrist and finger movements during typing

This study aims to elucidate the relationship between the wrist and finger movements and forearm muscle activation of twelve young people (age: 21.1 ± 0.76 years, nine men and three women) with different typing skills. We hypothesize that skilled typists (STs) could move their wrist and finger joints faster than unskilled typists (UTs) because they could efficiently use their muscles according to the activity characteristics of the flexors and extensors of the wrist joint. We measured wrist and finger movements using a 3D motion capture system and forearm muscle activation using surface electromyography during the typing task. We analyzed the entire task and the time when the U key was entered during the same task. The angular velocity of the wrist and finger flexion/extension and the muscle activation of the wrist flexors was higher in the STs than in the UTs, while the muscle activation of the wrist extensors was higher in the latter than that in the former. Our results showed that STs may have used their forearm muscles to take advantage of the physical characteristics of the keys and the spring characteristics of their muscles and tendons. It was suggested that they placed less mechanical stress on their finger muscles and tendons when pressing and releasing the keys.     

A postural workload evaluation system based on a macro‐postural classification

Many Korean workers are exposed to repetitive or prolonged poor working postures, which are closely related with pains or symptoms of musculoskeletal disorders. Poor working postures in Korea were reviewed and an observational method to assess the postural load was developed. A computer‐based postural workload evaluation system based on a macro‐postural classification scheme was developed. The macro‐postural classification is based on the perceived discomforts for various joint motions. On the basis of the perceived discomfort, postural stress levels for the postures at each joint were also defined in a ratio scale to the standing neutral posture. A neural network approach was used to predict the whole‐body postural stresses from the body joint motions. A computer‐based postural stress evaluation system was designed to automate the procedure for analyzing postures and enhance the usability and practical applicability. © 2002 Wiley Periodicals, Inc.     

The development and validation of equations to predict grip force in the workplace: contributions of muscle activity and posture

The inherent difficulty of measuring forces on the hand in ergonomic workplace assessments has led to the need for equations to predict grip force. A family of equations was developed, and validated, for the prediction of grip force using forearm electromyography (six finger and wrist muscles) as well as posture of the wrist (flexed, neutral and extended) and forearm (pronated, neutral, supinated). Inclusion of muscle activity was necessary to explain over 85% of the grip force variance and was further improved with wrist posture but not forearm posture. Posture itself had little predictive power without muscle activity (<1%). Nominal wrist posture improved predictive power more than the measured wrist angle. Inclusion of baseline muscle activity, the activity required to simply hold the grip dynamometer, greatly improved grip force predictions, especially at low force levels. While the complete model using six muscles and posture was the most accurate, the detailed validation and error analysis revealed that equations based on fewer components often resulted in a negligible reduction in predictive strength. Error was typically less than 10% under 50% of maximal grip force and around 15% over 50% of maximal grip force. This study presents detailed error analyses to both improve upon previous studies and to allow an educated decision to be made on which muscles to monitor depending on expected force levels, costs and error deemed acceptable by the potential user.     

The development and validation of equations to predict grip force in the workplace: contributions of muscle activity and posture

The inherent difficulty of measuring forces on the hand in ergonomic workplace assessments has led to the need for equations to predict grip force. A family of equations was developed, and validated, for the prediction of grip force using forearm electromyography (six finger and wrist muscles) as well as posture of the wrist (flexed, neutral and extended) and forearm (pronated, neutral, supinated). Inclusion of muscle activity was necessary to explain over 85% of the grip force variance and was further improved with wrist posture but not forearm posture. Posture itself had little predictive power without muscle activity (<1%). Nominal wrist posture improved predictive power more than the measured wrist angle. Inclusion of baseline muscle activity, the activity required to simply hold the grip dynamometer, greatly improved grip force predictions, especially at low force levels. While the complete model using six muscles and posture was the most accurate, the detailed validation and error analysis revealed that equations based on fewer components often resulted in a negligible reduction in predictive strength. Error was typically less than 10% under 50% of maximal grip force and around 15% over 50% of maximal grip force. This study presents detailed error analyses to both improve upon previous studies and to allow an educated decision to be made on which muscles to monitor depending on expected force levels, costs and error deemed acceptable by the potential user.     

Changes in mechanical load and extensor muscle activity in the cervico-thoracic spine induced by sitting posture modification

The influence of whole body sitting posture on cervico-thoracic posture, mechanical load and extensor muscle activity was examined in 23 asymptomatic adults. Cervical and upper thoracic extensor muscle activity measured in guided slouched and lumbo-pelvic neutral postures was normalised to that measured in a self-selected habitual posture. Head and neck posture and gravitational load moment measurements were obtained in each posture. Sagittal head translation, upper cervical extension and load moment were significantly greater in the slouched posture (p < 0.001). Contrasting patterns of cervical and thoracic extensor activity were observed in the slouched and neutral postures, with cervical extensor activity 40% higher in the slouched posture (p < 0.0001). Thoracic extensor activity was significantly higher in the lumbo-pelvic neutral posture than the habitual posture (p = 0.002). The significant changes in extensor muscle activity with postural modification appear to be induced by the associated change in mechanical load moment of the head. STATEMENT OF RELEVANCE: More neutral sitting postures reduce the demand on the cervical extensor muscles and modify the relative contribution of cervical and thoracic extensors to the control of head and neck posture. Postures that promote these patterns of muscular activity may reduce cervical spine loading and the development of posture-related neck pain.     

Prediction of forearm muscle activity during gripping

Occupational exposure is typically assessed by measuring forces and body postures to infer muscular loading. Better understanding of workplace muscle activity levels would aid in indicating which muscles may be at risk for overexertion and injury. However, electromyography collection in the workplace is often not practical. Therefore, a set of equations was developed and validated using data from two separate days to predict forearm muscle activity (involving six wrist and finger muscles) from grip force and posture of the wrist (flexed, neutral and extended) and forearm (pronated, neutral, supinated). The error in predicting activation levels of each forearm muscle across the range of grip forces, using the first day data (root mean square error; RMSEmodel), ranged from 8.9% maximal voluntary electrical activation (MVE) (flexor carpi radialis) to 11% MVE (extensor digitorum communis). Grip force was the main contributor to predicting muscle activity levels, explaining over 70% of the variance in flexor activation levels and up to 60% in extensor activation levels, respectively. Inclusion of gender as a variable in the model improved estimates of flexor but not extensor activity. While posture itself explained minimal variance in activation without grip force (< 10% MVE), wrist and forearm posture were required (with grip force) to explain over 70% of the variance of all six muscles. The validation process indicated good day-to-day reliability of each equation, with similar error for flexor muscle models but slightly higher error in the extensor models when predicting activity levels for the second day of data (RMSEvalid ranging from 8.9% to 12.7% MVE). Detailed error analysis during validation revealed that inclusion of posture in the model effectively decreased error at grip forces above 25% maximum, but was detrimental at very low grip forces. This study presents a potential new tool to estimate forearm muscle loading in the workplace using grip force and posture, as a surrogate to use of a complex biomechanical model.     

Prediction of forearm muscle activity during gripping

Occupational exposure is typically assessed by measuring forces and body postures to infer muscular loading. Better understanding of workplace muscle activity levels would aid in indicating which muscles may be at risk for overexertion and injury. However, electromyography collection in the workplace is often not practical. Therefore, a set of equations was developed and validated using data from two separate days to predict forearm muscle activity (involving six wrist and finger muscles) from grip force and posture of the wrist (flexed, neutral and extended) and forearm (pronated, neutral, supinated). The error in predicting activation levels of each forearm muscle across the range of grip forces, using the first day data (root mean square error; RMSE_model), ranged from 8.9% maximal voluntary electrical activation (MVE) (flexor carpi radialis) to 11% MVE (extensor digitorum communis). Grip force was the main contributor to predicting muscle activity levels, explaining over 70% of the variance in flexor activation levels and up to 60% in extensor activation levels, respectively. Inclusion of gender as a variable in the model improved estimates of flexor but not extensor activity. While posture itself explained minimal variance in activation without grip force (<10% MVE), wrist and forearm posture were required (with grip force) to explain over 70% of the variance of all six muscles. The validation process indicated good day-to-day reliability of each equation, with similar error for flexor muscle models but slightly higher error in the extensor models when predicting activity levels for the second day of data (RMSE_valid ranging from 8.9% to 12.7% MVE). Detailed error analysis during validation revealed that inclusion of posture in the model effectively decreased error at grip forces above 25% maximum, but was detrimental at very low grip forces. This study presents a potential new tool to estimate forearm muscle loading in the workplace using grip force and posture, as a surrogate to use of a complex biomechanical model.     

Evaluation of handgrip force from EMG measurements

A series of experiments were performed in order to investigate whether estimates of handgrip force could be derived with fair accuracy from surface EMG levels recorded on the finger flexors of the forearm, taking into account the position of the wrist in the flexion-extension plane and in the ulnar-radial deviation plane. Handgrip forces (on a JAMAR dynamometer) and corresponding surface EMG levels (on the finger flexors of the forearm) were recorded for 20 subjects in 11 postures of the wrist and for 30% and 70% of the MVC in neutral posture. A mathematical empirical model was developed using multiple non-linear regression analysis. Although quite simple, it provides very reliable results, the correlation coefficient between predicted and observed forces being 0.895. Its use must, however, be restricted to work situations where: (a) the hand efforts are of the same type and involve the same muscles as those exerted on the dynamometer; (b) the hand is in neutral pro-supination; and (c) no voluntary effort is exerted by the wrist flexors except for maintaining the wrist posture.     

The effect of upper-extremity posture on maximum grip strength

The objective of this study was to determine and document the position of peak grip strength in different shoulder, elbow, and wrist posture combinations. Fifteen (15) male subjects performed maximal exertions with their dominant hands in nine wrist postures, three elbow postures, and two shoulder postures. Analysis of the data revealed that shoulder and elbow angles had significant effect upon the grip strength. Similarly, it was seen that grip strength at elbow at 135 degrees flexion was significantly different from those with elbow at 90 and 180 degrees. Further, the results revealed that peak grip strength occurred at a combined posture of shoulder abducted 0 degrees, elbow flexed 135 degrees, and the wrist in the neutral posture. Decrements of up to 42% in grip strength could be seen as elbow and wrist angles deviated. This means that use of handtools at deviated postures of shoulder, elbow, and wrist would decrease the percent of MVC at which a worker operated. Hence, the implementation of the finding of this study might be a reduction in the risk of injury, increase in productivity, and well-being of the workers.     

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.     

The effect of viewing angle on wrist posture estimation from photographic images using novice raters

Observational assessment of wrist posture using photographic methods is theoretically affected by camera view angle. A study was conducted to investigate whether wrist flexion/extension and radial/ulnar deviation postures were estimated differently by raters depending on the viewing angle and compared to predictions using a quantitative 2D model of parallax. Novice raters (n = 26) estimated joint angles from images of wrist postures photographed from ten different viewing angles. Results indicated that ideal views, orthogonal to the plane of motion, produced more accurate estimates of posture compared to non-ideal views. The neutral (0°) posture was estimated the most accurately even at different viewing angles. Raters were more accurate than model predictions. Findings demonstrate a need for more systematic methods for collecting and analyzing photographic data for observational studies of posture. Renewed caution in interpreting existing studies of wrist posture where viewing angle was not controlled is advised.     

Effect of head-mounted displays on posture

OBJECTIVE: The aim of the present study was to determine if a wearable system based on a head-mounted display (HMD) causes users to alter their head position and adopt postures that place greater stress on the musculoskeletal system. BACKGROUND: HMDs are common output devices used with wearable computers. HMDs provide the wearer with visual information by projecting computer-generated virtual images in front of the eyes. Deviations of neck posture from a neutral upright position increase the stresses on the musculoskeletal system of the head and neck. METHOD: Seven paramedics simulated the treatment of a patient under a normal condition and when using an HMD wearable computer system. During the simulations a posture analysis was performed using the Rapid Upper Limb Assessment method. RESULTS: The postures adopted when wearing an HMD, as compared with a normal condition, scored significantly higher for the neck (z = 2.463, p < .05) and for overall body posture (left side of the body: z = 2.447, p < .05; right side of the body: z = 2.895, p < .05). CONCLUSION: Wearing an HMD can force the wearers to modify their neck posture. As such, the musculoskeletal system of the head and neck may be placed under increased levels of stress. APPLICATION: Potential users should be made aware that HMDs could dictate modifications in neck posture, which may have detrimental effects and may compound the weight effect of the HMD.     

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 interacting effects of forearm rotation and exertion direction on male and female wrist strength

The accurate estimation of wrist strength is an important component of ergonomics task evaluation, as a vast majority of occupational tasks involve use of the hands to generate forces and moments. The purpose of this study was to examine the interacting effects of forearm rotation (pronation/supination) and wrist exertion direction on strength at the wrist joint in males and females. A total of 24 male and female participants performed maximum isometric wrist exertions while maintaining a non-deviated wrist posture (no flexion/extension or radial/ulnar deviation) and an open hand. Maximum wrist moments were obtained in combinations of three forearm rotations (90° pronation, neutral, 90° supination) and four exertion directions (flexion, extension, radial and ulnar deviation). A greater effect of forearm rotation was observed for males, as strength in the neutral forearm posture was significantly different than pronated and supinated postures in 5 of 8 comparisons. For females, both wrist flexion and extension strengths were higher in neutral, compared to supinated forearm postures. The findings of this study suggest that wrist strength does depend on forearm rotation, and this interaction between axes needs to be accounted for in future strength capability estimates.Relevance to industryThis study shows that wrist strength estimates, currently used by ergonomics software packages in industry, can be improved to more accurately reflect the actual wrist strength capabilities of workers during hand-intensive tasks.     

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.     

Guidelines for wrist posture based on carpal tunnel pressure thresholds

OBJECTIVE: To develop work guidelines for wrist posture based on carpal tunnel pressure. Background: Wrist posture is considered a risk factor for distal upper extremity musculoskeletal disorders, and sustained wrist deviation from neutral at work may be associated with carpal tunnel syndrome. However, the physiologic basis for wrist posture guidelines at work is limited. METHODS: The relationship of wrist posture to carpal tunnel pressure was examined in 37 healthy participants. The participants slowly moved their wrists in extension-flexion and radioulnar deviation while wrist posture and carpal tunnel pressure were recorded. The wrist postures associated with pressures of 25 and 30 mmHg were identified for each motion and used to determine the 25th percentile wrist angles (the angles that protect 75% of the study population from reaching a pressure of 25 or 30 mmHg). RESULTS: Using 30 mmHg, the 25th percentile angles were 32.7 degrees (95% confidence interval [CI] = 27.2-38.1 degrees) for wrist extension, 48.6 degrees (37.7 -59.4 degrees) for flexion, 21.8 degrees (14.7-29.0 degrees) for radial deviation, and 14.5 degrees (9.6-19.4 degrees) for ulnar deviation. For 25 mmHg, the 25th percentile angles were 26.6 degrees and 37.7 degrees for extension and flexion, with radial and ulnar deviation being 17.8 degrees and 12.1 degrees, respectively. CONCLUSION: Further research can incorporate the independent contributions of pinch force and finger posture into this model. APPLICATION: The method presented can provide wrist posture guidelines for the design of tools and hand-intensive tasks.     

Quantitative postural load assessment for whole body manual tasks based on perceived discomfort

Many Korean workers are exposed to repetitive manual tasks or prolonged poor working postures that are closely related to back pain or symptoms of musculoskeletal disorders. Workers engage in tasks that require not only handling of heavy materials, but also assuming prolonged or repetitive non-neutral work postures. Poor work postures that have been frequently observed in the workplaces of shipbuilding shops, manufacturing plants, automobile assembly lines and farms often require prolonged squatting, repetitive arm raising and wrist flexion and simultaneous trunk flexion and lateral bending. In most manufacturing industries, workers have to assume improper work postures repetitively, several hundreds of times per day depending on daily production rate. A series of psychophysical laboratory experiments were conducted to evaluate the postural load at various joints. A postural load assessment system was then developed based on a macro-postural classification scheme. The classification scheme was constructed based on perceived discomfort for various joint motions as well as previous research outcomes. On the basis of the perceived discomfort, postural stress levels for the postures at individual joints were also defined by a ratio scale to the standing neutral posture. Laboratory experiments simulating automobile assembly tasks were carried out to investigate the relationship between body-joint and whole-body discomfort. Results showed a linear relationship between the two types of discomfort, with the shoulder and low back postures being the dominant factor in determining the whole body postural stresses. The proposed method was implemented into a computer software program in order to automate the procedure of analysing postural load and to enhance usability and practical applicability.