The impact of work configuration,target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work

Overhead work has established links to upper extremity discomfort and disorders. As many jobs incorporate working overhead, this study aimed to identify working conditions requiring relatively lower muscular shoulder load. Eleven upper extremity muscles were monitored with electromyography during laboratory simulations of overhead work tasks. Tasks were defined with three criteria: work configuration (fixed, stature-specific); target angle (?15°, 0°, 15°, 30° from vertical); direction of applied hand force (pulling backwards, pushing forwards, downwards, sideways, upwards). Normalised electromyographic activity was greater for fixed configurations, particularly when pulling in a backward direction (total activity = 108.3% maximum voluntary exertion (MVE)) compared to pushing down or forward (total activity ranging from 10.5 to 17.3%MVE). Further, pulling backwards at angles of –15° and 0° showed the highest muscular demand (p < 0.05). These results suggest that, if possible, positioning overhead work in front of the body with exertions directed forwards will result in the lowest upper extremity muscle demand.

Statement of Relevance: Overhead work pervades occupational settings and is associated with risk of upper extremity musculoskeletal disorders. The muscular intensity associated with performing overhead work was assessed in several combinations of work placement and hand force direction. These findings should have utility for designing overhead work tasks that reduce muscular exposure.     

Handle displacement and operator responses to pneumatic nutrunner torque buildup

The objective of this study is to investigate the workstation and tool effects on the responses of the powered hand tool operator reacting against the impulsive reaction forces that may be associated with upper extremity musculoskeletal disorders. The study demonstrated a means of direct measurement of force at the interface between the tool and the operator. Fifteen experienced male operators performed three independent work configurations: pistol grip and right angle tools used on the horizontal surface and pistol grip tools used on the vertical surface, in the laboratory. A full factorial experiment consisting of 36 conditions was designed to examine the effects of working height, distance, tool, and fastener joint hardness on handle displacement and grip forces. The results indicate that operator responses were affected by different factors depending on the work configuration. When pistol grip tools were used on the vertical surface, the mean handle displacement decreased from 9.9 degrees to 7.3 degrees as the working height increased from 30 cm below shoulder to 30 cm above shoulder. When right angle tools were used, the greatest handle displacement (51.1mm) and grip force (84.7% MVC) during torque reactions were measured at 30 cm below elbow and 40% forward reach away from the operator. This study provides quantitative information that can be used for workstation design and tool selection to reduce the torque reaction experienced by powered nutrunner operators.     

Inward torque and high-friction handles can reduce required muscle efforts for torque generation

OBJECTIVE: The effects of handle friction and torque direction on muscle activity and torque are empirically investigated using cylindrical handles. BACKGROUND: A torque biomechanical model that considers contact force, friction, and torque direction was evaluated using different friction handles. METHODS: Twelve adults exerted hand torque in opposite directions about the long axis of a cylinder covered with aluminum or rubber while grip force, torque, and finger flexor electromyography (EMG) were recorded. In addition, participants performed grip exertions without torque, in which they matched the EMG level obtained during previous maximum torque exertions, to allow us to determine how grip force was affected by the absence of torque. RESULTS: (a) Maximum torque was 52% greater for the high-friction rubber handle than for the low-friction aluminum handle. (b) Total normal force increased 33% with inward torque (torque applied in the direction fingertips point) and decreased 14% with outward torque (torque in the direction the thumb points), compared with that with no torque. Consequently, maximum inward torque was 45% greater than maximum outward torque. (c) The effect of torque direction was greater for the high-friction rubber handle than for the low-friction aluminum handle. CONCLUSION: The results support the proposed model, which predicts a large effect of torque direction when high-friction handles are gripped. APPLICATION: Designing tasks with high friction and inward rotations can increase the torque capability of workers of a given strength, or reduce required muscle activities for given torque exertions, thus reducing the risk of fatigue and musculoskeletal disorders.     

Forces associated with pneumatic power screwdriver operation: statics and dynamics

The statics and dynamics of pneumatic power screwdriver operation were investigated in the context of predicting forces acting against the human operator. A static force model is described in the paper, based on tool geometry, mass, orientation in space, feed force, torque build up, and stall torque. Three common power hand tool shapes are considered, including pistol grip, right angle, and in-line. The static model estimates handle force needed to support a power nutrunner when it acts against the tightened fastener with a constant torque. A system of equations for static force and moment equilibrium conditions are established, and the resultant handle force (resolved in orthogonal directions) is calculated in matrix form. A dynamic model is formulated to describe pneumatic motor torque build-up characteristics dependent on threaded fastener joint hardness. Six pneumatic tools were tested to validate the deterministic model. The average torque prediction error was 6.6% (SD = 5.4%) and the average handle force prediction error was 6.7% (SD = 6.4%) for a medium-soft threaded fastener joint. The average torque prediction error was 5.2% (SD = 5.3%) and the average handle force prediction error was 3.6% (SD = 3.2%) for a hard threaded fastener joint. Use of these equations for estimating handle forces based on passive mechanical elements representing the human operator is also described. These models together should be useful for considering tool handle force in the selection and design of power screwdrivers, particularly for minimizing handle forces in the prevention of injuries and work related musculoskeletal disorders.     

Forces associated with pneumatic power screwdriver operation: statics and dynamics

The statics and dynamics of pneumatic power screwdriver operation were investigated in the context of predicting forces acting against the human operator. A static force model is described in the paper, based on tool geometry, mass, orientation in space, feed force, torque build up, and stall torque. Three common power hand tool shapes are considered, including pistol grip, right angle, and in-line. The static model estimates handle force needed to support a power nutrunner when it acts against the tightened fastener with a constant torque. A system of equations for static force and moment equilibrium conditions are established, and the resultant handle force (resolved in orthogonal directions) is calculated in matrix form. A dynamic model is formulated to describe pneumatic motor torque build-up characteristics dependent on threaded fastener joint hardness. Six pneumatic tools were tested to validate the deterministic model. The average torque prediction error was 6.6% (SD = 5.4%) and the average handle force prediction error was 6.7% (SD = 6.4%) for a medium-soft threaded fastener joint. The average torque prediction error was 5.2% (SD = 5.3%) and the average handle force prediction error was 3.6% (SD = 3.2%) for a hard threaded fastener joint. Use of these equations for estimating handle forces based on passive mechanical elements representing the human operator is also described. These models together should be useful for considering tool handle force in the selection and design of power screwdrivers, particularly for minimizing handle forces in the prevention of injuries and work related musculoskeletal disorders.     

A systematic exploration of distal arm muscle activity and perceived exertion while applying external forces and moments

The purpose of this study was to systematically explore and describe the response of selected hand and forearm muscles during a wide range of static force and moment exertions. Twenty individuals with manual work experience performed exertions in power grip, pulp pinch and lateral pinch grips. Electromyography (EMG) from eight sites of the hand and forearm, grip force as well as ratings of perceived exertion (RPE) were monitored as each participant exerted approximately 350 short (5 s) static grip forces and external forces and moments. As expected, strong relationships were found between grip force alone without other actions and muscle activation. When the hand was used to grip and transmit forces and moments to the environment, the relationships between grip force and muscle activation were much weaker. Using grip force as a surrogate for forearm and hand tissue loading may therefore be misleading.     

A systematic exploration of distal arm muscle activity and perceived exertion while applying external forces and moments

The purpose of this study was to systematically explore and describe the response of selected hand and forearm muscles during a wide range of static force and moment exertions. Twenty individuals with manual work experience performed exertions in power grip, pulp pinch and lateral pinch grips. Electromyography (EMG) from eight sites of the hand and forearm, grip force as well as ratings of perceived exertion (RPE) were monitored as each participant exerted approximately 350 short (5 s) static grip forces and external forces and moments. As expected, strong relationships were found between grip force alone without other actions and muscle activation. When the hand was used to grip and transmit forces and moments to the environment, the relationships between grip force and muscle activation were much weaker. Using grip force as a surrogate for forearm and hand tissue loading may therefore be misleading.     

Constrained handgrip force decreases upper extremity muscle activation and arm strength

Many industrial tasks require repetitive shoulder exertions to be performed with concurrent physical and mental demands. The highly mobile nature of the shoulder predisposes it to injury. The purpose of this study was to determine the effects of simultaneous gripping, at a specified magnitude, on muscle activity and maximal arm force in various directions. Ten female subjects performed maximal arm exertions at two different heights and five directions using both specified (30% maximum voluntary grip) and preferred (self-selected) grip forces. Electromyography was recorded from eight muscles of the right upper extremity. The preferred grip condition produced grip forces that were dependent on the combination of arm height and force direction and were significantly greater (arm force down), lower (to left, up and push forward), or similar to the specified grip condition. Regardless of the magnitude of the preferred grip force, specifying the grip resulted in decreased maximal arm strength (by 18–25%) and muscle activity (by 15–30%) in all conditions, indicating an interfering effect when the grip force was specified by visual target force-matching. Task constraints, such as specific gripping demands, may decrease peak force levels attainable and alter muscle activity. Depending on the nature of task, the amount of relative demand may differ, which should be considered when determining safety thresholds.     

Muscle response to pneumatic hand tool torque reaction forces

Surface electromyography was used for studying the effects of torque reaction force acting against the hand, on forearm muscle activity and grip force for five subjects operating right angle, air shut-off nutrunners. Four tools having increasing spindle torque were operated using short and long torque reaction times. Nutrunner spindle torque ranged between 30 Nm and 100 Nm. Short torque reaction time was considered 0.5 s while long torque reaction time was 2 s. Peak horizontal force was the greatest component of the reaction force acting against the hand and accounted for more than 97% of the peak resultant hand force. Peak hand force increased from 89 N for the smallest tool to 202 N for the largest tool. Forearm muscle rms EMG, scaled for grip force, indicated average flexor activity during the Torque-reaction phase was more than four times greater than the Pre-start and Post Shut-off phases, and two times greater than the Run-down phase. Flexor EMG activity during the Torque-reaction phase increased for increasing tool peak spindle torque. Average flexor rms EMG activity, scaled for grip force, during the Torque-reaction phase increased from 372 N for the 30 Nm nutrunner to 449 N for the 100 Nm nutrunner. Flexor rms EMG activity averaged during the Torque-reaction phase and scaled for grip force was 390 N for long torque reaction times and increased to 440 N for short torque reaction times. Flexor rms EMG integrated over the torque reaction phase was 839 Ns for long torque reaction times and decreased to 312 Ns for short torque reaction times. The average latency between tool spindle torque onset and peak initial flexor rms EMG for long torque reaction times was 294 ms which decreased to 161 ms for short torque reaction times. The average latency between peak tool spindle torque, just prior to tool shut-off, and peak final rms EMG for long torque reaction times was 97 ms for flexors and 188 ms for extensors, which decreased for short torque reaction times to 47 ms for flexors and 116 ms for extensors. The results suggest that right angle nutrunner torque reaction forces can affect extrinsic hand muscles in the forearm, and hence grip exertions, by way of a reflex response.(ABSTRACT TRUNCATED AT 400 WORDS)     

A study of the difference between nominal and actual hand forces in two-handed sagittal plane whole-body exertions

Given a task posture, changes in hand force magnitude and direction with regard to joint locations result in variations in joint loads. Previous work has quantified considerable vertical force components during push/pull exertions. The objective of this work was to quantify and statistically model actual hand forces in two-hand, standing exertions relative to the required nominal horizontal and vertical hand forces for a population of widely varying stature and strength. A total of 19 participants exerted force on a fixed handle while receiving visual feedback on the magnitude of force exerted in the required horizontal or vertical direction. A set of regression equations with adjusted R(2) values ranging from 0.20 to 0.66 were developed to define actual hand force vectors by predicting off-axis forces from the required hand force magnitude. Off-axis forces significantly increase the overall magnitude of force exerted in two-hand push/pull and up/down standing force exertions. STATEMENT OF RELEVANCE: This study quantifies and statistically models actual hand forces in two-hand, standing exertions. Inaccuracies in hand force estimates affect the ability to accurately assess task-oriented strength capability. Knowledge of the relationship between nominal and actual hand forces can be used to improve existing ergonomic analysis tools, including biomechanical simulations of manual tasks.     

Measurement of prehensile grasp capabilities by a force and moment wrench: Methodological development and assessment of manual workers

Prehensile grasp capability is typically quantified by pinch and grasp forces. This work was undertaken to develop a methodology to assess complex, multi-axis hand exertions through the measurement of forces and moments exerted by the hand along and about three orthogonal axes originating at the grip centre; termed an external wrench. Instrumentation consisting of a modified pinch/grip dynamometer affixed to a 6?df force cube was developed to simultaneously measure three forces, three moments and the pinch/grip force about the centre of the grip. Twenty right hand dominant manual workers (10 male and 10 female), free of hand or wrist disorders, completed a variety of maximal strength tasks. The randomized block design involved three separate grips?–?power grip, lateral pinch and pulp pinch. Randomized within each block were three non-concurrent repetitions of isolated maximal force and moment generations along and about the three principle orthogonal axes and a maximal grip force exertion. Trials were completed while standing, with the arm abducted and elbow flexed to 90° with a wrist posture near neutral. Where comparable protocols existed in the literature, forces and moments exerted were found to be of similar magnitude to those reported previously. Female and male grip strengths on a Jamar dynamometer were 302.6?N and 450.5?N, respectively. Moment exertions in a power grip (female and male) were 4.7 Nm and 8.1 Nm for pronator, 4.9 Nm and 8.0 Nm for supinator, 6.2 Nm and 10.3 Nm for radial deviator, 7.7 Nm and 13.0 Nm for ulnar deviator, 6.2 Nm and 8.2 Nm for extensor, and 7.1 Nm and 9.3 Nm for flexor moments. Correlations with and between maximal force and moment exertions were only moderate. This paper describes instrumentation that allows comprehensive characterization of prehensile force and moment capability.     

Measurement of prehensile grasp capabilities by a force and moment wrench: methodological development and assessment of manual workers

Prehensile grasp capability is typically quantified by pinch and grasp forces. This work was undertaken to develop a methodology to assess complex, multi-axis hand exertions through the measurement of forces and moments exerted by the hand along and about three orthogonal axes originating at the grip centre; termed an external wrench. Instrumentation consisting of a modified pinch/grip dynamometer affixed to a 6 df force cube was developed to simultaneously measure three forces, three moments and the pinch/grip force about the centre of the grip. Twenty right hand dominant manual workers (10 male and 10 female), free of hand or wrist disorders, completed a variety of maximal strength tasks. The randomized block design involved three separate grips--power grip, lateral pinch and pulp pinch. Randomized within each block were three non-concurrent repetitions of isolated maximal force and moment generations along and about the three principle orthogonal axes and a maximal grip force exertion. Trials were completed while standing, with the arm abducted and elbow flexed to 90 degrees with a wrist posture near neutral. Where comparable protocols existed in the literature, forces and moments exerted were found to be of similar magnitude to those reported previously. Female and male grip strengths on a Jamar dynamometer were 302.6 N and 450.5 N, respectively. Moment exertions in a power grip (female and male) were 4.7 Nm and 8.1 Nm for pronator, 4.9 Nm and 8.0 Nm for supinator, 6.2 Nm and 10.3 Nm for radial deviator, 7.7 Nm and 13.0 Nm for ulnar deviator, 6.2 Nm and 8.2 Nm for extensor, and 7.1 Nm and 9.3 Nm for flexor moments. Correlations with and between maximal force and moment exertions were only moderate. This paper describes instrumentation that allows comprehensive characterization of prehensile force and moment capability.     

Relationships between observational estimates and physical measurements of upper limb activity †

This study examined the internal validity of observational-based ergonomic job analysis methods for assessing upper limb force exertion and repetitive motion. Six manual tasks were performed by multiple ‘workers’ while direct measurements were made to quantify force exertion and kinematics of the upper limb. Observational-based analyses of force and upper limb motion/repetition were conducted by 29 professional ergonomists. These analysts overestimated the magnitude of individual force exertions – temporal aspects of force exertion (duty cycle) were estimated more accurately. Estimates of the relative severity of repetitive motions among the jobs were accurate. Absolute counts of repetitive motions were less accurate. Modest correlations (r² = 0.28 to r² = 0.50) were observed between ratings of hand activity level and measured joint velocities. Ergonomic job analyses relying on systematic observation should be applied and interpreted with consideration given to the capabilities and limitations of analysts in estimating the physical risk factors. These findings are relevant to a better understanding of the internal validity of ergonomic job analysis methods based on systematic observation.     

Wrist strength is dependent on simultaneous power grip intensity

The effect of grip activities on wrist flexion/extension strength was examined. Twelve healthy subjects performed maximum wrist flexion/extension exertions with one of five levels of simultaneous grip effort: minimum effort; preferred effort; 30%, 60% and 100% maximum voluntary contraction. As grip force increased from the minimum to the maximum effort, average wrist flexion strength increased 34% and average wrist extension strength decreased 10%. It appears that the finger flexor tendons on the volar aspect of the wrist act agonistically in wrist flexion and act antagonistically to wrist extension. When an object gripped by the hand is fragile or uncomfortable, the reduced finger flexor activity will limit wrist flexion strength. Gripping a slippery object that requires high grip effort will result in reduced wrist extension strength. Grip force should be controlled during measurement of wrist flexion or extension strength. When analysing a task that involves both grip and wrist exertions, use of grip/wrist strength values that were measured during grip exertions only, or wrist exertions only, may incorrectly estimate the true grip/wrist strength, as grip and wrist activities significantly interact with each other as demonstrated in this paper.     

Predicting subjective perceptions of powered tool torque reactions

Powered hand tools have the potential to produce reaction forces that may be associated with upper extremity musculoskeletal disorders. In this study, subjective ratings of discomfort and acceptability of reaction forces were collected in an attempt to identify their associations with factors such as work location, and response covariates such as grip force and tool handle displacement. Three work configurations using pistol grip and right angle pneumatic nutrunners on horizontal and vertical surfaces were set up in the laboratory. Twenty healthy right-handed male participants operated four tools at nine locations and the corresponding subjective responses were collected. The results indicate that normalized grip force during the torque buildup period was a significant factor for both subjective ratings. For the unacceptable torque reactions across the three tool configurations, the ratio of hand moment impulse over tool torque impulse was significantly greater than for the acceptable reactions. For pistol grip tools used on the vertical surface, as the working height increased 30 cm, the odds of an unacceptable rating over an acceptable rating increased 1.6 times. Prediction models for subjective ratings of discomfort and acceptability provide insight regarding either workstation design or exposure control. These models can further be used to establish exposure limits based on handle displacement and grip force.     

Development and application of a multi-axis dynamometer for measuring grip force

Objective: This paper describes the development and application of a novel multi-axis hand dynamometer for quantifying 2D grip force magnitude and direction in the flexion-extension plane of the fingers. Methods: A three-beam reconfigurable form dynamometer, containing two active beams for measuring orthogonal forces and moments regardless of point of force application, was designed, fabricated and tested. Maximum grip exertions were evaluated for 16 subjects gripping cylindrical handles varying in diameter. Results: Mean grip force magnitudes were 231 N (SD = 67.7 N), 236 N (72.9 N), 208 N (72.5 N) and 158 N (45.7 N) for 3.81 cm, 5.08 cm, 6.35 cm and 7.62 cm diameter handles, respectively. Grip force direction rotated clockwise and the centre of pressure moved upward along the handle as handle diameter increased. Conclusions: Given that the multi-axis dynamometer simultaneously measures planar grip force magnitude and direction, and centre of pressure along the handle, this novel sensor design provides more grip force characteristics than current sensor designs that would improve evaluation of grip characteristics and model-driven calculations of musculoskeletal forces from dynamometer data.     

Interobserver repeatability and validity of an observation method to assess physical loads imposed on the upper extremities

Interobserver repeatability and validity were assessed for a new semiquantitative, time-based observation method for the estimation of physical loads imposed on the upper extremities. Six risk factors of upper extremity disorders were included in the method: repetitive use of hand, use of hand force, pinch grip, non-neutral wrist posture, elevation of upper arm, local mechanical pressure. Two occupational health nurses were trained to use the method. They observed 127 work cycles at a food-processing plant and a paper mill. The method was validated against expert observations from the video, continuous recordings of myoelectric activity (EMG) of forearm muscles, and wrist posture measured with goniometers. Interobserver repeatability was good or moderate for repetitive use of hand, hand force, pinch grip (range κ = 0.58–0.71 on the right; 0.60–0.61 on the left side). Interobserver repeatability was moderate or poor for non-neutral wrist posture, elevation of the upper arm and local mechanical pressure. Validity ranged from moderate to good for repetitive use of hand, use of hand force, pinch grip and non-neutral wrist posture when expert observation was used as reference standard. When observations were validated against force estimations (EMG) and wrist goniometer data, validity was poor. In the absence of generally accepted reference values, arbitrarily chosen limits were used for the proportional duration of some physical load factors. Studies should be carried out to assess the limits that best diVerentiate between safe and hazardous jobs.     

Effects of user experience,working posture and joint hardness on powered nutrunner torque reactions

Powered hand tools produce reaction forces that may be associated with upper extremity musculoskeletal disorders. The handle displacement, grip force and upper limb muscle activity (electromyography (EMG)) due to the effects of operator experience, working height and distance, type of tool and fastener joint hardness were measured in this study with 15 experienced and 15 novice nutrunner users. The results show that when pistol grip handles were used to work on a horizontal surface, experienced users allowed an average handle displacement of 7.9°, while novice users allowed 11.5°. Average EMG scaled by reference voluntary contraction (RVC) at forearm flexors, forearm extensors and biceps were greater for experienced users (318% RVC, 285% RVC, 143% RVC, respectively) than for novice users (246% RVC, 219% RVC, 113% RVC, respectively). Experienced users exerted more grip force than novice users when using right angle handles, but less force when using pistol grip handles. The results suggest that it is possible to minimize tool handle displacement by adapting the workplace layout to permit different working postures for each user group.     

Optimal work–rest cycles for an isometric intermittent gripping task as a function of force,posture and grip span

The aim of this study was to investigate the maximum acceptable contraction frequencies (i.e. work–rest cycles) for an isometric-intermittent handgrip task as a function of grip span, applied force and shoulder posture using psychophysical and physiological approaches. Twelve healthy males served as subjects. The three grip spans investigated were the optimal, 2 cm narrower than the optimal, and 2 cm wider than the optimal. The grip force levels studied were 15% and 30% of maximum voluntary grip force and the two shoulder postures were 25° flexion and 30° abduction. The psychophysical results indicate that subjects work faster with the narrower grip span at 15% of maximum voluntary grip force level in comparison to working with the optimal and the wider spans. However, when the task required 30% of maximum grip force level, the subjects worked faster with the optimal grip span. These findings were supported by the results of electromyography, heart rate, blood pressure and perceived discomfort. The study suggests that grip span of a tool is an important factor to be considered in predicting optimal work–rest cycles for hand grip tasks, and the optimum setting of grip span of the hand-tool depends on the required task force level. That is, the optimality is relative rather than absolute. In addition, it appears that weaker subjects can work at a higher rate than stronger ones at the same relative force level.     

Interobserver repeatability and validity of an observation method to assess physical loads imposed on the upper extremities

Interobserver repeatability and validity were assessed for a new semiquantitative, time-based observation method for the estimation of physical loads imposed on the upper extremities. Six risk factors of upper extremity disorders were included in the method: repetitive use of hand, use of hand force, pinch grip, non-neutral wrist posture, elevation of upper arm, local mechanical pressure. Two occupational health nurses were trained to use the method. They observed 127 work cycles at a food-processing plant and a paper mill. The method was validated against expert observations from the video, continuous recordings of myoelectric activity (EMG) of forearm muscles, and wrist posture measured with goniometers. Interobserver repeatability was good or moderate for repetitive use of hand, hand force, pinch grip (range kappa = 0.58-0.71 on the right; 0.60-0.61 on the left side). Interobserver repeatability was moderate or poor for non-neutral wrist posture, elevation of the upper arm and local mechanical pressure. Validity ranged from moderate to good for repetitive use of hand, use of hand force, pinch grip and non-neutral wrist posture when expert observation was used as reference standard. When observations were validated against force estimations (EMG) and wrist goniometer data, validity was poor. In the absence of generally accepted reference values, arbitrarily chosen limits were used for the proportional duration of some physical load factors. Studies should be carried out to assess the limits that best differentiate between safe and hazardous jobs.