Postures adopted when using a two-wheeled cylinder trolley

Upper limb and trunk positions were investigated when subjects exerted force to start and move a two-wheeled cylinder trolley with four different handles. Three of the handles had orientations 35 degrees, 50 degrees and 70 degrees in the sagittal plane (relative to the back of the trolley). The fourth was the 50 degrees handles fitted with a link bar. Measurements were made of x (horizontal), y (lateral) and z (vertical) coordinates of the body joints in space throughout the motion, using a CODA MPX30 optical 3-D measurement system, and both linear and angular displacements were computed. The linear and angular positions of the body joints were found to change through the stages of the task. Orientation of the handle influenced the arm postures adopted in the task, and showed to produce different patterns of arm displacement in force exertion to tilt the trolley from standing position. For steady movement, the sagittal plane 50 degrees handle proved most desirable.     

Effects of handle orientation,gloves, handle friction and elbow posture on maximum horizontal pull and push forces

Biomechanical models were evaluated for effects of handle orientation, handle material, gloves and arm posture on maximal pull/push force. Eight healthy subjects performed maximum pull/push exertions on handles with two different orientations and two different surface materials, using bare hand and two types of glove as well as two arm postures. The empirical data supported the proposed biomechanical models: Pull/push forces for the bare hand on a rubber handle decreased 10% when the handle was parallel to the pull/push direction, compared with when perpendicular to it. For parallel handles, pull/push forces further decreased with decreasing hand–handle friction coefficient (simulated by different handle materials and gloves). Pull force exerted by the bare hand was 29% greater when the elbow was extended than when flexed. Pull force was greater than push force (with bare hand and flexed elbow). The biomechanical models suggest that friction between the hand and handle limits pull/push forces for parallel handles. Elbow strength may be responsible for decreased pull force for the flexed elbow posture and decreased force for pull compared with push in the postures examined.

Statement of Relevance: Biomechanical models presented in this paper provide insights for causes of upper extremity strength limitations during pull/push tasks. Findings in this paper can be used directly in the design of workstation and objects to reduce fatigue and risk of musculoskeletal disorders.     

A comparative investigation of the influence of certain arm positions on hand pinch grips in the standing and sitting positions of dentists

Hand pinch grips in the standing and sitting positions on a group of 46 healthy males of 20 to 26 years old were measured. The results were as follows: 1. Hand pinch grip forces are higher when the subject is standing than when he is sitting. 2. Hand pinch grip forces, depending on the position of the arm in the working space, are higher when the arm is supported than the corresponding forces of the unsupported arm. 3. There is an effect due to the position of the arm in relation to the frontal position of the subject's thorax. In the standing position, the forces are maximum when the forearm has a 60 degrees angle towards the frontal position, while in the sitting position pinch grip forces are maximum when the forearm is perpendicular (90 degrees ) to the frontal position. 4. A handle which permits all fingers to be spread in a pinch grip is capable of having an applied force 50% greater than if the thumb and either forefinger or middlefinger grips the handle. In such a handle each finger is required to apply less force to contribute to the total needed for the task, and therefore there is a diminished likelihood of the onset of fatigue.     

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 degrees, while novice users allowed 11.5 degrees. 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.     

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.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a force-transducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45 degrees (SD 3.3 degrees) downward to near horizontal, while the pulling force direction changed from pulling upward by 14 degrees (SD 15.3 degrees) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

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.     

Effects of standardized foot positions during the execution of a submaximal pulling task

This study examined how experimentally controlled foot positions could affect the temporal and spatial parameters of a load (20% of subject mass) during a one-handed repetitive submaximal pulling activity. Foot positions standardized relative to a frontal and sagittal plane defining a pull force vector were derived based on the preferences of 45 volunteer subjects. In general, the subjects assumed asymmetrical foot positions and on average the big toe of the foot contralateral to the hand exerting the pull was located 19% (SD = 4.4) of stature in front of the frontal plane containing the pull origin and 8.6% (SD = 4.5) of stature laterally from the sagittal plane through which the load was displaced. The big toe of the foot ipsilateral to the hand exerting the force was located at a distance of 46.7% (SD = 6.3) of stature in front of the frontal plane containing the pull origin and 0.4% (SD = 3.9) of stature laterally from the sagittal plane through which the load was displaced. The left and right feet were orientated at angles of 56.8 degrees (SD = 20.2 degrees ) and 25.9 degrees (SD = 22.7 degrees ), respectively, relative to a right horizontal of a frontal plane parallel to the plane containing the origin of pull. These foot positions were subsequently employed in a second experiment to investigate how dictating foot positions would affect the way in which 15 newly recruited subjects exerted a pull force on the same load. Results from this experiment showed that the load velocities and forces were not affected by standardized foot positions when compared to those collected when subjects were free to choose foot orientations. It is suggested that future researchers should consider the benefits of employing standardized foot positions in studies of pull exertions, particularly for methodologies similar to that described in this study.     

The influence of arm position on the pinch grip strength of female dentists in standing and sitting positions

The objective of this study was to measure and compare hand pinch grip forces in standing and sitting postures for a group of 77 healthy female dentists of 25 to 55 years old. The equipment used consisted of a gripping device with strain gauges, a measuring device and a printer. The results can be described as follows. With respect to absolute values the hand pinch forces are higher when the subject is standing with her arm supported, than when she is sitting. In the standing position the measured forces were maximum when the forearm was at 60 degrees angle towards the frontal position, while in the sitting position the pinch grip forces were maximum with the forearm vertical (90 degrees ) with respect to the frontal position. The above data can be used to design the dentist's working area.     

Pouring liquid from a pot-kinematics of an everyday task

An experiment was carried out to investigate the effects of handle design on postures in the use of pots. Six combinations of handle (sloping, vertical and curved) and spout designs (long and short) were tested during grasping, lifting and pouring. The fill load was 1000 ml of water at the prevailing room temperature (about 20 degrees C) and three males and three females were involved as subjects. Measurements were made of x- (horizontal), y- (lateral) and z- (vertical) co-ordinates of the wrist, elbow shoulder and hip joints in space throughout the motions, using a CODA MPX30 optical 3D measurement system and both linear and angular displacements of the arm joints and trunk were computed. Very different configurations of arm posture were found to be adopted at the moment of pot lift, and when pouring commences and the subjects' movement patterns during transfer of the pot to fill vessel showed to be influenced by the configuration of arm posture adopted at the moment of pot lift. For lifting the pot, the vertical handle design proved to be more desirable than either the curved or sloping design, while for pouring from the pot the short spout proved to be more desirable than the long spout, at least when the pot was full. The results are however, not clear on which handle or spout design is best for transferring the pot.     

An investigation on the relationship between grip, push and contact forces applied to a tool handle

Owing to the strong dependence of the health risks associated with vibration exposure of the human hand and arm on hand force, a laboratory study was conducted to develop a methodology for measurement of the contact force at the tool handle–hand interface, and to identify the relationship between the contact force and the hand grip and push forces. A simulated tool handle fixture was realized in the laboratory to measure the grip and push forces using compression/extension force sensors integrated within the handle and a force plate, respectively. The contact force was derived through integration of the interface pressure over the contact area. These were measured using a capacitive pressure-sensing grid. The measurements were performed with 10 male subjects and three circular cross-section handles of different sizes under different combinations of grip and push forces. The hand–handle interface pressure data were analyzed to derive the contact force, as functions of the constant magnitudes of the grip and push forces, and the handle size. The results suggest that the hand–handle contact force is strongly dependent upon not only the grip and push forces but also the handle diameter. The contact force for a given handle size can be expressed as a linear combination of grip and push forces, where the contribution of the grip force is considerably larger than that of the push force. The results further suggest that a linear relation can characterize the dependence of the contact force on the handle diameter. The validity of the proposed relationship is demonstrated by evaluating the magnitudes of errors between the estimated contact forces with the measured data for the range of handle diameters, and grip and push forces considered in the study.

Relevance to industry

The methodology proposed in this study can be applied to measure the effective hand–handle contact force at workplaces for assessing the health risks associated with exposure to hand-transmitted vibration exposure and hand–wrist cumulative trauma. The relationship proposed in the study could be effectively applied for estimating the hand–handle contact force from known grip and push forces that are conveniently and directly measurable in laboratory studies involving vibration analyses of the human hand, power tools and relevant vibration attenuation devices. It is expected to be most useful in field applications, where it could provide an estimate of the range of magnitudes of the hand-grip force applied to the handle of an actual tool, which is quite difficult and expensive to measure. The relationship is also expected to contribute to the on-going standardization efforts for defining a correction factor to account for the effects of hand force on the vibration transmission and hand injuries.     

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.     

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.     

Effects of handle orientation and between-handle distance on bi-manual isometric push strength

Hand-handle interface is seldom considered in contemporary upper limb biomechanical analyses of pushing and pulling strength. A laboratory study was designed to examine if handle rotation in the frontal plane (0°-horizontal, 45°, and 90°-vertical), anterior tilt (0°-parallel to the frontal plane, and 15°), and distance between two handles (31 and 48.6 cm) affect pushing strength and subjective rating of handle preference. A special testing station was constructed to elicit upper limb push exertions that involved minimal contribution of the torso and legs. Within the station, four load cells were used to measure the horizontal (forward pushing) and vertical components of the pushing forces. Thirty-one participants performed seated bi-manual pushing strength tests. Comparing to the reference handle configuration (horizontal, straight, and a 31-cm between-handle distance), the 45°-rotated and tilted handles with a 31-cm between-handle distance allowed 6.7% more pushing output, while the horizontal and tilted handles with a 31-cm between-handle distance resulted in 2.8% less. Subjective preference was correlated with normalized pushing strength (r=0.89). Tilted handles, at 45°-rotated and vertical positions received highest subjective ratings of preference among all handle configurations. Men exerted greater pushing strength with the 48.6-cm handle distance while women's capacity was greatest with the 31-cm distance. The results demonstrated that handle rotation and tilt angles affected pushing strength and should be taken into consideration when evaluating or designing pushing 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 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.     

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.     

Effect of horizontal position of the computer keyboard on upper extremity posture and muscular load during computer work

The distance of the keyboard from the edge of a work surface has been associated with hand and arm pain; however, the variation in postural and muscular effects with the horizontal position have not been explicitly explored in previous studies. It was hypothesized that the wrist approaches more of a neutral posture as the keyboard distance from the edge of table increases. In a laboratory setting, 20 adults completed computer tasks using four workstation configurations: with the keyboard at the edge of the work surface (NEAR), 8 cm from the edge and 15 cm from the edge, the latter condition also with a pad that raised the work surface proximal to the keyboard (FWP). Electrogoniometers and an electromagnetic motion analysis system measured wrist and upper arm postures and surface electromyography measured muscle activity of two forearm and two shoulder muscles. Wrist ulnar deviation decreased by 50% (4 degrees ) as the keyboard position moved away from the user. Without a pad, wrist extension increased by 20% (4 degrees ) as the keyboard moved away but when the pad was added, wrist extension did not differ from that in the NEAR configuration. Median values of wrist extensor muscle activity decreased by 4% maximum voluntary contraction for the farthest position with a pad (FWP). The upper arm followed suit: flexion increased while abduction and internal rotation decreased as the keyboard was positioned further away from the edge of the table. In order to achieve neutral postures of the upper extremity, the keyboard position in the horizontal plane has an important role and needs to be considered within the context of workstation designs and interventions.     

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.     

Keyswitch orientation can reduce finger joint torques during tapping on a computer keyswitch

OBJECTIVE: To examine the effects of keyswitch orientation on joint torques. BACKGROUND: The fingertip produces primarily vertical forces during single-finger tapping on a computer keyswitch. However, horizontal force components within the sagittal plane of the finger could reduce net joint torques. METHOD: Eleven participants tapped on a keyswitch oriented in three directions: vertical, tilted 30 degrees such that when pressed it moved away from the user (similar to a positive-tilt keyboard), and tilted 30 degrees such that when pressed it moved toward the user (similar to a negative-tilt keyboard). Participants also tapped on a prototype cantilever keyswitch design in which the key cap moves along the arc of a bending beam gradually away from the user. Miniature electro-optical goniometers measured the finger posture, and a two-axis force sensor measured fingertip forces. RESULTS: Tapping on a keyswitch oriented such that it moves away from the user when pressed reduced net joint torques by 47% relative to tapping on a vertically orientated keyswitch and by 56% relative to tapping on a keyswitch oriented toward the user, whereas the cantilever design resulted in 14% decreases in net joint torque relative to the vertical orientation. CONCLUSION: Reductions of torques resulted from decreasing the moment arm of the fingertip force about the joints. APPLICATION: Keyboard design should incorporate keyswitch mechanism angles along with other postural and geometric constraints to reduce exposure of the finger joints and muscles to force during typing.