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.     

Ergonomic design and evaluation of new surgical scissors

The purpose of this study is to design a new surgical scissors handle and determine its effectiveness with various usability indices. A new scissors handle was designed that retains the professional grip but has the shapes of the eye rings modified to fit the thumb and ring finger and finger rests for the index and little finger. The newly designed scissors and traditional scissors were compared by electromyography, subjective evaluation and task performance in experiments using cutting and peeling tasks. The newly designed scissors reduced muscle load in both hand during cutting by the closing action, and reduced the muscle load in the left hand during peeling by the opening action through active use of the right hand. In evaluation by surgeons, task performance improved in addition to the decrease in muscle load. The newly designed scissors used in this study demonstrated high usability.

Practitioner Summary: A new scissors handle was designed that has the eye rings modified to fit the thumb and ring finger. The newly designed scissors reduced muscle load and enabled active use of the right hand. In evaluation by surgeons, task performance improved in addition to the decrease in muscle load.     

IEA Newsletter

Five grip spans (45 to 65 mm) were tested to evaluate the effects of handle grip span and user's hand size on maximum grip strength, individual finger force and subjective ratings of comfort using a computerised digital dynamometer with independent finger force sensors. Forty-six males participated and were assigned into three hand size groups (small, medium, large) according to their hands' length. In general, results showed the 55- and 50-mm grip spans were rated as the most comfortable sizes and showed the largest grip strength (433.6 N and 430.8 N, respectively), whereas the 65-mm grip span handle was rated as the least comfortable size and the least grip strength. With regard to the interaction effect of grip span and hand size, small and medium-hand participants rated the best preference for the 50- to 55-mm grip spans and the least for the 65-mm grip span, whereas large-hand participants rated the 55- to 60-mm grip spans as the most preferred and the 45-mm grip span as the least preferred. Normalised grip span (NGS) ratios (29% and 27%) are the ratios of user's hand length to handle grip span. The NGS ratios were obtained and applied for suggesting handle grip spans in order to maximise subjective comfort as well as gripping force according to the users' hand sizes. In the analysis of individual finger force, the middle finger force showed the highest contribution (37.5%) to the total finger force, followed by the ring (28.7%), index (20.2%) and little (13.6%) finger. In addition, each finger was observed to have a different optimal grip span for exerting the maximum force, resulting in a bow-contoured shaped handle (the grip span of the handle at the centre is larger than the handle at the end) for two-handle hand tools. Thus, the grip spans for two-handle hand tools may be designed according to the users' hand/finger anthropometrics to maximise subjective ratings and performance based on this study. Results obtained in this study will provide guidelines for hand tool designers and manufacturers for designing grip spans of two-handle tools, which can maximise handle comfort and performance.     

Evaluation of total grip strength and individual finger forces on opposing (A-type) handles among Koreans

The present study evaluated the effect of grip span on finger forces and defined the best grip span for maximising total grip strength based on the finger forces and subjective discomfort in a static exertion. Five grip spans (45, 50, 55, 60 and 65 mm) of the opposing (A-type) handle shape were tested in this study to measure total grip strength and individual finger force among Korean population. A total of 30 males who participated in this study were asked to exert a maximum grip force with two repetitions, and to report the subjective discomfort experienced between exertions using the Borg's CR-10 scale. The highest grip strength was obtained at 45 mm and 50 mm grip spans. Results also showed that forces of all fingers, except for the middle finger force, significantly differed over the grip spans. The lowest subjective discomfort was observed in the 50 mm grip span. The results might be used as development guidelines for ergonomic opposing (A-type) hand tools for Korean population.     

Study on the grip spans of combination pliers in a maximum gripping task

A newly developed system was applied in this study to evaluate the effects of the grip spans (45–80 mm) of combination pliers on the total grip strength, individual finger force, resultant force, and subjective discomfort. A total of twenty-six males participated and were asked to exert their maximum grip strength with two repetitions. The highest and the lowest total grip strength and resultant force (311.8 N and 737.9 N vs. 210.1 N and 501.7 N) were obtained at a 60 mm and 80 mm grip spans, respectively. In general, the participants considered the 50 and 60 mm grip spans as being the least discomfort, whereas the 80 mm grip span was considered as the most discomfort grip span in a maximum grasping task. The results can be utilized as basic data for the manufacturing and design industries of two-handle hand tools, such as pliers and wrenches.Practitioners summaryCustom-made combination pliers were applied in this study to evaluate grip strength, resultant force, and subjective discomfort, relative to five grip spans. The authors expect that the results of the present study will provide valuable information for the designers and users of pliers.     

Ergonomic evaluation of an alternative tool for cake decorating

AimCake decorating involves several hand intensive steps with high grip force during the application of icing. The purpose of this laboratory study was to evaluate forearm muscle activity, discomfort, productivity, and usability of an alternative tool for cake decorating compared to decorating with the traditional piping bag.MethodsParticipants (n = 17) performed 2 h of cake decorating tasks using the two tools. Subjective hand and arm fatigue, usability, upper extremity posture, and muscle activity from three forearm muscles were assessed for each tool. Outcome measures were evaluated using the Wilcoxon Signed Rank test and the paired t-test.ResultsLess fatigue was reported in the dominant hand (p = 0.001), forearm (p = 0.003) and shoulder (p = 0.02) for the alternative tool when compared to the piping bag. Average median (APDF 50%) and peak (APDF 90%) muscle activity was significantly less for the alternative tool across all three forearm muscles. The alternative tool significantly reduced grip force, an important risk factor for distal upper extremity pain and disorders. Participants rated usability of the alternative tool superior for refill and comfort but the traditional method was rated better for accuracy, stability, positioning and control.ConclusionsThe alternative tool was associated with less dominant arm fatigue, muscle activity, and grip force when compared with the piping bag. However, the alternative tool did not receive the best overall usability rating due to problems with accuracy and overflow, especially with smaller decorating tips. Recommendations were made for addressing these problems with the alternative tool.     

Ergonomic evaluation of the musculoskeletal risks in a banana harvesting activity through qualitative and quantitative measures,with emphasis on motion capture (Xsens) and EMG

The objective of this study was to evaluate the risk of musculoskeletal injuries in a banana processing task. A sample of three workers were evaluated with the Nordic questionnaire as well as according to effort, discomfort and usability levels, hand grip dynamometry, inertial sensor motion capture (Xsens) and surface electromyography in the right arm. The workers had musculoskeletal complaints throughout the spine and hips, low effort and discomfort levels, and a satisfactory usability level. The hand grip strength values (dominant: 54.8 Kgf; non-dominant: 54.2 Kgf) were above the reference values for the Brazilian population. Regarding movement analysis, we identified risks related to cervical protrusion, forward and lateral trunk flexion, abduction and flexion in the shoulders, flexion, extension, radial and ulnar deviation in the wrists, as well as repetitive movements. Surface electromyography showed that the greatest demand during the task was on the extensor carpi radialis, although the trapezius and flexor carpi radialis showed a tendency toward fatigue. Thus, the set of evaluations showed that the tool used and worker adaptation to the position of the bunches, together with the repetitious movement, led to inadequate postures and musculoskeletal risks.Relevance to industryThe banana processing activity generates employment and income in several regions of the world and in Brazil. However, it is still rudimentary and may present risks to the worker's health. This study seeks to elucidate ergonomic risks through technological instrumentation in order to have a quantitative assessment of the ergonomic risks.     

Effects of target location, stature and hand grip type on in-vehicle reach discomfort

In order to improve car interior design, data of perceived discomfort and reach posture were collected for 75 different target locations. Altogether, 24 males and females of different statures participated in the experiment. In addition to three-finger grip, index fingertip reach and five finger grip were also compared. The effects of target location, stature and hand grip on reach discomfort were analysed. Predictive regression equations were provided. In addition to the confirmation of target location effects, the results showed that seat back and steering wheel affected discomfort. Their effects differed according to the subject's anthropometry. A detailed analysis of possible interference between the car interior and reach movement showed that short females were more likely impeded by the seat back when a target was close to the body. A significant difference between three hand grip types could be explained by the change of hand reach distance when changing hand grip type. STATEMENT OF RELEVANCE: The present study analysed the effects of target location, stature and hand grip type on reach discomfort, based on the statistical analysis of subjective ratings when reaching a target in a vehicle. The results would help to optimise the location of automotive controls for improving car interior design.     

Individual finger contribution in submaximal voluntary contraction of gripping

The objective of this study was to evaluate individual finger force and contribution to a gripping force, the difference between actual and expected finger forces and subjective discomfort rating at 10 different submaximal voluntary contraction (%MVC) levels (10–100% in 10 increments). Seventy-two participants randomly exerted gripping force with a multi-finger force measurement system. The individual finger force, gripping force and discomfort increased as %MVC levels increased. The middle and ring fingers exerted more force and contributed to a gripping force more than the index and little fingers due to their larger mass fractions of the digit flexor muscles. It was apparent at <50% MVC; however, the index finger increased its contribution and exerted even more force than expected at more than 50% MVC. Subjective discomfort supported the results of the objective measures. This could explain the conflicting findings between index and ring fingers in previous finger contribution studies.

Statement of Relevance: Hand tool design is of special interest in ergonomics due to its association with musculoskeletal disorders in the hand. This study reveals a different contribution pattern of the fingers in submaximal voluntary contraction of gripping exertion.     

Hand strength: the influence of grip span and grip type

The maximal force from each of the fingers II-V (FF) and the resultant force between the jaws of the tool (RF), due to contribution from all fingers, were measured using a pair of modified pairs. The RF was measured at 21 handle separations and the FF was measured at seven handle separations for each finger. A traditional grip type was compared with a 'reversed' grip where the little finger was closest to the head of the tool. Sixteen subjects (8 females and 8 males) participated in the study. Both the RF and FF varied according to the distance between the handles. For both grip types, the highest RF was obtained at a handle separation of 50-60 mm for females and 55-65 mm for males. For wide handle separations, the RF was reduced by 10% (cm increase in handle separation). The force-producing ability of the hand was influenced by the grip type and the highest RF was obtained when using the traditional grip. An interaction was found between the fingers, i.e., the maximal force of one finger depended not only on its own grip span, but also on the grip spans of the other fingers. About 35% of the sex difference in hand strength was due to hand size differences.     

Power grip strength as a function of tool handle orientation and location

Thirty male volunteers participated in a study evaluating the effect of workspace envelope (work height and reach distance) and handle orientation on grip force capacity. Maximum voluntary power grip exertions were recorded using instrumented tool handles under three conditions: a pistol grip tool handle oriented horizontally and vertically and a right angle tool handle oriented horizontally. Significant main effects of handle height and reach location on normalized grip force capacity were observed with the horizontally oriented pistol grip and right angle handles, whereas only an interaction effect was observed with the vertically oriented pistol grip handle. Comparison of results to scores produced with a job assessment tool (RULA) is included as an appendix. The proposed methodology can provide information useful to job, workstation or tool design directed toward best accommodating the physical capacities of workers performing hand tool tasks.     

Power grip strength as a function of tool handle orientation and location

Thirty male volunteers participated in a study evaluating the effect of workspace envelope (work height and reach distance) and handle orientation on grip force capacity. Maximum voluntary power grip exertions were recorded using instrumented tool handles under three conditions: a pistol grip tool handle oriented horizontally and vertically and a right angle tool handle oriented horizontally. Significant main effects of handle height and reach location on normalized grip force capacity were observed with the horizontally oriented pistol grip and right angle handles, whereas only an interaction effect was observed with the vertically oriented pistol grip handle. Comparison of results to scores produced with a job assessment tool (RULA) is included as an appendix. The proposed methodology can provide information useful to job, workstation or tool design directed toward best accommodating the physical capacities of workers performing hand tool tasks.     

Ergonomics evaluation of a foam rubber grip for tool handles

An important consideration in tool design is the avoidance of hand discomfort and a reduction of work efficiency by using proper grip designs. To measure grip force distribution on a tool handle, Force Sensing Resistors were calibrated, placed on two different types of grip (wood and foam) and interfaced to a personal computer. The results, using six common garden tools - loppers, hedge shears, shovels, leaf rakes, hoes, garden rakes - indicated a very uneven distribution in grip force, but with the foam grip providing a more uniform distribution. Unfortunately, in most cases, the tool grip force was greater for the foam grip due to the deformation of the foam and a 'loss of control' feeling in the subjects. However, most subjects strongly preferred the foam grips.     

Characterisation of forces exerted by the entire hand during the power grip: effect of the handle diameter

The objective of this study was to analyse the effect of the handle diameter on the grip forces exerted by the hand during a maximal power grip task. A handle ergometer, combining six instrumented beams and a pressure map, was used to determine the forces exerted by the palm side of the hand regrouping data from 10 anatomical sites (fingertips, phalanges, thumb, palm…). This methodology provided results giving new insight into the effect of the handle diameter on the forces exerted by the hand. First, it appeared that the relationship between the hand length/handle diameter ratio and the maximal grip force fit a U-inverted curve with maximal values observed for a handle diameter measuring 17.9% of the hand length. Second, it was showed that the handle diameter influenced the forces exerted on the anatomical sites of the hand. Finally, it was showed that the handle diameter influenced the finger force sharing particularly for the index and the little fingers. Practitioner Summary: This study analysed the effect of the handle diameter on the grip forces exerted by the hand during a maximal power grip force. This study showed that measurement of the totality of the forces exerted at the hand/handle interface is needed to better understand the ergonomics of handle tools. Our results could be re-used by designers and clinicians in order to develop handle tools which prevent hand pathologies.     

Individual finger contribution in submaximal voluntary contraction of gripping

The objective of this study was to evaluate individual finger force and contribution to a gripping force, the difference between actual and expected finger forces and subjective discomfort rating at 10 different submaximal voluntary contraction (%MVC) levels (10-100% in 10 increments). Seventy-two participants randomly exerted gripping force with a multi-finger force measurement system. The individual finger force, gripping force and discomfort increased as %MVC levels increased. The middle and ring fingers exerted more force and contributed to a gripping force more than the index and little fingers due to their larger mass fractions of the digit flexor muscles. It was apparent at <50% MVC; however, the index finger increased its contribution and exerted even more force than expected at more than 50% MVC. Subjective discomfort supported the results of the objective measures. This could explain the conflicting findings between index and ring fingers in previous finger contribution studies. STATEMENT OF RELEVANCE: Hand tool design is of special interest in ergonomics due to its association with musculoskeletal disorders in the hand. This study reveals a different contribution pattern of the fingers in submaximal voluntary contraction of gripping exertion.     

Using pliers in assembly work: short and long task duration effects of gloves on hand performance capabilities and subjective assessments of discomfort and ease of tool manipulation

The present study investigated the effects of wearing typical industrial gloves on hand performance capabilities (muscle activity, wrist posture, touch sensitivity, hand grip and forearm torque strength) and subjective assessments for an extended duration of performing a common assembly task, wire tying with pliers, which requires a combination of manipulation and force exertion. Three commercially available gloves (cotton, nylon and nitrile gloves) were tested and compared with a bare hand condition while participants performed the simulated assembly task for 2 h. The results showed that wearing gloves significantly increased the muscle activity, wrist deviation, and discomfort whilst reducing hand grip strength, forearm torque strength and touch sensitivity. The combined results showed that the length of time for which gloves are worn does affect hand performance capability and that gloves need to be evaluated in a realistic working context. The results are discussed in terms of selection of gloves for industrial assembly tasks involving pliers.     

Effects of two ergonomic aids on the usability of an in-line screwdriver

We assessed the effects of two ergonomic aids on the usability of an in-line screwdriver. The simple aids considered were a hand support and a sleeve which softened the handle. In an experiment, 15 subjects drove screws into horizontal plywood plates with four combinations of a screwdriver and an ergonomic aid. The activities of four forearm muscles (EMG) and the force acting on the screw were measured. The subjects rated the properties of the tools as regards physical exertion, the ease of taking hold of the tool, comfort of the grip, and the surface texture. The hand support and the sleeve had positive effects on the subjective perception of exertion and the surface texture, respectively. Differences in EMG and thrust force along the shaft of the screwdriver handle were found to be too small for reliable deductions in the effects of the ergonomic aids.     

Hand tool handle size and shape determination based on hand measurements using a contour gauge

The purpose of this study is to provide a novel approach to tool handle design and development based on measurements of hand shape using a contour gauge. In general, traditional design techniques, designing based on anthropometric data, and derived mathematical models do not incorporate enough subject data to design a customized product. First, anthropometric measurements on the right hand of 60 participants were collected with a contour gauge to manufacture matching handles. A curved handle fitting the human hand was constructed with common computer‐aided design software, and cylindrical handles and elliptical handles were added for comparison. All of the handles were used to record the participants' grip force to evaluate the operating efficiency of the handles. Finally, the participants completed a comfort‐rating questionnaire. The results show that contours based on the hand provided the highest operating performance and the best overall comfort‐rating compared to cylindrical handles and elliptical cylindrical handles. The newly developed handles in the grip force tasks have the highest push performance and the best comfort ratings compared to traditional cylindrical and elliptical handles. The developed handles could provide the hand tool industry information on developing and manufacturing many other similar handle designs (such as those for saws and electric screwdrivers).     

Ergonomic design and evaluation of the handle for an endoscopic dissector

The purpose of this study was to design an endoscopic dissector handle and objectively assess its usability. The handles were designed with increased contact area between the fingers and thumb and the eye rings, and the eye rings were modified to have a more perpendicular insertion angle to the finger midline. Four different handle models were compared, including a conventional product. Subjects performed dissection, exclusion, grasping, precision manipulation and precision handling tasks. Electromyography and subjective evaluations were measured. Compared to conventional handles, the designated handle reduced the muscle load in the extensor and flexor muscles of the forearm and increased subjective stability. The activity of the first dorsal interosseous muscle was sometimes influenced by the shape of the other parts. The ergonomically designed endoscopic dissector handle used in this study achieved high usability. Medical instrument designs based on ergonomic concepts should be assessed with objective indices.

Practitioner Summary: The endoscopic dissector handles were designed with increased contact area and more suitable insertion angle between the fingers and thumb and the eye rings. Compared to conventional handles, the designated handle reduced the muscle load in the extensor and flexor muscles of the forearm and increased subjective stability.     

Short and longer duration effects of protective gloves on hand performance capabilities and subjective assessments in a screw-driving task

The study investigated short and longer duration effects of gloves on hand performance capabilities (muscle activity, dexterity, touch sensitivity, finger pinch and forearm torque strength) and subjective assessments of discomfort and ease of manipulation when performing a light assembly task. The independent variables were hand condition with four levels (wearing cotton, nylon or nitrile gloves as well as barehanded) and point of time within the 2 h duration of the task (with measurements taken at 0, 30, 60, 90 and 120 min). Participants worked with a screwdriver to fit two components together using screws. Wearing gloves significantly increased the muscle activity, pinch strength and discomfort but reduced the dexterity and touch sensitivity. There was also a significant effect of task time on the muscle activity, dexterity, forearm torque strength and touch sensitivity, which indicates that the duration of the task should be an important consideration in glove evaluation studies and in the selection of work gloves.

Statement of Relevance:It is important to evaluate the effects of gloves on hand performance capabilities in a working context so that job demands can be taken into account and the most appropriate type of glove be chosen for each task. This study gives recommendations regarding the evaluation and use of gloves for screw-driving tasks.