Tool-handle Design Based on a Digital Human Hand Model

um

Laboratory for Intelligent CAD Systems, Faculty of Mech

a r t i c l e i n f o

Article history:Received 7 September 2012Received in revised form16 January 2013Accepted 7 May 2013Available online 19 June 2013

Keywords:Product developmentErgonomicsTool-handle designHandles shapeDigital human hand-model3D hand model

. All rights reserved.

Ergonomic principles should be included in the phase of in-dustrial/mechanical product design before the engineers tackle theproblem, because the main function of the product and the form ofthe product are usually strongly connected (Hogberg et al., 2008;Shuxing et al., 2008). The whole human-product system

e a designer has toe expected system

efciency and prevent cumulative trauma disorders (CTD) of theusers (Hogberg et al., 2008).

A signicant part of manual work is still done with hand-tools,despite the automation in many industries. Badly-designed hand-tools can induce upper-extremity musculoskeletal disorders; suchas carpal tunnel syndrome, hand-arm vibration syndrome (HAVS),tendonitis, etc. These CTDs account for about one third of the sickleaves of workers, resulting in high workers compensations claims(Punnett and Wegman, 2004).

* Corresponding author. Tel.: 386 2 220 76 93; fax: 386 2 220 79 94.E-mail addresses: [email protected] (G. Harih), [email protected]

Contents lists available at

International Journal of

.e l

International Journal of Industrial Ergonomics 43 (2013) 288e295(B. Dolsak). 2013 Elsevier B.V

1. Introduction performance is also human-dependent, thereforconsider the ergonomics in order to achieve thincreases the contact area and the subjective comfort-rating, thus increasing user performance andlowering the risk of CTD.0169-8141/$ e see front matter 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.ergon.2013.05.002anical Engineering, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia

a b s t r a c t

A signicant part of manual work is still done using hand-tools. Therefore, a correct design is crucial forpreventing upper-extremity musculoskeletal disorders, such as carpal tunnel syndrome, hand-arm vi-bration syndrome, tendonitis, etc. When considering the ergonomics of a hand-tool, in addition to itsmain functionality, the most important part is the tools handle. Most of the authors have consideredcylindrical handles and provided guidelines and mathematical models for determining optimal di-ameters in order to maximise nger-force exertion, comfort, contact area, thus minimising the chances ofcumulative trauma disorders (CTD). However, they have not taken into account the shape of the handduring optimal power-grasp posture when determining the tool-handles shapes, which could addi-tionally improve the handles ergonomics. In order to overcome this limitation, we have developed ananatomically accurate static digital human-hand model (DHHM). The developed DHHM allows directtool-handle modelling and does not require an iterative design process when designing a tool-handlewith improved ergonomics. In order to develop DHHM, anthropometric measurements on ten subjectswere performed for the manufacturing of corresponding optimal cylindrical pre-handles with variablediameters for each nger. Outer hand moulds were manufactured based on the pre-handles for obtainingthe shape of the hand with skin and subcutaneous tissue undeformed. Magnetic resonance imaging wasconducted with the outer hand moulds attached, and segmentation and 3D reconstruction were per-formed on the images to obtain the DHHMs for each subject. Tool-handles based on DHHM were thenobtained within common Computer-Aided Design software. Measurements on the handles based on theDHHM have shown that they provide; on average; an over 25% higher contact area compared to thecorresponding cylindrical handle. With higher contact area and anatomical shape of the handle,extensive deformation of the soft tissue can be avoided, thus preventing excessive load on the hand.Subjects also compared these DHHM handles with cylindrical handles regarding perceived subjectivecomfort-rating. It was shown that those tool handles based on the DHHM provided a higher overallcomfort-rating compared to cylindrical handles. It has also been demonstrated that anatomically shapedtool-handles based on the developed DHHM can improve user performance and lower the risk of CTD.Relevance to industry: This paper introduces methods for developing a static DHHM for an optimal power-grasp posture by directly modelling a tool-handle with improved ergonomics. It also demonstrates thatanatomically-shaped tool-handles based on the developed DHHM with optimal power-grasp postureGregor Harih*, Bojan DolsakTool-handle design based on a digital h

journal homepage: wwwAll rights reserved.an hand model

SciVerse ScienceDirect

Industrial Ergonomics

sevier .com/locate/ergon

of InThe broad variety of powered and non-powered hand-tools hassteered many authors; when researching the topic of tool-handledesign; into dening the optimal sizes and shapes of tool-handles. A correctly-designed handle can provide safety, comfortand increased performance (Eksioglu, 2004). Most authors havefocused on the cylindrical or elliptical shapes of the handles, butnone of them have considered the anatomical shape of the handwhen in the optimal power-grasp posture during the designing oftool-handles. It has been shown that the maximum voluntarynger contraction force is diameter-dependent, therefore handlesshould vary in size according to hand and nger sizes (Kong andLowe, 2005). Therefore the authors Garneau and Parkinson (2011)suggested that any further research into this topic shouldconsider the shape of the hand at its optimal power-grasp posturein order to obtain maximum grip-force, with lowest stresses onligaments, tendons, and soft tissue, thus lowering the risk of CTDs(Khalil, 1973).

The mechanical behaviour of the skin and subcutaneous tissueis crucial during gripping tasks, since forces and moments aretransferred from the tool to the whole hand-arm system. Skin andsubcutaneous tissue have non-linear viscoelastic properties,where the skin is stiffer than the subcutaneous tissue (Wu et al.,2007). Both have low stiffness region at small strains; followedby a greater increase in stiffness when increasing the strain. Apower-grasp can also yield a contact pressure on the ngertip of80 kPa, which creates excessive loading for the skin and subcu-taneous tissue (Gurram et al., 1995). It has been shown, that anyhigher contact pressures than allowed for over a specic time canresult in discomfort, pain, and ischemia. Excessive loading can alsoresult in other CTDs; such as carpal tunnel syndrome (Eksioglu,2004). Handehandle contact-force and therefore the contactpressure; as well as the grip and push-forces, are also handlediameter dependant (Welcome et al., 2004). The smallest inves-tigated cylindrical handle (30 mm) has shown to yield in highestmagnitude of contact-force, which also suggests highest contactpressure. Aldien et al. (2005) have shown that the higher grip andpush-forces on a cylindrical handle can produce concentratedcontact-forces and pressures that exceed the limit of pressurediscomfort (PDT) and sustained pressure (SP) values for preservingwork efciency over a working day. Therefore authors havealready suggested that further research should identify a handlesize and shape that distributes the contact-forces and pressuresmore evenly with PDT and SP within acceptable values. Many ofpowered hand-tools produce vibrations, which are transferredfrom the handle to the hand. Deformations of skin and subcu-taneous tissue whilst holding the tool; plus the vibration inducedby the tool; can lead to HAVS that may cause vascular, sensori-neural and musculoskeletal disorders (Bernard et al., 1998;Youakim, 2009).

This extensive ergonomic knowledge that is necessary duringthe design phase of a tool-handle; and its poor integration withexisting, well-established CAD software, has affected companiesthat do not or on very low scales address ergonomic principlesduring the design phase (Kaljun and Dolsak, 2012). In order toovercome this issue, several digital-human models (DHM) havebeen developed over recent decades. Within DHMs, the human isrepresented digitally inside a virtual environment, where analysescan be performed without physical prototypes (Demirel and Duffy,2007a; 2007b). Based on these analyses, safety and performancecan be predicted and design errors can be identied and correctedduring the design phase.

Usually those hand-arms of Digital HumanModels that are partsof a whole digital body models are used to evaluate the vision andclearance. Nowadays DHMs based on kinematics and biomechanics

G. Harih, B. Dolsak / International Journalare also used for evaluation of tasks; such as lifting or pushing(Chafn and Andersson, 1999). However; most of the DHMs do notincorporate anthropometric and anatomically-correct humanhands, thus preventing ergonomic analyses; and product and tooldevelopment where the grip is the main ergonomic design attri-bute. The level of accuracy regarding the ergonomic analyses ofhand-grip based on DHM relies on the models level of accuracy.Therefore those hands of DHM that only consider the kinematicsand biomechanics of the hand, but neglect the anatomical shape ofthe hand and soft tissue deformation whilst gripping, cannot beused for realistic ergonomic analyses; and product shape deter-mination and optimisation (Nierop et al., 2007).

In order to overcome this issue, a few authors have recentlydeveloped stand-alone anatomically-accurate Digital Human HandModels (DHHMs) for ergonomic evaluation of hand-held products.They are mostly anthropometric models that are modelled basedon magnetic resonance imaging (MRI) or computed tomography(CT) that utilise mathematical models to predict a viable humangrasp for a target product (Endo et al., 2007; Pea-Pitarch et al.,2009). However the complexity behind the phenomena ofgrasping can also lead to non-viable grasping of the product by themathematical models.

These DHHMs are designed for ergonomic analyses of existingvirtual 3D models of products, therefore designers still have topossess comprehensive knowledge of ergonomics in order to lowerthe design iterations and to obtain a product containing the desiredergonomics. DHHMs do not allow for direct development of theproducts shape and size that is within their interactions with thehumans.

Grasps generated by the mathematical model are usually eval-uated by the operator visually or by calculating grasp quality usingdifferent methods within the software. This kind of evaluation canbe unreliable, since real-world grasping is very complex and is alsodependents on the subjective comfort rating of the user (De Loozeet al., 2003). It has been shown that perceived subjective comfort isstrongly correlated with user performance, therefore it is necessaryto incorporate this aspect of product evaluation during the designphase (Kuijt-Evers et al., 2007). Comfort is affected by physical,physiological, and psychological factors; and is a subjectively-dened feeling that differs from person to person. Therefore itcannot be simply predicted neither by objective methods (such asgrip-force and pressure measurement, electromyography, biome-chanical hand-models, nite element analyses, etc.) nor by theresulting mathematical models that can only predict the physicalaspects on the perceived comfort (De Looze et al., 2003). Thus usingsubjective measurement methods is preferred when evaluating ahandle. The usages of hand-tools are mostly accompanied by feel-ings of discomfort that can be considered as a contradiction ofcomfort. Therefore designers have to optimise the size and theshape of the handle in order to reduce the discomfort (Kuijt-Everset al., 2004).

Therefore the aim of this paper was to overcome those limi-tations of current DHHMs regarding the tool-handle design thatrequire extensive ergonomic knowledge and iterative design pro-cess. Thus we propose methods for developing a static DHHM inits optimal power-grasp posture for directly modelling a corre-sponding tool-handle with improved ergonomics. The objectivewas to evaluate whether the developed DHHM based on optimalpower-grasp posture can lower the risk of CTD whilst increasingthe subjective comfort-rating. Therefore our approach also in-cludes user-testing and thereby real-world verication and vali-dation of the proposed methods and the resulting tool-handle;that would allow for the future optimisation of the tool-handlessizes and shapes; in order to dene optimised handles for a widerpopulation with lower risks of CTD and higher subjective comfort-

dustrial Ergonomics 43 (2013) 288e295 289ratings.

University Medical Centre Maribor. The MRI machine was a GE

5.3.3. (Visage Imaging) was used for the segmentation and 3D

necessary. Segmentation was done with a Label/Voxel module anda threshold value of 200, which proved to be the best value forobtaining the required segmentation. Small inclusions and seg-mentation errors were corrected by remove islands and ll holescommands. In order to achieve a smoother surface, a resamplemodule was added to the segmentation and 3D reconstructionprocess. The result was a smooth 3D representation of the subjectshand in the power-grasp posture (Fig. 3).

2.6. Power-grasp tool-handles shape determination

STL les obtained within Amira software were imported intoCATIA V5R20 (Dassault Systems). The mathematically-denedvolumetric model was dened based on generated surface modelout of the STL le. In order achieve the optimal handle for power-grasp posture, an elliptical cylinder was modelled in CATIA. The

of I2. Methods

2.1. Determination of the optimal cylindrical pre-handle

Different authors have used different criteria for determiningthe optimal cylindrical handle: subjective comfort-rating (Yakouet al., 1997; Hall and Bennett, 1956); nger-force measurement(Amis, 1987; Chen, 1991); muscle force minimisation (Sancho-Bruet al., 2003), and hand anthropometrics (Grant et al., 1992; Ohand Radwin, 1993; Johnson, 1993; Yakou et al., 1997; Blackwellet al., 1999; Garneau and Parkinson, 2010; Seo and Armstrong,2008). A few studies have also used two or more criteria: nger-force measurement and muscle activity (Ayoub and Presti, 1971;Grant et al., 1992; Blackwell et al., 1999); subjective comfort rating,nger-force measurement, and electromyographic efciency dur-ing muscle activity (Kong and Lowe, 2005). The broad varieties ofcriteria used for determining optimal cylindrical handle have alsodictated the usages of different methods.

In our study tenmale subjects with no hand injuries or disorderswere used to obtain the DHHMs for each subject. In order tocalculate the optimal diameters for the pre-handle, the followinganthropometric measurements were performed on the subjects:lengths of the index, middle, ring, and little ngers from the handswrist crease; ngertip lengths of the thumb, the index, and middlengers together with the inside grip breadth between the indexand middle ngers. Additionally the widths of each nger weremeasured for obtaining the corresponding section sizes for the pre-handle.

We evaluated and compared recent mathematical models fordetermining optimal diameters for the development of the pre-handle. The mathematical model from authors Seo andArmstrong (2008) was extended into the equation for a variablehandle diameters of the index and middle ngers according toGarneau and Parkinson (2010). It was assumed, that optimal handlediameters could also be obtained also for the ring nger and littlengers using the relationship between the length of the indexnger and the ring nger and little ngers. The obtained diameterswere veried according to the equation for optimal handle diam-eter from the study of Kong and Lowe (2005). The diameter dif-ference were calculated and proved to be negligibly small.

2.2. Optimal cylindrical pre-handle with variable diameters

Calculated diameters were used to manufacture customisedoptimally-cylindrical pre-handles with variable diameters; madefrom hard polyurethane. These cylindrical pre-handles with vari-able diameters were tested and it was shown that the calculateddiameters were correct. This was because; as calculated; there wasan overlap of the thumbs ngertip with the index ngertip andmiddle ngers (Fig. 1).

2.3. Manufacturing the outer hand mould

In order to obtain the shape of the hand in its optimal powergrasp posture with undeformed soft tissue, outer hand mouldswere manufactured to maintain the diameters and shape of thehand when softly holding the corresponding optimal cylindricalhandle with variable diameters. The outer-hand moulds weremanufactured by two physiotherapists at The Institute of Physicaland Rehabilitation Medicine of the University Medical CentreMaribor. The orthotic material Orlight (Ort Industries, Belgium)was used with thickness of 2.5 mm and micro perforation, whichhas the ability of moulding to anatomical contours. The mouldswere shaped on the dorsal side of a hand softly holding the cor-

G. Harih, B. Dolsak / International Journal290responding optimal cylindrical handle with variable diameters. Thereconstruction of the DICOM images. Segmentation was performedusing the threshold technique, since only the surface of the hand isneeded and no differentiation in anatomical structure of the hand ismedical systems Signa HDxt 3.0T. The subjects scanning positionwas HFDR (Head First-Decubitus Right) with the extended hand.The used coil was a one-channel HD Knee/Foot Coil that allowed forthe best positioning of the hand during the scanning. Prior to thescanning, the optimal cylindrical pre-handle with variable di-ameters had been used to nely adjust the correct size of the cor-responding outer-handmould. This optimal cylindrical handle withvariable diameters was removed during the scanning in order toobtain undeformed soft tissue. The slice thickness was set at 1 mmto avoid any unnecessary small anatomical structures and surfacedetails. The image area was 512 512 121 pixels. The scanningtime was about 10 min. The subjects were told to hold their handsin open-positions touching themould during the scanning; in orderto maintain the proper diameters and shape of an optimal power-grasp. The scanned images were provided in the DICOM format.

2.5. Segmentation and 3D reconstruction

A professional medical imaging and editing software Amirahand was in a neutral position according to ergonomic recom-mendations. After the shapes of the moulds were satisfactory,straps were added for hand and hand-opening xation (Fig. 2).

2.4. MRI

MRI was performed at the Radiological Department of the

Fig. 1. Testing the optimally cylindrical pre-handle with variable diameters.ndustrial Ergonomics 43 (2013) 288e295size and the position of the cylinder were determined so as to fully

Fig. 2. Palmar and top view of the outer hand mould attached to the hand.

G. Harih, B. Dolsak / International Journal of Industrial Ergonomics 43 (2013) 288e295 291overlap the palmar empty volume created by the hand during theoptimal power grasp posture (Fig. 4).

To get the handle based on DHHM, Boolean operation Removewas used that removed the cylinder model volume, and whichoverlapped with the hand-model volume. Additional smoothing ofthe sharp edges was performed to prevent injury on the handle.The resulting handle can be seen in Fig. 5.

2.7. Manufacturing of tool handles

In order to evaluate the optimally-cylindrical handles againstthose handles based on DHHM, both types were manufactured foreach subject using rapid prototyping technology. The diameters ofthe cylindrical handles were determined based on the mathemat-ical model that was also used for determining the shape of thehandle based on the DHHM. All the handles were manufacturedwith a 3D printer using black ABS plastic with a smooth surfacenish.

2.8. Task and measurement of subjective comfort rating

The subjects were instructed about the measurement proce-dure. They were told to stand comfortably with elbows at ninetydegrees and wrists in neutral positions. They were asked toperform ve tasks of gripping the DHHM-based handle for 1 mineach time using their preferred normal grip-force whilst applying apush-force of 50N on the handlemounted into a force-gauge. In thisway a standardised and more generalised simulation of a commontask using hand-tools was performed. This was assumed, as most

tasks that require power or pistol-grips cause normal forces on thehands surface. The same task was also performed by each subject

Fig. 3. Obtained 3D hand in Amirausing a corresponding cylindrical handle. In order to compare andevaluate the newly-developed and manufactured handles based onDHHM with the cylindrical handles, the subjects were given asubjective questionnaire regarding comfort-rating immediatelyafter gripping both handles. This questionnaire was adapted basedon a paper of Kuijt-Evers et al. (2007), as a continuum scale tends todeliver the best results in terms of sensitivity Kong et al. (2012). Thesubjects rated each handles comfort descriptors and overallcomfort-rating on a scale containing 7 discrete levels (from1 totally disagree to 7 totally agree) based on their perceivedsubjective responses for each handle. The questionnaire used forcollecting the subjective data can be viewed in Tables 1 and 2.

3. Results and discussion

3.1. DHHM methods verication

Measurements on the obtained handles based on DHHM inCATIA showed, that the optimal diameters for each nger werewithheld by the outer-hand moulds within small deviations duringthe MRI. Therefore, according to Seo and Armstrong (2008), themaximum grip-force can be exerted that can increase the userperformance whilst using the handles designed with DHHM. Sub-jective comfort-rating based on the subjects preferences regardinggrip-diameter size is also increased since there are small deviationsaccording to study from Kong and Lowe (2005).

The manufactured tool-handles based on the DHHM were alsocompared to the corresponding optimally-cylindrical handlesregarding the contact area. The mean contact area of the optimally-

cylindrical handles were Aoptjcir 80.80 cm2. On the other hand, thecontact area measured on the handles designed based on DHHM

in optimal power-grasp posture.

ratings. The mean values with standard deviations, together with

of Ithe statistical signicances of the comfort predictors and overallcomfort-rating, can be seen in Fig. 6.was Aoptjcust 101.34 cm2. An increase in contact area of 20.54 cm2could be observed that was an increase of over 25%.

3.2. User evaluation e subjective comfort rating

The mean values and standard deviations were calculated basedon the data obtained from the subjective comfort-rating ques-tionnaire. A dependent samples T-test was conducted to examinewhether there was a signicant difference between the cylindricalhandles and the handle based on the DHHM design in relation tosubjective comfort predictors and overall subjective comfort-

Fig. 4. 3D hand and cylinder in overlapping position.

G. Harih, B. Dolsak / International Journal292The T-test revealed that comfort descriptors Fits the hand andOffers a nice grip feeling are statistically signicant different at thep < .05 between the cylindrical handles and handles based onDHHM. Both comfort predictors were rated higher for the handlebased on DHHM than the cylindrical handle. This can be explainedby the anatomical shape of the handle based on DHHM, because itconsiders the optimal power-grasp posture; with optimal di-ameters being achieved for each nger which assures themaximum voluntary contraction of ngers. Therefore the handlebased on DHHM provided better tting for the tested subjects. Thiswas impossible with the cylindrical handle, since it took only onengers optimal diameter determination into account.

The cylindrical handle is axle-symmetrical and therefore pro-vides several feasible gripping positions, whilst the handle basedon DHHM provides only one feasible gripping position. Neverthe-less, the rating of the comfort predictor Is easy in use is statisticallynot signicant different between both handles.

The stability of the tool-handle based on DHHMhas been greatlyincreased because the majority of the forces and moments havebeen transferred over to the anatomical handle-shape and muchless with the friction between the handle material and skin. In or-der to provide stability whilst holding the cylindrical handle, thenormally exerted nger-force has to be reasonably high to preventslippage, and rotation in the direction of the handles axis. Highlocal and overall contact pressures occur from highly exertednormal forces that can cause discomfort and also acute disordersndustrial Ergonomics 43 (2013) 288e295and CTD (i.e. blisters, inamed skin, cramped muscles, .). Whenusing tool-handle based on DHHM; lower normal gripping-forcecan be exerted in comparison to the cylindrical handle, and thetool can be stabler held in the hand. Therefore, the handle based onthe DHHM also prevents excessive tensile and shear stresses on theskin and subcutaneous tissue, because the forces and moments aretransferred with the shape of the handle and not by the soft tissue.This is clearly evident from the comfort predictors Has a good forceand moment transmission, Needs a low grip force for stable grip.Both comfort predictors showed signicant difference between thecylindrical handles and those handles based on DHHM at p < .01.

In comfort predictor Has a good friction between the handleand hand, it is evident that the subjects were referring to thefriction caused by the forms of the handles and not the materialfriction between hand and handle, since both handles were man-ufactured with same material and same surface nish. A statisti-cally signicant difference could be observed between thecylindrical handles and those handles based on DHHM for thiscomfort predictor.

Fig. 5. Tool-handle based on a DHHM.

Table 1Subjective comfort descriptors rating questionnaire.

Totallydisagree

e Disagreesomewhat

e Agreesomewhat

e Totallyagree

Fits the hand 1 2 3 4 5 6 7Is functional 1 2 3 4 5 6 7Is easy in use 1 2 3 4 5 6 7Has a good force transmission 1 2 3 4 5 6 7

G. Harih, B. Dolsak / International Journal of Industrial Ergonomics 43 (2013) 288e295 293Subjective comfort-rating that describes the handles visualappearance and quality (Is a high quality handle) showed a sig-nicant difference between both handles at p < .01, and was ratedhigher for the handle based on the DHHM than the cylindricalhandle. The higher rating of the handle based on the DHHM for thissubjective comfort predictor can be explained by past user expe-riences and expectations because hand-tools; and consequentlythose handles with good t for the user; have higher functionalityand thus performance. Although the handles appearance onlyindirectly affects the comfort, it has signicant impact on buyingdecisions.

The causes of numbness and lack of tactile feeling usually occurswhen high contact pressure on a nerve is present or high contactpressure prevents the blood-ow in the underlying soft tissue, thuscausing ischemia. However this effect is strongly time-correlated.Therefore; the short gripping times of a handle that produce highcontact pressure on soft tissue do not evoke numbness and lack oftactile feeling. Therefore also comfort predictors Causes numbnessand lack of tactile feeling and Causes cramped muscles werestatistically not signicant different between the cylindrical handleand the handle based on DHHM. This can be explained by themeasurement procedure, where the test subjects gripped the tool-handle ve times each time for 1 min. Therefore it can be

Has a nice-feeling 1 2Can offer a high task performance 1 2Provides a high product quality 1 2Looks professional 1 2Needs low hand grip force supply 1 2Has a good friction between the handle and hand 1 2Causes an inamed skin of hand 1 2Causes peak pressure on the hand 1 2Causes blisters 1 2Feels clammy 1 2Causes numbness and lack of tactile feeling 1 2Causes cramped muscles 1 2concluded, that longer gripping times or real hand-tools with tasksthat require longer gripping times should be used when evaluatingthe comfort predictors.

The subjects also evaluated the overall subjective comfort-rating whilst gripping both handles. The T-test revealed thatthere was a statistically signicant difference between the cylin-drical handle and the handle based on DHHM at p < .01. Thus theanatomically-shaped handle based on the DHHM can be consideredas more comfortable overall in comparison with the cylindricalhandle. This was also expected, since most subjective comfortpredictors indicate that those tool-handles based on DHHM aremore comfortable than the cylindrical handles.

Table 2Overall subjective comfort rating questionnaire.

Overall comfort:

Veryuncomfortable

e A littleuncomforta

I think this handle is: 1 2 33.3. DHHM evaluation

The 25% increase in the contact area for those handles designedon DHHM can be attributed to the fact that these handles follow theshape of the hand during its optimal power-grasp posture andthereby provide greater contact area. According to Seo andArmstrong (2008) the greatest contact area in their study was ob-tained with diameters of 51 mm and 58 mm with cylindrical han-dles, which is in contradiction with the size of optimal diametersfor grip-force and comfort maximisation that suggest smaller di-ameters. It is obvious, that contact area maximisation is impossiblewith cylindrical handles when considering an optimal diameter formaximising grip-force exertion and comfort-rating. Since thecontact pressure depends not only on the grip-force but also on thecontact area, it is reasonable to provide greater contact area in orderto lower the contact-pressure. The non-linear visco-elastic prop-erties of skin and subcutaneous tissue lead to an exponential rise incontact-pressure with a higher degree of tissue deformation. Highoverall and local contact pressure can be avoided with ananatomical shape of the handle and, therefore, also a greater con-tact area. Research into tissue deformation under mechanicalstresses using nite element analyses has shown that the shapethat follows anatomical shape of the hand results in much lower

3 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 73 4 5 6 7contact pressures. Therefore the handle introduced during thisstudy is more likely to prevent those CTDs that are pressure-dependent, and provide a greater comfort-rate than a cylindricalhandle, as has been shown by user evaluation.

Some heavy tasks require the use of protective or anti-vibrationgloves to avoid acute disorders and CTD. The thicknesses of thegloves can vary, and thus the resulting effective grip diameter ofeach nger varies also when gripping the tool-handle using barehands. However the relatively small thicknesses of the glovescompared to the optimal grip diameters effects the overall gripdiameters only slightly, which therefore has a small effect on themaximum voluntary contraction forces of the ngers. On the other

blee A little

comfortablee Very

comfortable

4 5 6 7

of IFig. 6. Overall handle subjective comfort predictors rating and overall subjectivecomfort-ratings.

G. Harih, B. Dolsak / International Journal294hand, all benets using gloves (peak contact-pressure reduction,vibration reduction, shear-force reduction,.) would still bemaintained. Nevertheless, the subjective comfort-rating would beaffected, since the gloves reduce the tactile sensing and; therefore;also subjective feelings. In order to compensate for the grips di-ameters when using gloves, the DHHM could be adapted with ascaling function that would allow the designer to choose whetherthe tool-handle would be used bare-handed or using gloves. Thepredicted mean thickness of the glove could be the input data forthe scaling function of the tool-handle. In order to fully investigatethe importance of using gloves with tool-handles based on DHHMthis issue should be the subject of broader investigation in thefuture.

Many of existing biomechanical DHMs or DHHMs are complexregarding usage, thus preventing broader dissemination. The staticshape DHHM developed during this study can be used by designersand engineers as it enables simple manipulation inside CAD soft-ware of choice. It also does not require an ergonomics specialist,since the obtained DHHM is based on an optimal grasping-posturethat allows for the direct modelling of anatomically-shaped tool-handles. Most of the dynamic DHHMs with autonomous graspingare intended for evaluating and analysing an existing CAD designsinside the virtual environment. If the produced autonomousgrasping of the DHHM is feasible and realistic, ergonomics designerrors can be identied and design solutions can be proposed. Thisiterative design process is repeated until the design does not meetthe desired level of ergonomics. On the other hand, direct devel-opment of tool handles based on the obtained DHHM withinexisting CAD tools becomes an integrated process, resulting inincreased time efciency and tool handles with improved ergo-nomics, without the need for comprehensive knowledge of ergo-nomics by the designer. Unnecessary become also physicalprototypes for the purpose of ergonomic analyses. Followingimproved product ergonomics, the market value of the product isincreased, thus enhancing the competitiveness of the product onthe market.

Future work should also consider user-testing of those tool-handles based on the DHHM using a pressure-mapping systemfor identifying those contact forces and pressure zones that exceedthe PDT and SD values. Based on the obtained data, optimisation ofhandle-size and shape could be conducted for limiting thosepressure-peaks below acceptable limits. Correlations could also bedetermined between subjective comfort-rating and local andoverall pressures. The resulting methods for the development ofthe DHHM also provide the possibility for future tool-handle shapedeterminations and optimisation for a broader population. In re-gard to that purpose; more subjects should be considered whenmodelling a parametric DHHM, which would allow for a directgeneration of tool-handles for a targeted population. The ndingsof this research could also be combined with a dynamic DHHM forproviding verication and validation of the proposed power grasp.In this way a comprehensive DHHM could be developed for per-forming ergonomic analyses and enabling direct designing forimproved ergonomics.

4. Conclusion

This paper presented a different approach to tool-handle design.An anatomically-accurate static DHHM was developed based onMRI and an optimal power-grasp posture with undeformed soft-tissue. The proposed methods allow for the direct development oftool-handles with anatomical shapes and sizes that increases themaximum voluntary contraction of ngers, maximises the contactarea, and, thereby lowers the local and overall contact pressures,and increases the subjective comfort-rating.

References

Aldien, Y., Welcome, D., Rakheja, S., Dong, R., Boileau, P.E., 2005. Contact pressuredistribution at handehandle interface: role of hand forces and handle size. Int.J. Ind. Ergon. 35, 267e286.

Amis, A.A., 1987. Variation of nger forces in maximal isometric grasp tests on arange of cylinder diameters. J. Biomed. Eng. 9, 313e320.

Ayoub, M.M., Presti, P.L., 1971. The determination of an optimum size cylindricalhandle by use of electromyography. Ergonomics 14, 509e518.

Bernard, B., Nelson, N., Estill, C.F., Fine, L., 1998. The NIOSH review of hand-armvibration syndrome: vigilance is crucial. J. Occup. Environ. Med. 40, 780e785.

Blackwell, J.R., Kornatz, K.W., Heath, E.M., 1999. Effect of grip span on maximal gripforce and fatigue of exor digitorum supercialis. Appl. Ergon. 30, 401e405.

Chafn, D.B., Andersson, G., 1999. Occupational Biomechanics. Wiley, New York.Chen, Y., 1991. An Evaluation of Hand Pressure Distribution and Forearm Flexor

Muscle Contribution for a Power Grasp on Cylindrical Handles. Ph.D. Disser-tation. University of Nebraska, Lincoln, Nebraska.

De Looze, M., Kuijt-Evers, L., Van Dien, J., 2003. Sitting comfort and discomfort andthe relationships with objective measures. Ergonomics 46, 985e997.

Demirel, H.O., Duffy, V.G., 2007a. Applications of digital human modeling in in-dustry. Lecture Notes Comput. Sci., 824e832.

Demirel, H.O., Duffy, V.G., 2007b. Digital human modeling for product lifecyclemanagement. Lecture Notes Comput. Sci., 372e381.

Eksioglu, M., 2004. Relative optimum grip span as a function of hand anthropom-etry. Int. J. Ind. Ergon. 34, 1e12.

Endo, Y., Kanai, S., Kishinami, T., Miyata, N., Kouchi, M., Mochimaru, M., 2007.A computer-aided ergonomic assessment and product design system usingdigital hands. In: Duffy, V. (Ed.), Digital Human Modeling. Springer, Berlin/Heidelberg, pp. 833e842.

Garneau, C.J., Parkinson, M.B., 2010. Optimization of tool handle shape for a targetuser population. In: Proceedings of the ASME International Design EngineeringTechnical Conferences and Computers and Information in Engineering Confer-ence, vol. 5, pp. 1029e1036.

Garneau, C.J., Parkinson, M.B., 2011. Optimization of product dimensions for discretesizing applied to a tool handle. Int. J. Ind. Ergon., 1e9.

Grant, K.A., Habes, D.J., Steward, L.L., 1992. An analysis of handle designs forreducing manual effort: the inuence of grip diameter. Int. J. Ind. Ergon. 10,199e206.

Gurram, R., Rakheja, S., Gouw, G.J., 1995. A study of hand grip pressure distri-

ndustrial Ergonomics 43 (2013) 288e295bution and EMG of nger exor muscles under dynamic loads. Ergonomics38, 684e699.

Hall Jr., N.B., Bennett, E.M., 1956. Empirical assessment of handrail diameters.J. Appl. Psychol. 40, 381e382.

Hogberg, D., Backstrand, G., Lamkull, D., Hanson, L., Ortengren, R., 2008. Indus-trial customisation of digital human modelling tools. Int. J. Serv. Oper.Inform. 3, 53e70.

Johnson, S.L., 1993. Ergonomic hand tool design. Hand Clin. 9, 299e311.Kaljun, J., Dolsak, B., 2012. Ergonomic design knowledge built in the intelligent

decision support system. Int. J. Ind. Ergon. 42, 162e171.Khalil, T.M., 1973. An electromyographic methodology for the evaluation of indus-

trial design. Hum. Factors 15, 257e264.Kong, Y., Lowe, B., 2005. Optimal cylindrical handle diameter for grip force tasks.

Int. J. Ind. Ergon. 35, 495e507.Kong, Y.K., Kim, D.M., Lee, K.S., Jung, M.C., 2012. Comparison of comfort, discomfort,

and continuum ratings of force levels and hand regions during gripping exer-tions. Appl. Ergon., Special Section on Product Comfort 43, 283e289.

Kuijt-Evers, L.F., Groenesteijn, L., de Looze, M.P., Vink, P., 2004. Identifying factors ofcomfort in using hand tools. Appl. Ergon. 35, 453e458.

Kuijt-Evers, L.F.M., Vink, P., de Looze, M.P., 2007. Comfort predictors for differentkinds of hand tools: differences and similarities. Int. J. Ind. Ergon. 37, 73e84.

Nierop, O.A.v., Helm, A.v.d., Overbeeke, K.J., Djajadiningrat, T.J.P., 2007. A naturalhuman hand model. Vis. Comput. 24, 31e44.

Oh, S., Radwin, R.G., 1993. Pistol grip power tool handle and trigger size effects ongrip exertions and operator preference. Hum. Factors 35, 551e569.

Pea-Pitarch, E., Yang, J., Abdel-Malek, K., 2009. Virtual human hand: grasping andsimulation. In: Duffy, V. (Ed.), Digital Human Modeling. Springer, Berlin/Hei-delberg, pp. 140e149.

Punnett, L., Wegman, D.H., 2004. Work-related musculoskeletal disorders: theepidemiologic evidence and the debate. J. Electromyogr. Kinesiol. 14, 13e23.

Sancho-Bru, J.L., Giurintano, D.J., Prez-Gonzlez, A., Vergara, M., 2003. Optimumtool handle diameter for a cylinder grip. J. Hand Ther. 16, 337e342.

Seo, N.J., Armstrong, T.J., 2008. Investigation of grip force, normal force, contact area,hand size, and handle size for cylindrical handles. Hum. Factors 50, 734e744.

Shuxing, D., Aimei, Z., Xihui, Y., Yanhua, Z., 2008. The study on intelligent tech-nology integrating ergonomics into industrial design. In: Computer-aided In-dustrial Design and Conceptual Design, 2008. CAID/CD 2008. 9th InternationalConference on, pp. 21e25.

Welcome, D., Rakheja, S., Dong, R., Wu, J.Z., Schopper, A.W., 2004. An investigationon the relationship between grip, push and contact forces applied to a toolhandle. Int. J. Ind. Ergon. 34, 507.

Wu, J.Z., Cutlip, R.G., Andrew, M.E., Dong, R.G., 2007. Simultaneous determination ofthe nonlinear-elastic properties of skin and subcutaneous tissue in unconnedcompression tests. Skin Res. Technol. 13, 34e42.

Yakou, T., Yamamoto, K., Koyama, M., Hyodo, K., 1997. Sensory evaluation of gripusing cylindrical objects. JSME Int. J. C e Mech. Syst. Mach. Elem. Manuf. 40,730e735.

Youakim, S., 2009. Hand-armvibration syndrome (HAVS). Br. ColumbiaMed. J. 51,10.

G. Harih, B. Dolsak / International Journal of Industrial Ergonomics 43 (2013) 288e295 295

Tool-handle design based on a digital human hand model1 Introduction2 Methods2.1 Determination of the optimal cylindrical pre-handle2.2 Optimal cylindrical pre-handle with variable diameters2.3 Manufacturing the outer hand mould2.4 MRI2.5 Segmentation and 3D reconstruction2.6 Power-grasp tool-handle's shape determination2.7 Manufacturing of tool handles2.8 Task and measurement of subjective comfort rating

3 Results and discussion3.1 DHHM method's verification3.2 User evaluation subjective comfort rating3.3 DHHM evaluation

4 ConclusionReferences