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TheAssessmentofaTwo-HandedPinchForce:QuantifyingDifferentAnthropometricPinchGraspPatternsforMales...

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DOI:10.1016/j.ergon.2017.02.006

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International Journal of Industrial Ergonomics 58 (2017) 38e46

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International Journal of Industrial Ergonomics

journal homepage: www.elsevier .com/locate/ergon

The assessment of a two-handed pinch force: Quantifying differentanthropometric pinch grasp patterns for males and females

Mohammed Shurrab a, *, Nabeel Mandahawi b, M.D. Sarder c

a Department of Mechanical Engineering, Kyushu University, Fukuoka, Japanb Department of Industrial Engineering, Hashemite University, Amman, Jordanc Department of Industrial Engineering, University of Southern Mississippi, Hattiesburg, MS, USA

a r t i c l e i n f o

Article history:Received 16 June 2016Received in revised form21 November 2016Accepted 16 February 2017

Keywords:Two hands pinch forcePinch grip widthChuck pinchLateral pinchPulp-2 pinch

* Corresponding author. 744 Motooka, Nishi-ku, FuE-mail address: mshurrab@live.com (M. Shurrab).

http://dx.doi.org/10.1016/j.ergon.2017.02.0060169-8141/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

In industrial applications, it is essential to describe and estimate the distinctive nature or features of gripforce so as to optimize tool, machine grip, and/or handle designs. Most of the industrial machine'shandles require two hands pinch grip force exertion, however, most of the existing research focused onone hand pinch grip force. Each different machine handle's design requires definitely different anthro-pometric grasp types based on the machine handle shape. This study is therefore aims at examining andinvestigating the influence of pinch grip pattern, pinch grip width, gender, lean body mass (LBM), bodymass index (BMI), and hand dimensions on pinch grip forces by conducting two-hand experiment usinga custom-designed measuring tool. Three different types of anthropometric pinch grasp patterns weretested, which are: lateral (key), chuck, and pulp-2. Pinch grips were tested for static maximal voluntarycontraction (MVC) forces using a two hands in a snap-type action at two different widths (3.8 cm and6.8 cm) among forty-six volunteers. The two-handed pinch grip force was also quantified by developingregression models for each anthropometric pinch grasp pattern. The results showed that the pinch gripforce was affected by: the pinch grasp pattern, pinch grip width, gender, and hand dimensions. Chuckand lateral pinch forces were not significantly different from each other. Pulp-2 pinch had the lowestpinch forces for males and females. Individuals' medical indexes were calculated to study their signifi-cance on the pinch grip force. It was noticed that the LBM index has a significant effect on the pinch gripforce compared to the BMI.Practitioner summaryThe study investigates the influence of pinch type, pinch width, gender, body mass, and hand dimensionson pinch grip forces by conducting two-hand experiment using a custom-designed measuring tool. Pinchgrip force was affected by pinch type and width. Two hands produce twice the force of a one-handedpinch grip.

© 2017 Elsevier B.V. All rights reserved.

1. Introduction

Grip force studies are considered one of the substantial issuesthat are being under focus in applied ergonomics science. They areabsolutely necessary in designing the industrial machines handlesor any daily life equipment at work or leisure activities. Ergonomicas a concept is the measurement, analysis, evaluation, and design ofsystem involving human machine task environment interaction forthe purpose of enhancing performance, safety and health(Grandjean, 1988; Imrhan, 1996; Mandahawi et al., 2008a). In order

kuoka 819-0395, Japan.

to enhance this interaction, knowledge about: human body di-mensions, physical strength, limitations, and capabilities arerequired. Pinch grip force is an important aspect that must beconsidered when designing tools and equipment, which people usein their technical or social systems. The forceful exertions andrepetitiveness are considered essential and major causes of theCumulative Trauma Disorder (CTDs) in upper extremities (NationalInstitute for Occupational Safety and Health (NIOSH), 1997).

Gripping or squeezing tasks with both hands at different gripwidths are used in industry, activities of daily living, or leisure ac-tivities. For instance, some industrial tasks require the two handlesto be gripped while they are close together with a certain gripwidth. The grip force may result from the thenar eminence and

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e46 39

thumb opposing the other fingers (Imrhan, 1999). In carmanufacturing industry, gripping and snapping seat belt retractorsinto their housing is another example. Leisure activities include;climbing, horse racing, canoe polo, Brazilian jiu-jitsu, and uppersuspension exercises.

In applied industrial operations, pinch gripping tools are usedfrequently. Hence, it is important to quantify the pinch grip forcethat is exerted by workers to evaluate effectively the job or the tooldesign (Ayoub, 1973; McGorry et al., 2010; Dale et al., 2011).Investigating the pinch exertion forces is highly needed in jobdesigning purpose. This could enhance the job performance andlower the CTDs occurrence probability. Many studies pointed that ifhand tools have poor ergonomics, then workers’ hands and fore-arms become increasingly subject to a variety of work relatedmusculoskeletal disorders by cumulative trauma such as: tendon-itis, strainedmuscles, carpal tunnel syndrome, nerve impingement,and many others (Kelly et al., 1995; Kattel et al., 1996; Wells andKeir, 1999; Sande et al., 2001; Mirka et al., 2002; Boyles et al.,2003). Therefore, pinch grip force evaluation in the workplacemay help in three main aspects, which are: identifying individualsat risk for work related musculoskeletal disorders of the hands andforearms, determining the improvement made over the process oftreatment or rehabilitation, and assessing feigned injury (Boisseyet al., 1999; Greeves et al., 1999; Abbott et al., 2001; Peolssonet al., 2001; Tredgett and Davis, 2000; Westbrook et al., 2002;Kong et al., 2012).

Generally speaking, the human hand could be in different po-sitions and shapes to grip or touch the materials/objects. It usuallydepends on how the hand will be used for the desired action. Typesof grip include: the crush grip, the pinch grip, and the support grip(Magee, 2014). The pinch grips and support grips (e.g. chuck,lateral, and pulp-2) are widely used in the industrial applications(Aghazadeh, 1994; Shivers et al., 2002). However, the crush gripsare commonly used in sport exercises machines (Spindler andHeslep, 2013).

Most of the studies on hand pinch grip force have tested one-handed contraction, usually, the preferred hand at a single fixedpinch grip width. Imrhan and Rahman (1995) tested the pinchwidth and pinch force for chuck, lateral, and pulp-2 pinch grasptypes. However, many industrial tasks and activities are performedusing both hands with muscular forces applied simultaneouslywith both hands (Imrhan and Mandahawi, 2010; Imrhan, 2003;Imrhan, 1999). Therefore, there is a need to measure the two-hand pinch grip force which is one of the objectives of this study.

In Jordanian industry, tools and equipment are used widely forvarious types of operations. Most of these tools are imported fromcountries which do not take into account the gripping force forJordanians populations. Hence, there is a need to evaluate thesuitability of these tools for Jordanian operators/workers.

Therefore, the objective of this study is to consider not only onehand pinch grip force, but also to consider studying two handspinch grip force. The pinch grip force takes into account three typesof anthropometric pinches at two different pinch grip widths. Thisresearch paper evaluates three different pinch grip forces consid-ering both genders in one experiment. The experiment was con-ducted on a Jordanian population to enrich the literature about thepinch grip force investigations and to make improvements thatmay be necessary for comfort, health, and safety in the Jordanianindustry. The experimental data and results are uniquely analysedwith an insight from the statistical point of view, and regressionmodels have been developed to quantify the two hands maximalvoluntary contraction (MVC) pinch force based on different factors.The following hypotheses will be tested:

� The gender, pinch grasp patterns, and pinchwidth affect the twohands MVC pinch force.

� The hand dimensions (i.e. length, breadth, and thickness) affectthe two hands MVC pinch force for each pinch width and eachpinch grasp pattern.

� The BodyMass Index (BMI) and Lean BodyMass (LBM) affect thepinch force for males and females

2. Methods

2.1. Subjects

In this study, forty-six healthy college students (i.e. 25males and21 females) participated voluntarily. To our knowledge, no subjecthas any history of musculoskeletal problems. Permission was ob-tained from the Hashemite University to conduct the force tests andgather the demographic and anthropometric information from thesubjects. Letter of consent was signed by all the participants for thepurpose of participating this study's experiment.

2.2. Apparatus

A special two-hand measuring force tool has been designed tocollect the two hands force data (i.e. pinch grip force). The appa-ratus is similar to the one that was designed by Imrahn (1999,2003). It consists of a custom-designed handle attached to a Dil-lon digital push-pull force gauge (Dillon GL). The Dillon GL is a self-contained measurement device that monitors forces and/orweights (i.e. push/pull) on its built-in load cell. The force capacity ismeasured in 500� 0.02 N (50� 0.02 kg,110� 0.05 lb), which is thehighest available capacity in the GL Force Gauge Series. Dillon GLForce gauges are affordable digital force measurement deviceswhich include a full set of handy accessories that assist in tension/compression testing applications. Dillon's digital force gaugesprovide 120% overload protection and a rugged metal die castenclosure, backlit LCD display, and serial output as shown in Fig. 1.

Fig. 1. Dillon force gauge (Dillon GL).

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e4640

In this experiment, Dillon GL was calibrated by the manufacturerbefore being used.

Furthermore, a special designed handles was combinedwith theDillon's digital force gauges. The handles consist of two oval Teflonplates (long axis ¼ 12.7 cm; short axis ¼ 10.2 cm; andthickness ¼ 1 cm). The force gauge is connected to the lowergalvanizedbeam inorder to get thehighest sensitivity. Furthermore,the distance between the oval plates could be adjusted manuallyover a wide range to provide different grip widths. The handle andforce gauge were attached to a sturdy adjustable tripod stand toprovide flexibility for the experimenter to adjust to the height of thetool tomake sure that he is in the upright position as shown in Fig. 2.The height of the handle (measured frommid-distance between theTeflon plate pair) was set at 5.1e10.2 cm below subjects' standingelbowheight. This distance value is slightly the recommendedworksurface height for tasks requiring significant hand forces(Mandahawi et al., 2008b). Furthermore, the orientation and theangleof thehandle isdesigned so that the forearmsand thewrist canbe kept in their natural mid-orientation and will not be bent duringthe MVC contractions. A Vernier caliper was used to measure thehand length, breadth and thickness within 0.1 mm accuracy.

2.3. Procedures

Subjects were given demonstrations and basic concept educa-tional session about the experiment in general and the graspingpatterns (i.e. pinch grip type) in particular. Each subject was givensimulated gripping tests to make him/her familiar with the appa-ratus's use and the required gripping positions. During the testsessions, the main researchers observed and provided the requireddirection for each subject to make sure that the experiment isperformed in the correct manner. A large set of clarification photoshave been posited around the experiment area to guide the subjectsthroughout the whole experiment.

The height of the instrument was adjusted according to thestature of subjects to insure the proper standing of the subject, andthen the pinch grip width was set to the required size by the

Fig. 2. Custom-designed apparatus for measuring snap type two-handed pinch gripforce.

experimenter. The experiment requires that the subject should standcomfortably, and then squeeze the Teflon plates (handle) by using acertain grasp pattern (i.e. pinch type) with both hands. The subjectshould squeeze as hard as he/she can, to get a maximal voluntarycontraction effort (MVC)without jerking. The subjects were asked toperformthepinchgrip on thepinchhandles bystanding comfortablyand to not use their upper weight to make sure that the grippingvalue is only related to the human grip force capabilities.

Three different anthropometric pinch grasp patterns wereinvestigated: chuck (three jaw chuck), lateral (key), and pulp-2pinches as shown in Fig. 3. The main researchers demonstratedwhere to locate the thumb and the rest of the hand fingers to eachsubject before conducting the experiment.

Each subject was then observed by the researcher to make surethat the obtained results are correct when performing theexperiment.

Pinches were tested at two pinch width sizes, which are 3.8 cmand 6.8 cm. The subjects performed two trials for each pinch grasptype. The two consecutive trials were separated to prevent anymuscular fatigue that could happen in such experiments. Theapparatus was set to display the peak value of theMVC of pinch gripand themaximumvalue of the two trialswas obtained and recorded.

2.4. Theory and calculations

The Body Mass Index (BMI) is a medical index that is related tothe human body. BMI is defined as the body mass divided by thesquare of the body height, and is expressed in kg/m2. It is an act oftrying to quantify the amount of tissue mass (i.e. muscle, fat, andbone) in any person. Based on this index value, the individuals areclassified as underweight, normal weight, overweight, or obese(Eknoyan, 2008). Furthermore, Lean Body Mass (LBM) is also amedical index that is related to the human body, it can be calculatedby subtracting the body fat weight from total body weight, and isexpressed in unit of kg. To calculate the LBM, the body fat per-centage (body fat %) is calculated first. LBM is then calculated bysubtracting body fat percentage from total body weight percentage(i.e. which is 100%). The body fat % of an individual is the total fatmass divided by bodymass, and is expressed in unit of kg. There aredifferent formulas for the body fat % depending on the individual'sage. In this study, it is calculated for adults using Eq. (1)(Deurenberg et al., 1991).

Body fat%for adults ¼ ð1:2�BMIÞ þ ð0:23�ageÞ � ð10:8�genderÞ� 5:4

(1)

where gender is 1 for males and 0 for females.

2.5. Statistical analysis

The experiment was conducted based on certain factors, whichare: gender, stature, body weight, hand length, inner hand breadth,hand thickness, BodyMass Index (BMI), and Lean BodyMass (LBM).Analysis of Variance (ANOVA) was conducted to examine the effectof different factors on the MVC for different pinch grips. Then, aninvestigation for the relation between individual's body mass in-dexes (i.e. BMI and LBM) and the pinch force is presented.

The multivariate hypotheses tests were then adopted for twopurposes: the first purpose, is to investigate the effect of hand di-mensions (i.e. hand length, breadth, and thickness) on the MVC formales and females. The second purpose, is to show the relationbetween MVC and both, BMI and LBM. Multivariate post-analysiswas then applied to show the difference between the hand

Fig. 3. Anthropometric pinch grasp patterns used in the experiment.

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e46 41

dimensions on multivariate MVC for each pinch grasp pattern.Furthermore, in some industrial applications the designers need toknow the estimated MVC for certain type of pinch grasp patterns;thus regression model was developed for each type of pinch grasppatterns based on gender, pinch grasp patterns, and pinch width. AStatistical Package for Social Science known as SPSS with live-linkmodule of Microsoft Access were used to perform the statisticalanalysis.

3. Results and analysis

3.1. Anthropometric subjects’ parameters

Twenty-five males and twenty-one females participated in thisexperiment, they are students at Jordanian universities. A summaryfor the parameters’ description are shown in Table 1. The averageage, weight, and stature of the subjects (i.e. 46 participants) are 21.1years, 65.7 kg, and 170.5 cm respectively. Average hand relatedparameters for the subjects are 17.5 cm for hand length, 7.7 cm forinner hand breadth, and 3.3 cm for hand thickness. The averagevalues for the body mass related parameters are 22.3 kg/m2 for BMIand 51.8 kg for the LBM.

3.2. Main and interaction effects on MVC

3.2.1. Test the hypothesis that the gender, pinch grasp patterns,pinch width affect the pinch force

To examine the effect of gender, anthropometric pinch grasppattern, and pinch width on the MVC ANOVA analysis has beenperformed. ANOVA results are shown in Table 3. Simple main

Table 1Parameters description for participating subjects.

Parameter Gender: Male

Mean SD Min

Age [years] 21.4 1.9 19.0Body Weight [kg] 78.2 18.4 50.0Stature [cm] 177.8 6.2 167.0Hand Length [cm] 18.6 0.7 17.0Inner Hand Breadth [cm] 8.3 0.4 7.3Hand Thickness [cm] 3.8 0.4 3.1BMI [kg/m2] 24.6 5.1 17.1LBM [kg] 62.9 9.7 45.4

effects analysis showed that the gender type (males or females), theanthropometric pinch grasp pattern (check, lateral, or pulp-2), andthe pinch grip width (3.8 cm or 6.8 cm) were significantly affect theMVC (p < 0.001). There was also a statistically significant interac-tion between the effects of both pinch grasp pattern and handpinch grip width with gender, F (2, 262) ¼ 110.747, p < 0.001, and F(1, 262)¼ 16.066, p < 0.001 respectively. In contrast, the interactionbetween the three factors (gender, anthropometric pinch grasppattern, and pinch width) is not significant, p ¼ 0.446.

3.2.2. Two hands pinch grip MVC forceThe measured values of the pinch force for each pinch grasp

pattern for males and females are shown in Fig. 4 (a) and 4 (b). Formales, the MVC values are 156 N, 172 N, and 36 N for chuck, lateral,and pulp-2 patterns at 3.8 cm pinch grip width. However, the MVCis slightly higher at 6.8 cm pinch grip width, where the MVC valuesare 187, 186, 83 N for chuck, lateral, and pulp-2 patterns respec-tively. On the other hand, for females, the MVC values are 21 N,27 N, and 9 N for chuck, lateral, and pulp-2 patterns at 3.8 cm pinchgrip width. The MVC is also slightly higher at 6.8 cm pinch gripwidth, where the MVC values are 45 N, 42 N, 17 N for chuck, lateral,and pulp-2 patterns. In general, the MVC for females was signifi-cantly lower than that one of males by ~80% at 3.8 cm and 6.8 cmpinch grip width. The minimum, maximum and the average valuesfor MVC are shown in Fig. 4 for both males and females for eachpinch grip type at each pinch grasp width.

3.2.3. MVC values for the pinch grasp patternsThe mean and the standard deviation of the MVC values for the

46 participants are summarized in Table 2, based on gender. It is

Gender: Female

Max Mean SD Min Max

26.0 20.8 0.5 20.0 22.0126.0 53.3 5.4 43.0 64.0190.0 163.4 5.8 150.0 173.019.6 16.4 0.8 15.0 18.09.1 7.1 0.4 6.3 7.75.2 2.8 0.2 2.5 3.341.1 20.0 1.5 15.5 22.180.5 40.8 3.5 33.4 47.6

Fig. 4. The MVC values for males and females at a certain pinch grip width.

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e4642

noticed that regardless of the pinch grip width, males are stillhaving higher MVC values than females.

T-test found that chuck and lateral pinch grasp patterns havestatistically similar means (mean ¼ ~117 ± 32 N) between eachother for males regardless of the pinch grip width, p ¼ 0.85. Incontrast, t-tests found that chuck pinch grasp has amean ¼ ~23 ± 11 N which is higher than the pinch grasp lateralmean ¼ ~19 ± 8 N for females regardless the pinch grip width andthey have statistically different means, p¼ 0.04. Pulp-2 pinch grasppattern has the lowest MVC values for males (mean ¼ ~34 ± 17 N)and females (mean ¼ ~8 ± 3 N).

3.2.4. Relationship between the pinch width and the pinch forceThe relationship between the pinch grip width and the MVC is

shown in Fig. 5 (a) for males and Fig. 5 (b) for females. Chuck andlateral pinch grasp patterns dominate the highest MVC values for

Table 2MVC values of all participants for all the pinch grasp patterns.

Gender: Grasp pattern Statistic

Mean SD

Male: Chuck 116.6 35.7Male: Lateral 117.8 28.3Male: Pulp-2 34.1 17.3Female: Chuck 23.5 11.6Female: Lateral 19.1 8.5Female: Pulp-2 8.8 3.7

Table 3ANOVA results to examine the effects on the MVC.

Source Type III sum of squares

Corrected Model 628 978.155a

Intercept 776 969.234Gender 357 547.468Hand Pinch Grip Width 24 630.297Grasp Pattern 135 453.244Gender * Hand Pinch Grip Width 5372.311Gender * Grasp Pattern 74 063.851Hand Pinch Grip Width * Grasp Pattern 4388.260Gender * Hand Pinch Grip Width * Grasp Pattern 541.947Error 87 608.234Total 1 595 675.440Corrected Total 716 586.389

a R Squared ¼ .878 (Adjusted R Squared ¼ .873).

males (between ~100 and ~140 N) and for females (between ~15and ~35 N). Pulp-2 pinch grasp pattern's MVC values are the lowestfor both males and females. The results of this experiment showsthat when the pinch grip width increases the MVC value also in-creases for males and females, this relationship is the same for allthe pinch grasp patterns used in this study.

3.3. Test of hypotheses

3.3.1. Test the hypothesis that the hand dimensions (i.e. handlength, breadth, and thickness) affect the pinch force for each pinchwidth and each pinch pattern

There was a significant dependence between the hand di-mensions and the MVC for males and females as shown in Table 4,where the p value was <0.0005 for all effects. Wilks's lambda dis-tribution (Wilk's L), which is a probability distribution used inmultivariate tests, was less than 0.5 for all effects as well.

The “Test Between-Subjects Effects” analysis was adopted(Table 5) to show the detailed effect of hand length, hand breadth,and hand thickness on each MVC for different pinch grasp patterns.The hand length, hand breadth, and hand thickness are significant(p < 0.05) for all the three gasp patters (chuck, lateral, and pulp-2).It is concluded that the MVC has a significance dependence withthe hand dimensions.

3.3.2. Test the hypothesis that BMI and LBM affect the pinch forcefor males and females

The multivariate hypotheses tests were also used to show the

Df Mean square F Sig.

11 57 179.832 171.001 0.0001 776 969.234 2323.594 0.0001 357 547.468 1069.277 0.0001 24 630.297 73.659 0.0002 67 726.622 202.542 0.0001 5372.311 16.066 0.0002 37 031.925 110.747 0.0002 2194.130 6.562 0.0022 270.973 0.810 0.446262 334.383274273

Fig. 5. The MVC values against the grip width.

Table 4Multivariate hypotheses test for anthropometric hand dimensions for each gender.

Effect Value F Hypothesis df Error df Sig.

Hand Length Pillai's TraceWilks' LambdaHotelling's TraceRoy's Largest Root

1.042 1.441 72.000 195.000 0.0260.174 2.093 72.000 189.136 0.0003.601 3.084 72.000 185.000 0.0003.279 8.881a 24.000 65.000 0.000

Inner Hand Breadth Pillai's TraceWilks' LambdaHotelling's TraceRoy's Largest Root

1.048 1.541 69.000 198.000 0.0110.142 2.561 69.000 192.054 0.0004.757 4.321 69.000 188.000 0.0004.496 12.901a 23.000 66.000 0.000

Hand Thickness Pillai's TraceWilks' LambdaHotelling's TraceRoy's Largest Root

0.991 2.434 45.000 222.000 0.0000.170 3.887 45.000 214.674 0.0003.965 6.226 45.000 212.000 0.0003.735 18.424a 15.000 74.000 0.000

a The statistic is an upper bound on F that yields a lower bound on the significance level.

Table 5Multivariate post analysis “Tests of Between-Subjects Effects for MVC vs hand dimensions”.

Source Dependent variable Type III sum of squares Df Mean square F Sig.

Hand Length Chuck MVC 160 939.251a 24 6705.802 4.730 0.000Lateral MVC 196 850.430 b 24 8202.101 8.749 0.000Pulp-2 MVC 12 847.584 c 24 535.316 2.163 0.007

Inner Hand Breadth Chuck MVC 183 507.854 23 7978.602 7.568 0.000Lateral MVC 209 112.416 23 9091.844 12.328 0.000Pulp-2 MVC 13 857.583 23 602.504 2.637 0.001

Hand Thickness Chuck MVC 181 750.823 15 12 116.722 12.569 0.000Lateral MVC 201 134.952 15 13 408.997 17.515 0.000Pulp-2 MVC 14 297.121 15 953.141 4.818 0.000

a Avg. R Squared ¼ .81.b Avg. R Squared ¼ .75.c Avg. R Squared ¼ .54.

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e46 43

relation between MVC and both, BMI and LBM. Wilks's lambdadistribution indicated that BMI has no correlation with the MVC(Wilk's L ¼ 0.998). In contrast, it indicated a correlation betweenMVC and the LBM (Wilk's L ¼ 0.002).

3.4. Two hands pinch grip MVC regression models

3.4.1. A regression model for the MVC based on gender, pinch grasppatterns, and pinch width

Based upon the ANOVA results, this study has shown thatgender, pinch grasp patterns, and pinch width are all significantand affect the pinch force. Hence, MVC regression model helps inquantifying the pinch grip force based on these parameters. Theanalysis show that the R of the regression model is 0.85 and the R2

is 0.7. Table 6 and Eq. (2) show the regression model and equation

for the MVC as a function of gender, pinch grasp pattern and pinchgrip width.

MVC ¼ 177:8� 72:8 Gender � 25:3 Pinch grasp pattern

þ 6:7Pinch grip width

(2)

where “male” is substituted by 1, “female” is substituted by 2,“chuck” is substituted by 1, “lateral” is substituted by 2, and “pulp-2” is substituted by 3.

3.4.2. A regression model for the pinch force of each anthropometricpinch grasp pattern based on gender and pinch width

A regression model was developed for the chuck, lateral, and

Table 6Regression model details for MVC.

Model Unstandardized coefficientsa Standardized coefficients T Sig.

B Std. error Beta

(Constant) 177.840 8.855 20.083 0.000Gender �72.841 3.365 -0.710 �21.650 0.000Hand Pinch Grip Width 6.725 1.118 0.197 6.015 0.000Pinch Grasp Pattern �25.306 2.058 -0.403 �12.299 0.000

a Dependent Variable: MVC.

Table 7Summary of regression models for each MVC pinch grasp pattern.

Model Unstandardizedcoefficients(MVC “Chuck”)

Unstandardized coefficients(MVC “lateral”)

Unstandardized coefficients(MVC “pulp-2”)

Sig.(MVC “Chuck”)

Sig.(MVC “lateral”)

Sig.(MVC “pulp-2l”)

B B B

(Constant) 155.625 197.258 28.730 0.000 0.000 0.006Gender �93.076 �98.751 �25.570 0.000 0.000 0.000Grip width 10.196 3.651 5.906 0.000 0.015 0.000R/R2 0.95/0.85 0.97/0.88 0.85/0.75 e e e

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e4644

pulp-2 grasp patern to get more robust regression model that maysatisfy the industrial needs. The R and R2 of the regression modelsand the models summary are shown in Table 7 and Eqs. (3) (4) and(5) as a function of gender and pinch grip width.

MVC}chuck} ¼ 155:6� 93:1 Gender

þ 10:2 Pinch grip width (3)

MVC}lateral} ¼ 197:3� 98:7 Gender

þ 3:7 Pinch grip width (4)

MVC}pulp� 2} ¼ 28:7� 25:6 Gender

þ 5:9 Pinch grip width (5)

where “male” is substituted by 1 and “female” is substituted by 2.

4. Discussion

The present study combined the two hands force data for bothmales and females for a sample of a Jordanian population. Test ofhypotheses confirmed that there was a statistically significant ef-fect of the hand pinch grip width and gender on MVC (p < 0.05).There was also a statistically significant interaction between theeffects of both pinch grasp pattern and hand pinch grip width withgender (p < 0.001). Imrhan (1999, 2003) presented two studies thatmeasure the two handsMVC considering two factors (i.e. pinch gripwidth and gender). He confirmed that there is a significant effectfor both gender and hand pinch grip width on MVC and there is asignificant interaction between gender and pinch grip width(p < 0.001). In the present study, the significance of the interactionbetween both pinch grasp pattern and hand pinch grip width withgender on the MVC means that, the MVC absolute value for eithermales or females is affected by a certain hand pinch grip width (3.8,6.8 cm) and a certain anthropometric pinch grasp pattern (chuck,lateral, or pulp-2). The ongoing influence between the anthropo-metric pinch grasp type and gender is pointing out that males havehigher differences between MVC than females within the pinchgrasp patterns, this conclusion agreed with the findings of theprevious literature (Dempsey and Ayoub, 1996).

Chuck and lateral MVC values were considerably higher than thepulp-2 MVC value at any pinch grip widths. A possible explanationof this, is that the strength retrogression is due to the use of onlytwo fingers in case of pulp-2 pinch grasp. However, the pinch gripforce is generated by almost all the figures in chuck and lateralpinch grasps. On the other hand, the pinch force can be influencedby many biomechanical factors such as fingers deviation, which inreturn affects the finger's bending effect by muscle relaxation orcontraction (i.e. musculotendinous fingers' flexors) (Kumar, 1999).From the functional bone-anatomy point of view, the force values insuch experiments are strictly controlled by the process of deviation(i.e. excursion) of the extensor and flexor tendons to the figuresinside the compartments of the rest (Cooper, 2013). This study isalso confirming the previous research regarding the fact that thechuck and the lateral pinch grasp patterns are slightly tending to beequal (Dempsey and Ayoub, 1996).

The present study concentrated on confirming the literaturefindings about the effect of certain hand dimensions on the pinchforce for each pinch grasp pattern. Multivariate analysis showedthat hand length, hand breadth, and hand thickness have a signif-icant effect on the pinch force (Wilk's L < 0.5). Previous researchpapers showed that hand length and hand breadth affect the MVCof one hand dynamometer experiment (Shurrab et al., 2015).

Imrhan and Sundararajan (1992) and Fallahi and Jadidian (2011)found considerable effects between hand length, hand breadth andpinch force as a result of their experiments samples in USA andIranian population. McDowell et al. (2012) pointed out that there isan increasing relation between the hand length and the measuredpinch grip force as well. However, Dempsey and Ayoub (1996)mentioned that further research is still needed to support the ev-idence that the hand thickness has an effect on the pinch force.Hence, this study considered this factor and found it to be signifi-cant on pinch force similar to Imrhan (1999) conclusions. Generallyspeaking, including the hand thickness factor will be subsidiarywithin the pinch force prediction models.

Furthermore, the pinch grip width and the pinch grip forcerelevance existed in many studies. The relevance tends to dependon the samples’ population, hand dimensions, and the anthropo-metric pinch grasp type. The results of the current experiment in aJordanian population showed that when the pinch grip width in-creases the MVC value also increases for males and females. This

M. Shurrab et al. / International Journal of Industrial Ergonomics 58 (2017) 38e46 45

current study results are depending on a small range of pinchwidth(~3.8e6.8 cm). Imrhan and Rahman (1995) presented a one handstudy for USA population. The study has slightly constant trendconcerning the relation between the pinch width and pinch forcefor chuck, lateral, and pulp-2 pinch grasp types for ~2.0e6.8 cmpinch width range in comparison to the current Jordanian popu-lation two-hand experiment results. Imrhan (1999, 2003) foundthat there is a decreasing relationship between pinch force andpinch width in his two-hand experiments for the USA population.However, the increasing or decreasing trends depend on certainfactors that affect this relevance as mentioned previously. Dempseyand Ayoub (1996) mentioned in their review that “the quantifica-tion of the effect of pinch width on strength capability and a set ofdata that reflect sustained, rather than peak pinch strength”.Anywise, Shivers et al. (2002) concluded that strength could in-crease or decrease monotonically depending on the grip span orwidth depends on the anthropometric data and sample population.

Designers should usually take into consideration the two-handed machine's handles interface design to support the ma-chine's pinch operation. In industrial applications, to our knowl-edge, the two hands have interaction effect between them. Thepinch forces exerted by the two hands (i.e. dominant and non-dominant hands) are not the same. However, in order to makemachine operation smooth, the two hands need to exert similarpinch force to maintain equilibrium. In contrast, if the machineoperation requires high pinch force exertions then the differencebetween the dominant and the non-dominant handwill be obviousand significant. The machine operation is still smooth unless highexertion forces are needed to be exerted to operate the machine'shandles using the two hands.

Based on male's comparison of one-hand grip strength study(Dempsey and Ayoub (1996) and the current study, the two handsproduce twice the force of a one-handed pinch grip of chuck andlateral patterns. Furthermore, in this study, the Jordanian femalesshowed 19% lower than Jordanian male's pinch strength. Thereduction of female's grip strength compared to male's gripstrength is similar to the results of previous research studies byDempsey and Ayoub (1996), Imrhan (1999, 2003).

Furthermore, in this research paper, two medical body indexeswere uniquely calculated in the current two-hand experiment forthe Jordanian subjects. The analysis showed that, BMI has no cor-relation with the MVC (Wilk's L ¼ 0.998). However, there is a sig-nificant correlation betweenMVC and the LBM (Wilk'sL¼ 0.002). Apossible explanation for this, is that BMI is a body components'measure that depends on the subject's body weight and his/herlength. Generally, body weight and length do not have main effecton the pinch forces based upon the research findings. In contrast,LBM which includes he muscular body components and considersthe amount of weight human carry on his/her body that is not fat.Therefore, this measure is logically related to the force more thanthe BMI because muscles (i.e. lean body) are usually related to theforce exertion. Previous research papers that studied the grip force(i.e. NOT pinch grip force) on one hand experiment results (Hardyet al., 2013; Shurrab et al., 2015) showed that BMI has a very loweffect that can be negligible on the MVC regression model. Fallahiand Jadidian (2011) studied some anthropometric characteristicseffects on the handgrip force for the Iranian population. They founda significant relevance between LBM and the pinch grip force. Thoseresults confirm our current experiment results.

5. Conclusions

An experiment was conducted to measure the two hands pinchgrip force for three different pinch grasp patterns (Chuck Pinch,Lateral Pinch and Pulp-2 Pinch) and two different pinch widths

using a custom-designed two-hand pinch grip strength tool. Theanalysis showed that Chuck and lateral pinch grasp patterns areslightly similar to each other for males and females, while Pulp-2pinch grasp pattern has the lowest MVC values for males and fe-males. ANOVA showed that both pinch width and pinch type weresignificant to the pinch force. The results showed that there is asignificant relationship between the anthropometry variables andthe pinch forces, where hand dimensions (i.e. length, breadth, andthickness) have an effect on the pinch force for each pinch widthand each pinch pattern. It is concluded that when the pinch gripwidth increases theMVC value also increases for males and femalesfor all the pinch grasp patterns. A significant correlation betweenMVC and the LBM was noticed by the multivariate analysis results.Future studies could consider other types of anthropometric pinchgrasp patterns (e.g. pulp to pulp pinch for each finger opposing thethumb) with a wider range of pinch widths.

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