Development of a Finger-Shaped Muscle Hardness Tester and Its Measurement Cases

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Journal of Mechanics Engineering and Automation 3(20 1 3)405-4 1 3Development of Finger Shaped Muscle Hardnessa -5Tester and Its Measurement CasesM itsuo Nagao ，Kotaro Yatabe2
，
Shin—ichi Konno3
，
Tokuo Endo4 and Osamu Yokota』，College ofEngineering,Nihon University，Koriyama 963—8642，Japan2．Graduate School ofEngineering,Nihon University,Koriyama 963—8642，Japan3．Department ofOrthopedic Surgery,Fukushima Medical University，Fukushima 960-1295，Japan4．Endo—Osteopathic Clinic．Motomiya 969-】129,JapanReceived：M ay 2，20 1 3／Accepted：June 6，20 1 3／Published：July 25，20 1 3Abstract：As the background of our study，we requested that practitioners use muscle hardness testers to conduct a digital assessmentof muscle hardness layers that they can feel by palpation．W e developed muscle hardness testers to assess muscle hardness digitallyfrom the reaction force and the depth in pushing a finger-shaped indenter，thereby simulating palpation．To assess muscle hardnessdigitaly，we proposed this means using the reaction force and depth that are measured when the indenter is pushed，along with theelastic constant，an d the differential elastic modulus．The tester is designed to be useful to ascertain efects of,or follow the course of,muscle layer treatment applied for shoulder stiffness and other conditions．As described herein，we confirmed the effectiveness ofdigital assessment using foam rubber consisting of an upper layer and a lower layer．respectively simulating the cortical and musclelayers of a human body．Additionally，monitoring six subjects，we digitally assessed the change of hardness of the trapezius muscleby chan ging the position of the upper extremity．Next，we were able to measure the chan ge of hardness before and after treatment for2 l subjects with shoulder stiffness.

Key words：Muscle hardness tester，finger-shaped，trapezius muscle，shoulder stiffness，palpation1．IntroductionThe objective of this study is the development of amuscle-hardness measuring device【1]that convertsmuscle hardness into digital signals after sensing bypressing a finger-shaped indenter，simulating palpation.

The muscle hardness detection methods used bycommercially available devices and those described inthe literature are of five categories， respectivelyassessing displacement[2—5】，inclination angle[6-7]，viscoelasticity【8—10]，mechanical impedance【1 1-12]，and impulse[1 3]．These devices have their respectivebenefits and shortcomings， but none has beencompletely accepted by practitioners who mustdiagnose patients 【4-5，10，14—17]．This fact isC0rresponding author：M itsuo Nagao，associate professor，Ph．D．，research fields：measurement and diagnosis systems，biomechanics，bionics．E-mail：nagao###mech．ce．nihon-u．ac．jp.

atributable to a lack of knowledge about the fatty layerthickness and differences of its flexibility．Furthermore，the reliability of measurements and operability needimprovement.

The authors are striving to realize a digitalassessment method that is insensitive to theseconditions and to develop a measuring device that canaccommodate small changes in hardness，hardness ofdeep muscle layers，enhanced effects of treatment，andwhich allows follow—up monitoring．In other words，muscle layer hardness caused by shoulder stiffness，lumbago，and fatigue can be digitalized.

This paper presents an outline of the proposedmeasuring device，its method of digitalization，and itseffectiveness and validity as confirmed throughexperiments．First，digitalization of hardness of thelower layer of foam rubber consisting of theupper／lower two layers are checked．Second，changes
Development of a Finger—Shaped Muscle Hardness Tester and Its Measurement Cases 407displacement is x 0—15 mm．Its total height is 155mm．Its mass is 650 g．The body casing is stainlesssteel(Fig．3 1．It is possible to change the indenter andthe keep plate depending on the figure ofan object site.

Additionaly，rr 0 is the value obtained when the testeron the steel plate l 0 mm thickness is pushed into at apushing stroke of 9 mm.

2．2Methods ofQuantifcationHardness of a stiff area or lump．as defined for thisstudy，is the amount of resilience or repulsion thatOCCURS when the surface part of the stifness or lump ispushed into the surface’s thickness，as presented in Fig.

1[1 8—1 9]．This measurement is correlated with hardnesssensed by palpation．Fig．4 shows an exam ple of W-x inwhich the palpated stiffness became softer after theoperation than before it．The hardness is shown as that ofthe surface layer including the fat layer in the earlyperiod of pushing，then as the difference of hardnessbetween the fat layer and the muscle layer，and finally asthe diference of hardness betw een regions．To take adigital measurement of hardness，four methods areproposed as shown in Eq．(1)：Wb>Wo，Xb< d， >lea，Ob> (1)= W／x．tan0=AW／AxWith this example，let us explain Eq．(1)．Thepushing reaction force is described as Wb> (N)，thepushing depth，Xb< 口(mm)，the elastic constant，W／x，Kb> (N／cm)，and differential elastic modulus，tan0=A W／Ax， > (deg)．The method to describethe hardness with W or is similar to that of theexisting muscle hardness tester[2-3，6-9]，which iseffective when the targeted site is restricted or for thesite of which the hardness varies widely．If the methoddoes not alow judgment of the hardness of the site，then is added for judgment．Even so，if it is difcult，then the difference of hardness can be quantified byadding the gradient at the end of pushing tan0.

Alternatively，because it is possible to adjust thepushing force according to the surface thickness or thehardness of the targeted site as presented in Fig．2，amethod to obtain measurements that are approximatelyequal to those of the hardness felt by palpation is alsopossible.

3．Experiments and Results3． Pushing ForceAssessing the Lower Layer HardnessW e confirmed the effectiveness of our proposeddigital measurement method using pushing force，which can assess the change of the lower layerhardness，by maintaining the upper layer thicknessconstant for two layers consisting of the upper layer ofthe surface layer(1)and the lower layer of the musclelayer(3)presented in Fig．1．Conditions of the twolayers are shown in Table 1.

Theupperlayerismadeoffoam rubberA rf×W ×5×100×50 mm．E21 4-2／5)．The lower layers withdiferent hardness are made offour kinds offoam rubberA-D (to× W × l0× 100×50 mm)．Thecombinations ofcomposition are symbolized by AA，AB，AC，and AD．The foam rubber hardness is represented bythe values of Durometer Type E．The pushing forces aredivided into eight levels of4 N to 36 N.

Results of the experiments show that the correlationbetween pushing reaction force W and pushing depthare as portrayed in Fig．5a．The elastic constant andthe differential elastic modulus 0 are in Fig．5b．Allresults are mean values obtained from 1 0 pushingrepetitions．The“4 Mono”presented in Fig．5 meansthe values in directly pushing it into the samples A-Dwith pushing force of 4 N．To clarify the differences inmeasurements obtained before and after the sample，thesymbols are connected with a line.

The amount of W0 by which the differences betweenAA and AB．between AB and AC，and AC and AD canTable 1 Composition of the samples。of two layers
． Foam rubber dimensions：f×W × =t×50 x l00 mmFoam rubber hardness of from A to D408 Development of a Finger—Shaped Muscle Hardness Tester and Its Measurement Cases234 AA AB AC ADComposition of the object(a)Quantifcationby and WAA AB AC AD3020Composition ofthe object(b)Quantification by and 0Fig．5 Effect of pushing force to assess the lower layerhardness.

be judged，was obtained using t-tests．Assuming thatthe two samples are homoscedastic，the conditionswere obtained with two—sided t boundary values，significance level of P 0．000 1．and degrees offreedom of 1 8．The result was f( ，p) t(1 8，0．0001)= 4．97．In terms of W0，with the test statistic ofto>t，X，W and K had W0> 20 N and 0 had > 8 N．“ p<0．O001”is added to Fig．5.

Consequently，if each pushing force equivalent tothe thickness ofthe upper layer is appropriate，then it ispossible to assess the different hardness of the lowerlayers digitaly[1 8—1 9，2 1]．It is applicable directly tothe thickness of(1)and (2)in Fig．1．In thedigitalization of O，even if W0 is smaller than the others，the differences can be discriminated as the advantageof this equipment.

3．2 Comparison between and 0 by the Upper Layer刀2fc essW e confirm ed the effect of the upper layer thicknesson the sensitive reference in digitalization of K and 0when foam rubber A with thickness oft=1．2．or 3mmwas put on the lower layer shown in Table 1．Theresults are depicted in Fig．6．Indications“Mono”arevalues of A．D．In digitalization of elastic constant，asthe upper layer thickens，the difference between objectsbecomes indistinct．Therefore，with f= 3 mm，itbecomes more di~ cult to determine it than with f= 1mm．However，in digitalization of 0，unlike in K，without difference of an upper layer，we conducted anANOVA <0．0 1，one·way analysis，level of factor 4，and repetition number of times l 01 for every differentcomposition ofthe objects(e．g．，in the case ofAA，fourM ono 户 1 户2 t=3Composition of the obj ect(a)Quantifcation by K∽ 2l三0gbD苎161lMono 户 l户2 t=3Composition of the obj ect(b)Quantification by 0Fig．6 Sensitivity comparison betweenⅣand 0.

8 6 4 2 一 D ．IoJI10 0对0一量量一 暑Q l二【1 5 9 2 l 一吕 )f I】§∞I。。。一 叫一 0 一 ∞t1一j 0—IIu一 ∞对一 一对一 Io．I 口一uIu 工J 如 加 O暑 I10Q 0一0一 一盘兰Q 一l二【Development of a Finger-Shaped Muscle Hardness Tester and Its Measurement Cases 409kinds of 0 including single layer A，and layers with f=1，2，and 3 mm)．The discriminant standard is thatshown below.

F0( ，CE；0．01) (3，36；0．01) 4．38The values F of the objects were the folowing：AA(1．69)， AB(1．02)， AC(2．64)， and AD(1．57).

Consequently，because F

3．3 Contraction Hardness of Trapezius Muscle byEversion ofthe ArmThis experiment confirms the possibility of digitallyassessing the hardness that exists before and after atrapezius muscle contracts．The contents are shown inTables 2-3 and Fig．7．Examinees were six universitystudents with different body shapes and features ofphysical constitution：they were designated as A 1一A6in order of increasing BAli(Body Mass Index)values.

The hardness of contracting muscle was obtainedfrom the posture in which the position of upperextremity was different at two sites．First，the musclewas not tense and soft when the upper extremity wasput on the armrest．Second，the muscle contracts andbecomes tight when the first position of the upperextremity turns outward by 90。[22]．The site on whichthe intender is placed is midway between the seventhcervical spine and the acromion，where the hardnesscan be felt by fingers.

In Fig．8．data for examinees A 1．A6 are shown onthe X axis；the hardness of the armrest position and90-deg turning outward position in term s of x and 0 areshown on the Y axis．Along with the mean value，therange between the maximum value and the minimumvalue is also shown．According to W／x．thedifference between conditions before and after musclecontraction are evident．so it is possible to assess themuscle hardness digitally．The hardness decreases，Table 2 Features and measurement condition 0f 6examinees：university students，age 21-22.

Table 3 A position，posture，measurement condition andBMI value to test.

Item Contents0bjectpositionContractionaction ofthe armTrapezius muscle of the right shoulder，theposition which guesses the indenter is amidpoint of cervical spine C7 and acromialprocess.

It sits in the armchair,and positions of the armare armchair and 90。eversion.

Indentation set load W0= 9．8 N．keep plateMeasurement diameter 40 mm．indentation seting hour 5 s.

condition the tip is a globe on the indenter diameter at 8mm，and pushing frequency 5(convolution)Thinness type l9BMIvalue 19

except for examinee A4，because the BMI valuesbecome greater from A1 to A6.

Although the experiment did not assess the fat layerand muscle layer thickness digitally，to assume thecorrelation between BMI and the thickness of thoselayers，it is reasonable to expect that the value shouldbe smaller．Examinee A4，a tennis player，has a hardand thick neck and shoulder muscles，which arerepresented by the value．Examinee A6 has thickmuscles．Therefore，the difference between the armrest4 1 0 Development of a Finger-Shaped Muscle Hardness Tester and Its Measurement CasesExaminees n[person】Fig． 8 Comparison of muscle hardness by musclecontraction of the trapezius muscle.

position and a 90一degree outward tuming position isbest．In contrast，A 1，who is thin，has a large valuewhen his upper extremity was put on the armrest．Toassume that his muscle is also proportionally thin，itshould be true because the value also represented thebone layer hardness as well as muscle layer or thepushing force was strong.

However，even though the tendency in digitalizationby 0 is like that shown by results show that thevariation of values is greater than that by ．Thattendency might be attributed to the high sensitivitydepicted in Fig．6．Showing the effectiveness of ourproposed digitalization，when the difference in，forexample，A5 is evident by ，but it is dif cult todiscriminate by one can discriminate it using W／x.

Next，using the t-test，we confirm ed that the musclehardness in turning to an outward position can bediscriminated to be higher than that in arm rest position.

As test conditions to ascertain the values，two samplesare assumed to be homoscedastic．The f boundary valueis one—way．The significance standard is P 0．0 1.

Degrees offeedom are 8．In digitalization by and0，t(e，2p) t(8，0．01) 2．90 was found．Indigitalization by ．the test statistic to was to> r in a1lexaminers．W ith digitalization by 0 it was so in only A2And，with 0，A 1 and A4 have no significant difference.

It is shown as“ <0．01”in Fig．8.

The experiment demonstrated the following．Fig．1and Table 2 show that it is important to assess thehardness of each rather than the differences of hardnessamong individuals because human bodies haveindividual differences related to the thickness andhardness of the object sites．Furthermore，as shown inTable 1 and Fig．5，it is necessary to imitate themovement of strong pushing when the object site isthick for measurement with good sensitivity becausethickness is a factor holding the key to the sensitivity ofhardness．When the thickness is slight，even a smallchange can be grasped if one pushes the indenterlightly，as in palpation.

3．4MuscleHardnessRelatedtoShoulderStifnessThe examinees were 2 1 male university studentsaged 21—4(B1-B21)，of whom we had conducted asurvey in the foITI1 of a questionnaire．W e obtainedtheir consent for the experiment in advance．The objectsites were the sites of shoulder stiffness，which welocated by palpation and near which the indenter waspushed， as portrayed in Fig． 7， using the samemeasurement conditions as those presented in Table 2.

Before and after a practitioner gave an examinee amassage，we measured the hardness with our musclehardness tester five times，with the mean considered asthe measured value．The practitioner gave a massageby hand and used an ultrasonic therapy apparatus forabout four minutes.

For the amount of change before and after thetreatment practice，we conducted a t-test，assuming thatthe two samples should be homoscedastic．With atwo—sided t-boundary value <0．05)，we obtained ，P)=t(8，0．05) 2．3 1．The examinees were divided intothree groups based on this test：a group ofthe examineesofwhom muscles softened 03 by K and 9 by 0 among21)，a group showing no diference(5 by K and 1 1 by 0among 2 1)，and a group of examinees whose musclesbecameharderf3 by Kand 1 by 0among21).

In Fig．9，with K represented on the left vertical axis，the three groups are shown from right to left(represented by two bar charts)．They are aranged inascending order of muscle hardness．The horizontalaxis represents the values of examinee numbers and一 pJ。∞Tl—Ip0骞 加 m u一 sB 一菪3J Q
_黾 lS 0u 3I ％ 一Development of a Finger-Shaped Muscle Hardness Tester and Its Measurement Cases 4 1 1●K：Beforc O ：ARCl" ●●0：Before 0 0：Atier是 ； 》 · ! 1。· 圣+： +t●4035O2
2O15their BMI．On the other hand，with 0 represented on theright vertical axis(by signs of circle)，each 0 of theexaminees，divided into three groups by t-test，ismarked using a reference mark[ ]( <0．05).

They were divided into three groups because thepractitioner explains that physical stimulus was givento a stiffmuscle for which the tissues’blood circulationwas facilitated，thereby softening it．Others showed nodifference．Other muscles showed increasing innerpressure of the muscle，thereby hardening it．B 1 6(nodifference by ，hardening by ；T)and B2 1(hardeningby ，softening by ； )showed differentjudgment by Kand 0．W ith the boundary value ofp < 0．O 1．they havethe same judgment by and 0．Therefore，it is possibleto assess the muscle hardness digitally based onchanges that occur during physical treatment forpatients with complaints of shoulder stiffness．Resultsshow the capability of digital assessment of the effectsof practical treatment and palpation.

Next，targeting the group of examinees whosemuscle was softened(among 21，with ，13；with 0，9)，Fig．1 0 shows BMI on the horizontal axis in increasingorder，with a regression line．After operation，theamount of softening was likely to be greater becausethe BM1 was smaller in a lean body frame．In contrast，the softening was likely to be smaller in an obese bodyframe，with a downward—sloping regression line．Thethickness presented in Fig．1 might be correlated to theamount ofBAli．Fig．1O shows that，as BMIdecreases，the procedure is more likely to have the effect ofsoftening on it.

Fig．103015 20 25 3OBody mass index BMI 【kg／m 】Correlation of m uscle hardness and BMI。

4035302520l5Body mass index BMI[kg／m 】Fig． 1l Relation between BMI and muscle hardnesssoftening rate.

Defining the levels of softening digitalized by and0 as the softening rate，they were gained by Eq．(2)，aspresented in Fig．1 1．The examinees were 13，whosemuscles were softened with ．and to which 0 was alsoaccorded．Sufix a and sufix b respectively denotevalues obtained after oper~ion and before operation.

Furthermore，0c and a0 respectively represent the elasticconstant and softening rate by the differential elasticmodulus.

=
l_K_
q_a
， 1一 Ob (2)6The horizontal axis shows the order of BMI values.

The vertical axis shows the softening rate，6c and。c目.

The bar charts show digitalized levels of softening.

W ith this extent ofBMI,the level of softening does notdepend on the amount of BMI．In other words，becausethe levels of softening had individual differences evenwith the same procedure，it is necessary to conduct面 一 ∞三np0骞ul1跨一IB—luu．I毫口如 加 ：2 m一lI3／ )f看 营 0：一莒一叫一 一 ∞；一l1 0—I 呐霄一。一时一 I10．1∞磕 口加一暑3／NJ 们I10u ％一一o／0一。 。 ．I∞uII。 0∞如 加 0加 m O一0／0一 Q BJ∞uIIu om412 Development of a Finger-Shaped Muscle Hardness Tester and Its Measurement Casesprocedures suited for each condition of stifiness．Thesign“ ”in the figures like Fig．9．shows a significantdifference between and 0．Sign“×”denotes a lack ofsignificant difference．The formula is helpful toobiectify the results of procedures because it caldigitalize the results of softening by procedure.

4．ConclusionsProposing an outline of the muscle hardness testerthat can imitate hardness assessments done bypalpation and presenting a mode of to digitalassessment，we were able to confirm their effectivenessand validity by conducting several experiments．Theobtained results are summarized as follows：· W e proposed four methods to assess musclehardness digitally．Using them singly or multiply，thetester presents high reliability．K and 0 are used intypical methods；· It is suggested that the levels of softening beforeand after operation should be able to be digitalized bysoftening ratio，which can also be useful to objectifythe effectiveness of operation；· Discriminating the lower layer hardness fordifferent surface thickness is possible if the pushingforce is appropriate to thickness；· For a thin surface layer，such as a few millimeters，0 has less effect on the lower layer hardness than does，and is therefore validated．Furthermore，0 is moresensitive than ；· W hen a difference between hardening andsoftening is visible as in the contracted and stiftrapezius muscle ofa shoulder，digital assessment by，or is suficient．Even ifthe object sites are the same，they have individual differences reflected in the values，which suggests the need for measurement conditionsand data management tailored to respective examinees；· The muscle hardness before and after practicaltreatment for shoulder stiffness are categorizable intothree groups：softening，no difference，or hardening.

They are highly correlated to K and 0，with someexceptions；。 As BMI becomes larger，the hardness valuebecomes smaller．That tendency suggests that thesurface layer thickness can be expected to corelate totheBMIamount；· In future studies，we wil aim at improving theprocess for practical application by examining thepossibilities of digitally assessing effects according topractices，substitution of palpation or self-palpation，and follow—up.

AcknowledgmentsPart of this study is conducted with assistanceprovided by a Grant-in·-Aid for Scientific Research C(No．22560225)ofthe Japan Society for the Promotionof Science．W e express our gratitude for that support.

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