SCIENTIFIC/CLINICAL ARTICLE

JHT READ FOR CREDIT ARTICLE #138.

The Concurrent Validity of a Hand-held versusa Stationary Dynamometer in Testing Isometric

Shoulder Strength

Jean-Sebastien Roy, PT, PhDSchool of Rehabilitation Sciences, McMaster University,Hamilton, Ontario, Canada

Joy C. MacDermid, PT, PhD*School of Rehabilitation Sciences, McMaster University,Hamilton, Ontario, CanadaHand and Upper Limb Centre, St. Joseph’s Health Centre,London, Ontario, Canada

Brock Orton, PTThomas Tran, PT

School of Rehabilitation Sciences, University of WesternOntario, London, Canada

Kenneth J. Faber, MD, FRCSCDarren Drosdowech, MD, FRCSCGeorge S. Athwal, MD, FRCSC

Hand and Upper Limb Centre, St. Joseph’s Health Centre,London, Ontario, Canada

*Funded by a New Investigator Award, Canadian Institute ofHealth Research. Joy C. MacDermid was funded by a NewInvestigator Award.

Correspondence and reprint requests to Jean-Sebastien Roy, PT, PhD,School of Rehabilitation Science, McMaster University, IAHS, 1400Main Street West, Hamilton, Ontario L8S 1C7, Canada; e-mail:<jean-sebastien.roy.1@ulaval.ca>.

0894-1130/$ e see front matter � 2009 Hanley & Belfus, an imprintof Elsevier Inc. All rights reserved.

doi:10.1016/j.jht.2009.04.008

320 JOURNAL OF HAND THERAPY

ABSTRACT:Study Design: Clinical Measurement-Validity.Introduction: Validity of the JTech PowerTrack II hand-held

dynamometer (JTech; JTech Medical, Salt Lake City, UT) for mea-suring shoulder strength has yet to be established.

Purpose of the Study: To examine the concurrent validity ofisometric strength scores obtained with the JTech PowerTrack II,and on a stationary dynamometer, the LIDO WorkSET (LIDO;LoredanBiomedical, West Sacramento, CA).

Methods: Thirty-eight subjects performed three maximal effortsof shoulder flexion, abduction, and external rotation on a singleoccasion on the two dynamometers. Two testers were randomlyassigned to administer the tests.

Results: Pearson correlations between the scores on the twodynamometers (r.0.81) indicated a good concurrent validity. Corre-lations were similar when the results were subdivided by tester orgender.

Conclusions: This study suggests that either the JTech Power-Track II or LIDOWorkSET provide comparable scores for shoulderstrength. Although not interchangeable because of the differencesin units of measurement, the relative conclusions about strengthshould be similar, regardless of which instrument is used.

Level of Evidence: Not applicable.

J HAND THER. 2009;22:320–7.

Strength testing is a fundamental component ofassessing physical status in upper extremity rehabil-itation. This is particularly true in the shoulder jointwhere bony congruence is insufficient to ensurestability and muscle forces are important to bothstability and motion. Shoulder strength assessmentscan be performed using manual muscle testing(MMT), hand-held dynamometry (HHD), or station-ary dynamometers (SDs). MMT was introduced inthe early 1900s for the evaluation of patients withpoliomyelitis. Its application was later extended tothe evaluation of orthopedic patients. MMT although

technically easy, has several limitations.1,2 Beasley1

suggested that MMT is inefficient at differentiatingbetween strength levels; this is especially true whenfull antigravity movement has been attained. Forexample, a grade 4 on MMT is obtainable with only20% of maximal shoulder strength.2

Hand-held dynamometers were introduced in19493 to help quantify antigravity muscle strength.Since then, normative values have been established4,5

and HHDs are an accepted method in clinical evalu-ation.6 Furthermore, studies have indicated that fair-to-excellent inter-rater reliability is attainable usingthese instruments.7e11 Despite promising resultsfrom these studies, other research indicates that prob-lems such as tester strength, joint position, forceapplication, and stabilization of the patient can occurwith HHD.11 All these factors can influence reliabilityof results.

In the 1960s, large SDs capable of measuring eitherdynamic or static muscle strength were introducedinto the health care setting. These are typicallyexpensive systems, but provide the ability to test

muscle performance using different speeds or modesof muscle contraction (isokinetic, isotonic, concentric,and eccentric). The LIDO WorkSET� (LIDO; LoredanBiomedical, West Sacramento, CA) is a SD that isfrequently selected by clinicians dealing with upperlimb pathology because it has numerous tools thatcan be used to simulate upper extremity activities.The LIDO permits concentric, eccentric, isotonic, andisometric muscle assessment. In previous research,the LIDO has been found to produce reliable andvalid scores.12,13

Despite research supporting the use of SD, there aresome drawbacks that limit the application of thesedevices. SDs are costly and the setup time required totest patients is lengthy as compared with many otherphysical evaluation devices. Due to their size, SDsoccupy substantial space and are not portable, limit-ing their use to a single clinic with sufficient space.Additionally, some providers of isokinetic equipmenthave withdrawn from the market so that the technicalsupport for some SDs can be sporadic.

SDs produce strength measures that are moreprecise than MMT, but require greater financial,time, and technical resources. It is possible thatcomputerized HHD could provide a viable compro-mise. A reliable, valid, and commercially availabledevice that could be quickly administered wouldpotentially enhance shoulder strength measurementin a broad spectrum of clinical situations. JTechMedical (JTech Medical, Salt Lake City, UT) hasintroduced a computerized HHD as a component ofits assessment system (Tracker ME�). However, be-fore new assessment procedures are accepted intopractice, it is important to establish measurementproperties. Therefore, the purpose of this study wasto determine the concurrent validity of isometricshoulder strength scores obtained with the JTechPowerTrack II� hand-held dynamometer (JTech)compared with a SD, the LIDO. We hypothesizedthat the measurement provided by the two dyna-mometers should reach a correlation coefficient (r)above 0.70.14

METHODS

Subjects

Thirty-eight men (n¼ 21) and women (n¼ 17),recruited through convenience sampling, voluntarilyparticipated in the study. Eleven subjects were neededto have more than 80% power to detect a Pearsonproduct coefficient of 0.70. With 38 subjects, the powerwas 99%. Inclusion criteria were 1) age 18 years andabove; and 2) no wrist pain or pathology. Wrist pathol-ogy was an exclusion criterion due to the need for theshoulder strength to be transmitted by gripping theshoulder test device on the LIDO. Subjects were

young to middle-aged adults (mean age¼ 27.6 6 12.7years; range: 19e67 years). This study was approvedby our Institutional Review Board.

Volunteers included subjects with no or mildshoulder symptoms. Before the testing of isometricshoulder strength, the Shoulder Pain and DisabilityIndex (SPADI)15,16 was completed. The SPADI is aself-report questionnaire developed to measure thepain and disability associated with shoulder pathol-ogy.15 This questionnaire was used to classify subjectsas experiencing or not experiencing shoulder painand disability. A SPADI score $18 points was consid-ered to indicate a subject with shoulder symptoms ofpain and disability. We choose 18 points, because theminimal detectable change (95%) for the SPADI is18 points.17 Therefore, we considered that this cut-off reflected an appropriate designation for symp-toms. Ten out of the 38 subjects showed mild shouldersymptoms by this criterion. These subjects were notseeking any care for shoulder pathology and wereable to perform their activities of daily life normally.No attempt was made to define the nature of theirshoulder pathology.

Instruments

The maximal isometric strength of shoulderabductors, external rotators, and flexors was mea-sured using the JTech Powertrack II� computerizedhand-held dynamometer (JTech) and the LIDO. Forsubjects without shoulder symptoms, the dominantarm was evaluated; whereas, the affected arm wasevaluated for the subjects with mild shouldersymptoms.

Instead of the application of force through a griphandle at the end of a long lever arm as recommen-ded in the LIDO manual, a customized padded upperarm contact attachment was used for flexion strengthtesting. This customized attachment allowed mea-surement of shoulder flexion strength directly. Bothdevices (JTech and LIDO) were calibrated againstprecision weights at the start and conclusion of thestudy. No differences in calibration were detectedduring the study interval.

Testing Position

A variety of shoulder positions have been describedfor isometric strength testing. Some authors18 havetested in the scapular plane instead of the coronalplane, suggesting it decreases stress on the capsulareligamentousetendinous complex, increases the con-gruity between the humeral head and glenoid andkeeps an optimal lengthetension relationship for theabductors and rotators. The scapular rotator planehas been defined as the arm at 458 of forwardelevation in the sagittal plane and at 308 anterior tothe frontal plane.19 Some studies20,21 have suggested

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FIGURE 2. Test positioning for shoulder flexion with theJTech.

that higher torques are produced by subjects with andwithout shoulder pathology in this position; althoughothers have found that there is minimal22 or no differ-ence23 in subjects without shoulder pathology.

In the present study, all subjects had their armspositioned in the plane of the scapula during strengthtesting. They were seated on a straight back stablechair that allows to isolate joint motions and tostabilize the trunk. For shoulder abduction, theresistance arm for both instruments was applied tothe lateral aspect of the mid-humerus, at the insertionof the deltoid on the humerus (Figure 1). Duringshoulder flexion, the custom attachment was usedon the LIDO, so that the resistance arm could be ap-plied to the anterior aspect of the upper arm, at thelevel of the insertion of the deltoid. This same loca-tion was used for testing with the JTech (Figure 2).During lateral rotation, the resistance arm was ap-plied through the handgrip device for the LIDO(Figure 3) and applied on the dorsal aspect of theforearm just proximal to the ulnar head for theJTech (Figure 4).

Procedures

Two male physiotherapists who had not previouslyused either of the two dynamometers performed thetesting (tester 1: height¼ 1.73 m, weight¼ 75 kg;tester 2: height¼ 1.80 m, weight¼ 79.5 kg). Bothtesters received training before collecting data andhad time to practice on both dynamometers. Eachsubject was tested by one of the testers. The choiceof the tester was made through randomization; 16subjects were evaluated by tester 1 and 22 subjectsby tester 2. The dynamometer used first for the evalu-ation of maximal isometric strength was also random-ized; 18 subjects were evaluated first on the LIDO and20 subjects on the JTech. The scores for each test wererecorded by a computer. Evaluators and subjects wereblinded to the test results during the strength testing.

FIGURE 1. Test positioning for shoulder abduction withthe JTech.

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Subjects performed one practice trial and three testtrials in each of the three test positions. Subjectswere asked to perform maximal muscular contrac-tions. The subjects in the symptomatic subgroupwere instructed that the testing could be stopped ifthey experienced pain. However, no subject had tostop the testing or complained of disabling pain.This confirms that these subjects had mild shouldersymptoms. The intensity of pain during these testswas not measured. Each trial consisted of a 3-secondisometric maximal contraction. The average maximalisometric strength of the three trials was used as thecriterion measure. A rest period of 10 seconds wasgiven between each trial. Shoulder abduction maxi-mal isometric strength was tested first, followed byflexion and lateral rotation.

FIGURE 3. Test positioning for shoulder external rotationwith the LIDO.

FIGURE 4. Test positioning for shoulder external rotationwith the JTech.

Data Analysis

The LIDO calculated average peak torque values(Nm) from the three trials in each of the test positions.The JTech displayed maximal isometric strengthproduction in kilogram for each trial. The averagemaximal isometric strength was manually calculated.Final values were entered into SPSS software (Version12; SPSS Inc., Chicago, IL). Independent t-tests wereused to compare the men’s and women’s maximalisometric strength. The concurrent validity betweenthe scores produced by these two different dyna-mometers was estimated by calculating Pearsonproduct correlation coefficient (r) and its 95% confi-dence interval (95% CI). Pearson correlations indicatethe association between scores obtained on the twodifferent instruments for each shoulder musclegroups. As the JTech instrument measures strengthin kilogram and the LIDO measures torque in

TABLE 1. Subject Characteristics and Mean Max

Characteristics Muscle group

Age (years)Dominant side (n)LIDO (Nm) Abduction strength

Flexion strengthExternal rotation strength

JTech (kg) Abduction strengthFlexion strengthExternal rotation strength

Age (years)Gender (n)Dominant side (n)LIDO (Nm) Abduction strength

Flexion strengthExternal rotation strength

JTech (kg) Abduction strengthFlexion strengthExternal rotation strength

Newton meter, Pearson correlation coefficient was re-quired. Coefficients of determination (R2) were alsocalculated to determine the variation in score onone instrument that could be explained by the varia-tion in the score on the other instrument. Pearsonproduct correlation coefficients (and 95% CI) werealso calculated for specific subgroups of the data toevaluate whether order of instrument, tester, gender,or shoulder symptoms impact the results.

RESULTS

All patients completed the test protocol withoutdifficulty. The mean isometric maximal strengthobtained with the LIDO and the JTech for men andwomen and for subjects with or without shouldersymptoms are shown in Table 1. In all three musclegroups tested (flexors, abductors, and external rota-tors), men were stronger than the women(p , 0.001) with each dynamometer.

Correlations between the two dynamometer werehigh in all three shoulder muscle groups withPearson coefficients (r) ranging from 0.81 to 0.93and a 95% CI ranging from 0.66 to 0.92 (Table 2).All the correlations were significant (p , 0.001). Thecoefficient of determination (R2), which rangedfrom 0.66 to 0.76 (or 66% to 76%), indicated that a ma-jority of the score variances recorded by one instru-ment were explained by variation in the scores onthe other instrument (Table 2).

When data were subdivided according to thetester, there were no significant discrepancies be-tween results obtained by the two testers (tester 1:0.81 # r # 0.85; tester 2: 0.81 # r # 0.91) (Table 3). Thetest order also did not affect the correlations (LIDOperformed first: 0.77 # r # 0.86; JTech performed

imal Isometric Strength (Mean 6 SD or n)

Men (n¼ 21) Women (n¼ 17)

29.7 6 13.4 24.9 6 11.617 right/4 left 15 right/2 left57.5 6 18.9 29.5 6 8.753.4 6 20.8 26.5 6 7.428.4 6 7.2 15.8 6 4.320.5 6 6.8 12.3 6 2.720.5 6 7.3 11.8 6 2.711.4 6 3.6 7.3 6 1.8

Symptomatic(n¼ 10)

Asymptomatic(n¼ 28)

38.1 6 18.7 25.3 6 8.16 men/4 women 15 men/13 women8 right/2 left 24 right/4 left35.0 6 26.8 50.4 6 19.238.3 6 19.0 51.9 6 21.318.7 6 8.4 26.2 6 7.813.2 6 7.3 19.6 6 5.914.1 6 9.1 18.6 6 6.4

7.7 6 4.6 10.5 6 2.7

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TABLE 2. Correlation between Instruments Scores(n¼ 38)

Motion Pearson r 95% CI p-Values R2

Abduction 0.81 [0.66e0.90] ,0.0001 0.66 or 66%Flexion 0.87 [0.77e0.93] ,0.0001 0.76 or 76%Externalrotation

0.85 [0.72e0.92] ,0.0001 0.72 or 72%

Pearson r ¼ Pearson product correlation coefficients; 95%CI¼ 95% confidence interval; R2¼ coefficients of determination.

first: 0.83 # r # 0.88) (Table 3). Pearson correlationsfor subgroup classified as having a symptomaticshoulder were slightly higher than those who wereasymptomatic (symptomatic: 0.87 # r # 0.94; asymp-tomatic: 0.76 # r # 0.83) (Table 3). This difference,however, was not significant because the 95% CIoverlapped. Finally, the correlations for the menand women were similar (men: 0.65 # r # 0.80;women: 0.65 # r # 0.85) (Table 3).

DISCUSSION

This study demonstrated a high correlationbetween scores obtained for isometric shoulderstrength in men or women, with no or mild shouldersymptoms, using the LIDO SD and the JTech hand-held dynamometer. This is in agreement with thecorrelation established between static strength scoresobtained with the HHD and SD when measuringstrength of the knee24 and elbow.25 Correlationsbetween scores obtained on a HHD and the LIDOduring isometric elbow flexion and extension rangedfrom 0.72 to 0.85, similar to the correlations observedin the present study for shoulder movements.25

The reliability of the LIDO has already beenestablished for a variety of test protocols.12,13 Theapplication of this device to shoulder isometricstrength testing using scapula plane positioning;however, has not been previously reported. TheJTech is a newer device with little peer-reviewed liter-ature to document its performance during assessmentof shoulder strength. Reliability testing of the currenttest protocol using both instruments would furtherenhance our understanding of the relative utility ofthese two devices. A previous study comparing the re-test reliability of the Biodex, Nicolas hand-held dyna-mometer and Isodex found excellent reliability for allthree devices.10 Because inter-rater reliability has notbeen established for our specific test procedures, wetested the generalizability of our conclusions basedon variations in tester or instrument order. The corre-lations between these pairs of subgroups tended to besimilar. This suggests that tester or instrument orderhad a minimal impact on the relationships and im-plies a level of consistency in the conclusions.

Other factors that have been reported to influencethe validity of HHD are the dynamometers

324 JOURNAL OF HAND THERAPY

themselves, the tester involved (their strength andexperience), and the testing procedures.11 The twodynamometers used in this study have definite dif-ferences. The LIDO requires that for some shoulderassessments, such as external rotation, the forcemust be applied through a grip device. This can beproblematic when patients have wrist pathology.Thus, the comparison of external rotation strengthusing the two devices used in this study required dis-similar patient set-ups. In this study, subjects did nothave serious pathology of the upper limb; so theeffect of this difference was minimized, but remainsa consideration for certain clinical populations.Despite some inherent differences in the applicationarms and set-up procedures, a high correlationbetween the measures obtained suggests that theyprovided similar information on external rotationstrength.

Factors affecting the comfort of the subject canoften create nonmaximal efforts and, therefore,increase instrument error. The interface of the JTechconsisted of a hard, flat, circular rubber platform witha 10-cm diameter. When discomfort was felt, theprevious recording was disregarded and an addi-tional trial was performed, including appropriate restperiods. A slightly larger more conforming interfacepad may avoid these test inefficiencies.

Tester strength has been demonstrated to lead todifferent results when performing dynamometrymeasurements.11 Stronger therapists can create largerforces due to their greater ability to resist forces,whereas some therapists may be weaker than theirpatients. An understanding of how to use biome-chanical advantage to assess patients with properstability is required to minimize these tester errors.In the majority of clinical populations, most thera-pists should be able to provide sufficient resistanceto match the capability of the patient using thesebiomechanical principles. Athletic populations mayhave high strength, even in an injured state and there-fore stabilization for this type of patient may be moredifficult. These patients are often assessed usingisokinetic dynamometry because of greater stabilityand the ability to assess muscle performance at vary-ing speeds. Clinicians wishing to assess isometricstrength in athletic populations could consider theenhanced stabilization provided by SD or more stabi-lized isometric strength assessment systems.

This study tested a subgroup of subjects whoreported mild shoulder symptoms because we feltthese subjects would represent mildly disabledpatients. However, normative data are often estab-lished on subjects without shoulder pathology.Furthermore, when testing the correlation betweendifferent instruments, it is important to cover a widespectrum of relevant results, that is, low to highstrength. For these reasons, subjects with no or mildshoulder symptoms were included in the study

TABLE 3. Pearson Correlations between Instrument Scores in Subgroups

Flexion Abduction External rotation

Subgroup n Pearson r 95% CI Pearson r 95% CI Pearson r 95% CI

Tester #1 16 0.85 [0.61e0.95] 0.81 [0.53e0.93] 0.82 [0.55e0.94]#2 22 0.91 [0.79e0.96] 0.81 [0.59e0.92] 0.90 [0.77e0.96]

First test on LIDO 18 0.86 [0.66e0.95] 0.84 [0.61e0.94] 0.77 [0.47e0.91]JTech 20 0.88 [0.72e0.95] 0.83 [0.61e0.93] 0.88 [0.72e0.95]

Subgroup Symptomatic 10 0.94 [0.76e0.99] 0.92 [0.69e0.98] 0.87 [0.53e0.97]Asymptomatic 28 0.83 [0.66e0.92] 0.76 [0.54e0.88] 0.83 [0.66e0.92]

Gender Men 21 0.65 [0.30e0.84] 0.80 [0.56e0.92] 0.80 [0.56e0.92]Women 17 0.85 [0.63e0.94] 0.82 [0.56e0.93] 0.65 [0.25e0.86]

Pearson r¼ Pearson product correlation coefficients; 95% CI¼ 95% confidence interval.

sample. Our results demonstrated that the correlationbetween instruments was higher in subjects with mildshoulder symptoms. We expect that this reflects thegreater spectrum of responses provided by patientsreporting shoulder symptoms. This is supported bylower coefficients within male and female subgroups,where variability was minimized because of the morehomogeneous sample.

There are limitations in this study. The applicationarm of the LIDO used for shoulder flexion wascustomized and the devices, while calibrated, wereonly single representatives of their class of instru-ments. The fact that the moment arm was not calcu-lated for the JTech could be seen as a limitation.Torque could have been calculated for the JTech bymultiplying by the moment arm, allowing for directcomparison of the absolute amount of torquemeasured by both tools. We elected not to take thisapproach because, in our experience, moment armsare rarely measured by clinicians who are moreinterested in efficient measurement and assessmentof change over time. Our question was correlative: dothese measures give similar rankings. It does notsuggest that scores from one instrument could besubstituted for the other. The comparison of strengthin kilogram and Newton meter brings additionalsources of error because subjects with longer armsshould obtain a lower strength in kilogram, when thesame torque production is being produced by themuscles. This would tend to lower our observedcorrelations. We chose to standardize test positions asthis reduced positional measurement error andremains a good practice when comparing measuresover time. Mostly young and asymptomatic subjectswere recruited for the present study; therefore, thiscould reduce the generalizability of the results toolder or more symptomatic populations. Finally, thisstudy did not compare the reliability of the devicesand so their relative precision is unknown.

The results of this study suggest that HHD canprovide isometric strength scores that are qualita-tively similar to strength estimates obtained with aSD. Absolute quantitative difference are inherent inthe equipment and test methods. Future studies

should focus on enhancing our understanding ofbest practices when using HHD by evaluating thereliability and responsiveness of shoulder strengthmeasures in specific clinical populations, providingappropriate comparative data (normative, differentclinical subgroups, recovery milestones and endpoints), and describing the relationship betweenstrength and function.

CONCLUSION

The JTech computerized hand-held dynamometerprovided isometric shoulder strength scores that areclosely related to those obtained on the LIDO whentesting shoulder abductors, flexors, and externalrotators in the scapular plane. Both instruments canprovide valid measures of isometric shoulderstrength. Although scores between the instrumentscannot be interchanged because of differences inunits of measurements, determination of impairmentlevels should be related regardless of which instru-ment is selected.

REFERENCES

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5. Bohannon RW. Reference values for extremity muscle strengthobtained by hand-held dynamometry from adults aged 20 to79 years. Arch Phys Med Rehabil. 1997;78:26–32.

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18. Kuhlman JR, Iannotti JP, Kelly MJ, Riegler FX, Gevaert ML, ErginTM. Isokinetic and isometric measurement of strength of external

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JHT Read for CreditQuiz: Article # 138

Record your answers on the Return Answer Formfound on the tear-out coupon at the back of this is-sue. There is only one best answer for eachquestion.

#1. The purpose of the study was to test the

a. reliability of the JTech Power Track II in mea-

suring shoulder strengthb. validity of the JTech Power Track II in measur-

ing isotonic shoulder strengthc. validity of a hand held dynamometer in mea-

suring isometric shoulder strengthd. specificity of the JTech Power Track II in mea-

suring shoulder external rotation

#2. The subject population consisted of

a. entirely studentsb. half students and half active shoulder patientsc. entirely active shoulder patientsd. a sample of convenience recruited in a busy mall

#3. When comparing the JTech Power Track II to theLIDO WorkSET the study showed

a. no correlation for shoulder strength

b. a weak correlation for shoulder strengthc. virtually identical scores for shoulder strengthd. comparable scores for shoulder strength

#4. The procedure for using the JTech Power Track II

a. was fraught with technical complications, but

they were overcome by the experienced testingstaff

b. was mastered by the subjects’ reading of theinstructional manual

c. was sufficiently straight forward to allow allsubjects to successfully complete the batteryof tests

d. made available in three languages besidesEnglish

#5. The JTech Power Track II is currently only avail-able to academic settings in Canada

a. trueb. false

When submitting to the HTCC for re-certification,please batch your JHT RFC certificates in groups of 3or more to get full credit.

OctobereDecember 2009 327