A Method for Checking Jamar Dynamometer Calibration

Elaine Ewing Fess, MS, OTR, FAOTA

Hand Research Zionsville, Indiana

H aving instruments that measure accurately and consistently is critical to establishing

excellence in therapeutic intervention and research endeavors. I

-3 Quantitative measurement permits as­

sessment of baseline pathology, prediction of reha­bilitative potential, planning and evaluation of treat­ment programs, and definition of final functional capacity. By means of research reporting, quantitative measurement also serves as a vehicle for professional communication. The importance of utilizing the best possible assessment instruments cannot be overstat­ed.

Developed by Bechtol in 19544 and subsequently recommended by three profes8i6nal societies,5-7 the Jamar dynamometer has become a staple for those involved in treating upper extremity dysfunction. Serving both clinicians and researchers, the Jamar is used not only to assess patient progress and the ef­ficacy of treatment, but also has contributed signifi­cantly to a better understanding of normal hand func­tion. It has been the foundation of studies correlating grip strength with height, weight, age, and hand dominance,8 size of object grasped,4,9 and. motiva­tion.I O,11 Unfortunately, because of inconsistency in handle position,t2 altered instruments,8 an4 small numbers of subjects in age subgroups,t3 normative values for the dynamometer are inconsistent. Both individuals14 and professional groups6,7 have taken important steps to standardize the use of the Jamar, and two recent studies1s

,16 have addressed its reli­ability as a testing instrument. The latest of these studiesl6 found that the Jamar dynamometer can be a highly reliable assessment instrument, with corre­lation coefficients of 16 of 20 new dynamometers

Reprint requests to Ms, Fess, 635 Eagle Creek Court, Zionsville, IN 46077.

28 JOURNAL OF HAND THERAPY

ABSTRACT: When calibrated correctly, the Jamar dynamometer is a highly reliable assessment instrument, Utilizing an adjustable­top workbench, positioning blocks, force collar, standardized test weights, and a careful testing procedure, the ability of a Jamar dynamometer to measure accurately and consistently may be eval­uated. Resulting data may be plotted on graph paper for a general estimate of measurement capacity, whereas calculation of the cor­relation coeffiCient, mean of the standard weight applied, and mean of the dynamometer readings provide a more exacting as­sessment of instrument capacity. Dynamometers with a correlation coefficient of 0.9994 or better and a difference between the means of 1,5 pounds (0.68 kg) or less do not need recalibration. If the correlation coefficient is acceptable but the difference between the means is more than 1,5 pounds, the dynamometer may be adjusted by the calibration screw on the face plate. When the correlation coefficient is less than 0.9994, the instrument must be returned to the manufacturer for recalibration.

ranging from 0.9999 to 0.9994 (l.0 equals perfect cor­relation). It was also discovered in the same study that of the 51 dynamometers evaluated (20 new, 31 used), 65% needed to be recalibrated.

While the theory of utilizing test instruments that measure consistently and accurately is uncon­testable, in practice clinicians and researchers alike have been faced with a difficult dilemma. An accurate method for checking the calibration of dynamome­ters without sending them back to the manufacturer has not been available. The purpose of this paper is to present a simple and reliable method of assessing the calibration of the Jamar dynamometer. To date this method has been used to successfully evaluate over 50 dynamometers.

EQUIPMENT A commercially available adjustable split-top

workbench with stress tolerance to 350 pounds (158.76 kg) serves as the main structure for the work station (Fig. 1).

Two blocks of wood position the dynamometer handle posts in an attitude perpendicular to the table surface (Fig. 2A). The vertical handle position is crit­ical to accurate dynamometer testing. If the handles are not vertical, a perpendicular application of force (weights) cannot be attained, causing measurements to be skewed. While the length and width of the pOSitioning blocks remain constant (7.5 cm x 15 em), the height of the blocks must be matched to the spe­cific design of the dynamometer handle. Older dy­namometers with offset handles require blocks with heights of 2 cm and 5.5 em, respectively. Two blocks, each 2 cm in height, are needed for the new £lat­handle design dynamometers. In order to stabilize the positioning blocks in the table-top vice, a 2

FIGURE 1. A split-top bench serves as the main structure for the work station.

cm x 2.5 cm x 7.5 cm piece of wood is bonded to the center bottom surface of each block (Fig. 2B). Par­alleling the shorter length, these pi~ces allow the two positioning blocks to be held securely when the hor­izontal surfaces of the workbench are brought to­gether. Final increments of height adjustment are made by drilling a shallow indentation, 3.25 cm in diameter, into the top middle portion of each block approximately 1.5 cm from the edge. The "heels" of the dynamometer handle posts fit into these inden­tations (Fig. 2C), ensuring consistent dynamometer placement on the blocks. The vertical orientation of the handle posts should be checked with a bubble level to verify that the depths of the indentations are correct. Note: the longitudinal axis of the dynflmom­eter must be parallel and center to the parallel grip­ping surfaces of the table.

Constructed of two layers of thermoplastic ma­terial with a central safety pin guide, a force collar (Fig. 3) is fitted to the adjustable handle. This rigid collar maintains the application of the weights in a constant'position and distributes forces evenly along the handle.

Standardized slotted interlocking metal test weights, class F tolerance, provide accurate and con­sistent forces against which dynamometer readings

I I ~

FIGURE 2. Top, Two blocks of wood position the dyna­mometer handle posts in an attitude perpendicular to the table surface; Middle, In order to stabilize the positioning blocks in the table-top vice, an additional wood piece is bonded to the center bottom surface of each block; Bottom, The "heels" of the dynamometer handle posts fit into shallow drilled in­dentations in the top middle portion of each block.

October-December 1987 29

flGUR.E 3. Fitted to the adjustable handle, a force collar standardizes the point of application of the weights and dis­tributes pressure.

may be compared. These weights, in either avoir­dupois pound or metric units, should provide a test­ing range from 10 to 120 lb in 10-lb increments, or 5 to 55 kg in 5-kg increments. For stability and ease of loading, a 52 cm hook extension is welded to one of the metal weights at the time of purchase. To com­pensate for the added weight of the hook, this welded unit should be drill-calibrated to 10 lb or 5 kg to match the other weights. Since these test weights are finely calibrated, they should be handled and stored with care, and they should not be used as therapeutic tools.

With the adjustable handle in position 5 and the force collar in place, a double loop of 170 lb test nylon cord is threaded through the force collar guide and tied so that the top of the hook, when suspended from the loops, falls 2 to 3 cm below the under surface of the workbench top. As the cord loops pass through the space between the horizo~tal platforms, they should not contact the adjacent edges of the plat­forms. Check to be certain that the weights are freely suspended when the adjustable handle is in position 1 (smallest handle span). If the weights touch the floor in this position, the double loop must be shortened.

TESTING PROCEDURE After making certain that the workbench is on a

level plane and the positioning blocks are tightly secured, place the dynamometer on the blocks and check the handle posts with a bubble level to ensure that they are perpendicular. With the force collar in place, lock the adjustable handle in position 5 and turn both the red and black dynamometer needles to "0." Place the weighted hook through the double loop and record the resultant needle reading on a calibration check sheet (Fig. 4). Add a weight and again record the resultant needle reading. Continue adding and recording until all weights are suspend­ed. Unload the hook, remove the hook from the dou­ble loop, move the adjustable handle down one notch to position 4, reset the needles, and begin the process of adding weights and recording again. This proce-

30 JOURNAL OF HAND THERAPY

SERIAL NUMB£R, ______ _ [JlHE' ______ _

CONDIT ION, US£O DA Tf OF PURCHASE , __ _

HANDlE POSITION

2 __ 3_ __5_

1o __ 1o __ 1o __ 1o __ 10 __

20 __ 2o __ 20 __ 20 __ 20 __

30 __ 30 __ 30 __ 30 __ 30 __

40 __ 40 __ 40 __ 40 __ 40 __

5o __ 5o __ 5o __ 5o __ 5o __

60 __ 60 __ 60 __ 60 __ 60 __

70 __ 7o __ 70 __ 70 __ 70 __

8o __ 80 __ 80 __ 80 __ 80 __

9o __ 9o __ 9o __ 9o __ 9o __

10o __ 10o __ 10o __ 100 __ 10o __

110 __ 110 __ 110 __ 110 __ 110 __

120 __ 120 __ 12o __ 120 __ 120 __

I£AN OF STANDARD WEIGHTS, ~

I£AN OF OYNAMIHTER VALUES,

CORRELATION COfFFICIENT,

graph at tached

FIGURE 4. Jaymar dynamometer calibration check sheet.

dure is carried out for all handle positions, working down from the largest handle span to the smallest. To maintain examiner objectivity, completed columns (handle positions) on the flow sheet should be cov­ered with a blank sheet of paper, and from reading to reading it is critical that both the examiner's po­sition and method be consistent.

ANALYSIS OF DATA To organize test data in an understandable man­

ner, three factors must be derived from the infor­mation on the flow sheet: (1) the correlation coeffi­cient, (2) the mean of the standard weights, and (3) the mean of the dynamometer readings.!7

It is important to define the correlation between the amount of weight applied and the measurements indicated by the dynamometer. This relationship may be computed mathematically as a correlation coeffi­cient. For the Jamar dynamometer it has been shown that 0.9994 is a minimum level of tolerance.!6 Dyna­mometers whose correlation coefficients are below this standard cannot be adjusted to an acceptable level and must be returned to the manufacturer for recal­ibration. Either one or both of two problems may cause the correlation coefficient to drop. The most common problem occurs when portions of a given test range (0-120 lb; 0-55 kg) vary markedly from the remaining range. Although sometimes encountered mid-range, those segments most frequently discrep­ant may be found at either the lower or higher end of the range, or both. Once the problem occurs, it is often mirrored throughout all five handle positions.

Unfortunately, adjustment by a constant number of units throughout the range cannot compensate for the defection from a linear progression. The second problem that results in a low correlation coefficient occurs when there is a marked discrepancy between handle readings. The dynamometer handle position most often found out of calibration is the first posi­tion. 16 Since all handle readings are affected equally, adjustment by a constant number cannot restore the problematic range to those of the other handles.

Both the standard mean (weights) for all trials and the dynamometer reading mean for all trials must also be calculated. The comparison of these two means allows the examiner to identify the average amount of discrepancy between the actual amount of weight applied and the amount recorded by the dynamom­eter.

Knowing the correlation coefficient and the dif­ference between the two means provides an accurate understanding of the calibration status of a given dynamometer. If the correlation coefficient is 0.9994 or better and the difference between the means is 1.5 pounds (0.68 kg) or less, the dynamometer is mea­suring at a very high level of accuracy and does not need to be recalibrated. If the correlation coefficient is good but the difference between the means is more than 1.5 lb, the dynamometer may be adjusted (by the amount of the difference between the means) us­ing the calibration screw on the face plate. Finally, when the correlation coefficient does not meet the minimum standard, the dynamometer is unreliable in its ability to measure. This problem cannot be ad­justed with the calibration screw on the face plate and the instrument must be returned to the manu­facturer for recalibration. Note: the criteria of 1.5 lb difference between the means is based on a trial to trial mean standard deviation for those dynamome­ters with correlation coefficients of 0.9994 or better, of 0.545 lb (±0.1758 lb) or 0.25 kg (±0.08 kg).

The relationship describedJty.a correlation coef­ficient may be plotted on graph. paper to provide a quick non-mathematical visual interpretation (Fig. 5). To fully understand the data obtained from a given dynamometer, the ranges of all five handle positions must be plotted on the graph. If all five lines follow a similar pattern, then it is apparent that there is little difference between the abilities of the different han­dles to measure forces; and if these lines duplicate the line of the standardized weights, the correlation coefficient is within acceptable limits and the dyna­mometer does not need recalibration. If, however, one or more lines diverge from the others, the cor­relation coefficient will be below acceptable levels. As noted above, when one or more handles measure differently than the others, the dynamometer should be returned to the manufacturer for recalibration. If the five lines remain together and parallel the course of the standard weights, the correlation coefficient is high but the values of the dynamometer readings are off by a consistent amount throughout the test range. As noted previously, when the readings are consis­tently off by a given amount, the dynamometer may be recalibrated by adjusting the calibration screw on

120 --- WEIGHT (pounds) APPLIED

110 ----- DYNAMOMETER 'A'

100 - • - • - DYNAMOMETER'S'

c 90 •••••••••• DYNAMOMETER 'C'

!!! ....I 80 0.. 0.. < UJ 70 () cr: 0 60 u. u. 0 50 (fj C 2 40 :::> 0 0..

30

20

10

o 10 20 30 40 50 60 70 80 90 100 110 120

MEAN OF 5 HANDLE POSITION READINGS

FIGURE 5. Comparison of mean values of three dynamom­eters indicates that instrument "A" may be recalibrated with the calibration screw on the face plate, whereas "8" and "c" must be returned to the manufacturer for recalibration. When analyzing a single dynamometer, mean values are not used. Real values are used to individually plot each of the five handle positions.

the face plate, and the instrument does not need to be returned to the manufacturer for recalibration.

Careful examination of the plotted values for the five handle positions is important. While lines that duplicate or parallel the track of the standardized weights are easily apparent, how much divergence is permissible for a line that gradually angles away but remains within a O. 9994 level of reliability? Although not infallible, in general if the mean value of the handles does not diverge more than 3 lb (1.8 kg) at any point from the standardized weight line, the cor­relation coefficient for the dynamometer is probably within acceptable limits. If, however, the mean value moves from the standard line by more than 3 lb, the reliability of the dynamometer is suspect and the cor­relation coefficient must be mathematically computed to determine the reliability of the instrument.

TIMING Depending upon use, dynamometers should be

checked a minimum of once a year. If they are used on a daily basis, calibration checks should be done at least every four to six months. Instruments used for research require even more careful monitoring. Al­though this may seem time-consuming at first, with two experienced examiners (reader /recorder; weight loader) the test process requires approximately 15 to 20 min, and calculating the data or plotting a graph takes another 20 to 30 min. To put the issue in per­spective, in a society oriented toward accountability, an investment of one hour or less several times a year is a small price to pay to ensure the accuracy of ther­apeutic decisions based on grip-strength assessment.

October-December 1987 31

SUMMARY When calibrated correctly the Jamar dynamom­

eter is a highly reliable assessment instrument. Uti­lizing an adjustable-top workbench, positioning blocks, force collar, and standardized test weights, the ability of a dynamometer to measure accurately and consistently may be evaluated. Resulting data may be plotted on graph paper for a general estimate of measurement capacity, and calculation of the cor­relation coefficient, mean of the standard weight ap­plied, and mean of the dynamometer readings pro­vide a more exacting assessment of instrument capacity.

REFERENCES

1. Fess EE: The need for reliability and validity in hand assess­ment instruments. J Hand Surg llA:621-623, 1986.

2. Payton OD: Research: The Validation of Clinical Practice. Philadelphia, FA Davis Co., 1984.

3. Fess EE: In Hunter J, Schneider L, Mackin J, Callahan A (eds): Rehabilitation of the Hand, 2nd ed. St. Louis, CV Mosby Co., 1984, pp 49-78.

4. Bechtol CD: Grip test: Use of a dynamometer with adjustable handle spacing. J Bone Joint Surg 36A:820, 1954.

5. Kirkpatrick JE: Evaluation of grip loss: Factor of permanent

disability in California. Summation and conclusions of Sub­committee for Study of Grasping Power of Committee on In­dustrial Health and Rehabilitation of California Medical As­sociation. Calif Med 85:314-320,1956.

6. American Society for Surgery of the Hand: The Hand: Exam­ination and Diagnosis, 2nd ed. New York, Churchill Living­stone, 1983.

7. American Society of Hand Therapists: Clinical Assessment Rec­ommendations. ASHT, Garner, North Carolina, 1981.

8. Schmidt R, Toews J: Grip strength as measured by the Jaymar dynamometer. Arch Phys Med Rehabil51:321-327, 1970.

9. Fess, EE: The effects of Jaymar handle position and test protocol on normal grip strength. J Hand Surg 7:308,1982.

10. Murray J: The patient with the injured hand. Presidential ad­dress, American Society for Surgery of the Hand. J Hand Surg

. 7:543,1982. 11. Hildreth, D: The Jaymar dynamometer. J Hand Surg llA:768,

1986. 12. Kellor M, Frost J, Silberg N, et al: Hand strength and dexterity.

Am J Occup Ther 25:77-83,1971. 13. Mathiowetz V, Kashman N, et al: Grip and pinch strength:

Normative data for adults. Arch Phys Med Rehabil 66:69-74, 1985.

14. Mathiowetz V, Rennells MS, Donahoe L: Effect of elbow po­sition on grip and key pinch strength. J Hand Surg 10A:694-696,1985.

15. Mathiowetz V, Weber K, et al: Reliability and validity of grip and pinch strength evaluations. J Hand Surg 9A:222-226, 1984.

16. Fess EE: Reliability of the Jaymar dynamometer. Submitted for publication.

17. Swinscow TDV: Statistics at Square One. London, British Med­ical Association, 1983.

Medical Problems of Performing Artists In tune with your

information needs . ..

Medical Problems of

Pertonnlng Artists

Don't miss a beat. Subscribe today!

32 JOURNAL OF HAND~THERAPY

Please enter my subscription for Medical Problems of Performing Artists (published quarterly): Mar., June, Sept., Dec. o 1986 (Volume 1) 0 1987 (Volume 2)

o Individual subscription: $39.00 per year, US; $49.00, elsewhere

o Library/Institution subscription: $49.00 per year, US; $59.00 per year, elsewhere

I enclose payment: o Check 0 Visa o Bill me 0 MasterCard

Credit Card # ____________ Exp. Date, ___ _ Signature,--_~ ________________ _

'Name _____________________ _ Title, ____________________ _

C~mpany/Hospita'-I _______________ _ Street Address, _________________ _

City/State/Zip.-----___________ _

FOREIGN POSTAGE: Add $20.00 for air mail.