A standard method of shoulder strength measurement for the Constant score with a spring balance

A standard method of shoulder strength measurement for the Constant score with a spring balance Marcus

J. K. Bankes,

FRCS, John E. Crossman,

FRCS

The strength component of the Consfant score has been criticized for its lack of a precisely defined measurement method. A series of experiments was performed to compare three different methods in normal and pathologic shoulders with the use of a standard test position. These were /II the lsobex isometric dynamometer, (2) Constant’s unsecured spring balance, and (3) a new modification in which the spring balance is fixed at one end and the reading is taken after 5 seconds of maximum effort. The results suggest that this simple modification with a lowcost spring balance can give similar values to those from the Isobex. The need for precision of terms and a definition of the method is discussed, and recommendations for the standardization of the many variables in making this measurement are made. [J Shoulder

Elbow

Surg

1998;7:

J 16-2

I .I

A method

widely used in functional assessment of the shoulder is that devised by Constant and Murley.* In this score 35 points are allocated for subjective assessments of pain and activities of daily living, and 6.5 points are available for obiective measures of shoulder strength and range of movement. Although it has gained wide accep tance for initial assessment and follow-up of all types of shoulder pathologic conditions, this numeric evaluation has been criticized for its failure to standardize the method used to measure the strength of lateral elevation, for which 25 points out of a total of 100 are allocated.3 The method used for strength evaluation within the Constant score was originally devised by Moseley.9 He described use of either an unsecured cable tensiometer or a spring balance held at arm’s length in both 90” forward flexion and 90” abduction in the coronal plane. The maximum force that the patient can resist as the examiner From the Department of Orthopaedics, Reprint re uests: RogerJ. H. Emery, Hospita 9 , Prezd St., London W2 CopyrIght Board

0 1998 of Trustees.

1058-2746/98/$5.00+0

116

by Journal

St. Mary’s

Hospital.

FRCS (Ed), 1 NY, Great

St. Mary’s Britain

of Shoulder 32/l/81318

and

Elbow

Surgery

and RogerJ.

H. Emery, MS,

FRCS(Ed),

London,

U.K.

pulls down on the device is then measured. Constant’s only modification was to incorporate a cuff attachment for the arm or wrist in those with poor grip strength from other disease. ’ Constant fails to specify the plane of elevation, the duration of each measurement, or indeed the number of repetitions required, and the lack of fixation at both ends of the spring balance makes accurate measurement difficult. Gerber3 has drawn attention to these deficiencies and has questioned the validity of a spring balance as a measuring device. In an attempt to resolve these difficulties the lsobex isometric dynamometer (Cursor A.G., Berne, Switzerland) was developed, and a range of normal values for the strength of lateral elevation in the scapular plane measured at the wrist has been defined.4 No comparable data exist for the spring balance under similar carefully defined conditions. Furthermore, although the discrepancy in measurements between the two devices has been recognized,4 they both have yet to be tested on the same subject sample. This study aims to overcome some of these difficulties by defining a simple standardized measurement method for use with a spring balance that has been fixed at one end and goes on to perform a within-subiects comparison between this method, the lsobex and Moseley’s unsecured spring balance in both normal and pathologic shoulders. MATERIAL

AND

METHODS

Comparison of the fixed spring balance and lsobex in normal shoulders. The subjects included 25 male (age range 2 1 to 34 years, mean 27.2 years) and 25 female volunteers (age range 20 to 34 years, mean 26.6 years), none of whom had previous or current upper limb pathologic conditions. The test position used was the subject standing with the arm in 90” of elevation in the scapular plane, the elbow extended, and the forearm pronated. An adjustable strap was placed around the forearm just proximal to the radiocarpal joint and attached to either a 30-pound spring balance or the Isobex. Both devices were firmly attached to a solid surface at waist height. Subjects were instructed to pull

Bankes, Crossman,

J Shoulder Elbow Surg Volume 7, Number 2

Table I Mean values of the largest measurement obtained each measurement method in normal shoulders Mean

Men isobex Isobex Spring Women lsobex lsobex Spring

+ SD/kg

and Emery

from the three maximum voluntary efforts and the range for Points

Range/kg

Points

3 set 5 set balance

10.52 10.18 11.45

+ 2.05 ? 1.83 ? 1.8

23 22 25

6.6-l 4.3 6.8-l 3.5 8.2-l 4.5

14-31 15-30 18-32

3 set 5 set balance

6.20 6.05 6.60

? 1.29 5 1.11 k 1.13

14 13 15

4.5-9.2 4.3-9.1 4.5-9.5

1 O-20 9-20 1 o-2 1

upwards with maximum effort until requested to stop. No constraining devices were used to prevent elbow flexion, although attention was drawn to subjects with this tendency so that they could make the necessary adjustments to keep their elbows straight. Three series of measure ments were obtained with the dominant arm, each series separated by a l-minute rest period. A single series comprised three measurements, each separated also by 1-minute rest periods with the Isobex in 3-second mode, then in 5-second mode, followed by the measurement from the spring balance after 5 seconds. Nine measure ments were therefore obtained per subiect. The Isobex is a microprocessor controlled device whose measurement cycle is triggered by a minimum force of 1 kg. It disregards the first second to exclude the initial period when there is a rapid increase in strength. It then measures the relatively constant upward force that follows 10 times per second for either 3 or 5 seconds depending on sampling mode and calculates a mean value. For each of the three measurement modalities the highest of the three values recorded per subject was used for making the direct comparison. Mean values from each measurement method were compared with the use of paired Student’s t tests, because the data closely conformed to a normal distribution. Comparison of the fixed unsecured spring balance

spring balance, Isobex, in pathologic shoulders.

and

Sub jects included 16 male and 10 female (age range 20 to 70 years) patients attending a dedicated shoulder clinic. For the unsecured spring balance the same standard test position was used with the device attached to the wrist. The subject was instructed to resist the downward pull on the other end of the spring balance produced by the single examiner, who simultaneously recorded the maximum value obtained. Three series of measurements were obtained with the affected arm, each series separated by a l-minute rest period. A single series comprised five measurements, each separated by rest periods of at least 1 minute with first the Isobex in 5-second mode, followed by the secured spring balance at 5 seconds, and finally the maximum value from the unsecured spring balance. Fifteen measurements were there-

117

fore obtained per subiect. A mispull was automatically awarded zero. For each of the three measurement modalities the highest of the five values recorded per subject was used for the comparison. Mean values from each measurement method were compared with the use of paired Student’s t tests, with findings checked with analysis of variance (ANOVA). Repeated measures ANOVA was performed to investigate the relationship among the five successive values obtained with each measurement method. Simple regression analyses were aerformed for each measurement method to examine the relationship between the mean and range of the five values obtained per individual. RESULTS Comparison lsobex

in

of normal

the

fixed

shoulders

spring

balance

(Table

I).

For

and male

was 10.52 + 2.05 kg with the 3-second lsobex mode, 10.18 + 1.83 kg with the 5second lsobex mode, and 11.45 + patients

1.80

the mean

kg with

value

the spring

balance.

The difference

between the 3-second and 5-second lsobex values was not significant (p > O.l), whereas the difference between the spring balance and the lsobex at either sampling mode was highly significant (p < 0.001). For female patients the values were 6.20 + 1.29 kg, 6.05 c 1 .l 1 kg, and 6.60 f 1.13

kg, respectively.

Although

the difference

be-

tween the 3-second and 5-second lsobex values is not significant (p > O.l), the difference between the spring balance and the lsobex at either sampling mode is again highly significant (p < 0.001). Effect of fatigue in normal shoulders. As expected, with each successive effort a progressive fall of values occurred in both men and women for each measurement method. For men the mean values (LSD) for the 3-second lsobex mode ob tained for the first, second, and third efforts were

118

Bankes, Crossman,

Table

II Mean efforts for three shoulders

First Second Third Fourth Fifth F-Value p-value

and Emery

v&es of successive measurement methods

maximum voluntary in pathologic

lsobex mean f SD/kg

Fixed spring balance mean f SD/kg

Unsecured spring balance mean f SD/kg

3.71 + 2.54 3.51 -C 2.40 3.60 k 2.68 3.62 ? 2.71 3.63 ? 2.85 0.227 0.92

4.21 1- 2.62 3.73 ~fr2.49 3.85 2 2.63 3.48 2 2.19 3.53 2 2.20 6.587 <0.0001

4.52 + 2.77 4.81 k 3.03 4.57 2 2.88 4.54 ? 2.71 4.51 2 2.53 1.002 0.41

10.18 -+ 2.19, 9.64 + 2.06, and 9.07 + 1.65, respectively. The differences between the means from the first and third (p < 0.01) and the second and third efforts (p < 0.02) were highly significant, but the difference between the first and second was not (p < 0.1). The mean values for the 5-second lsobex mode obtained for the first, second, and third efforts were 9.71 + 2.09, 9.43 + 1.77, and 8.85 ? 1.88, respectively. A similar pattern emerges with the differences between the means from the first and third (p < 0.01) and the second and third efforts (p < 0.02) again being highly significant, but no significant difference between the first and second efforts (p < 0.4) was seen. The mean values for the spring balance obtained for the first, second, and third efforts were 1 1.26 + 2.01, 10.78 ? 1.6, and 10.16 ? 1.83, respectively. The differences between these values were highly significant, with p < 0.05 between the first and second and p < 0.001 for both between the first and third and between the second and third efforts. Similar trends were found in women, with mean values for the 3-second lsobex mode obtained for the first, second, and third efforts 5.77 t 1.54, 5.72 -C 1.39, and 5.56 t 0.93, respectively, although the difference between any of the pairs of results failed to reach significance (p > 0.1). The mean values for the 5-second lsobex mode obtained for the first, second, and third efforts were 5.64 2 1.42, 5.42 + 1 .Ol, and 5.324 + 1.1 1, respectively. Again, a nonsignificant downward trend (p > 0.1 between any pair) was seen. The mean values obtained from the spring balance for the first, second, and third efforts were 6.47 & 1.16, 6.14 + 1.19, and

J Shoulder Elbow Surg March/April I998

5.94 ? 1.13, respectively. Some of the differences between these values were significant, however, with p < 0.05 between the first and second and p < 0.001 for the first and third efforts. The difference between the second and third efforts approached significance with p < 0.1. It therefore appears that the effect of fatigue is more clearly demonstrated with the spring balance. Comparison of the maximum value obtained for the fixed spring balance, lsobex, and unsecured spring balance in pathologic shoulders. The mean

value was 4.45 + 2.7 kg with the fixed spring balance, 4.35 t 2.6 kg with the 5-second lsobex mode, and 5.18 + 3.0 kg with the unsecured spring balance. The difference between the fixed spring balance and 5-second lsobex values was not significant (p < 0.76), whereas a significant difference was seen between the unsecured spring balance and either the lsobex (p < 0.002) or fixed spring balance (p < 0.003) (F-value 8.752, p = 0.0006). The maximum value was obtained within the first three pulls in 24 (92%) subjects with the fixed spring balance, in 2 1 (81%) subjects with the Isobex, and in 16 (62%) subjects with the unsecured spring balance. When the maximum value was provided in the final two pulls, it was never equivalent to more than one point greater than the maximum provided from the first three pulls. Effect of fatigue in pathologic shoulders. In contrast to the normal shoulders, a progressive downward trend could be demonstrated only for the fixed spring balance (Table II). No statistically significant differences were found for the lsobex data, with only the difference between the first and second efforts for the unsecured spring balance reaching significance (p = 0.026). However, the first effort with the fixed spring balance was significantly higher than the following four (p < 0.03), and the third effort was significantly higher than the subsequent two (p < 0.05). All other comparisons failed to reach statistical significance. ter

Effect of measurement method on relative scatin pathologic shoulders. For any individual

relative scatter can be expressed as the ratio of the absolute scatter (i.e., the lowest value subtracted from the highest) to the mean value of the five efforts. The absolute scatter was correlated with the mean value of all 26 subjects for each measurement method, producing three simple regression analyses, two of which are shown (Figures 1 and 2). The gradient of the regression line indicates the

Bankes, Crossman,

I Shoulder Elbow Surg Volume 7, Number 2

4.5 4

1

and Emery

119

0

9” , b

2.5

1.5

.5

5

0

Meanlkg

Figure 1 Scatter diagram showing relatIonship range and mean strength values for 26 abnormal with Isobex (R = 0 137, gradient = 0.047)

4

8

12

10

MMIllkg

between shoulders

scatter. For both spring balance methods the absolute scatter correlated well with the mean values, with correlation coefficients of 0.693 and 0.537 and gradients of 0.28 and 0.21 for the fixed and unsecured methods, respectively. Therefore for these two methods the scatter of values obtained from an individual seems to increase linearly with their absolute strength. No such linear relationship was found for the Isobex, because absolute scatter and mean strength were poorly correlated (R = 0.137). relative

DISCUSSION

The Constant-Murley Score for functional assessment of the shoulder has been deliberately developed for use in a clinical setting.2 In accordance with this philosophy this study investigates the measurement of shoulder strength within the context of this score and makes no attempt to undertake time-consuming controls for many of the variables involved in shoulder strength measurement, most notably length of the lever arm and movements of the rest of the body and at the elbow.14 lsokinetic dynamometers and measurements of torque can therefore have no role to play in this context.‘O-l3 It is important to appreciate that the lsobex and spring balance measure strength or force. They do not measure work, which is the product of force and distance, or power, which is the rate of work.15 It is therefore apparent that any method suitable for use in the Constant score would amount to little more than pseudoscientific quantification of manual muscle testing that could never be truly objective. It is important to appreciate that each method

Figure 2 Scatter range and mean with fixed spring

diagram showing relationship strength values for 26 abnormal balance (R = 0 693; gradient

between shoulders = 0.281.

measures a different component of force generated by the shoulder, and this explains many of the differences in the pattern of results obtained by the three methods. For the groups with weaker shoulders such as the normal female and pathologic groups, it was difficult to demonstrate a clear fatigue effect with the Isobex, because the device tends to smooth out the fluctuations and decline in strength that occur during the course of a single pull by calculating the mean of 50 readings taken over the Second period that follows the excluded first second. Normal male shoulders seem to have greater consistency in maintaining the upward force for the whole test duration, although the magnitude of that force may fall with subsequent tests. The fixed spring balance demonstrated fatigue in all subject groups far more clearly because it measures the force at a single point in time, namely after 5 seconds of maximum effort. This makes it more sensitive to the progressively earlier decline in the force generated within a single pull with each successive effort despite the rest periods. The unsecured spring balance also measures force generated at a point in time, but because this is a maximum force, it tends to be measured earlier than the 5 seconds of the fixed method and so produces a higher value, although the longer learning curve of this method may have masked fatigue effects. This explanation is supported by our data and may explain the discrepancy between Constant and ourselves on the figures from normal female shoulders (Table Ill). Despite the fact that each method is measuring something slightly different, lsobex readings fortuitously coincide with those from an inexpensive fixed spring balance in

120

Bankes,

Crossman,

Table III Strength scores with a spring balance

he

and Emery

for normal

shoulders

measured

Male

(yd

J Shoulder Elbow March/April

Table IV Strength scores for normal shoulders measured with an lsobex (5second mode, mean of three efforts) and their equivalent points wlthln the Constant score

Female

Age (yr) Constant, 21-30 31-40 Bankes, Emery, 21-34 21-30

Surg 1998

Male

Female

7.9 k 1.8 (rl $9)

4.3 -c- 0.9 (n = 39) 9

9.2 5 1.9 (n ,,‘5)

5.5 ? 1.0 (n = 25) 12

1986 24 22 Crossman 1997

(n = 45) (n = 45)

23 20

(n = 45) (n = 45)

Gerber 20-29

& Arneberg,

1992

& 25 (n = 25) 25(n=20)

15 (n = 25) 15(n=21)

Points Bankes, Crossman Emery, 1997 21-34 Points

pathologic shoulders, with only very small differences encountered in normal shoulders (Table I). Measurements were taken with the humerus in the plane of the scapula to provide comparison with other studies using the Isobex4* ‘, * and because this position provides certain biomechanical advantages such as optimum length-tension relationships for the humeral abductors, a relaxed inferior capsule, and maximum conformity be tween glenoid and humerus.5, 6 We strongly believe that patients who are unable to elevate their shoulders to the test position have very little functional shoulder strength and therefore should be automatically designated a strength score of zero. The reading from the fixed spring balance was taken after 5 seconds, because this was the shortest test duration that could be reliably measured after the marked oscillations that occur at the beginning of a maximal effort have subsided. It is also of comparable length to the lsobex test cycles. Difficulties were encountered with the unsecured spring balance, because patients found resisting the downward pull of the examiner harder to accommodate than pulling up against a fixed device. It was also troublesome for the examiner to coordinate the downward pull with the patient while simultaneously attempting to record a maximum value from a markedly fluctuating spring balance dial. As a result of this factor, individual readings from the unsecured spring balance were often little more than conjecture. The highest value per measurement modality was used to exclude mispulls and reduce the likelihood of including a less than maximal effort.’ ’ It has been shown that data from true maximum voluntary efforts are extremely reproducible, whereas those from deliberate submaximum efforts are not.14 Furthermore a maximum value may be more likely to reflect best function in real terms

&

rather than a mean value that has been reduced by a wide range of submaximal values from mispulls, earlier fatigue, and pain. lntraobserver and interobserver reliability have been shown to be good for the lsobex in normal shoulders with the same test position but with the use of the mean of three values rather than the maximum.4, 7 Reanalysis of the 5-second lsobex data with the mean value as opposed to the maximum value that we favor showed our subjects to be slightly stronger than Gerber’s4 (Table IV). A formal assessment of the reliability of the fixed spring balance and lsobex with the maximum values, for example by repeating the experiments on a different day with another observer, was not performed, although the similarity of the results between methods is reassuring. However, demonstration of reliability on an individual basis, which is the important issue when following a patient’s progress, would be intrinsically difficult for any measurement method be cause shoulder disease may so fundamentally affect consistency of the patient’s own performance. This is shown by the large scatter of values produced by an individual, and, unlike the spring balance, this scatter seems independent of the absolute strength when tested with the lsobex (Figures 1 and 2). We believe that the measurement method for the strength component of the Constant score should be standardized. We propose a quick and simple method with a fixed spring balance and a standard test position that consists of the subject standing with the arm in 90” of lateral elevation in the scapular plane, the elbow extended, and the forearm pronated. Patients unable to reach the test position as a result of pain or deformity are given a value of zero. The highest value at 5 seconds ob

I Shoulder Elbow Volume 7, Number

Bankes,

Surg 2

tained from three successive maximum pulls provides the strength score. The Isobex in 5-second mode is an alternative measuring device but is more expensive and less easily available. Moseley’s method with an unsecured spring balance should be rejected be cause of its complexity and lack of definition. REFERENCES Constant CR Age related tlon after inlury (MCh) Ireland, 1986 Constant CR, Murley functional assessment Res 1987;2 14.160-4

recovery University

of shoulder College,

funcCork,

AHG. A clInIcal method of the shoulder Clan Orthop

Greenfield BH, Donatelll R, Wooden MJ, Wilkes J Isoklnetlc evaluation of shoulder rotatlonal strength between the plane of scapula and the frontal plane Am J Sports Med 1990; 18.124-8 Kuhlman JR, lannottl JP, Ke(l MJ, Rlegler FX, Gevaert measurement of Ml, Ergin TM Isokinetlc an J lsometrlc strength of external rotation and abductlon of the shoulder. J BoneJoint Surg Am 1992,74A 1320-33

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Machesney MR, Emey RJH The method for the oblective measurement of muscular strength and presentation of reference ranges for the Constant-Murley score J Shoulder Elbow Surg 1994,3 S26

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Moseley HF Sh ou Id er Iesions 3rd E & S LIvingstone, 1969 p 28-9

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Murray MP, Gore DR, Gardner GM, Mollinger LA. Shoulder motion and muscle strength of normal men and women in two age groups Clan Orthop Rel Res 1985; 192 268-73

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Otis JC, Godbold torque to lsometrlc 71.

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Otis JC, Warren RF, Backus Sl, Santner TJ, Mabrey JD. Torque productlon In the shoulder of the normal young adult male. The interaction of function, domInance, lolnt angle, and angular velocity. Am J Sports Med 1990,18 1 19-23.

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Rabin SI, Post M A comparative study of clinical muscle testing and Cybex evaluation after shoulder operations Clan Orthop Rel Res 1990;258,147-56.

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Gerber C Integrated scoring systems for the functlonal assessment of the shoulder In: Matsen FA, Fu FH, Hawklns RJ, editors The shoulder a balance of mob& and stablllty. Rosemont (It) American Academy o 7 Orthopaedlc Surgeons, 1993 p. 53 l-50 Gerber C, Arneberg 0 Measurement of abductor strength with an electronlcal device (Isobex) J Shoulder Elbow Surg 1992;2 S6

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