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Enhancing the Examiner's Resisting Force Improves the Validity of Manual Muscle Strength Measurements: Application to Knee Extensors and Flexors

Lu, Tung-Wu1; Chien, Hui-Lien1; Chang, Ling-Ying2; Hsu, Horng-Chaung3

Journal of Strength and Conditioning Research: September 2012 - Volume 26 - Issue 9 - p 2364–2371
doi: 10.1519/JSC.0b013e31823db080
Original Research
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Lu, TW, Chien, HL, Chang, LY, and Hsu, HC. Enhancing the examiner's resisting force improves the validity of manual muscle strength measurements: Application to knee extensors and flexors. J Strength Cond Res 26(9): 2364–2371, 2012—The purposes of this study were to test whether an examiner's strength may affect the validity of the knee muscle strength measurements using a hand-held dynamometer (HHD) and whether enhancing the forces applied by an examiner using a resistance-enhanced dynamometer (RED) would improve measurement validity. Twenty-five young male volunteers (mean [±SD] age: 22.5 ± 1.7 years) without a history of injury to the test limb and 6 male and 6 female experienced examiners participated in this study. Maximum resisting forces of the knee flexors and extensors were measured using RED, HHD, and a dynamometer (Kin-Com). For all testing conditions, poor to moderate associations were found between the HHD and Kin-Com, whereas there was a good to excellent relationship between RED and Kin-Com. The systematic variations between RED and Kin-Com were also smaller than those between HHD and Kin-Com. The force values measured by RED were very close to those measured by Kin-Com. An examiner's strength affects the validity of the measurements using HHD. Enhancing the forces applied by the examiner to the tested segment using RED appeared to improve the validity of muscle strength measurements.

1Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan

2Department of Physical Therapy, China Medical University, Taichung, Taiwan

3Department of Orthopaedic Surgery, China Medical University Hospital, Taichung, Taiwan

Address correspondence to Dr. Tung-Wu Lu, twlu@ntu.edu.tw.

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Introduction

Muscle strength is an important factor in the normal function of joints and in the efficient motor performance during activities of daily living and strenuous athletic pursuits (3,22,31,40). Measurement of muscle strength is thus an integral part of athletic training and the rehabilitation process, playing a critical role in the evaluation and prediction of functional capacity (4,6,18,30,35). Objective strength measurements provide data for quantifying the rate of change and efficacy of an intervention or training program (13,18,26,39). Several muscle strength measurement methods are available, ranging from simple manual muscle testing (MMT) to the use of more complicated isokinetic dynamometry (8,9,14,15,19,29,30,32,37). Manual muscle testing is easy to apply and has been the most widely used clinical method for categorizing muscle strength using an ordinal scale. However, MMT is less responsive in monitoring strength changes over time and less reliable than a hand-held dynamometer (HHD) (12,21,38). Although the HHD provides objective, quantitative, and more accurate values of the maximum muscle strength, its measurements can be affected by factors such as the sensitivity and construction of the dynamometer, the subject's test position and stabilization, and the examiner's strength (10,25). Poor validity and reliability of the HHD were also found when measuring strong muscle groups, such as the knee extensors and flexors. Much research effort has been directed at improving the reliability of the HHD (1,11), suggesting that an examiner's muscle strength is a determinant of the interexaminer reliability of the HHD (11,28,39). However, no study has addressed the problem of validity for HHD.

Validity assesses how well an instrument measures what it is intended to measure (accuracy) and determines whether it is applicable in accurate decision making. High reliability but low validity reveals that the indicators measure something consistently but not the intended concept. Validity and reliability are of equal importance because if an instrument does not accurately measure what it is supposed to, there is no reason to use it even if it measures consistently (reliably). According to previous studies, it appears that eliminating the effects of the examiner's muscle strength is essential for achieving reliable and valid muscle strength measurements. Using a newly developed resistance-enhanced dynamometer (RED), Lu et al. (25) showed that enhancing the examiner's resisting force significantly improves the intraexaminer, interexaminer, intrasession, and intersession reliabilities when measuring the strength of the knee extensors and flexors. The RED uses a leverage mechanism for assisting an examiner when applying resisting forces—which can be many times greater than the examiner's strength—to the tested limbs (25). Because the knee flexors and extensors are among the strongest muscles of the human body and are critical to the normal functioning of the lower limbs during daily and athletic activities, reliable and valid strength evaluation of these muscles is essential. However, the validity of the RED in measuring these muscles is yet to be established against a gold standard.

Fixed isokinetic dynamometry is considered to be better than HHD because the tester's strength is not an issue and the patient is stabilized during tests. Measurements from a variety of isokinetic dynamometers have been found to be both mechanically reliable (20,36) and valid (5,20,24). Isokinetic dynamometers, such as Kin-Com, are thus considered the gold standard in muscle strength measurements (27,32,33). However, despite its high reliability and validity, fixed isokinetic dynamometry has not been widely used routinely in clinical practice mainly because the apparatus is relatively large, immobile, expensive, and complicated to use (24). Therefore, a manual measurement method that eliminates the effects of the examiner's muscle strength may be helpful for achieving reliable and valid muscle strength measurements.

The purposes of this study were to use an isokinetic dynamometer as the gold standard to test the hypotheses that (a) an examiner's muscle strength would affect the validity of the measurements of the knee extensors and flexors using an HHD and (b) enhancing the forces applied by the examiner to the tested segment using RED would improve measurement validity.

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Methods

Experimental Approach to the Problem

An HHD that did not enhance the forces applied by the examiner to the tested segment and a RED that did enhance the forces applied by the examiner to the tested segment were used in this study to test the hypotheses. The instrument validity of HHD and RED on the measurement of knee muscle strength was assessed on a sample of 25 male subjects by 2 typical female and male examiners with very different strengths. The test was performed by comparing each selected measure (RED and HHD) with a criterion (i.e., highly reliable Kin-Com dynamometer), that is, HHD vs. Kin-Com and RED vs. Kin-Com, using Pearson correlation analysis and the Bland-Altman approach. The strengths of the relationship between the instruments for these comparisons were also indicated by the corresponding effect sizes. Because the male and female examiners were very different in their strengths, differences in the validity of HHD (i.e., correlation coefficient and effect size) between the 2 examiners indicated the effects of an examiner's strength on the validity of the knee muscle strength measurements using HHD. Differences between the validity of HHD and RED for both examiners would indicate the effect of enhancing the forces applied by an examiner on measurement validity.

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Subjects

Twenty-five male volunteers (age: 22.5 ± 1.7 years; mass: 69.7 ± 10.5 kg; height: 172 ± 6.8 cm) without a history of injury to the lower limbs participated in this study. Six male and 6 female healthy examiners with extensive clinical experience in MMT were also recruited. The maximal push and pull capabilities of the upper extremities of the examiners were measured as the maximum forces applied by the examiner to the testing pad of a Kin-Com dynamometer, which was configured to simulate a knee extensor-flexor strength test of a subject sitting with the knee and hip flexed to 90°. Male examiners' push and pull strength were 416.7 ± 47.8 and 552.7 ± 40.8 N, respectively, whereas the corresponding values for the female examiners were 252.3 ± 16.5 and 207.3 ± 23.7 N. One male and 1 female average-strength examiner were then chosen for the subsequent validity studies. Institutional Review Board approval for this study and written informed consent from all the subjects were obtained.

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Procedures

Instruments

The design and construction of the resistance-enhanced dynamometer (RED) used in this study has been described by Lu et al. (25). Basically, the RED system consists of a leverage mechanism, a fixation apparatus, a load cell (capacity 2,000 N; precision 0.0045 N; Sensotec Inc., Columbus, OH, USA) and a data acquisition system (Figure 1). The leverage mechanism amplifies the examiner's applied force. The amount of force amplification is controlled by adjusting the length of the handle (input lever arm length), the magnification factor being the ratio of the input lever arm and the fixed output lever arm length. The output force measured by the load cell then quantifies the subject's muscle strength (Figure 1). The RED has been shown to have good intraexaminer reliability (intraclass correlation coefficient [ICC] = 0.91–0.94), interexaminer reliability (ICC = 0.98; SEM = 24.7 N), intrasession reliability (ICC = 0.93–0.99), and intersession reliability (ICC = 0.91–0.92; SEM = 16.7–24.7 N) (25). The output force is amplified and A/D converted (DAQ card-516, National Instrument Co., Austin, TX, USA) before being registered by a software program developed in LabVIEW 5.0.1 (National Instrument Co.). The software is a stand-alone program with a user-friendly graphics user interface, so further programming or installation of LabVIEW are not needed to use the RED. The fixation apparatus allows the RED to be fixed to the ground or to a fixed object such as a chair or a bed. The RED is portable and easy to apply in clinics because it has a small mass of approximately 4.5 kg with an assembled volume of approximately 46.6 × 30 × 50 cm3 and can be assembled in <5 minutes. It costs <1% of a Kin-Com dynamometer. A HHD (GT-10, OG Giken, Inc., Okayama, Japan) with a capacity of 1,000 N and an accuracy of 0.98 N was also used.

Figure 1

Figure 1

The Kin-Com 500H (capacity 2,000 N; precision 1 N; Chattecx Corporation, Chattanooga, TN, USA) was taken as a valid tool to assess the strength of the knee flexors and extensors. Before each test session, the Kin-Com dynamometer was self-calibrated, and the calculation of the moment of force was verified with a known weight according to the manufacturer's instruction. The axis of the dynamometer was aligned with that of the knee, and the eccentric mode with an angular speed set at 2° s−1 was chosen to simulate a break test.

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Measurement Procedure

All the subjects underwent evaluation of knee extensor and flexor strength by the 2 examiners with the RED, HHD, and Kin-Com dynamometer. Each subject was tested 3 times by each examiner, for each muscle group using each device. A random testing order of direction (knee flexion and extension) and evaluators (HHD, RED, and Kin-Com) was established before the experiment. Before the tests, each subject performed mild to moderately intense knee stretches for 10 minutes as a warm-up. All the tests were carried out with the subject sitting with their trunk and thighs stabilized using straps, the knee and hip flexed to 90° and the leg hanging free, while holding the sides of the seat with their hands. The dynamometer pad was placed on the anterior surface of the lower leg, approximately 1 cm proximal to the ankle joint. The examiner sat in front of the subject and his or her hand held the handle to provide a resistance force. For knee extensor tests, the examiner applied force to the lower leg by pushing the handle of the RED. For knee flexor tests, the examiner pulled the handle of the RED to apply force to the posterior surface of the lower leg. All the measurements were performed according to the break method, in which the examiner gradually increased the force, trying to overcome the maximal isometric resistance produced by the subject until the subject gave way (a “break”). The maximal static contraction was maintained for a minimum of 5 seconds, with a 2-minute rest period between contractions and 5-minute rest periods between test conditions. Standardized verbal encouragement was provided during all testing.

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Statistical Analyses

Instrument validity was assessed by comparing each selected measure (RED and HHD) to a criterion (Kin-Com dynamometer) using Pearson correlation analysis. For each examiner, Pearson's correlation coefficients and the coefficient of determination (R squared) for each selected measure were calculated using the averaged force values measured over the 3 trials, for both knee flexors and extensors. T-tests for correlated samples were also calculated to ensure that correlations did not provide a false indication. Because the male and female examiners were very different in their strengths, differences in the validity of HHD between the 2 examiners indicated the effects of an examiner's strength on the validity of the knee muscle strength measurements using the HHD. Differences between the validity of HHD and RED for both examiners would indicate the effect of enhancing the forces applied by an examiner on measurement validity. In comparing the validity levels between devices and examiners, a correlation coefficient of ≥0. 75 was defined as high to excellent correlation; 0.50–0.75 as moderate correlation, 0.25–0.5 as fair correlation, and 0.00–0.25 as poor or no correlation (34). All significance levels were set at α = 0.05. Effect sizes (Cohen's d) between the HHD and RED and the Kin-Com dynamometer were also calculated to indicate the strength of the relationship between the instruments. Generally, for Cohen's d, an effect size of ≥0.8 was defined as large, 0.5 as medium, and 0.2–0.3 as small (17). The smaller the effect size, the stronger the relationship between the instruments. All statistical analyses were performed using a statistical software package (SPSS v. 13; SPSS Inc., Chicago, IL, USA).

Measurement agreement between each selected measure (RED and HHD) and Kin-Com was also assessed using the Bland-Altman 95% limit of agreement method. The means and SDs of the differences in measurements for HHD vs. Kin-Com and RED vs. Kin-Com were calculated. The bias (mean difference), the 95% confidence intervals of the bias, and the 95% limits of agreement (bias ± 1.96SD of the differences) were also calculated (7).

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Results

Poor to moderate associations were found between the HHD and Kin-Com in measuring knee extensors and flexors for both the female and male examiners (knee extensors: r = 0.405 for the male examiner and r = −0.086 for the female examiner; knee flexors: r = 0.664 for the male examiner and r = 0.214 for the female examiner) (Table 1 and Figure 2). Medium to large effect sizes were found between the HHD and Kin-Com (Table 1). Bland-Altman analysis showed that the HHD tended to underestimate the knee extensor and flexor strength when compared with the Kin-Com dynamometer (Figure 3). The mean differences ± SD for the knee extensors (male = −404.78 ± 139.16 N; female = −546.04 ± 156.27 N) were much greater than those for the knee flexors (male = −8.78 ± 46.53 N, female = −78.30 ± 64.73 N) (Figure 3).

Table 1

Table 1

Figure 2

Figure 2

Figure 3

Figure 3

In contrast, good to excellent associations were revealed between the RED and Kin-Com dynamometer for both the female and the male examiner (knee extensors: r = 0.899 for the male examiner and r = 0.886 for the female examiner; knee flexors: r = 0.937 for the male examiner and r = 0.948 for the female examiner) (Table 1 and Figure 4). Small to medium effect sizes were found between the RED and Kin-Com (Table 1). As shown by the Bland-Altman analysis, the systematic variations between the RED and the Kin-Com dynamometer were smaller than those between the HHD and the Kin-Com dynamometer. The force values measured by RED were very close to those measured by the Kin-Com dynamometer (Figure 5). The mean differences ± SD for the knee extensors measured by the male and the female examiner were −39.92 ± 66.40 and −53.23 ± 70.10 N, respectively, whereas those for knee flexors were –3.77 ± 22.02 and −13.24 ± 19.93 N, respectively.

Figure 4

Figure 4

Figure 5

Figure 5

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Discussion

This study aimed to test the hypotheses that (a) an examiner's muscle strength would affect the validity of the measurements of the knee extensors and flexors using HHD and (b) enhancing the forces applied by the examiner to the tested segment using RED would improve measurement validity. Through comparisons with the measurements of the Kin-Com dynamometer, in contrast to the HHD, the RED system was found to have excellent validity in measuring knee extensor and flexor strength for both the male and the female examiner, supporting the hypotheses. The design of the RED provides an example of a portable, easy-to-use, and inexpensive device offering high validity and reliability for muscle strength measurements, especially of strong muscles, for physical therapists and strength and conditioning specialists, regardless of their own strength.

Poor to moderate validity of HHD appeared to be a result of the limited upper limb strengths of the examiners during measurements. The female examiner generated more or less the same force outputs for both the knee flexors and extensors, which were close to her upper limb strength, whereas the true values as measured by the Kin-Com dynamometer increased beyond her strength (Figures 2C and D). Similar results were also found for the male examiner when measuring knee extensors. Moderate validity was achieved for the male examiner when measuring the knee flexors because his strength was greater than the strength of the knee flexors of most subjects (Figure 2B). These findings were in agreement with the findings of previous studies (1,23,28,30,39). Hyde and Goddard (23) indicated that regardless of their experience, most physical therapists are able to offer resistance only up to a maximum force of 30 kg, whereas Mulroy et al. (28) reported that the utility of the HHD is limited by the strength of the examiner. The maximum vertical push force of the female examiner (235 ± 54.3 N) was only 60% of the normal knee extension value for women and 40% for men, whereas the male tester's vertical push force (357.0 ± 93.4N) was 96% for women and 61% for men. Agre et al. (1) demonstrated that although the dynamometer is reliable for testing upper extremity muscle groups, it is unreliable for testing lower-extremity muscle groups. This may be because some of the muscle groups tested were so strong that the examiner had great difficulty holding the specific position for isometric testing. Wikholm et al. (39) also showed that there was a significant difference in the magnitude of force measured by 3 examiners and that the strongest tester recorded higher maximal force readings than did the other examiners. Above 120 N, tester strength appeared to be a major determinant of the magnitude and reliability of the forces measured with an HHD. From the results of the current and previous studies, it appears that owing to the limited strength of the examiners, HHD may not be able to obtain valid measurements for strong muscles, which limits this device's ability in detecting differences in these muscles.

Enhancing the forces applied by the examiners to the tested limb using RED was found to improve the validity of the measured strength of the knee flexors and extensors. As shown in Figure 4, although the examiner's strength was in general smaller than that of the knee muscles, through the leverage design of the RED both examiners were able to apply resisting forces greater than their own ability and thus obtain accurate measurements. As indicated by the results of the Bland and Altman analysis, this advantage of RED ensured that no systematic bias existed between RED and Kin-Com measurements, as opposed to HHD vs. Kin-Com measurements (Figure 5). The mean measurement bias of HHD was approximately 10 times larger than that of RED for knee extensor strength and 5 times larger for knee flexor strength. The variation of the differences between HHD and Kin-Com measurements was approximately 2 times greater than that between RED and Kin-Com, for both knee flexors and extensors. Although significant correlations existed between HHD and Kin-Com measurements, the large amount of variance between these 2 methods should not be overlooked. In an individual subject, this variation of differences suggests a relatively big threshold for strength evaluation using an HHD in clinical practice and research.

Another advantage of the RED over the HHD is that the measurement validity of RED was not affected by the strength of the tested muscles, as indicated by the small R2-value of the linear regression line in the Bland and Altman plots (Figure 5). Compared with the HHD, the force-enhancing mechanism of RED enabled the examiners to measure the muscle strength that it is supposed to measure, independent of the examiners' own muscle strengths.

The measurements of the HHD tests were shown to be affected by the physical strength of the examiners in this study. This is important because most female physical therapists have less physical strength than male therapists, and about 74.6% of active registered American Physical Therapy Association physical therapists are women (2). Other factors, such as the skill of the examiner in holding the HHD and the stabilization of the segments proximal to the tested one, may also affect the results (10,16). These factors should be weighed carefully when considering its clinical applications, even though HHD offers advantages related to cost, portability, and simplicity of use. As patients or athletes improve in strength, they may reach the point of becoming untestable using an HHD. The force-enhanced RED offers a more advantageous alternative to HHD, especially with better reliability and validity. Nonetheless, like the HHD, the RED is also limited to isometric tests and cannot measure dynamic muscle strength. Although the RED theoretically should offer the same advantage in measuring other strong muscles, such as the hip muscles, further study is needed to establish the relevant reliability and validity for practical purposes.

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Practical Applications

The strength of muscles is an important factor in the normal functioning of a joint and in the efficient motor performance during activities of daily living and strenuous athletic pursuits. Appropriate muscle strength is also crucial for preventing work- and sport-related injuries. An examiner's muscle strength affects the validity of the strength measurements of strong muscles, such as the knee extensors and flexors. Incorrect muscle strength data as a result of insufficient examiner strength will lead to inaccuracies in the evaluation and prediction of functional capacity after rehabilitative or athletic training. Although the HHD provides objective and quantitative values of the maximum muscle strength, the validity of its measurements can be affected by the examiner's strength. This shortcoming has not been addressed in the literature. This study showed that this limitation can be improved by enhancing the forces applied by the examiner to the tested segment using a device such as the RED. The design of the RED offers an example of a portable, easy-to-use, and inexpensive device of high validity and reliability for muscle strength measurements, especially of strong muscles, for physical therapists and strength and conditioning specialists, regardless of their own strength.

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Keywords:

knee muscles; manual muscle test; examiner's strength strength; hand-held dynamometer; resistance-enhanced dynamometer

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