Hand‐held Dynamometer Positioning Impacts Discomfort During Quadriceps Strength Testing: A Validity and Reliability Study

Abstract

Purpose/Background

A belt‐stabilized hand‐held dynamometer (HHD) offers the ability to quantify quadriceps muscle strength in a clinical environment, but a limitation is participant discomfort at the interface between the HHD and the tibia. The purpose of this study was to quantify the level of discomfort associated with a modified belt‐stabilized HHD configuration compared to a standard belt‐stabilized configuration and an isokinetic dynamometer. The secondary purpose of this study was to determine the validity and reliability of a modified configuration used to measure quadriceps strength compared to the “gold‐standard” isokinetic dynamometer.

Methods

Twenty healthy participants (5 males, 15 females; age=24.7±2.2 years, height=171.1±8.8 cm, mass=72.0±18.7 kg) performed maximal knee extension isometric contractions during each of three testing conditions: isokinetic dynamometer, standard configuration with HHD placement on the tibia, and an alternative configuration with the HHD interfaced with the leg of a table. Discomfort was quantified using a Visual Analog Scale (VAS). Differences in discomfort and torque (N•m) associated with the testing positions were determined using Friedman test or repeated measures analysis of variance. Validity was quantified using Pearson correlations and within‐session intrarater reliability was determined using an intraclass correlation coefficient (ICC2,1) and associated confidence intervals (95% CI).

Results

The isokinetic dynamometer configuration resulted in the least discomfort (p< .01) and the modified configuration was more comfortable than the standard configuration (p= .003). There was a significant correlation between measures from the isokinetic dynamometer and the standard configuration (r=.87) and modified configuration (r=.93). Within‐session intrarater reliability was good for both the standard configuration (ICC_2,1=0.93) and modified configuration (ICC_2,1=0.93) conditions.

Conclusions

The use of the modified belt‐stabilized HHD configuration, where the HHD was interfaced with the leg of a table, offers a more comfortable alternative compared to the standard belt‐stabilized configuration to obtain isometric quadriceps strength measures in a clinical environment. This configuration is also a valid and reliable alternative to the “gold standard” isokinetic dynamometer when testing isometric quadriceps strength at 90° of knee flexion.

Level of Evidence

Diagnostic, Level 3

INTRODUCTION

Sufficient strength of the quadriceps muscle group is necessary for functional tasks including walking, sit‐to‐stand transfers, and climbing stairs.^1-3 Quadriceps strength is positively associated with physical activity levels and quality of life.^4,5 Assessment of quadriceps strength is used to guide rehabilitation progression and return to play decisions following anterior cruciate ligament injury⁶ and can be used to predict performance on functional tasks, such as jumping.⁷ Due to the importance of quadriceps strength, objective strength assessment is often sought in clinical and laboratory settings.

Isokinetic dynamometers are considered the “gold standard” of strength testing⁸ but cost, space requirements, and portability are barriers for use among clinicians. A hand‐held dynamometer (HHD) offers a relatively inexpensive, light, and portable option to obtain objective measures of muscle strength. Specific to testing quadriceps strength, a belt‐stabilized HHD configuration is considered a valid (r>0.86) and reliable (ICC=0.88) estimate of strength, but this configuration tends to produce measures which are approximately 23% lower measurements compared to an isokinetic dynamometer.⁹ and an isokinetic dynamometer. Differences in quadriceps strength measures may be due to discomfort of the HHD against the relatively bony tibia and decreased stability of the HHD testing setup. It may be possible to modify previously described methods⁹ in order to reduce discomfort and to improve stability of the HHD during testing. This could be accomplished by placing the HHD against a rigid surface and allowing a padded belt to interface with the tibia. The primary purpose of this study was to quantify the level of discomfort associated with a modified belt‐stabilized HHD configuration compared to a standard belt‐stabilized configuration⁹ and an isokinetic dynamometer. The secondary purpose of this study was to determine the validity and reliability of a modified belt‐stabilized HHD configuration to measure quadriceps strength compared to the “gold‐standard” isokinetic dynamometer.

METHODS

Participants

Twenty healthy participants (5 males, 15 females; age=24.7±2.2 years, height=171.1±8.8 cm, mass=72.0±18.7 kg) volunteered for the study. The participants were fully informed of the study purpose, procedures, and risks prior to enrollment. Inclusion criteria included age between 19 and 45 years. Exclusion criteria included history of traumatic spine or lower extremity injury within the past six months or inability to give consent or understand the procedures of the experiment. The Creighton University Institutional Review Board approved the study (IRB 14‐17059) and informed consent forms.

Procedures

All participants completed an approved informed consent form, standardized health history form, and a self‐reported form related to physical activity level (Tegner Activity Scale). Quadriceps strength (N•m) for the right leg was then measured using an isokinetic dynamometer using a refurbished Biodex System 3 Isokinetic Dynamometer (Computer Sports Medicine Inc.; Stoughton, MA). Participants were seated with their hips flexed at 85° and knees flexed at 90° (Figure 1). The chest and pelvis were secured to the chair using Velcro straps and a padded ankle strap was placed approximally five centimeters proximal to the distal aspect of the lateral malleolus. The isokinetic dynamometer was interfaced with an external data acquisition system (MP150; Biopac Systems, Inc.; Goleta, CA). Participants performed a warm‐up consisting of five submaximal and maximal isometric knee extension contractions (50‐100% effort).¹⁰ Participants then performed three maximal isometric contractions with one minute of rest between each contraction. All maximal trials were accompanied with verbal encouragement to ensure maximal effort was achieved. Torque signals were low pass filtered at 50 Hz to remove noise. Peak torque was calculated for each maximal isometric contraction by averaging a 100 ms epoch surrounding the identified peak torque value. The highest peak torque average from a single trial was used for data analysis. Following the isokinetic dynamometer testing, patients were asked to rate their level of discomfort during testing by marking a vertical line on a 100 mm Visual Analog Scale (VAS).

Figure 1.

Isokinetic dynamometer set‐up..

Next, participants transitioned to one of two HHD configurations using the microFET2, (Hoggan Scientific, LLC; West Jordan, UT): standard belt‐stabilized HHD configuration or a modified standard belt‐stabilized HHD (Figure 2A) configuration (Figure 2B). The order of HHD configuration was randomized. Participants were placed in a seated position with legs hanging from the edge of a wooden treatment table, with the knee in approximately 90° of flexion. A small wedge‐type bolster was placed at the posterior aspect of the distal thigh to minimize posterior thigh discomfort and a strap (standard gait belt) was used to stabilize the thighs to the table (Figure 2). A gait belt attached to the leg of the table was used to stabilize the HHD during testing and was positioned approximately 5 cm proximal to the distal aspect of the lateral malleolus, consistent with isokinetic dynamometer pad placement. The standard belt‐stabilized configuration positioned the HHD on the tibia, using a curved attachment (Figure 2A), which was consistent with previous methods.⁹ The a modified standard belt‐stabilized configuration positioned the HHD against the back of a treatment table leg using a flat attachment (Figure 2B). A foam pad was placed across the anterior aspect of the tibia and the gait belt was looped through the pad and secured the HHD against the leg of the table. When the participant extended his or her knee, the HHD was compressed against the table leg and force could be measured.

Figure 2.

A) Standard belt‐stabilized HHD configuration B) Modified belt‐stabilized HHD configuration.

For both HHD configurations, participants were asked to extend their knees in a similar manner as the isokinetic dynamometer testing to obtain three separate maximal isometric contractions. One minute of rest was provided between each contraction. All maximal trials with the HHD were also accompanied by verbal encouragement to ensure maximal effort was achieved. Maximal isometric force (N) was recorded for each trial. To calculate torque, a measure was taken, in centimeters, from the lateral knee joint line (estimated knee axis of rotation) to the point five centimeters proximal to the distal aspect of the lateral malleolus. This value was then multiplied by the amount of force (N) recorded by the HHD to obtain torque (N•m). The highest peak torque from a single trial for each HHD configuration was used for data analysis. After each HHD configuration, participants recorded their level of discomfort during testing using a VAS.

Participants were then provided a 10‐minute rest break. After the rest break, both HHD configurations were repeated using the same methods. This data was used to estimate within‐session intrarater reliability for both HHD configurations.

Statistical Analysis

The independent variable was testing method (isokinetic dynamometer, standard configuration, modified configuration) and the outcome variables were discomfort (VAS) and torque (N•m). Differences in discomfort (VAS) between the testing positions was determined using a Friedman test. Validity was quantified using Pearson correlations and examining differences in torque (N•m) between testing conditions using a repeated measures analysis of variance. Bland‐Altman Plots with Limits of Agreement (LOA) calculations¹¹ were used to graphically display differences between conditions and provide insights into systematic bias between measurement techniques. Within‐session intrarater reliability was determined using the repeated measures ANOVA, intraclass correlation coefficient (ICC_2,1), and associated confidence intervals (95% CI). To provide better interpretation of reliability estimates, the authors also calculated the standard error of measurement (SEM) (SEM=SD √1‐ICC)¹² and minimal detectable change (MDC) (MDC=SEM * 1.96* √2).¹² Within‐session intrarater reliability for HHD measures was interpreted as follows: <0.50 poor, 0.50 to 0.75 moderate, and >0.75 good.¹³ Alpha level was set a priori at p < 0.05. When significance was achieved relevant post‐hoc tests were performed to determine differences between conditions. Statistical analyses were performed with SPSS Version 20.0 (SPSS Inc., Chicago, IL).

RESULTS

Discomfort

There was a significant difference in discomfort between the three configurations (p<.001). The standard belt‐stabilized (p<.001) and modified belt‐stabilized configurations (p=.01) were significantly more uncomfortable than the isokinetic dynamometer (Table 1). The modified belt‐stabilized configuration was significantly (p=.003) more uncomfortable than the standard belt‐stabilized configuration.

Table 1.

Discomfort measured using a Visual Analog Scale (cm). Values are mean ± standard deviation.

Isokinetic Dynamometer	0.5 ± 0.6
Standard Belt‐Stabilized HHD	3.1 ± 2.0 *
Modified Belt‐Stabilized HHD	1.1 ± 1.3 †

^*significantly greater discomfort than the isokinetic dynamometer (P <.001) and the modified belt‐stabilized HHD (P= .003)

^†significantly greater discomfort than the isokinetic dynamometer (P = .01) position

Torque‐Generating Capacity

There was a significant difference in torque‐generating capacity between the three configurations (p<.001). The standard belt‐stabilized configuration resulted in significantly less torque than the isokinetic dynamometer (p<.001) and the modified belt‐stabilized HHD configuration (p<.001) (Table 2). There was no significant difference (p=.46) in torque between the isokinetic dynamometer and the modified belt‐stabilized HHD configuration. There was a significant correlation (p<.001) between measures from the isokinetic dynamometer and the standard belt‐stabilized (r= .87) and modified belt‐stabilized HHD configurations (r=.93). Bland‐Altman plot LOA (Figure 3) indicated standard belt‐stabilized configuration had an average difference of 52 N•m (SD=43) and a lower and upper LOA range between ‐31 to 136 N•m. The modified belt‐stabilized HHD configuration had an average difference of 13 N•m (SD=32) and a lower and upper LOA range between ‐50 to 77 N•m.

Table 2.

Torque‐Generating capacity (N·m) differences between the three HHD configurations. Values are mean ± standard deviation.

	Trial 1
Isokinetic Dynamometer Torque (N·m)	252.6 ± 82.8
Standard Belt‐Stabilized HHD Torque (N·m)	208.9 ± 69.2*
Modified Belt‐Stabilized HHD Torque (N·m)	248.5 ± 79.2

^*significantly less torque (p < .001) than isokinetic dynamometer and the modified belt‐stabilized configuration.

Figure 3.

Bland‐Altman Plot A) isokinetic dynamometer versus standard belt‐stabilized configuration (mean difference 52 ± 43 N•m). B) isokinetic dynamometer versus modified belt‐stabilized HHD configuration (mean difference 13 ± 32 N•m).

Reliability

There were no significant differences (p>.64) between the first and second trials for the standard belt‐stabilized and modified belt‐stabilized HHD configurations (Table 3). Furthermore, within‐session intrarater reliability (Table 3) was good for both the standard belt‐stabilized (ICC_2,1=0.93; 95% CI 0.83 to 0.97) and modified belt‐stabilized (ICC_2,1=0.93, 95% CI 0.84 to 0.97) HHD configurations. The standard belt‐stabilized configuration SEM was 5.1 N•m with a MDC of 14.1 N•m. The modified belt‐stabilized configuration SEM was 5.4 N•m with a MDC of 15.1 N•m.

Table 3.

Within session intrarater reliability for quadriceps strength using a belt‐stabilized HHD.

	Trial 1	Trial 2	ANOVA p‐value	ICC2,1 (95% CI)	SEM	MDC
Standard Belt‐Stabilized HHD Torque (N·m)	208.9 ± 69.2	208.8 ± 66.7	.98	.93 (.83‐.97)	5.1 N•m	14.1 N•m
Modified Belt‐Stabilized HHD Torque (N·m)	248.5 ± 79.2	251.6 ± 77.3	.64	.93 (.84‐.97)	5.4 N•m	15.1 N•m

DISCUSSION

The primary purpose of this study was to quantify the level of discomfort associated with a modified belt‐stabilized HHD configuration compared to a standard belt‐stabilized HHD configuration and an isokinetic dynamometer. The secondary purpose of this study was to determine the validity and reliability of a modified belt‐stabilized HHD configuration in order to measure quadriceps strength compared to the “gold‐standard” isokinetic dynamometer. The use of the modified belt‐stabilized HHD configuration provided a comfortable option to quantify quadriceps strength measures at 90° of knee flexion. This configuration is a valid and reliable option compared to the “gold standard” isokinetic dynamometer.

HHD positioning had an impact on levels of discomfort during testing. Although the modified belt‐stabilized HHD configuration (Figure 2B) resulted in a statistically greater level of discomfort compared to the isokinetic dynamometer (Table 1), this difference (mean difference of 0.6 cm on the VAS) is not thought to be clinically relevant since it is below an accepted clinically important difference of 1.3 cm.¹⁴ The standard method of testing quadriceps strength with a HHD utilizes a foam padded attachment, either curved or flat, that is in direct contact with the tibia (Figure 2A).^9,15,16 This configuration was less comfortable than modified belt‐stabilized configuration and the isokinetic dynamometer. These differences were both statistically significant (Table 2) and clinically significant as the difference exceeds an accepted clinically important difference of 1.3/10.¹⁴

Quadriceps strength values were also influenced by HHD testing position compared to the isokinetic dynamometer. The modified belt‐stabilized HHD configuration did not result in torque‐generating capacity that was significantly different than the isokinetic dynamometer, but was greater than the standard belt‐stabilized HHD configuration (Table 2). A similar configuration, in which the treatment table is used to stabilize the dynamometer, has been previously described.¹⁷ Comparison between studies is limited since Whitley et al¹⁷ used a less common push/pull dynamometer, did not quantify discomfort, and obtained isometric measures at 30° of knee flexion without comparison to an isokinetic dynamometer at the same joint angle. Thus the results of this study provide more comprehensive insights than the Whitley et al¹⁷ study and comparison to the “gold‐standard” isokinetic dynamometer.

The standard belt‐stabilized HHD configuration resulted in quadriceps strength values that were significantly lower than both the modified belt‐stabilized HHD configuration and the isokinetic dynamometer (Table 2). These results are in agreement with a previous study,⁹ which found the belt‐stabilized HHD produced reliable but statistically lower torque measurements compared to the isokinetic dynamometer.⁹ The standard belt‐stabilized HHD configuration in the current study was similar to previously described methods,⁹ but with the addition of the knee bolster and curved tibia attachment in an attempt to increase participant comfort and testing stability. In spite of these changes, the authors’ also found that the standard belt‐stabilized configuration used in the current study produced significantly lower torque readings and more discomfort than the isokinetic dynamometer. While there is some foam padding on the curved HHD attachment used in this study, the area of the tibia where the HHD is placed typically has little soft tissue coverage and is relatively bony. Thus, the minimal padding offered by the HHD curved attachment interfaced with the tibia may produce a significantly greater level of discomfort for some participants, thereby limiting the ability to produce a maximal isometric contraction.

The significant correlations between measures from the isokinetic dynamometer and the standard belt‐stabilized (r=.87) and modified belt‐stabilized (r=.93) HHD configurations further reinforce that the HHD is a valid measure of knee extensor strength. Both the standard and modified belt‐stabilized HHD configurations had good within‐session intrarater reliability with low SEM (5.1 to 5.4 N•m) and MDC (14.1 to 15.1 N•m) values. This indicates both configurations provide reliable estimates of quadriceps strength and evidence of the ability to detect changes in torque‐generating capacity near 15 N•m. These results are in agreement with previous studies that have examined within‐session reliability using a belt‐stabilized configuration.^15,17 Bland‐Altman plot LOA average differences for standard belt‐stabilized configuration (52 N•m) were greater than the modified belt‐stabilized HHD configuration (13 N·m). Both HHD placement configurations tended to underestimate torque‐generating capacity values obtained on the “gold standard” isokinetic dynamometer (Figure 3). These results are in agreement with previous studies.^9,15 Nearly all values from this study fell within two standard deviations of the mean (Figure 3). This indicates that, although there may be some underestimation of torque‐generating capacity for HHD configurations compared to the “gold‐standard” isokinetic dynamometer, either HHD configuration provides similar information.

A limitation of this study is that a relatively young, healthy population was utilized and that the testing angle for quadriceps strength only considered at 90° of knee flexion. Future studies should further examine individuals with knee joint or lower extremity pathology, torque‐generating capacity at different knee joint angles, and examine other muscles using a similar configuration.

CONCLUSION

The use of a modified belt‐stabilized HHD configuration offers a more comfortable alternative compared to the standard belt‐stabilized configuration to obtain isometric quadriceps strength measures in a clinical environment. This configuration also provides a valid and reliable alternative to the “gold standard” isokinetic dynamometer when testing isometric quadriceps strength at 90° of knee flexion. Placing a pad at the tibia and interfacing the HHD with a stable table leg resulted in less discomfort compared to the standard belt‐stabilized HHD configuration and produced measurements that were not statistically significantly different from the isokinetic dynamometer. These results have clinical relevance since this configuration could likely be reproduced in a clinical environment with an HHD, gait belt, and pad.