Reliability and measurement error of hip abductor muscle strength using a hand-held dynamometer in healthy adults

Abstract

[Purpose] Patients with hip osteoarthritis commonly exhibit weakened hip abductor (HA) muscles. Accurate assessment of muscle strength is critical; however, the reliability of HA strength measurements remains underreported. This study aimed to evaluate reliability and measurement error for HA muscle strength values measured with a hand-held dynamometer (HHD). [Participants and Methods] Two physical therapists measured HA muscle strength in 20 healthy adults (40 limbs) using an HHD. Participants were positioned supine, with lower limbs placed in a belt loop. Average and maximum values from the two trials were used for analysis. Reliability was assessed using intra-class correlation coefficients (ICCs). To evaluate changes in HA muscle strength, the minimal detectable change (MDC) of normalized strength values was calculated using an equation. [Results] Inter-rater reliability was excellent, with ICCs of 0.95 and 0.92 for average and maximum values, respectively. The associated standard errors of measurement and MDC values were 1.65 and 8.1 for average values, and 2.25 and 10.9 for maximum values, respectively. [Conclusion] Measuring hip abductor muscle strength using an HHD demonstrated excellent reliability. Using average value effectively reduced inter-rater variability and improved reliability.

INTRODUCTION

The hip abductor muscles play a key role in various movements, supporting the trunk and pelvis during standing and walking¹⁾. Patients with hip osteoarthritis often exhibit weakened abductor muscles. Accurately assessing muscle strength is essential for improving and maintaining muscular function²⁾. Manual Muscle Testing (MMT) offers a simple method for evaluating muscle strength without requiring specialized equipment. However, MMT is prone to overestimation, influenced by factors such as age, medical conditions, and the examiner’s experience³⁾. Recently, the hand-held dynamometer (HHD) has emerged as an objective alternative to MMT for measuring muscle strength. HHD measurements show significant correlation with grades 3, 4, and 5 on the MMT scale⁴⁾, offering a more objective assessment method. While many studies have reported the reliability of HHD in measuring knee extensor strength, the reliability of hip abductor strength measurements remains underreported, highlighting the need for further investigation.

Several previous studies have examined the intra- and inter-rater reliability of hip abductor muscle strength measurements using an HHD and reported high reliability^5,6,7,8,9). These reliability studies typically use either the average or maximum value from multiple measurements taken by each tester. Studies using average values have investigated the reliability of both manual sensor fixation^{6, 10)} and belt fixation⁶⁾. Other studies have reported on maximum values using manual sensor fixation in both healthy⁸⁾ and elderly⁵⁾ participants. In clinical practice, consistent testers are not always available and it remains unclear whether average or maximum values best minimize inter-rater variability.

Furthermore, existing methodologies frequently require specialized setups, such as securing fixation belts to walls or external structures, thereby possibly limiting their practical utility. In contrast, the method evaluated in this study secures the lower limbs using a simple belt stabilized exclusively by the examiner’s foot. Requiring no dedicated laboratory equipment, this approach minimizes environmental dependencies and enhances bedside clinical feasibility.

The aim of this study was to evaluate intra- and inter-rater reliability and measurement error for average and maximum hip abductor muscle strength values measured with an HHD in patients positioned supine.

PARTICIPANTS AND METHODS

Twenty healthy individuals (mean age: 25.1 ± 4.1 years; age range: 21–36 years), all regional hospital staff, participated in the study. Hip abductor muscle strength was measured in 40 limbs. The study was approved by the Ethics Committee of Nagai Hospital (No. 37) and informed consent was obtained from all participants.

An HHD (μTas F-1, Anima Corp., Tokyo, Japan) was used to measure muscle strength. Participants were positioned supine, with both lower limbs placed in a belt loop, and the hip joints maintained in a neutral position. The dynamometer, attached to the belt, was applied to the distal thigh of the measurement side, and the rater manually supported the device. The rater stabilized the pelvis on the non-measurement side to prevent trunk lateral bending, pelvic elevation, or hip abduction. The belt was anchored outside the non-measurement side using the rater’s lower limb (Fig. 1), positioned to avoid contact with the participant’s limbs.

Fig. 1.

Measurement method for hip abductor muscle strength. The participant is positioned supine, with both lower limbs in a neutral hip position placed within the belt loop. The dynamometer, attached to the belt, is applied to the distal thigh of the measurement side and the rater manually supports the device during measurement. The belt was anchored to the outside of the non-measurement side using the rater’s lower limb to stabilize the setup during muscle strength measurement.

Two physiotherapists served as raters: rater A (female, body mass index [BMI] 22.6, mean grip strength 37.0 kgf, knee extension strength 44.5% of body weight [%BW]) and rater B (female, BMI 21.2, mean grip strength 33.0 kgf, knee extension strength 46.5% BW), both with five years of clinical experience. Rater B performed measurements one day after rater A. To prevent bias, both raters were blinded to each other’s measurement sessions and data, which were subsequently managed and analyzed by an independent researcher. The procedure for measuring isometric hip abduction strength was as follows: (1) practice trial, (2) 1-min rest, (3) first measurement, (4) 1-min rest, and (5) second measurement. During each measurement, participants were verbally instructed to “push outward against the belt with maximal effort”, and isometric muscle contraction was maintained for approximately 5 seconds. The average and maximum values of the two trials were used for analysis. Muscle strength values were normalized to each participant’s % BW.

Intra-rater and inter-rater reliabilities of the normalized strength values were assessed using the intraclass correlation coefficient (ICC). To evaluate additive error between raters A and B, the standard deviation (SD) of the difference scores and the 95% confidence interval (CI) were calculated. To assess the presence of proportional error, a Bland–Altman plot was generated, a regression equation was calculated, and a significance test was performed. To evaluate changes in hip abductor muscle strength, the minimal detectable change (MDC) of the normalized strength values was calculated using Equation (1). To express the error relative to the measured value, the percentage of MDC (%MDC) was calculated using Equation (2).

For the analysis of ICC, systematic error, MDC95, and %MDC related to inter-rater reliability, the mean and maximum values from the two measurements by each rater were used. Statistical analyses were performed using a modified R commander for Windows version 4.3.1 (https://personal.hs.hirosaki-u.ac.jp/pteiki/research/stat/R/) with a significance level set at p<0.05.

MDC=SD×1.96·····(1)

%MDC=100×MDC/Average value·····(2)

RESULTS

The intra-rater reliability ICC for rater A was 0.94, with a standard error of the mean (SEM) of 1.86. For rater B, the ICC was 0.97, and the SEM was 1.20 (Table 1). The inter-rater reliability ICC for the average value was 0.95, with an SEM of 1.65. For the maximum value, the inter-rater ICC was 0.92, and the SEM was 2.25 (Table 2). Systematic inter-rater errors in normalized hip abductor muscle strength were analyzed using Bland–Altman plots (Figs. 2 and 3). The mean ± SD of the differences between raters were −0.012 ± 0.041 and −0.010 ± 0.056 for the average and maximum values, respectively. No additive errors were detected within the 95% CIs.

Table 1. Intra-rater reliability of hip abductor muscle strength measurements.

Intra-rater	Rater A	Rater B
ICC (1,1)	0.94	0.97
95% CI	0.90–0.97	0.95–0.99
ICC (1,2)	0.97	0.99
95% CI	0.95–0.98	0.98–0.99
SEM (kgf)	1.86	1.2

ICC: intraclass correlation coefficient; CI: confidence interval; SEM: standard error of the mean.

Table. 2. Inter-rater reliability of hip abductor muscle strength measurements.

Inter-rater	Average	Maximum
ICC	0.95†	0.92‡
95% CI	0.91–0.97	0.86–0.96
SEM (kgf)	1.65	2.25
MDC95 (%BW)	8.1	10.9
%MDC	18.27	23.67

^†: ICC(2,2), ^‡: ICC(2,1). ICC: intraclass correlation coefficient; CI: confidence interval; SEM: standard error of the mean; MDC: minimal detectable change.

Fig. 2.

Bland–Altman plot for inter-rater reliability of the average muscle strength value. The vertical axis represents the difference in average muscle strength values between the two raters while the horizontal axis shows the mean of the average muscle strength values from both raters. Muscle strength values were normalized to each participant’s % body weight (BW). The upper and lower dashed lines represent limits of agreement, respectively. The 95% confidence interval (CI) for the mean difference was −2.5 to 0.1.

Fig. 3.

Bland–Altman plot for inter-rater reliability of the maximum muscle strength value. The vertical axis represents the difference in maximum muscle strength values between the two raters, while the horizontal axis shows the mean of the maximum muscle strength values from both raters. Muscle strength values were normalized to each participant’s % body weight (BW). The upper and lower dashed lines represent limits of agreement, respectively. The 95% confidence interval (CI) for the mean difference was −2.7 to 0.8.

Regression analysis showed no significant proportional error, with p-values of 0.317 and 0.118 for the average and maximum values, respectively. The MDC values for the average and maximum measurements were 8.1 and 10.9, respectively.

DISCUSSION

In this study, we examined the intra- and inter-rater reliability and systematic error of hip abductor strength measurements using an HHD. Both intra- and inter-rater reliability for raters A and B demonstrated high ICC values exceeding 0.90. According to the evaluation criteria by Landis and Koch¹¹⁾, ICC values between 0.81 and 1.00 are considered “almost perfect”, 0.61 to 0.80 as “substantial”, 0.41 to 0.60 as “moderate”, 0.21 to 0.40 as “fair” and 0.00 to 0.20 as “slight”. All reliability coefficients obtained in this study fell within the “almost perfect” range. Kato and Yamasaki¹²⁾ reported an inter-rater ICC of 0.88 using the maximum value, consistent with the results of this study. However, higher inter-rater reliability was observed when using the average value. The lower limit of the 95% CI was 0.91 for the average and 0.86 for the maximum, indicating greater precision with the average value. Additionally, the SEM was smaller for the average (1.65) compared to the maximum (2.25). These findings suggest that using the average of two HHD measurements may yield more reliable values for assessing hip abductor muscle strength.

For this study, we selected a measurement method in which the participants were placed in the supine position and the belt was fixed to the outside of the non-measurement side using the rater’s lower limb. Previous studies have examined the reliability of different measurement methods. Tsunoda and Sato¹³⁾ reported an inter-rater reliability coefficient of 0.93 when the belt was anchored to a fixed post rather than the rater’s contralateral leg. Kamiya et al.¹⁴⁾ used a wall-mounted dynamometer pressed during hip abduction and reported an inter-rater reliability coefficient of 0.90. Although these methods were developed to achieve high reliability, they primarily provide point estimates without 95% CIs. Moreover, they are limited by the need for specific equipment or setups. In clinical practice, measurement environments often vary by rater and location. The belt-only method used in this study is simple and well-suited to such variable conditions. It allows for reliable measurement when testers are proficient and the average of repeated trials is used.

The results of this study showed no additive or proportional error when using either the average or maximum value. The MDC of the normalized strength value was 8.1% BW using the average value and 10.9% BW using the maximum value, indicating lower measurement error when the average value was used. While it is generally recommended that maximum values be used in muscle strength assessments—and high inter-rater reliability was achieved in this study—the average value demonstrated even greater reliability, suggesting it is less affected by variability in rater conditions.

Inter-rater measurement error may result from differences in experience, gender, physical fitness, and muscle strength^{12, 13)}. In this study, both raters A and B achieved reliability coefficients above 0.90; however, rater B, who had more continuous experience using the HHD, showed slightly higher intra-rater reliability. Proper fixation of the lower limb is critical for accurate measurement of abductor strength and variations in technique may lead to error.

Previous research has noted that when two raters perform maximal strength measurements consecutively, the second rater often records higher values. This suggests a potential learning effect, where repeated attempts enhance performance and enable more efficient force exertion⁵⁾. Therefore, accurate assessment of true maximal muscle strength may require additional preparation, including practice. However, in clinical settings, time constraints and patient fatigue must be considered. Using the average value may be more practical and effective in reducing inter-rater variability and improving overall reliability.

This study has several limitations. Both raters were female physical therapists with similar years of experience, body size, and muscle strength. As the measurement of higher muscle strength requires greater fixation force and technical skill, factors such as rater gender, physical characteristics, and experience may influence measurement quality. Additionally, the participants were healthy adults; results may differ in adolescent students, older populations or in individuals with conditions that lead to compensatory movement patterns. Further research involving diverse participant groups is warranted.

In conclusion, measuring hip abductor muscle strength in the supine position using an HHD with simple belt stabilization is a highly reliable and clinically practical approach. Utilizing the average value effectively reduces inter-rater variability, further enhancing measurement reliability. Free from the requirement of specialized laboratory equipment or fixed structures, this method may facilitate reproducible and efficient hip abductor strength testing in clinical settings.

Conference presentation

An earlier version of this study was presented at 108th Rigakuryoho Kagaku gakkai.

Funding and Conflict of interest

The authors have no conflicts of interest directly relevant to the content of this article.