Abstract
Introduction
The prone position is generally regarded as the standard posture for back extensor strength testing. However, some older adults may have difficulty assuming this position, making alternative approaches more clinically feasible. It remains unclear whether measurements obtained in the sitting and semi-standing positions demonstrate sufficient validity compared with those in the prone position. This study aimed to evaluate the reliability and validity of isometric back extensor strength measurements using hand-held dynamometry (HHD) in the sitting position and the Biodex system in the semi-standing position. The results were compared with measurements in the prone position.
Methods
Thirty-four healthy adults (10 males and 24 females; mean age, 23.8 years) participated. Back extensor strength was assessed using three methods: HHD in the prone, HHD in the sitting position, and the Biodex system in the semi-standing position. Test–retest reliability was evaluated using two sessions. Convergent validity of HHD in the sitting position and the Biodex in the semi-standing position was examined and compared with prone HHD, which served as a reference standard.
Results
The semi-standing Biodex demonstrated excellent reliability and sufficient convergent validity compared with prone HHD, whereas HHD in the sitting position showed acceptable reliability but limited validity. Intraclass correlation coefficients (ICCs) indicated excellent reliability for prone HHD (ICC=0.95) and semi-standing Biodex (ICC=0.92), while sitting HHD showed acceptable reliability (ICC=0.76). Regarding convergent validity, the Biodex in the semi-standing position showed a moderate correlation with prone HHD (r=0.60), whereas sitting HHD showed a weaker correlation (r=0.45).
Conclusions
The Biodex system in the semi-standing position demonstrated excellent reliability and sufficient validity for assessing back extensor strength. In contrast, HHD in the sitting position showed acceptable reliability but limited validity, which may be influenced by the measurement protocol used. Further studies are needed to improve measurement protocols, particularly for use in older adults and clinical populations.
Keywords: back extensor strength, hand-held dynamometry, isokinetic dynamometry, prone sitting, semi-standing
Introduction
Age-related hyperkyphosis is a common postural change in older adults that is associated with impaired physical function, increased risk of falls, and higher mortality rates1). Among its multifactorial causes, age-related reductions in back extensor muscle strength are considered to play a particularly important role1). Cross-sectional studies have demonstrated that a smaller cross-sectional area and lower density of back extensor muscles are associated with greater thoracic kyphosis in older adults2,3). Therefore, accurate assessment of back extensor muscle strength is essential in both clinical and research settings to better understand and manage hyperkyphosis in this population.
Muscle strength can be assessed using manual muscle testing (MMT), hand-held dynamometry (HHD), and isokinetic dynamometry, such as the Biodex system. MMT is the most commonly used method in clinical settings and generally demonstrates substantial reliability and validity, except at higher strength levels, where its reliability is limited4). However, its responsiveness is notably poor due to the limited number of grading levels4), making it less suitable for detecting treatment effects, such as those resulting from exercise therapy. HHD involves a less expensive and more convenient device for measuring muscle strength compared with an isokinetic dynamometer. Although both test–retest and interrater reliabilities of HHD are limited when the device is held by hand, its reliability can be substantially improved when the device is stabilized using a belt5).
The prone position is typically used to assess both back muscle strength and endurance, particularly in MMT and the Biering–Sørensen test6). Electromyographic studies have confirmed the occurrence of back muscle fatigue during prone endurance testing7,8). Strength testing of the back extensor muscles in the prone position using fixed HHD has been shown to be reliable in women with osteoporosis and vertebral fractures9). However, because some older adults may have difficulty assuming the prone position, assessing strength in a more accessible position, such as sitting, would represent a practical alternative.
A few previous studies have examined the reliability and validity of HHD measurements of back extensor strength in the sitting position. However, it remains unclear whether measurements in the sitting position demonstrate sufficient validity compared with those obtained in the prone position due to the lack of validation against a commonly used or standard posture for back extensor strength assessment10,11). In contrast, measurements of back extensor strength in the sitting and standing positions using isokinetic dynamometry have been well studied, whereas those in the semi-standing position have not been sufficiently investigated. The purpose of this study was to evaluate the reliability and validity of isometric back extensor strength measurements using HHD in the sitting position and a Biodex system in the semi-standing position, compared with measurements obtained in the prone position. We hypothesized that measurements in both the sitting and semi-standing positions would demonstrate acceptable reliability and validity relative to those obtained in the prone position.
Materials and Methods
This methodological study aimed to evaluate the test–retest reliability and validity of HHD in the sitting position and the Biodex system in the semi-standing position for measuring back extensor muscle strength. The study received approval from the institutional review board of Wakayama Medical University and was conducted at our hospital from September 2023 to June 2025.
Participants
Healthy participants between 18 and 40 years of age were recruited from hospital employees and medical students. Exclusion criteria included sciatica, a history of spinal surgery, and pregnancy. Due to limitations of the HHD device, participants with test results exceeding 510 N were also excluded. All participants received an information leaflet and provided written informed consent prior to participation.
Study equipment
An HHD device (Mobie MT-100, Sakai Medical Co., Ltd., Tokyo, Japan) was used to measure back extensor strength in both the prone and sitting positions. In the prone position, participants were positioned prone on a hydraulic treatment table, and a parallel bar was used to stabilize the HHD device during the measurement. In the sitting position, participants were seated on a 40 cm high platform placed against a wall, which served to stabilize the HHD device during the measurement. A Biodex dynamometer (System 4, Biodex Medical Systems, Inc., Shirley, NY, USA) was used to measure back extensor strength in the semi-standing position. The Dual Position Back Ex/Flex Attachment was used to secure the legs and position the participants in a semi-standing posture (Figure 1).
Figure 1.
Isometric back extensor strength testing methods: a) HHD in the prone position, b) HHD in the sitting position, c) Biodex in the semi-standing position.
HHD: hand-held dynamometry
Study protocol
Three types of assessment orders were randomly assigned to the participants. The first measurement session was conducted to evaluate the convergent validity of the HHD test in the sitting position and the Biodex test in the semi-standing position by comparing results obtained with the HHD test in the prone position, which has previously demonstrated acceptable reliability and validity. One week after the first measurement session, the second session was conducted to assess the test–retest reliability of the three measurement methods.
For the HHD assessment in the prone position, participants were instructed to lie with their arms positioned alongside their bodies. The pelvis and ankles were secured using non-elastic bands. In the sitting position, participants were instructed to sit on a platform with their hips and knees flexed at 90 degrees, their feet flat on the floor, and their arms crossed in front of their chest. For the HHD tests conducted in both the prone and sitting positions, the center of the HHD device was placed at the level of the T7 spinous process. Upon the starting signal, participants were instructed to push against the device with their backs as forcefully as possible for five seconds. Each HHD test in the sitting and prone positions was performed three times, with a 30-second rest between trials, following one warm-up trial. The average force values (N) from the three trials in each of the two sessions were used for analysis.
A Biodex dynamometer protocol was configured to measure isometric contraction of back extensor strength in the semi-standing position. Isometric trunk extension was performed five times, each lasting five seconds, with a five-second rest between exertions. The peak torque (N.m) recorded across the five trials was used for analysis. Table 1 summarizes the characteristics of each assessment protocol.
Table 1.
Characteristics of the Three Back Extensor Strength Assessment Protocols.
| Characteristic | HHD prone | HHD sitting | Biodex semi-standing |
|---|---|---|---|
| Body position | Prone | Sitting | Semi-standing |
| Device | HHD | HHD | Biodex |
| Fixation (trunk) | No | No | Yes |
| Fixation (pelvic) | Yes | No | Yes |
| Fixation (lower extremity) | Distal lower legs | None | Thighs |
| Foot contact | No | Yes | Yes |
| Number of trials | Three | Three | Five |
| Contraction duration | 5 s | 5 s | 5 s |
| Rest between trials | 30 s | 30 s | 5 s |
| Outcome for analysis | Mean force of three trials | Mean force of three trials | Peak torque across five trials |
| Measurement unit | N | N | Nm |
HHD: hand-held dynamometry
Statistical analysis
The required sample size for this study was determined based on two statistical considerations: test–retest reliability and convergent validity. For reliability assessment, an intraclass correlation coefficient (ICC) of at least 0.70 was considered acceptable, in accordance with the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guidelines12). A minimum of 30 participants is generally recommended to obtain a reliable estimate of the ICC with sufficiently narrow confidence intervals (CIs). For convergent validity, assuming a Pearson correlation coefficient of 0.50, the required sample size was determined to be 29 using G*Power software (version 3.1.9.7; Heinrich Heine University, Düsseldorf, Germany) (exact test; correlation: bivariate normal model; α=0.05; power=0.80). Considering both analyses, we determined that a sample size of 30 participants would be sufficient for this study.
In accordance with the COSMIN guidelines, the test–retest reliability of each test was evaluated using the ICC. A two-way random-effects model with absolute agreement was applied. For the HHD measurements, the ICC for average measures [ICC(2,k)] was calculated, as the mean of repeated trials was used for analysis. For the Biodex measurements, the ICC for single measures [ICC(2,1)] was calculated, as only a single value was used for analysis. An ICC value of 0.70 or higher was considered acceptable, indicating sufficient reliability. Measurement errors were evaluated using the standard error of measurement (SEM) and limits of agreement. The SEM was calculated as SEM=SD×√(1−ICC), where SD represents the standard deviation of the pooled measurements. The smallest detectable change (SDC) was calculated as SDC=1.96×√2×SEM. Bland–Altman analyses were performed to assess agreement between the first and second measurement sessions for the HHD test in the prone and sitting positions and the Biodex test in the semi-standing position. Convergent validity of the HHD measurements in the sitting position and the Biodex measurements in the semi-standing position was evaluated by calculating Pearson correlation coefficients with the HHD test in the prone position, which has previously been reported to have acceptable reliability and validity9,11). Although the COSMIN guidelines do not specify a strict cutoff value, a correlation coefficient of approximately 0.50 or higher is generally considered indicative of sufficient convergent validity13). All statistical analyses were performed using IBM SPSS Statistics for Windows (version 27.0; IBM Corp., Armonk, NY, USA).
Results
A total of 34 healthy participants (10 males and 24 females; mean age, 23.8 years) were included in this study (Table 2).
Table 2.
Characteristics of Healthy Participants.
| Characteristic | N=34 |
|---|---|
| Age (years) | 23.8±5.3 |
| Sex, n female (%) | 24 (70.6) |
| Height (cm) | 162.4±7.6 |
| Weight (kg) | 56.2±8.5 |
| Body mass index (kg/m2) | 21.2±2.8 |
Values are mean±SD.
SD: standard deviation
Regarding test–retest reliability, HHD in the prone position (ICC=0.95, 95% CI: 0.91-0.98) and the Biodex in the semi-standing position (ICC=0.92, 95% CI: 0.85-0.96) demonstrated excellent reliability, whereas HHD in the sitting position (ICC=0.76, 95% CI: 0.53-0.89) showed acceptable reliability (Table 3).
Table 3.
Test–Retest Reliability and Measurement Error for Muscle Strength Assessments.
| Test 1 | Test 2 | Diff* | p | ICC | SEM | SDC | LoA | |
|---|---|---|---|---|---|---|---|---|
| HHD prone (N) | 377.5±82.6 | 382.0±78.1 | –4.5±34.5 | 0.14 | 0.95 (0.91-0.98) | 17.2 | 47.7 | –72.3, 63.2 |
| HHD sitting (N) | 208.6±53.2 | 219.9±48.5 | –11.4±44.1 | 0.45 | 0.76 (0.53-0.89) | 22.4 | 61.9 | –97.8, 75.0 |
| Biodex semi-standing (N.m) | 218.0±59.1 | 218.5±61.1 | –0.5±23.9 | 0.91 | 0.92 (0.85-0.96) | 16.4 | 45.3 | –47.2, 46.3 |
Values are mean±SD.
*Difference between Test 1 and Test 2.
HHD: hand-held dynamometry; ICC: intraclass correlation coefficient; SEM: standard error of measurement; SDC: smallest detectable change; LoA: limits of agreement
Regarding convergent validity, the Biodex in the semi-standing position showed a moderate correlation with prone HHD (r=0.60), whereas sitting HHD showed a weaker correlation (r=0.45) (Table 4).
Table 4.
Pearson Correlation Coefficients among Back Extensor Strength Measurements.
| HHD prone | HHD sitting | Biodex semi-standing | |
|---|---|---|---|
| HHD prone | – | 0.45, p<0.01 | 0.60, p<0.01 |
| HHD sitting | – | – | 0.38, p=0.03 |
| Biodex semi-standing | – | – | – |
HHD: hand-held dynamometry
Measurement error, as evaluated by SEM and SDC, was smaller for HHD in the prone position (SEM=17.2 N, SDC=47.7 N) than for HHD in the sitting position (SEM=22.4 N, SDC=61.9 N). The Biodex in the semi-standing position showed measurement error values of SEM=16.4 N.m and SDC=45.3 N.m. Figure 2 shows Bland–Altman plots for each muscle strength assessment, illustrating agreement between Test 1 and Test 2. The limits of agreement were −72.3 to 63.2 N for HHD in the prone position, −97.8 to 75.0 N for HHD in the sitting position, and −47.2 to 46.3 N.m for the Biodex in the semi-standing position.
Figure 2.
Bland-Altman plots: a) HHD in the prone position, b) HHD in the sitting position, and c) Biodex in the semi-standing position. Each plot displays the mean difference between Test 1 and Test 2 (solid line), along with the upper and lower limits of agreement (dashed lines).
HHD: hand-held dynamometry
Discussion
The main findings of this study are as follows. First, isometric back extensor strength measured using the Biodex system in the semi-standing position demonstrated excellent test–retest reliability and sufficient convergent validity compared with prone HHD. Second, HHD measurements in the sitting position showed acceptable reliability but limited convergent validity. Third, the lower reliability and validity observed for sitting-position HHD are likely attributable to the specific measurement protocol used in this study rather than to the sitting posture itself.
In this study, the test–retest reliability of HHD in the sitting position, based on our measurement method, was lower than that reported in previous studies10,11,14,15). Although previous studies demonstrated excellent test–retest reliability with ICC values ranging from 0.82 to 0.99 for HHD in the sitting position9,10,13,14), the ICC value in this study was 0.76. In the previous studies, the pelvis was stabilized with a belt, and the feet were not in contact with the ground. In contrast, the present study employed a different method in which the pelvis was not stabilized and the feet remained grounded during measurement. The difference in measurement protocols may have contributed to the lower test–retest reliability observed in the sitting position.
Taken together, these findings highlight the importance of standardizing sitting posture and ensuring adequate pelvic and trunk stabilization during HHD-based strength assessments. Without proper control of these biomechanical factors, measurements obtained in the sitting position may not accurately reflect the intended construct of back extensor muscle strength. Potential protocol modifications, such as pelvic strapping, restricting lower limb movement, standardizing foot contact, and controlling thigh and trunk support, may help improve the reliability and validity of sitting-position HHD measurements and should be examined in future studies.
Regarding convergent validity compared with HHD in the prone position, the Pearson correlation coefficient was 0.60 for the Biodex in the semi-standing position, whereas it was 0.45 for HHD in the sitting position, which is below the commonly accepted threshold of 0.5013). A previous study evaluating the convergent validity of three HHD measurements compared with an isokinetic dynamometer in the seated position reported Pearson correlation coefficients of 0.50-0.60 for HHD in the standing position, 0.54 in the prone position, and 0.32 in the sitting position11). Electromyographic studies have shown that trunk and lumbar erector spinae muscle activity varies depending on sitting posture16-18). A slumped sitting posture leads to decreased trunk erector spinae activity, similar to the flexion-relaxation phenomenon observed during standing forward bending16,17). Furthermore, seat depth influences lumbar erector spinae activity by altering the support area of the buttocks and thighs18). These findings suggest that the lower convergent validity observed in the sitting position may be influenced by postural and biomechanical factors unique to this testing posture.
Taken together, these findings highlight the importance of standardizing sitting posture and ensuring adequate pelvic and trunk stabilization during HHD-based strength assessments. Without proper control of these biomechanical factors, measurements obtained in the sitting position may not accurately reflect true muscle strength, particularly in comparison with more controlled positions such as prone or semi-standing. Future studies should explore whether modifications such as pelvic strapping, standardizing the thigh support surface, or using alternative sitting postures can enhance the reliability and validity of HHD measurements in this commonly used testing position.
The results of this study have important implications for both clinical practice and research. In clinical settings, where time, space, and equipment availability may limit the use of devices such as the Biodex system, HHD remains a practical alternative for assessing back extensor strength. However, clinicians should be aware that the sitting position may introduce variability due to posture-related factors and should therefore consider incorporating standardized support or alternative testing postures, such as prone or semi-standing, when feasible. In research settings, the semi-standing position using isokinetic dynamometry may provide a more stable and valid assessment of back extensor strength, especially in populations that cannot tolerate prone positioning. The findings also highlight the need to further develop HHD testing protocols that improve validity while ensuring feasibility across diverse patient populations, particularly older adults and individuals with musculoskeletal disorders.
This study has some limitations. First, participation was limited to healthy young adults, which restricts the generalizability of the findings to older adults or clinical populations. Second, the HHD test in the sitting position was performed without pelvic stabilization or consideration of the support area of the buttocks and thighs, which may have increased measurement variability and negatively affected reliability and validity. Third, the upper force limit of the HHD device (510 N) led to the exclusion of participants with higher muscle strength, potentially biasing the sample. Finally, differences in measurement units (N vs. N.m) between HHD and Biodex complicate direct comparisons. Future studies should investigate the reliability and validity of these measurements in older adults and clinical populations, employing rigorously standardized postures to reduce measurement variability.
In conclusion, the Biodex in the semi-standing position showed excellent reliability and sufficient validity for measuring back extensor strength, whereas the HHD in the sitting position demonstrated acceptable reliability but limited validity compared with the prone position. To enhance the clinical relevance of these findings, future studies should include older adults, particularly those with age-related hyperkyphosis, and employ rigorously standardized postures to minimize measurement variability.
Author Contributions: TN and MM initiated this study and designed it. TN and MM collected the data. TN and MM did the statistical analysis. YY and YN advised and monitored the progress. YY and YN supervised the research. TN and MM wrote the manuscript and all coauthors contributed to critically revising it. The authors read and approved the final manuscript.
Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.
Ethical Approval: The study received approval of the Institutional Review Board at Wakayama Medical University (No. 3959).
Informed Consent: All participants provided written informed consent before enrollment.
Funding Statement
This study was funded by Japan Society for the Promotion of Science KAKENHI (21K17533).
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