Skip to main content
NCBI home page
Journal of Physical Therapy Science logoLink to Journal of Physical Therapy Science
. 2025 Dec 1;37(12):595–599. doi: 10.1589/jpts.37.595

Reliability and validity of a low-cost luggage scale for hip external rotation strength measurements

Samuel Galligani 1, Austin Lopez 1, Justin Ilejay 1, Benjamin Cornell 1,*
PMCID: PMC12665391  PMID: 41328279

Abstract

[Purpose] The purpose of this study was to determine the intra- and inter-rater reliability of strength testing of the hip external rotators using a dynamometer and a luggage scale. Furthermore, the secondary purpose was to examine the validity of the luggage scale when compared to the dynamometer. [Participants and Methods] Twenty-two participants were strength tested bilaterally (n=44). Hip external rotation strength was tested using a Lafayette handheld dynamometer (HHD) and an analog luggage scale (LS) with the participant prone with their knee flexed to 90 degrees and hip maintained in neutral. [Results] The results indicated good to excellent intra-rater reliability for the HHD (ICC=0.921, 95% CI=0.864, 0.99, ICC=0.886, 95% CI=0.772, 0.923) and excellent intra-rater reliability for the LS (ICC=0.971, 95% CI=0.948, 0.984, ICC=0.926, 95% CI=0.871, 0.958) for each examiner. Inter-rater reliability was good for the HHD (ICC=0.891, 95% CI=0.837, 0.927) and excellent for the LS (ICC=0.938, 95% CI=0.907, 0.958). Strong correlations were found between the HHD and LS by both examiners (r=0.869, r=0.750, respectively). [Conclusion] Both an HHD and LS are reliable for measuring hip ER strength. The luggage scale is a valid, cost-effective means for reliable measurement of hip ER strength.

Key words: Hip strength, Dynamometry, Analog scale

INTRODUCTION

A deficit in hip external rotation (ER) muscle strength is a common impairment in numerous hip and lower extremity pathologies. Some of the most common include femoral acetabular impingement (FAI)1, 2), patellofemoral pain3), groin pain4), and chronic hip pain5). Furthermore, computational musculoskeletal models have identified increased anterior hip joint forces during squatting when hip external rotation strength is decreased6). Hip ER strength has been correlated with improved balance7) and improved dynamic control of the lower extremity during landing tasks, specifically with unanticipated single-leg landing and cutting tasks8). It has also been linked to improved hop test performance9) and a lower risk of worsening knee osteoarthritis following anterior cruciate ligament (ACL) reconstruction10). Strengthening the hip external rotator muscles is also effective in treating individuals with patellofemoral pain11). Due to its importance in lower extremity function, evaluating hip ER strength is crucial for understanding functional limitations, biomechanical health, and injury recovery from a rehabilitative perspective12). Reliable hip ER muscle strength assessments make it possible to objectively determine current status and change over time.

Conventional strength assessment methods used in a clinical setting include manual muscle testing and handheld dynamometry. Manual muscle testing occurs when a trained examiner applies resistance against the force exerted by a patient while performing a specific movement or muscle contraction. With this form of testing, scoring is conducted on a numerical scaling system (0–5), and the amount of force applied may vary between testers, leading to inter-rater reliability values that are comparatively lower than alternative methods13). In contrast, handheld dynamometry involves using a device to measure the force exerted, giving a more objective and quantifiable measurement than the manual muscle test. Considering handheld dynamometry’s ease of use, portability, and compact size compared to the gold standard for strength measurement, isokinetic devices, it has become a common tool in rehabilitation.

Handheld dynamometers (HHDs) have been extensively researched in assessing muscle strength across various clinical settings and patient populations14). There is evidence of strong inter-rater and intra-rater reliability of HHD use compared to isokinetic devices, even amongst experienced and novice clinicians, further highlighting its utility in clinical and research applications15,16,17). High levels of reliability have also been documented in the literature when testing across multiple muscle groups and varying degrees of patient strength and functional ability14). With regard to hip ER strength specifically, HHDs have been shown to be reliable for this measure18, 19).

Although the HHD demonstrates efficacy as a tool for collecting quantifiable strength data, it is still not as widely accessible or utilized as manual muscle testing due to the cost of equipment and training for appropriate clinical use. Recent studies have explored the viability of commercially available luggage scales as cost-effective alternatives for measuring the muscular strength of both the upper and lower extremities20, 21). Strong validity and excellent reliability have been demonstrated for using a luggage scale (LS) to measure lower extremity strength compared to the isokinetic dynamometer, BIODEX System Pro 4™22). However, the LS was attached to a fixed point rather than held manually, which could decrease the device’s ease of use and portability in the clinic.

The primary purpose of this study was to determine the intra- and inter-rater reliability of strength testing of the hip external rotators by novice clinicians using an HHD and an LS. Furthermore, the secondary purpose was to examine the validity of the LS when compared to the HHD.

PARTICIPANTS AND METHODS

A test-retest reliability design consisting of 22 participants were included in this study. Participants were recruited via email from Mount St. Mary’s University (MSMU) Doctor of Physical Therapy (DPT) program. Both hips of each of the 22 healthy physical therapy students (n=44) were strength tested by each examiner. Inclusion criteria required that the participant was a physical therapy doctoral student at MSMU and could lie prone comfortably with the knee flexed to 90 degrees. Participants underwent initial screening via a self-reported questionnaire to assess past medical history to rule out the following exclusion criteria: history of lower extremity injury or surgeries within the past six months, existing musculoskeletal conditions that could affect hip external/internal rotation, and persistent or current hip, back, or knee pain.

The study was approved by the Institutional Review Board at Mount St. Mary’s University (#00007935). Informed written consent was obtained before instruction on exam procedures. The two examiners practiced strength testing using the LS and HHD for one hour before examining the participants to standardize the subject setup and technique. Both examiners were third-year DPT students who had completed two, semester-long courses on manual muscle testing and had practiced them during twelve weeks of clinical internships.

Equipment for testing included a treatment table with adjustable height functionality, a Samsonite manual luggage scale (Samsonite, Luxembourg City, Luxembourg, Model 043202720031), an adjustable ankle cuff, and a hand-held dynamometer (Lafayette Manual Muscle Testing System; Model 01163; Instrument Company, Lafayette, IN, USA). Reliability of the Lafayette HHD for strength measures of the hip in healthy subjects and those with osteoarthritis has previously been established23, 24).

The participant was positioned in prone with the knee of the test limb flexed to 90 degrees. A gait belt was placed superior to the participant’s sacral border to secure their pelvis on the treatment table. This position was chosen as it keeps the hip in a position closer to where it functions in weightbearing25). Previous research has indicated that there is no significant difference in strength of the hip rotators when tested in prone versus sitting26), but the prone position has been shown to be “rated higher” than the seated position when the HDD was fixed manually14). Instruction was provided to the participant to hold onto the slides of the table for further stabilization (Figs. 1 and 2).

Fig. 1.

Fig. 1.

Open in a new tab

Testing position for Lafayette Handheld Dynamometer.

Fig. 2.

Fig. 2.

Open in a new tab

Testing position for luggage scale.

When testing with the HHD, the examiner placed the force plate of the HHD directly superior to the medial malleolus. The line of pull was into hip internal rotation (Fig. 1). When testing with the LS, an ankle cuff was fastened directly superior to the medial malleolus and the LS. The table height was adjusted and maintained so that the ankle cuff aligned with the examiner’s forearm with the elbow flexed to 90 degrees (Fig. 2). In each condition, a preliminary trial was completed. The participants were instructed to perform one isometric sub-maximal hip ER contraction to ensure the desired contraction was performed before testing.

Following the preliminary trial, participants were then asked to exert and hold a 5-second maximum voluntary contraction against the force of the examiner. Both examiners were blinded to the measures as a third investigator took and recorded the measures. A total of eight measurements were performed on each leg. A random number generator was utilized to determine the order of the test conditions (left versus right and HHD versus LS).

The data were analyzed using the Statistical Package for Social Sciences (SPSS, Version 25.0. Chicago, IL, USA) software. The significance level was set at p<0.05 for all analyses to minimize type I errors. Intraclass correlation coefficients (ICC2,1) were examined to determine the intra- and inter-rater reliability of the strength measures. Confidence intervals (CI) were set at 95% to account for the variation in sampling. Determination of the strength of the intraclass correlation coefficient utilized Koo’s classification, with values greater than 0.90, between 0.75 and 0.9, between 0.5 and 0.75, and less than 0.5, indicative of excellent, good, moderate, and poor reliability, respectively27). A Pearson correlation was calculated to determine the relationship between the HHD and LS measures.

RESULTS

A total of 10 women and 12 men participated, ranging in age from 25 to 43 years old (mean=30.8). Intrarater reliability measures for individual examiners are presented in Table 1. Intrarater reliability for examiner 1 was excellent for the HHD (ICC=0.921, 95% CI=0.864–0.955) and the LS (ICC=0.971, 95% CI=0.948–0.984). Good intrarater reliability was found for examiner 2 for the HHD (ICC=0.866, 95% CI=0.772–0.923) and excellent intrarater reliability for the LS (ICC=0.926, 95% CI=0.871–0.958).

Table 1. Intra-rater reliability for measurement of hip external rotation strength.

Examiner Device ICC 95% CI
1 HHD 0.921 0.864–0.955
2 HHD 0.866 0.772–0.923
1 LS 0.971 0.948–0.984
2 LS 0.926 0.871–0.958
Open in a new tab

HHD: hand held dynamometer; LS: luggage scale; ICC: intraclass correlation coefficient; 95% CI: 95% confidence interval.

Interrater reliability measures for both examiners are presented in Table 2. Interrater reliability measures showed good agreement between examiners for HHD (ICC=0.891, 95% CI=0.837–0.927) and excellent agreement for LS (ICC=0.938, 95% CI=0.907–0.958).

Table 2. Inter-rater reliability and confidence interval (CI) for measurement of hip external rotation strength.

Device ICC 95% CI
HHD 0.891 0.837–0.927
LS 0.938 0.983–0.958
Open in a new tab

HHD: hand held dynamometer; LS: luggage scale; ICC: intraclass correlation coefficient; 95% CI: 95% confidence interval.

Validity results for the comparison of the HHD to the LS are presented in Table 3. Examiner 1 showed a very strong correlation between the two measures (r=0.869, p<0.001). Examiner 2 showed a strong correlation between the two measures (r=0.750, p<0.001). A strong positive relationship was found when measurements were pooled from both examiners (r=0.822, p<0.001).

Table 3. Validity of luggage scale compared to hand held dynamometer as measured by Pearson Correlation.

Examiner Pearson correlation (r) Significance
1 0.869* p<0.001
2 0.750* p<0.001
1 & 2 0.822* p<0.001
Open in a new tab

p<0.001.

DISCUSSION

The results indicate that a handheld dynamometer is reliable when used by novice clinicians to measure hip ER strength. The implications of these findings validate previous studies that also demonstrated HHD’s efficacy in assessing hip strength in both experienced and novice clinicians3). Furthermore, this finding aligns with previous research demonstrating reliability among clinicians using HHD to measure hip ER strength in a prone position19).

The findings also indicate that novice clinicians can reliably use an LS device to measure hip ER strength. This is similar to other studies that found LS devices were valid alternatives for measuring the muscular strength of both the upper and lower extremities20, 21). This study demonstrates the practicality of using an LS to measure hip ER strength without attaching it to a fixed object. Whereas previous studies on LS devices required fixation, this is the first to utilize manual fixation20, 21). Although securing the LS to a fixed object provides a standardized testing environment, a comparable alternative is fixation by a clinician. This method reduces the setup time required for testing, thereby increasing portability and clinical efficiency.

Isometric hip ER strength assessed by the LS showed strong concurrent validity between the LS and HHD for both examiners. Given its lower cost and greater accessibility, the LS presents a viable substitute for the HHD for more widespread clinical use. The LS typically requires minimal setup and is more straightforward to use as there are fewer operative components with which to familiarize. Thus, inexperienced clinicians can quickly adapt to utilizing the LS in practice and reduce the time needed for training.

This study had several limitations in both design and execution. The protocol was designed to test a maximum of four participants in each testing session, allowing for a standardized rest period between tests. However, due to cancellations or inability to fill scheduled spots, the number of participants tested at any given time varied, resulting in non-standardized rest times. Although our findings did not appear to be significantly impacted by the variability in rest times, future studies should ensure standardization of rest breaks. It has been suggested that rest periods of 3–5 minutes between tests of maximal strength are optimal28).

Future research should also explore whether the inclusion of a gait belt for stabilizing the participant on the table significantly impacted reliability. Prior studies have demonstrated HHD reliability with belt stabilization24) and without stabilization23). One limitation for the clinical use of a gait belt for fixation is set-up time. This may not be clinically feasible. Future research should determine the reliability and validity of the HHD and LS when the participant is not fixed to the table. Finally, while the results demonstrate good overall reliability and validity, external factors, such as examiner strength and body positioning, can influence recorded muscle strength values13). In this study, both examiners were similar in stature. Future research should investigate the impact of these variables on the reliability of strength tests.

The findings of this study hold significant clinical relevance for practitioners assessing hip ER strength. Using an LS has proven both efficacious and reliable for evaluating hip ER strength and is recommended as an alternative to traditional manual muscle tests. It provides a simple, cost-effective means of objectively testing muscle strength compared to an HHD.

Conference presentation

Findings of this study was presented at the California Chapter of the American Physical Therapy Association (APTA) conference on 21 September 2025.

Conflict of interest

There are no conflicts of interest to report.

REFERENCES

  • 1.Malloy P, Wichman DM, Garcia F, et al. : Impaired lower extremity biomechanics, hip external rotation muscle weakness, and proximal femoral morphology predict impaired single-leg squat performance in people with FAI syndrome. Am J Sports Med, 2021, 49: 2984–2993. [DOI] [PubMed] [Google Scholar]
  • 2.Mentiplay BF, Kemp JL, Crossley KM, et al. : Relationship between hip muscle strength and hip biomechanics during running in people with femoroacetabular impingement syndrome. Clin Biomech Bristol, 2022, 92: 105587. [DOI] [PubMed] [Google Scholar]
  • 3.Prins MR, van der Wurff P: Females with patellofemoral pain syndrome have weak hip muscles: a systematic review. Aust J Physiother, 2009, 55: 9–15. [DOI] [PubMed] [Google Scholar]
  • 4.Bonello C, King MG, Crossley KM, et al. : The association between hip/groin pain and hip strength in football players: an exploratory analysis of the FORCe cohort. J Sci Med Sport, 2023, 26: 471–475. [DOI] [PubMed] [Google Scholar]
  • 5.Harris-Hayes M, Mueller MJ, Sahrmann SA, et al. : Persons with chronic hip joint pain exhibit reduced hip muscle strength. J Orthop Sports Phys Ther, 2014, 44: 890–898. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Tateuchi H, Yamagata M, Asayama A, et al. : Influence of simulated hip muscle weakness on hip joint forces during deep squatting. J Sports Sci, 2021, 39: 2289–2297. [DOI] [PubMed] [Google Scholar]
  • 7.Hislop A, Collins NJ, Tucker K, et al. : The association between hip strength, physical function and dynamic balance in people with unilateral knee osteoarthritis: a cross-sectional study. Musculoskelet Sci Pract, 2023, 63: 102696. [DOI] [PubMed] [Google Scholar]
  • 8.Malloy PJ, Morgan AM, Meinerz CM, et al. : Hip external rotator strength is associated with better dynamic control of the lower extremity during landing tasks. J Strength Cond Res, 2016, 30: 282–291. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Kline PW, Burnham J, Yonz M, et al. : Hip external rotation strength predicts hop performance after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc, 2018, 26: 1137–1144. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Girdwood MA, Patterson BE, Crossley KM, et al. : Hip rotation muscle strength is implicated in the progression of early post-traumatic osteoarthritis: a longitudinal evaluation up to 5 years following ACL reconstruction. Phys Ther Sport, 2023, 63: 17–23. [DOI] [PubMed] [Google Scholar]
  • 11.Khayambashi K, Mohammadkhani Z, Ghaznavi K, et al. : The effects of isolated hip abductor and external rotator muscle strengthening on pain, health status, and hip strength in females with patellofemoral pain: a randomized controlled trial. J Orthop Sports Phys Ther, 2012, 42: 22–29. [DOI] [PubMed] [Google Scholar]
  • 12.Bazett-Jones DM, Squier K: Measurement properties of hip strength measured by handheld dynamometry: reliability and validity across the range of motion. Phys Ther Sport, 2020, 42: 100–106. [DOI] [PubMed] [Google Scholar]
  • 13.Chamorro C, Armijo-Olivo S, De la Fuente C, et al. : Absolute reliability and concurrent validity of hand held dynamometry and isokinetic dynamometry in the hip, knee and ankle joint: systematic review and meta-analysis. Open Med (Wars), 2017, 12: 359–375. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Waiteman MC, Garcia MC, Briani RV, et al. : Can clinicians trust objective measures of hip muscle strength from portable dynamometers? A systematic review with meta-analysis and evidence gap map of 107 studies of reliability and criterion validity using the COSMIN methodology. J Orthop Sports Phys Ther, 2023, 53: 655–672. [DOI] [PubMed] [Google Scholar]
  • 15.Aramaki H, Katoh M, Hiiragi Y, et al. : Validity and reliability of isometric muscle strength measurements of hip abduction and abduction with external hip rotation in a bent-hip position using a handheld dynamometer with a belt. J Phys Ther Sci, 2016, 28: 2123–2127. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Arnold CM, Warkentin KD, Chilibeck PD, et al. : The reliability and validity of handheld dynamometry for the measurement of lower-extremity muscle strength in older adults. J Strength Cond Res, 2010, 24: 815–824. [DOI] [PubMed] [Google Scholar]
  • 17.Stark T, Walker B, Phillips JK, et al. : Hand-held dynamometry correlation with the gold standard isokinetic dynamometry: a systematic review. PM R, 2011, 3: 472–479. [DOI] [PubMed] [Google Scholar]
  • 18.Mentiplay BF, Perraton LG, Bower KJ, et al. : Assessment of lower limb muscle strength and power using hand-held and fixed dynamometry: a reliability and validity study. PLoS One, 2015, 10: e0140822. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Thorborg K, Petersen J, Magnusson SP, et al. : Clinical assessment of hip strength using a hand-held dynamometer is reliable. Scand J Med Sci Sports, 2010, 20: 493–501. [DOI] [PubMed] [Google Scholar]
  • 20.Lehecka BJ, Morris T, Nance T, et al. : The reliability and validity of a luggage scale for gluteal strength measurements. 2018. .
  • 21.Massey PA, Montgomery C, Paulos J, et al. : Are luggage scales a viable alternative to hand-held dynamometers for the measurement of shoulder scaption strength? JSES Int, 2023, 8: 212–216. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Romero-Franco N, Jiménez-Reyes P, Montaño-Munuera JA: Validity and reliability of a low-cost digital dynamometer for measuring isometric strength of lower limb. J Sports Sci, 2017, 35: 2179–2184. [DOI] [PubMed] [Google Scholar]
  • 23.Vaz GF, Freire FF, Gonçalves HM, et al. : Intra- and inter-rater reliability, agreement, and minimal detectable change of the handheld dynamometer in individuals with symptomatic hip osteoarthritis. PLoS One, 2023, 18: e0278086. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Florencio LL, Martins J, da Silva MR, et al. : Knee and hip strength measurements obtained by a hand-held dynamometer stabilized by a belt and an examiner demonstrate parallel reliability but not agreement. Phys Ther Sport, 2019, 38: 115–122. [DOI] [PubMed] [Google Scholar]
  • 25.Sah AP: How much hip motion Is used in real-life activities? Assessment of hip flexion by a wearable sensor and implications after total hip arthroplasty. J Arthroplasty, 2022, 37: S871–S875. [DOI] [PubMed] [Google Scholar]
  • 26.Bloom N, Cornbleet SL: Hip rotator strength in healthy young adults measured in hip flexion and extension by using a hand-held dynamometer. PM R, 2014, 6: 1137–1142. [DOI] [PubMed] [Google Scholar]
  • 27.Koo TK, Li MY: A guideline of selecting and reporting intraclass correlation coefficients for reliability research. J Chiropr Med, 2016, 15: 155–163. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.de Salles BF, Simão R, Miranda F, et al. : Rest interval between sets in strength training. Sports Med, 2009, 39: 765–777. [DOI] [PubMed] [Google Scholar]

Articles from Journal of Physical Therapy Science are provided here courtesy of Society of Physical Therapy Science

RESOURCES