Intrarater Test-Retest Reliability of Hip Abduction, Internal Rotation, and External Rotation Strength Measurements in a Healthy Cohort Using a Handheld Dynamometer and a Portable Stabilization Device: A Pilot Study.

OBJECTIVE: To investigate the within-day and between-day test-retest reliability of hip abduction, internal rotation, and external rotation strength measurements taken using a portable device externally stabilizing a handheld dynamometer in healthy participants.
DESIGN: Observational study.
SETTING: Institute of Technology Carlow, Ireland, a third-level education institute. PARTICIPANTS: Healthy participants (N=18; 11 male, 7 female) who participate in a field sport for more than 2 hours per week were recruited via convenience sampling.
INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Hip abduction, internal rotation, and external rotation peak force during a maximal voluntary isometric contraction (N). The 3 best values recorded for each movement for each day were used to analyze within-day and between-day test-retest reliability. Intraclass correlation coefficients (ICCs), coefficients of variance, standard error of measurement, and minimal detectable change statistics were also calculated.
RESULTS: External fixation of a handheld dynamometer produced excellent test-retest reliability for within-day (ICC>0.934) and between-day (ICC>0.802) contexts.
CONCLUSIONS: Clinical measurements of hip strength can be performed reliably, efficiently, and cost effectively using the methods described. Furthermore, the use of external fixation eliminates the influence of tester strength on the handheld dynamometry measurements.

Hip strength is commonly measured in sports and musculoskeletal medicine as part of an objective assessment or as a marker for recovery. Hip strength has also been associated with injury incidence rates. Athletes who sustained a lower limb injury during a 2-season period reported significantly lower hip abduction strength (P=.02, 3% body weight) and hip external rotation (ERot) strength (P=.001, 2.7% body weight) when compared with their counterparts who did not sustain an injury. Furthermore, when expressed as a percentage of body weight, hip abduction and ERot strength of less than 35.4% and 20.3%, respectively, classified an athlete as high risk for sustaining a noncontact anterior cruciate ligament injury. Deficits in hip strength have also been associated with many other conditions, such as ankle ligament sprains, patella-femoral pain syndrome, iliotibial band syndromes, groin strains, hip pain, and low back pain.3, 4, 5, 6, 7, 8, 9 Furthermore, a recent consensus statement recommends future research to “investigate, report and improve the measurement properties of tests of…muscle strength and functional performance.”(p3)

Lateral hip musculature, namely gluteus medius, is fundamental in hip abduction while also contributing to hip ERot and internal rotation (IRot) in varying proportions depending on hip position., Gluteus medius activity is notably high during single-leg tasks,, illustrating its important contribution to lumbopelvic hip or core stability, which, along with hip strength, is a major target of many neuromuscular training programs used for injury prevention purposes.14, 15, 16, 17 Therefore, reliable clinical strength measurements for all movements to which gluteus medius can contribute to are important for rehabilitation clinicians in assessment, tracking progress post injury, or monitoring the effects of interventions carried out, such as neuromuscular training programs.

The current and most commonly used strength measurement technique is manual muscle testing, which consists of a clinician’s subjective rating of force along the Oxford Muscle Grading Scale from 0-5, with 0 being no palpable muscle contraction and 5 being normal full muscle performance. Although widespread in clinical practice over a large array of professions, its subjective nature and inability to be used to truly express small strength differences are some of its limitations.

Previous research has led to the introduction and practice of handheld dynamometry (HHD) as an alternative to manual muscle testing, providing clinicians with an objective, numerical measurement of muscle generated force., HHD has also become more common in the scientific literature with normative HHD values reported for strength testing in the literature. HHD has previously been shown to be valid and comparable with the criterion standard in strength testing and isokinetic dynamometry, without sacrificing ease of use, portability, or cost.,

HHD is not without limitations. Research dating back to 1991 highlights the importance of tester strength in the accuracy of HHD measurements, particularly upper limb tester strength and its inverse relationship with strength values recorded by HHD. These reliability discrepancies are most common in stronger movements of >120N, as may be expected across lower limb movements or in highly trained individuals in particular.,

These findings have led to the development of externally stabilized dynamometers. Examples include belt fixation to an adjacent wall or fixation through the construction of cage-like structures around a treatment plinth. Both the aforementioned studies resulted in satisfactory reliability for hip strength values (intraclass correlation coefficient [ICC], 0.76-0.95 and 0.73-0.89, respectively), but these procedures may not be as time efficient as traditional handheld measurement methods.

A much simpler solution was recently proposed by using a polyvinyl chloride (PVC) pipe-like structure that could be placed between the limb being tested and a wall. One end was designed to accommodate the MicroFET2 dynamometer, and the other end, a flat plate, was designed to aid in its stability against the wall. Excellent reliability was established with ICCs for hip abduction and ERot strength measurements (ICC=0.96 and 0.98, respectively) across 30 limbs tested in 15 participants; however, researchers omitted IRot measurement and did not investigate the between-day reliability of these methods.

The aim of this current study was to establish intratester reliability when measuring the strength of hip abduction, IRot, and ERot by the use of a simple pipe-like stabilization device coupled with a MicroFET2 dynamometer and additionally to explore the between-day reliability of these strength values. This article was formulated in accordance to the Guidelines for Reporting Reliability and Agreement Studies.

Methods

Participants

Convenience sampling was used to recruit 18 (11 male, 7 female) participants from the Institute of Technology Carlow. Sample size requirements for ICCs were predetermined with R0=0.0, R1=0.7 (as established during pilot testing), and statistical power=0.9. The calculated sample size was 13; however, to allow for potential dropouts, 18 was the target sample size. Subjects were deemed eligible if they participated in a field sport for more than 2 hours per week. Subjects were excluded if they had any incidence of injury to the lower back, hip, knee, ankle, or foot of their self-selected preferred jumping leg (leg which they were most likely jump off) in the past 6 months.

Ethical considerations

Ethical approval for the study was granted by the Ethics in Research committee of the Institute of Technology Carlow, Ireland. Following a description of the study, individuals were recruited for participation. Written informed consent and medical screening questionnaires were also collected prior to the initiation of testing procedures. There was no financial inducement offered to participants, and no participants were in a dependent relationship to either the lead researcher or research supervisors at the time of testing. Participants were also free to withdraw from participation at any time. Personal information was protected in accordance to the Institute of Technology Carlow Data Protection Policy and General Data Protection Regulation guidelines. This study was conducted as part of a PhD research program funded by the President’s Fellowship Scheme at the Institute of Technology Carlow, Ireland.

Instrumentation

A MicroFET2 dynamometera was used to obtain all strength measurements. The stabilization device was constructed using a PVC pipe, 11 cm wide, and adjoining duct pieces that were bonded together with adhesive so that 1 side contained a 100-mm diameter circular opening that accommodated the shape of the handheld dynamometer securely, and the opposing end consisted of a flat surface that would lay against the wall during testing procedures (fig 1).

Fig 1

HHDstab construction. Abbreviation: HHDstab, HHD and stabilization device.

An adjustable treatment plinth (Plinth 2000b), which was sourced from a National Health Service approved supplier, was used for all participants.

Testing procedures

All measurements were performed on the participants’ self-reported preferred jumping leg by a single tester, a certified athletic therapist. A predefined script was used to describe the tests so as not to bias efforts exerted by participants. Testing took place on 2 occasions, 3 days apart, in the physiology laboratory at Institute of Technology Carlow. Procedures as outlined hereafter, plinth height and position in proximity to the wall, apparatus used, rest periods, and time of day were replicated between both testing days. Participants were also urged to abstain from high-intensity exercise for the 24 hours preceding both testing sessions.

Peak force in newtons over a 5-second maximal voluntary isometric contraction was recorded for each movement. For each measurement, the pad of the handheld dynamometer was positioned 5 cm proximal to the malleoli with the HHD and stabilization device (HHDstab) perpendicular to the wall and supported by the tester. All trials were separated by a 30-second rest period. Four trials were recorded for each movement with the best (highest) 3 scores tabulated for analysis.,

Hip abduction measure

Hip abduction strength was recorded with the participant lying supine on the treatment plinth, positioned parallel to the adjacent wall. A belt was secured around the participant and plinth, resting on both anterior superior iliac spines to limit lateral pelvic motion during testing. The HHDstab was positioned perpendicular to wall and the target leg, contacting the leg 5 cm proximal to the lateral malleolus (fig 2). The participant was then instructed to “cross your arms over your chest and push into the pad as hard as possible” for 5 seconds.

Fig 2

Hip abduction, internal rotation, and external rotation strength testing positions.

Hip internal rotation measure

Hip IRot strength was recorded with the participant seated on the end of the treatment plinth, thigh parallel to the adjacent wall and hip in neutral rotation. A belt was secured around participant and plinth, on the superior femur, with a standardized 11-cm wide piece of PVC positioned between the knees to maintain knee position. The HHDstab was positioned between the wall and the target leg, contacting the leg 5 cm proximal to the lateral malleolus (fig 2). The participant was then asked to “keep both hands on top of the pipe, squeeze both knees together and push into the pad as hard as possible” for 5 seconds.

Hip external rotation measure

Hip ERot strength was recorded with the participant seated on the opposite end of the treatment plinth to the IRot measurement position, with thigh parallel to the adjacent wall and hip in neutral rotation. For ERot, the target leg was the leg furthest away from the wall, and the longer length PVC device was used so that the plinth could remain in situ. A belt was secured around participant and plinth, on the superior femur, with a standardized 11-cm wide length of PVC positioned between the knees to maintain knee position. The HHDstab was positioned between the wall and the target leg, contacting the leg 5 cm proximal to the medial malleolus. The nontest leg was flexed so as to lie behind the HHDstab (see fig 2). The participant was then asked to “keep both hands on top of the pipe, squeeze both knees together and push into the pad as hard as possible” for 5 seconds.

Statistical analysis:

All data was tabulated and analyzed using SPSS version 23c and Microsoft Excel 2013.d Means, standard deviations, coefficients of variance percentage (CV%), ICCs, and respective 95% CIs were calculated within SPSS with α=0.05 and 1−β=0.95. ICC (3,1) was applied in within-day analyses, with ICC (3,k) applied in between-day analyses for intrarater reliability.33, 34, 35 ICC statistics were classified within the following ranges: poor (0-0.39), fair (0.4-0.59), good (0.6-0.74), or excellent (0.75-1). The standard error of measurement (SEM) and minimal detectable change (MDC95) were calculated for both within-day and between-day reliability analyses using the following formulas,,:

, (with r being the ICC value calculated prior)

Results

Participant sex, age, preferred jumping leg, and body mass are presented in table 1.

Table 1

Participant characteristics

Characteristics	Female (n=7)		Male (n=11)		Total (n=19)
Age (y), mean ± SD	22.9±2.7		21.4±1.6		21.9±2.2
Weight (kg), mean ± SD	73.2±17.1		75.4±12.2		74.6±13.9
Preferred jumping leg	L=4	R=3	L=8	R=3	L=12	R=6

Abbreviations: L, left; R, right.

Within-day test-retest reliability statistics for strength measurements were highly reliable with all ICC values>0.934, CV%<6.2%, and the largest MDC95 value was 5.09 N, which was recorded in IRot strength.

Similar to within-day reliability, between-day reliability statistics for strength measurements were excellent, with all ICC values>0.802, CV%<14.7 %, and the MDC95 value was 13.41 N for ERot strength (table 2).

Table 2

Within-day and between-day test-retest reliability statistics

Movement	Within-Day Reliability (n=18)
	Trial 1 (N)	Trial 2 (N)	Trial 3 (N)	ICC (3,1) (95% CI)	CV (%)	SEM (N)	MDC95 (N)
Abduction	117.37±43.78	115.90±41.96	117.72±41.74	0.947 (0.887-0.978)	6.2	1.75	4.85
Internal rotation	132.24±36.77	134.69±37.01	129.50±32.95	0.934 (0.863-0.973)	5.2	1.84	5.09
External rotation	74.44±24.96	76.36±25.16	74.48±26.07	0.961 (0.917-0.984)	6.1	0.85	2.36

Abbreviations: ICC (3,1), intraclass correlation coefficient, 2-way mixed-effects, single measures; ICC (3,k), intraclass correlation coefficient, 2-way mixed-effects, average measures.

Discussion

Findings from this current study suggest that external stabilization of a handheld dynamometer provided excellent reliability of measurements of hip abduction, IRot strength, and ERot strength in both within-day and between-day conditions. The methodologies conducted in this study took approximately 8 minutes to complete, including landmarking, positioning, and 4 repetitions of each specific movement with a minimum of 30 seconds rest allotted between repetitions, demonstrating its time efficient nature, which is ideal for clinical settings.

Within-day reliability for abduction and ERot strength was excellent (ICC=0.947 and 0.961, respectively) (fig 3). The abduction and ERot reliability observed in the current study was comparable with previous research using a similar stabilization device (ICC=0.96 and 0.98, respectively). In addition, IRot strength was measured with similarly excellent reliability (ICC=0.934) as the aforementioned movements. MDC95 values for within-day reliability were also low, the largest of which was in IRot at 5.09 N. Any change in hip strength seen immediately >5.09 N, or 3.85% of maximum muscle force, would suggest a change that cannot be attributed to measurement error alone. The outlined procedures are therefore more sensitive to detect change than nonstabilized HHD measurements taken in comparable positions for abduction, ERot strength, and IRot strength (MDC95=9.4N, 12.4N, and 26.6N, respectively), even when those nonstabilized measurements were taken by an experienced tester.

Fig 3

Within-day and between-day scatter plots for abduction, internal rotation, and external rotation.

Moreover, from previous research that only examined within-day reliability for a similarly stabilized handheld dynamometer, excellent between-day reliability was observed for abduction, IRot strength, and ERot strength (ICC=0.953, 0.928, and 0.802, respectively) by comparing the averages of the 3 best scores recorded on each day. The largest MDC95 value for between-day hip strength measurement was seen in ERot at 13.4 N, or 18.3% of maximum muscle force, indicating that if on measurement by a clinician, hip strength changed by greater than this MDC95 value between days, one cannot attribute this change to measurement error alone.

Through the addition of IRot strength measurement, the protocol in this current study aims to build on previous research conducted on abduction and ERot strength measurement without sacrificing portability, cost, or time. The addition of a standard 11-cm wide pipe section keeps femoral position consistent across all tests unlike the nonuniform towel used previously. The addition of IRot measurement to the already established abduction and ERot reliability provides clinicians with an accessible method to measure hip abduction and rotational strength, which may be of particular importance to rehabilitation clinicians.

Study limitations

The findings from the current study, although encouraging, should be considered with caution. The current procedures were only carried out on a healthy, physically active cohort. These same methodologies should be investigated in populations with pathologies prior to its adaptation to clinical practice.

Also, unlike the previous studies that validated HHD measurements by comparing its measurement to isokinetic dynamometry, this HHDstab method, to the authors’ knowledge, is yet to be validated or directly compared with measurements taken with HHD without external stabilization.

Future research

Future research should focus on directly comparing HHDstab to strength measurements taken with the HHD stabilized manually by the tester. Moreover, validating HHDstab by comparing it with isokinetic dynamometry, and assessing HHDstab reliability in populations with pathologies should be performed prior to its wide-scale adaptation to clinical practice.

Conclusions

The addition of external fixation to HHD addresses a previously documented limitation of HHD. The removal of individual tester strength is possible and provides a high level of consistency in strength assessments about the hip. Hip abduction, IRot strength, and ERot strength can be reliably measured with minimal additional time or financial costs to either clinicians or patients, allowing such objective markers to guide clinical decision making in rehabilitation settings.

Suppliers

MicroFET2 dynamometer; Hoggan Scientific.

Plinth 2000 treatment plinth; Sports Physio Supplies.

SPSS version 23; IBM.

Excel 2013; Microsoft.