ABSTRACT
Background:
Muscle length is a common component of the physical therapy examination, which may include the prone knee flexion (Ely) and active knee extension (AKE) tests. Clinicians using these tests should understand the clinimetric properties.
Purpose:
To investigate the reliability and minimal detectable change (MDC95) of the Ely and AKE tests.
Study Design:
Reliability analysis.
Methods:
Seventy-one asymptomatic adults (mean age 24.6 + /- 2.8 years) were recruited based on a convenience sample. Two examiners each performed the Ely and AKE test one time each in an intrasession design for the interrater reliability component, with one examiner repeating the tests one time 48 hours later to determine the intra-rater reliability. Results were recorded based on one trial per test and utilized a pelvic strap for the Ely test and an adjustable bolster for the AKE test. A separate researcher recorded measurements and results were blinded from the examiners.
Results:
The Ely test had excellent intra-rater and inter-rater reliability with an intraclass correlation coefficient (ICC) (3,1) of 0.900 and ICC (2,1) of 0.914 respectively. The intra-rater and inter-rater reliability of the AKE test was good with ICC (3,1) of 0.882 and ICC (2,1) 0.886 respectively. The MDC95 indicated that a change greater than or equal to 8° and 12° is required to exceed the threshold of error for the Ely and AKE test respectively.
Conclusion:
The Ely and AKE tests have good to excellent inter-rater and intra-rater reliability for measuring rectus femoris and hamstring muscle length when stabilization of the pelvis and hip is accounted for. The MDC should be considered as a threshold for true change in the asymptomatic adult population.
Levels of Evidence:
2b
Keywords: Active Knee Extension, Ely test, hamstring muscle tightness, lower extremity muscle length, Prone knee flexion test, rectus femoris
INTRODUCTION
Impaired muscle length or tightness is commonly observed in patients with lower extremity and lumbar injuries.1,2 Numerous variables have been implicated in the etiology of movement impairments with muscle tightness being a key factor. Muscular tightness may interfere with articular biomechanics and muscular function, potentially affecting athletic performance and predisposing an athlete to injury.3 Muscle length testing allows clinicians the ability to identify impairments that may assist in determining the most appropriate interventions and training programs. Moreover, muscle length is an outcome that is addressed among asymptomatic individuals participating in exercise and sports.4 Thus, in order to ensure accurate testing and appropriate interventions, it is essential to have reliable measurement methods and an understanding of the threshold for error within a measurement.
Tightness of the hamstrings and rectus femoris has been implicated as a risk factor for both lower extremity musculoskeletal injuries and mechanical low back pain.1,5 Researchers have linked decreased flexibility of both muscle groups with isolated muscle injury,5 patellofemoral pain syndrome,6,7 patellar tendonitis,2 and low back pain in both adults and adolescents.8 Radwan et al 1 found a direct relationship between hamstring tightness when measured via the AKE Test and self-reported severity of mechanical low back pain, especially when significant asymmetries were noted in hamstring excursion between limbs. However, a recent systematic review with meta-analysis by Hori, Hasegawa, and Tagasaki suggested that the low validity of the methods to assess flexibility may be problematic and negatively affect the strength of the research conclusions.9
Decreased hamstring excursion has also been associated with higher incidence of musculoskeletal injuries in Special Operations Forces trainees.10 Knapik et al 11 found that male military recruits with hamstring tightness were twice as likely to sustain an injury as compared to those with average flexibility; however, flexibility was not a risk factor for female recruits. Furthermore, evidence on Osgood Schlatter Disease suggests an increased incidence of the disorder in adolescent male soccer players who demonstrate rectus femoris tightness. 12,13 These findings suggest that assessment of bilateral hamstring and quadriceps extensibility is a key component of a physical examination and may provide insight into an individual's risk for a musculoskeletal lower extremity or lower back pathology.
One of the most common tests performed to assess rectus femoris tightness, the Ely Test, involves measuring the angle at the knee when the patient maximally flexes the knee actively or the therapist passively flexes the knee with the patient in a prone position.14 Peeler and Anderson implemented an objective assessment to the active Ely test by adding a goniometric measurement. This method displayed moderate reliability with an intra-rater reliability and inter-rater reliability ranging from of 0.50-0.83 (54 participants:37 males and 17 females).15 The mean score for all participants was an end range of 124° + /- 7 of knee flexion.15 One should note that the Ely test can also be performed with similar or slightly modified procedures to detect sacroiliac joint dysfunction (Yeoman test), femoral neural tension (Nachlas test) and rectus spasticity (Duncan-Ely test).16,17 The investigators of this study hypothesize that the reliability of the Ely test can be further improved by stabilizing the pelvis with a mobilization belt to further eliminate compensatory anterior pelvic tilt. This manuscript will primarily focus on the clinimetric properties of the Ely muscle length test using this methodology and a sample that more equitably represents the general population.
The reliability of the AKE test, for hamstring tightness, has been reported with intra-rater reliability between 0.78-0.94 and inter-rater reliability as high as 0.98-0.99.18-20 However, further research is warranted due to sample homogeneity, as the majority of the previous research was performed on males (55 of 61 total participants).18-20 Conner et al 20 reported the highest inter and intra-rater reliability for the AKE when using an inclinometer just distal to the tibial tuberosity to obtain an objective measurement. In their study, the untested lower extremity was supported in approximately 20° of knee flexion. The tested extremity was maintained in 90° of hip flexion supported by the participant's upper extremity. A vertical sidebar was placed adjacent to the table to give the participant a visual reference to ensure vertical alignment of the femur.20 While this is the most reliable hamstring muscle length test to date, this study had a small sample size (15 male athletes). Researchers have also suggested that the reliability of this test could also be improved with the addition of an external force to ensure maintenance of hip position throughout testing.20
The examination of muscle length may be objectively documented utilizing instruments such as a goniometer or inclinometer during the assessment of active and passive range of motion while in specific positions thought to capture a muscles end-range length. Both the goniometer and inclinometer have been widely employed clinically due to their relatively low cost and portability. However, both of these instruments require the clinician to use both hands which could make joint stabilization and accurate measurement difficult. With this in mind, this study hypothesized that the utilization of a mobilization belt and an adjustable bolster would reduce measurement error and ensure reproducibility of testing for both the Ely and AKE tests. To date there is no current research analyzing this potential modification of procedures. Thus, the purpose of this study was to investigate the inter-rater and intra-rater reliability and minimal detectable change for the Ely and AKE Tests.
METHODS
Participants
A convenience sample of seventy-one asymptomatic adult (142-knees) participants, 40 women and 31 men, were recruited for this investigation from a local University setting. All participants completed a questionnaire prior to testing including their age, height, and body mass. Height and body mass were used to determine body mass index (BMI). The mean and standard deviation (SD) for the participants’ age, height, weight, and BMI were 24.6 (2.8) years, 67 (4.1) inches, 155.4 (33.3) lbs., and 24.1 (3.1) kg/m2; respectively. Inclusion criteria included males and females, ages 18-50, with absence of lower extremity pathology at the time of testing, and ability to read the English language as required to provide informed consent. Exclusion criteria consisted of participants with lower extremity pain within 48 hours of testing or participants who were unable to tolerate testing positions. Those who were selected for testing completed an informed consent document approved by the Institutional Review Board at Nova Southeastern University.
Instruments
An adjustable Metron® Elite Aster 3-Section Table (Performance Health; Chicago, IL) was used for all testing. A standard BASELINE® 12-inch plastic goniometer (model 12-1000, Fabrication Enterprises; White Plains, NY) was used to obtain the measurement of knee flexion for the Ely Test and hip flexion for the AKE test. A Baseline® Bubble inclinometer (model 12-1056, Fabrication Enterprises; White Plains, NY) was used to determine the final position of knee extension for the AKE test. To ensure reproducibility of testing a mobilization belt, vertical sidebar, adjustable leg rest, and 6-inch foam roller were used.
PROCEDURES
Preparation
Prior to testing, and after formal consent, each participant was required to complete a questionnaire to ensure that inclusion criteria were met. Following completion of an IRB approved consent form, participants underwent landmark marking by one researcher. With participants standing in anatomic position, Rater C used palpation to distinguish bony landmarks of the greater trochanter, tibial tuberosity, and lateral malleolus bilaterally, marking each with the body marker to ensure accuracy of goniometer and inclinometer placement throughout testing. Rater A flipped a coin to determine which lower extremity would be tested first (heads = right lower extremity, tails = left lower extremity). In the initial session, participants underwent two trials of each test with the Ely test performed first followed by the AKE test. The first trial was performed by Rater A, and the second trial by Rater B. Each subject attended a follow up session 48 hours later. At the follow up session, Rater B reassessed each muscle length test to obtain data for assessment of intra-rater reliability one time. All measurements were obtained by Rater C with blinding of the results to the other researchers. Testing at the follow up session was conducted in accordance with the initial coin toss for lower extremity sequence, as well as maintaining the order of Ely test followed by AKE to ensure consistency and minimize the effects of confounding variables on the results.
All testing and measurement were performed by three third-year doctoral physical therapy students under the supervision of the primary author. The raters received additional training through video and in-person instruction.
Ely Test
Each participant was placed in the prone position where their pelvis was strapped down to the table by the mobilization belt to eliminate compensatory anterior pelvic tilt. They were instructed to actively flex the tested knee maximally, bringing their heel toward the ipsilateral buttock, with monitoring of the ASIS throughout testing by the researcher to ensure contact with the plinth. The end position was determined by the subject's inability to further flex their tested knee without any compensation. If a compensation was observed, such as hip hiking or elevation of the pelvis from the plinth, the participant was instructed to return to the starting position and repeat the test. The examiner obtained the measurement of knee flexion in this final position using the 12″ goniometer (Figure 1). The goniometer was aligned with the participant such that the axis was placed over the fibular head, the stationary arm was aligned with the greater trochanter, and the movement arm was aligned with the lateral malleolus. A separate researcher recorded the measurement.
Figure 1.
Measurement position for the Ely test
Active Knee Extension Test
The subject was instructed to lie in the supine position on the plinth with the tested leg placed over an adjustable leg rest. The contralateral leg placed on the plinth with a 6” foam roller beneath the popliteal fossa to maintain approximately 20° of knee flexion. The leg rest was adjusted to each participant to support the tested leg in 90° hip and knee flexion. The femur was then aligned with the vertical sidebar to ensure maintenance of 90° hip flexion throughout the duration of the test. A third researcher calibrated the inclinometer to zero against a vertical wall prior to each measurement to ensure consistency. The participant was then instructed to plantar flex their ankle to a comfortable position and to keep their head on the table throughout the test. The researchers monitored the participant position throughout the procedure to ensure no compensations occurred that might affect the results. A third researcher stabilized the adjustable leg rest during testing to avoid hip extension past 90° of flexion. The participant was then instructed to maximally extend the knee of the tested lower extremity while maintaining 90° hip flexion and a plantar flexed ankle. The angle of measurement was taken from the final position of knee extension with the inclinometer placed on the tibial tuberosity (Figure 2). The participant was asked to repeat the test in the presence of the following: alteration of hip position, ankle dorsiflexion, cervical spine flexion, or inability of the participant to hold the terminal position long enough for the researcher to obtain the final measurement.
Figure 2.
Measurement position the Active Knee Extension test
Statistical Methods
Data were recorded on a data collection sheet and transposed into Microsoft Excel and then imported into SPSS (SPSS Inc, Chicago, Illinois) Version 25.0 for analysis. Data was analyzed with SPSS software to determine standard deviation (SD), intraclass correlation coefficients (ICC), and mean knee angle. The inter-rater reliability of all tests was determined using the ICC Model 2,1 and the intra-rater reliability using the ICC Model 3,1. The standard error of measurement (SEM) and minimal detectable change (MDC95) were computed to determine the error threshold for both the intrarater reliability and interrater reliability with the following equations SEM = and MDC95 = 1.96 respectively. The standard error of measurement (SEM) was calculated to determine the precision of the mean values. The MDC95 was calculated to determine the magnitude of change needed to exceed the threshold of error at 95% confidence level. Numbers were rounded to the nearest degree to coincide with reporting values used for goniometry and inclinometer. All calculated values for inter-rater reliability and intra-rater reliability are presented in Tables 1 and 2 respectively.
Table 1.
Inter-rater reliability of the Ely and Active Knee Extension (AKE) Tests
| Rater A Mean Angle (SD) | Rater B Mean Angle (SD) | ICC (2,1) (95% CI) | SEM (˚) | MDC95(˚) | |
|---|---|---|---|---|---|
| Ely Test | 125.31 (8.2) | 126.68 (9.1) | 0.914 (0.882-0.938) | 3 | 7 |
| AKE Test | 23.35 (13.5) | 22.52 (12.4) | 0.886 (0.844-0.917) | 4 | 12 |
ICC – Intraclass Coefficient (2,1); CI – Confidence Interval; SD – Standard Deviation; SEM – Standard Error of Measurement; MDC95 – Minimal Detectable Change at the 95% confidence level; AKE – active knee extension
Table 2.
Intra-rater reliability of Ely and AKE Tests (Rater B)
| Session 1 Mean Angle (SD) | Session 2 Mean Angle (SD) | ICC (3,1) (95% CI) | SEM (˚) | MDC95(˚) | |
|---|---|---|---|---|---|
| Ely Test | 126.68 (9.1) | 125.85 (8.7) | 0.900 (0.864-0.927) | 3 | 8 |
| AKE Test | 22.52 (12.4) | 22.10 (13.6) | 0.882 (0.839-0.914) | 4 | 12 |
ICC – Intraclass Coefficient (3,1); CI – Confidence Interval; SD – Standard Deviation; SEM – Standard Error of Measurement; MDC95 – Minimal Detectable Change at the 95% confidence level; AKE – Active knee extension
RESULTS
The results of this study suggest that the Ely and AKE tests have good to excellent reliability as measurement tools corresponding to the length of the rectus femoris and hamstrings respectively. The Ely test had excellent inter-rater (0.914) and intra-rater (0.900) reliability. The Ely test SEM was 3°, which equated to about 2.5% of the average measurement. This indicates that when using this measurement tool, one can be confident that the angle obtained is within 2.5% of the participants’ true result. Additionally, with a minimal detectable change (MDC95) of 7-8°, one can be 95% confident that with an 8° increase or decrease in knee flexion over time, a true change in muscle length has been detected.
The AKE test demonstrated good inter-rater (0.886) and intra-rater (0.882) reliability. Mean measurements for all trials was 23° in the tested population which is comparable to previously established criteria of 20° of knee flexion remaining at the terminal position for normal adults.14 The standard error of measure was 4°, indicating that when using this measure one can be confident the angle obtained is within 4° of the subject's true result. With an MDC95 of 12 degrees, one can be 95% confident that with a 12° increase or decrease in knee extension over time, a true change in muscle length has been detected.
DISCUSSION
The findings of the current study demonstrate excellent inter-rater and intra-rater reliability for the Ely test, which suggests a high potential for achieving consistent results of rectus femoris length when tested by multiple clinicians or between multiple sessions. When assessing hamstring length, this study found the AKE test to have good inter-rater and intra-rater reliability. This suggests the ability for multiple therapists or a single therapist on multiple occasions to have a high potential for achieving consistent results when utilizing the AKE test for hamstring muscle length assessment. The authors of this study conclude both the Ely and AKE tests to be clinically appropriate and favorable due to their good to excellent reliability, efficiency of testing, and the requirement of minimal equipment, which can be found in a conventional rehabilitation setting.
As previously stated, the AKE test had good inter-rater and intra-rater reliability; however, the authors of this study considered potential explanations for decreased reliability of the assessment relative to the excellent reliability of the Ely test. The decrease in reliability of the AKE compared to the Ely test may be in part due to an increased room for error and compensation by the participant. For example, in the testing procedure for the Ely test when compared to the AKE, The Ely test utilizes the surface of the mat for stability of the femur, thereby allowing for only one plane of movement and in one direction (knee flexion) and almost no potential for compensations made by the participants throughout the testing. However, the AKE test has limited stability of the femur which allows participants to attempt to forcefully extend the testing lower extremity. In doing this, participants utilize momentum or deviate from the desired angle of hip flexion, potentially self-limited range of motion during testing. The authors believe that these variables contributed to the reliability of the AKE test when tested between raters and sessions. A bolster and foam roll were used to mitigate these changes. However, the possibility of these errors is feasible and likely concurrent to what would occur in clinical practice. Although there are studies published on the reliability of the AKE test, each utilized a different procedure and required different equipment. Therefore, future research should be conducted to determine the most reliable procedure for the AKE test while minding clinician ease in regard to time, accuracy, and equipment.
The clinical implications for these findings extend well-beyond reliability as the MDC95 in particular offer clinical utility. Specifically, for the Ely test a change of 8° for the better or worse would imply change beyond the threshold of error. For the AKE a change of 12° would be needed. Thus, an improvement of hamstring flexibility would need to be 12° or greater to exceed the threshold for error. Having an understanding of error offers the clinician information useful for both change and for establishing goals.
LIMITATIONS
The primary limitation of this study was granting participants the freedom to engage in their regular sport and recreational activity throughout the duration of the study. Sport and recreational activity can contribute to variability in undocumented subjective reports such as soreness, stiffness, and flexibility increases or deficits secondary to timing of fitness regimens relative to either testing session. This extraneous factor would be mitigated in a follow-up study. A further limiting factor of this study is population bias. Further research is also needed to eliminate potential population bias as the participants of this study were recruited from a single data collection site (college university). A limitation of the study is generalization as the results from an asymptomatic cohort may not be appropriately disseminated to a symptomatic population. Nevertheless, muscle length testing is not limited to symptomatic cohorts and standardization of procedures is often performed among asymptomatic individuals prior to being implemented in a clinical setting.
CONCLUSION
The researchers of this study found the Ely and AKE tests to have good to excellent inter-rater and intra-rater reliability suggesting that the tests are valid and reliable for measuring rectus femoris and hamstring muscle lengths in the asymptomatic adult population when appropriate stabilization and fixation of the extremities is performed. This study determined the MDC95 for the Ely and AKE tests to be 7-8° and 12°, respectively, providing a quantitative value to determining true change as related to the threshold for error. Clinicians may use both muscle length test procedures for reliable quantitative data to determine rectus femoris and hamstring tightness and changes in muscle length over time.
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