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doi: 10.2522/ptj.20060143 Originally published online January 23, 2007
2007; 87:337-349.PHYS THER. Greg K Alcock and J Robert Giffin Andrea Reid, Trevor B Birmingham, Paul W Stratford, Cruciate Ligament Reconstruction Measure During Rehabilitation After Anterior Hop Testing Provides a Reliable and Valid Outcome
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Hop Testing Provides a Reliable and Valid Outcome Measure During Rehabilitation After Anterior Cruciate Ligament Reconstruction Andrea Reid, Trevor B Birmingham, Paul W Stratford, Greg K Alcock, J Robert Giffin
Background and Purpose Although various hop tests have been proposed as performance-based outcome measures following anterior cruciate ligament (ACL) reconstruction, limited reports of their measurement properties exist. The purpose of this study was to investigate the reliability and longitudinal validity of data obtained from hop tests during reha- bilitation after ACL reconstruction.
Subjects Forty-two patients, 15 to 45 years of age, who had undergone ACL reconstruction participated in the study.
Methods and Measures The study design was prospective and observational with repeated measures. The subjects performed a series of 4 hop tests on 3 separate occasions within the 16th week following surgery and on a fourth occasion 6 weeks later. The tests were a single hop for distance, a 6-m timed hop, a triple hop for distance, and crossover hops for distance. Performance on the ACL-reconstructed limb was expressed as a per- centage of the performance on the nonoperative limb, termed the “limb symmetry index.” Subjects also completed the Lower Extremity Functional Scale and a global rating of change questionnaire.
Results Intraclass correlation coefficients for limb symmetry index values ranged from .82 to .93. Standard errors of measurement were 3.04% to 5.59%. Minimal detectable changes, at the 90% confidence level, were 7.05% to 12.96%. Changes in hop test scores on the operative limb were statistically greater than changes on the non- operative limb. Pearson correlations (r) between change in hop performances and self-reported measures ranged from .26 to .58.
Discussion and Conclusion The results show that the described series of hop tests provide a reliable and valid performance-based outcome measure for patients undergoing rehabilitation follow- ing ACL reconstruction. These findings support the use and facilitate the interpreta- tion of hop tests for research and clinical practice.
A Reid, PT, MSc, was a physiotherapist at the Fowler Kennedy Sport Medicine Clinic, 3M Centre, University of West- ern Ontario, London, Ontario, Can- ada, during the completion of this project. She is currently a physiother- apist at the Allan McGavin Sports Medicine Centre, John Owen Pavilion, University of British Columbia, Van- couver, Canada.
TB Birmingham, PT, PhD, is Associate Professor and Tier 2 Canada Research Chair in Musculoskeletal Rehabilita- tion, School of Physical Therapy, El- born College, University of Western Ontario, London, Ontario, Canada N6G 1H1, and Co-Director, Wolf Or- thopaedic Biomechanics Laboratory, Fowler Kennedy Sport Medicine Clinic. Address all correspondence to Dr Birmingham at: [email protected].
PW Stratford, PT, MSc, is Professor, School of Rehabilitation Science, and Associate Member, Department of Clinical Epidemiology and Biostatis- tics, McMaster University, Hamilton, Ontario, Canada, and a Scientific Af- filiate in the Department of Surgery, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada.
GK Alcock, PT, MSc, is Physiotherapist, Fowler Kennedy Sport Medicine Clinic.
JR Giffin, MD, FRCS(C), is Assistant Professor, Department of Surgery, University of Western Ontario, and Co-Director, Wolf Orthopaedic Bio- mechanics Laboratory, Fowler Ken- nedy Sport Medicine Clinic.
[Reid A, Birmingham TB, Stratford PW, et al. Hop testing provides a reliable and valid outcome measure during rehabili- tation after anterior cruciate ligament reconstruction. Phys Ther. 2007;87: 337–349.]
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The importance of using stan- dardized outcome measures in research and clinical practice
has been described repeatedly in the orthopedic and physical therapy lit- erature. For example, various out- come measures have been suggested for use when evaluating the effec- tiveness of different interventions being compared in clinical trials1,2
and when making clinical decisions about individual patients.3–5 Post- operative rehabilitation following an- terior cruciate ligament (ACL) recon- struction is the focus of numerous research studies6 and comprises a substantial portion of orthopedic physical therapist practice.7 Accord- ingly, standardized outcome mea- sures that are appropriate for assess- ing patients undergoing physical therapy following ACL reconstruc- tion are required for comparing dif- ferent postoperative rehabilitation strategies and for evaluating individ- ual patient progress.
Standardized outcome measures can be described as measures with ac- ceptable measurement properties that have been published with spe- cific procedures for administration, scoring, and interpretation. Dissemi- nation of this type of information has indeed occurred for a variety of self- report measures (questionnaires) and continues to progress. However, research reports focused on similar information for performance-based measures of physical function have not paralleled that for self-report measures. Specifically, although in- formation about the measurement error and ability to detect change has been reported in a clinically inter- pretable way for many self-report measures, this often is not the case for performance-based measures.
Some authors8–10 have suggested that self-report and performance- based measures quantify different as- pects of function and that using one type of measure alone does not suf-
ficiently capture the breadth of health concepts associated with the measure- ment of function. Researchers8,9,11,12
investigating the relationship be- tween self-report and performance- based measures have reported Pear- son correlations (r) ranging from .02 to .59. Other authors13 have empha- sized that there are situations in which performance-based measures may be preferable and have suggested that these measures also be included in re- search and clinical practice. Owing to the increased emphasis on incorporat- ing functional and sport-specific exer- cises into current ACL postoperative rehabilitation protocols, and the goal to have patients return to dynamic and potentially injurious activities, the in- clusion of outcome measures that are performance-based may be espe- cially important when evaluating these patients.
Hop testing has frequently been pro- posed as a practical, performance- based outcome measure that reflects the integrated effect of neuromuscular control, strength (force-generating ca- pacity), and confidence in the limb and requires minimal equipment and time to administer.14–17 Based on a re- view of the potential use of hop tests as measures of dynamic knee stability, Fitzgerald et al8 suggested that hop- ping may be appropriate for use as a predictive tool for identifying patients who may have future problems as a result of knee injury or pathology and as an evaluative tool to reflect change in patient status in response to treatment.
A combination of 4 different hop tests originally described by Noyes et al18 may be particularly suitable as a performance-based outcome mea- sure for patients who are undergoing rehabilitation after ACL reconstruc- tion. The tests incorporate a variety of movement principles (ie, direc- tion change, speed, acceleration- deceleration, rebound) that mimic the demands of dynamic knee stabil-
ity during sporting activities and are suggested to prepare the patient for return to such activities.7,19–22 This series of hop tests involves a single hop for distance, a 6-m timed hop, a triple hop for distance, and cross- over hops for distance. Measure- ments are obtained on both extrem- ities so that test performance on the operative limb can be expressed as a percentage of test performance on the opposite limb, termed the “limb symmetry index.”
Based on performance on these 4 hop tests, the limb symmetry index has been used to help differentiate individuals with and without dy- namic knee stability18,23–27 and to compare different rehabilitation strategies following ACL reconstruc- tion.19 Some authors7,20,21 also have advocated the use of these hop tests when monitoring progress in indi- vidual patients who are undergoing rehabilitation following ACL recon- struction. Various clinical practice guidelines include specific scores on the limb symmetry index that must be met in order for a patient to progress through phases of rehabilitation, to re- turn to sports, or to be discharged from physical therapy.7,20,21
Bolgla and Keskula28 evaluated the rel- ative reliability of scores on the limb symmetry index based on the de- scribed series hop tests in subjects who were healthy and suggested that it is a reliable measure of lower- extremity performance (intraclass cor- relation coefficient [ICC]�.95–.96). Intraclass correlation coefficients also have been reported for individual hop tests in patients following ACL recon- struction (ICC�.76–.97 for the single hop for distance test,11,29,30 ICC�.88– .97 for the 6-m timed hop test,11,31 and ICC�.94–.98 for the crossover hops for distance test31). However, we are unaware of any previous reports pro- viding estimates of the measurement error and minimal detectable change for the series of hop tests in patients
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following ACL reconstruction, or the ability of this performance-based mea- sure to detect change during postop- erative rehabilitation.
In order to facilitate the use of the described series of hop tests as a standardized performance-based out- come measure for patients who are undergoing rehabilitation following ACL reconstruction, further informa- tion regarding its measurement prop- erties should be provided. Specifically, further information regarding the reli- ability and longitudinal construct valid- ity of data obtained from these hop tests is necessary to more accurately plan future clinical trials and to more confidently make clinical decisions about individual patients. Therefore, the objective of the present study was to investigate the reliability and longitudinal validity of data from these hop tests during rehabilitation after ACL reconstruction.
Method Study Design The study design was prospective and observational with repeated measures (Fig. 1). Subjects per- formed the 4 hop tests and then completed self-report questionnaires on 4 different test occasions. The subjects were blinded to their hop test scores. The testing procedures were identical on each test occasion and were administered by the same investigator. The initial 3 test occa- sions occurred within the 16th week following ACL reconstruction, with a minimum of 24 hours between any 2 test occasions. The first test occasion was intended to allow motor learn- ing. The second and third test occa- sions were used to evaluate test- retest reliability. The fourth and final test occasion took place 6 weeks later and was used to evaluate longi- tudinal validity.
A construct validation process was based on 2 theories of change. First, validity was evaluated based on the construct that changes in the hop performances on the operative limb should be significantly greater than changes in the hop performances on the nonoperative limb. We consid- ered this comparison of limbs within individuals to be a form of known- groups validity, although it should be recognized that known-groups valid- ity traditionally has involved compar- isons among individuals. Second, convergent validity was evaluated based on the construct that change in limb symmetry index scores should be at least moderately corre- lated to changes in scores on self- report measures.
Participants Forty-two patients between the ages of 15 and 45 years participated in this study (Tab. 1). All patients had
Figure 1. Schematic diagram of study design. Subjects attended 3 test occasions within the 16th week following anterior cruciate ligament (ACL) reconstruction and a final test occasion 6 weeks later.
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undergone primary unilateral ACL re- construction at the Fowler Kennedy Sport Medicine Clinic using a semi- tendinosus and gracilis tendon au- tograft and were following the post- operative rehabilitation protocol used at that center. All patients had a stable contralateral knee (no injury or surgical interventions in the past 2 years), had full range of motion in the operative limb when compared with the nonoperative limb (flexion within 5°), and had only trace or no effusion. Patients with concomitant meniscal injury that required repair were included in the study, provided that they were permitted to undergo typical rehabilitation after ACL re- construction involving immediate full weight-bearing gait and unre- stricted non–weight-bearing range of motion.
Patients were excluded if they had concomitant posterior cruciate liga- ment or medial collateral ligament injury requiring treatment, had any concurrent musculoskeletal condi- tion (eg, back, hip, or ankle injury) rendering them unable to hop on either extremity, had advanced de- generative changes (ie, Kellgren and
Lawrence32 grade of III or greater based on the preoperative radio- graph or noted intraoperatively), or were unable to speak, read, write, or understand English. All participants provided informed consent prior to participation.
Sample size was based on parameter estimation of the reliability coeffi- cient for overall limb symmetry in- dex, with a lower confidence inter- val (CI) width of 0.1, an expected ICC of at least .85, and a one-tailed CI set to 1 � � (��.05).33 Using these parameters, the estimated sample size required was 36 subjects. Given that the study design involved 4 re- peated test occasions over a 6-week period, we conservatively recruited 50 subjects to account for a dropout rate of up to 25%.
One hundred seventeen patients were approached as potential partic- ipants. Those who did not enter the study were injured on their non- operative side (n�5), had under- gone revision surgery (n�4), had ex- perienced a superficial wound infection (n�2), had an associated fracture (n�2), had nontypical ACL
reconstruction (n�3), were away from home either traveling or attend- ing university (n�20), were outside of a reasonable driving distance (n�23), were unwilling to partici- pate (n�6), or failed to attend the scheduled appointment (n�4). Forty-eight patients were entered into the study.
During the course of the study, 6 patients withdrew from the study for the following reasons: 1 patient moved out of the area, 1 patient was diagnosed with pneumonia, 2 pa- tients had scheduling difficulties, and 2 patients had complaints of thigh pain after 2 consecutive days of testing. Of the remaining 42 patients, 8 patients could attend only 2 of the 3 sessions completed within 1 week. Three patients did not complete the final test day (1 patient had a back injury rendering her unable to hop, 1 patient had hernia surgery, and 1 pa- tient developed a knee effusion after playing ice hockey the previous day). As a result, the final sample consisted of 42 patients who at- tended either 3 or 4 test occasions and contributed data for summary statistics. Thirty-five patients contrib-
Table 1. Patient Characteristicsa
Female Subjects Male Subjects Total
Sample size (n) 19 23 42
Age (y)a 23.1�8.2 (15–40) 27.7�9.7 (15–45) 25.6�9.2 (15–45)
Height (cm)a 165.3�6.2 (155.0–175.0) 177.2�8.4 (165.0–192.5) 171.8�9.5 (155.0–192.5)
Weight (kg)a 64.5�10.6 (47.7–81.8) 84.4�17.1 (54.5–115.9) 75.4�17.5 (47.7–115.9)
Body mass indexa 23.1�3.2 (19–29) 26.7�5.3 (19–40) 25.2�4.8 (19–40)
Operative limb (right/left) 11/8 9/14 20/22
Dominant limb (right/left) 18/1 23/0 41/1
Meniscal repair (yes/no) 12/7 8/15 20/22
Self-rated activity level
Sedentary 0 0 0
Recreationally active 12 15 27
Competitive athlete 7 8 15
a Mean � standard deviation (minimum–maximum).
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uted data to the analysis of reliability, and 39 patients contributed data to the test of longitudinal validity.
Hop Testing Procedures The series of 4 hop tests was admin- istered in accordance with the pro- tocols outlined by Noyes et al,18 Bar- ber et al,34 and Daniel et al.35 The tests were a single hop for distance, a 6-m timed hop, a triple hop for distance, and a crossover hop for dis- tance (Fig. 2). In keeping with the original description,18 the tests were administered in that order on each test occasion, followed by the ad- ministration of the self-report mea- sures. The hop testing course was constructed on low-pile, rubber- backed carpet glued over concrete floor. The course consisted of a 6-m- long � 15-cm-wide marking placed on the floor.
For each hop test, the subjects per- formed one practice trial for each limb, followed by 2 measured and recorded trials. Consistent with the original description of the 4 hop
tests, no additional warm-up activity was performed. For each set of tests, the subjects were instructed to begin with the nonoperative limb. To min- imize fatigue, a rest period was of- fered between types of hop tests (up to 2 minutes) and between indi- vidual hop test trials if needed (typ- ically less than 30 seconds was suf- ficient). Subjects started each test with the lead toe behind a clearly marked starting line. No restrictions were placed on arm movement during testing, and no instructions were provided regarding where to look. Subjects were encouraged to wear the footwear they would nor- mally wear during their rehabilita- tion sessions.
For the hops for distance (single, tri- ple, and crossover) to be deemed successful, the landing must have been maintained for 2 seconds. An unsuccessful hop was classified by any of the following: touching down of the contralateral lower extremity, touching down of either upper ex- tremity, loss of balance, or an addi-
tional hop on landing. If the hop was unsuccessful, the subject was re- minded of the requirement to main- tain the landing, and the hop was re- peated. No further instructions were provided to the subjects. Typically, 1 or 2 extra trials were required.
The single hop for distance was per- formed as outlined by Daniel et al.35
The subjects stood on the leg to be tested, hopped, and landed on the same limb. The distance hopped, measured at the level of the great toe, was measured and recorded to the nearest centimeter from a stan- dard tape measure that was perma- nently affixed to the floor. The timed 6-m hop was performed as outlined by Barber et al.34 Subjects were in- structed to perform large one-legged hops in series over the total distance. A standard stopwatch was used to record time. The stopwatch was started when a subject’s heel lifted from the starting position and was stopped the moment that the tested foot passed the finish line. Measure-
Figure 2. Diagrammatic representation of the series of 4 hop tests: single hop for distance, 6-m timed hop, triple hop for distance, and crossover hop for distance. Adapted and reprinted by permission of Sage Publications Inc from: Noyes FR, Barber SD, Mangine RE. Abnormal lower limb symmetry determined by function hop tests after anterior cruciate ligament rupture. Am J Sports Med. 1991;19: 513–518. Copyright 1991 by Sage Publications Inc.
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ments were recorded to the nearest 10th of a second.
The triple hop for distance was per- formed as outlined by Noyes et al.18
Subjects were instructed to stand on one leg and perform 3 consecutive hops as far as possible, landing on the same leg. The total distance for 3 consecutive hops was recorded. Fi- nally, the crossover hop for dis- tance18 was performed over a 15-cm strip on the floor. The subjects hopped forward 3 times while alter- nately crossing over a marking. The total distance hopped forward was recorded. Subjects were instructed to position themselves such that the first of the 3 hops was lateral with respect to the direction of crossover. The series of hop tests took approx- imately 10 minutes to administer.
Self-Report Measures The Lower Extremity Functional Scale (LEFS) is a region-specific, self- report functional status measure.36
Individuals’ scores on this 20-item questionnaire range from 0 to 80, with higher scores indicating bet- ter functional status. Previous re- search37 has determined the mea- surement properties of the LEFS, in- cluding its standard error of measurement (SEM) (3.4–3.9 LEFS points), 90% CI for a given score (�6 LEFS points), minimal detectable change at the 90% confidence level (9 LEFS points), and minimal clini- cally important difference (9 LEFS points).
On the final test occasion, subjects also completed a global rating of change questionnaire that asked them how much they had changed over the last 6 weeks (ie, since first performing the hop tests).38 This tool was used to provide an indica- tion of the subjects’ perception of the size of the change experienced. The questionnaire asks patients to indicate whether they are better, worse, or the same, and, if appropri-
ate, how much they have changed on a 15-point scale (–7 to 7) that includes descriptors ranging from “a tiny bit, almost same” to “a very great deal.”39
Data Analysis On each test occasion, all hop test scores were recorded as absolute distance (in centimeters) or time (in seconds) and were calculated as the mean of the 2 recorded trials. Also using the mean of 2 trials, the limb symmetry index was calculated such that the score on the ACL- reconstructed limb was expressed as a percentage of the score on the non- operative limb. Limb symmetry in- dex scores were calculated for each of the 4 hop tests and for the overall combination of hop tests. Although the limb symmetry index scores were the outcome measures of most interest, absolute scores on each limb also were presented to better understand the behavior of the cal- culated index scores upon repeated assessments.
Hop test scores on each of the 4 test occasions were compared using repeated-measures analyses of vari- ance (ANOVAs). Separate ANOVAs were completed for the operative and nonoperative limbs using data from all subjects. Following a signif- icant main effect, Scheffé post hoc tests were used to compare scores for each test occasion.
Reliability. Test-retest reliability was assessed using the hop values obtained from test occasions 2 and 3. Reliability first was estimated using ICC(2,1).40 The ICC is a ratio of the variance between patients to the to- tal variance, it provides an indication of how well a measure can distin- guish among patients, and it there- fore can be considered a measure of relative reliability. Reliability then was estimated using the SEM.41 The SEM provided an expression of an individual subject’s hop test mea-
surement error in the original test units (eg centimeters, seconds, per- centage), and therefore can be con- sidered an absolute measure of reli- ability. An upper one-sided 95% CI for the point estimate of the SEM was constructed using the method de- scribed by Stratford and Goldsmith.5
The point estimate of the SEM then was used to estimate the error in an individual subject’s score at a given point in time, at the 90% confidence level, by multiplying the SEM by the z value for 90% confidence (1.64).
The point estimate of the SEM also was used to calculate an estimate of the minimal detectable change at the 90% confidence level by multiplying the SEM by the square root of 2 (this accounts for measurement error at 2 testing occasions) and the z value for 90% confidence (1.64).42 We used a different level of confidence when creating CIs for point estimates (95%) than when describing the in- terpretation of an individual’s score (90%), partly to emphasize that these concepts are indeed different and be- cause we believed that clinical inter- pretations based on a single subject’s score should be interpreted more lib- erally than estimates based on our study’s sample of subjects (n�35). We felt that the 90% level repre- sented that sentiment while still be- ing quite conservative.
Longitudinal validity. Change scores were calculated as the differ- ence between scores obtained on test occasion 4 and the mean of test occasions 2 and 3 (n�35). For the subjects without occasion 3 data, the values for test occasion 2 were used (n�4). For known-groups validity, we compared change scores on the absolute hop scores between limbs on each of the 4 hop tests using paired t tests. For convergent valid- ity, we evaluated the correlation be- tween change in limb symmetry in- dex scores and: (1) change in the LEFS and (2) the global rating of
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change. We calculated Pearson cor- relation coefficients (r) and lower one-sided 95% CI. Given that previ- ously reported correlations between performance-based and self-report measures have typically ranged from approximately 0 to 0.6,8,9,11,12 we decided on the following criteria for strength of evidence for longitud- inal validity: good, r�.5; moderate,
r �.36–.5; low, r �.2–.35; and no ev- idence, r�.2.
Results Summary statistics for hop test and LEFS scores on all test occasions are presented in Table 2 for the entire sample and in Tables 3 and 4 for female and male subjects, respec- tively. For all of the absolute hop test
scores on both the operative and nonoperative limbs (Tab. 2), the ANOVAs indicated a significant main effect for time (P �.001). For all tests completed on the operative limb, post hoc comparisons indicated that absolute hop scores on the first test occasion were significantly different from those on the second test occa- sion (P �.01). There was no signifi-
Table 2. Mean � Standard Deviation (Minimum–Maximum) for All Subjects for Hop Test Absolute Scores on the Operative and Nonoperative Limbs, the Limb Symmetry Index (Operative Limb Expressed as a Percentage of Nonoperative Limb), and the Lower Extremity Functional Scale Scores on 4 Separate Test Occasions
Test Day 1 (16 wk Postoperatively)
Day 2 (�24–48 hr) Day 3 (�24–48 hr) Day 4 (22 wk Postoperatively)
n 42 42 35 39
Single hop
Operative limb (cm)
112.0�32.5 (39.0–179.5) 127.4�32.3 (41.5–187.5) 128.9�32.4 (61.5–192.5) 141.4�28.1 (74.0–187.5)
Nonoperative limb (cm)
135.3�31.2 (71.5–204.0) 154.4�30.0 (77.0–213.5) 158.4�28.3 (92.5–215.0) 160.0�26.0 (100.5–212.0)
Limb symmetry index (%)
82.9�15.4 (33.8–110.1) 82.2�12.3 (47.2–103.2) 81.0�12.1 (51.6–103.7) 88.2�9.5 (63.8–103.2)
6-m timed hop
Operative limb (s) 3.4�2.1 (1.7–12.8) 2.9�1.2 (1.8–7.7) 2.9�1.2 (1.7–6.4) 2.6�0.8 (1.6–5.9)
Nonoperative limb (s)
2.5�0.71 (1.6–5.1) 2.3�0.5 (1.5–3.5) 2.3�0.6 (1.5–3.8) 2.3�0.5 (1.5–3.9)
Limb symmetry index (%)
81.7�16.3 (33.8–109.5) 81.8�13.4 (45.4–102.8) 83.2�12.7 (50.2–100.3) 89.6�9.5 (66.0–102.1)
Triple hop
Operative limb (cm)
344.8�91.4 (124.0–532.5) 363.5�89.0 (159.0–570.0) 371.7�96.5 (173.0–553.5) 393.2�88.9 (193.5–618.0)
Nonoperative limb (cm)
416.1�84.1 (247.0–576.5) 440.1�81.4 (271.5–606.5) 452.3�91.9 (249.0–633.5) 450.6�99.4 (239.0–666.5)
Limb symmetry index (%)
82.6�13.3 (45.1–99.6) 82.4�11.7 (48.4–99.7) 82.1�13.2 (54.4–102.7) 87.7�10.2 (68.0–102.3)
Crossover hop
Operative limb (cm)
303.3�90.7 (68.5–514.0) 328.0�92.3 (128.5–552.5) 330.9�98.7 (136.0–544.5) 358.6�89.3 (152.0–589.0)
Nonoperative limb (cm)
362.6�93.2 (140.0–534.0) 387.3�84.8 (204.5–602.0) 399.1�89.5 (220.5–604.5) 405.6�89.8 (194.0–618.5)
Limb symmetry index (%)
83.1�13.0 (48.9–106.1) 84.4�14.1 (46.0–112.5) 82.2�13.3 (47.5–103.4) 88.3�9.6 (68.2–105.7)
Overall combination of hops: limb symmetry index (%)
82.6�13.0 (41.8–99.6) 82.7�11.9 (47.3–100.8) 82.1�11.6 (55.4–102.1) 88.5�8.5 (70.0–101.7)
Lower Extremity Functional Scale
66.0�9.9 (24–79) 66.0�9.1 (28–79) 65.5�8.9 (26–78) 69.3�8.3 (30–80)
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cant difference in absolute scores completed on the second and third test occasions (P�.89). With the ex- ception of the timed hop (P�.17), there was a significant difference be- tween absolute scores obtained on the second and fourth test occasions (P �.001).
For all tests completed on the non- operative limb, post hoc compari- sons indicated that absolute hop scores on the first test occasion were significantly different from those on
the second test occasion (P �.05). There was no significant difference in absolute scores completed on the second and third test occasions (P�.1). Unlike the operative limb, there were no significant differences between absolute scores obtained on the second and fourth test occa- sions (P�.1), with the exception of the crossover hop (P�.035).
When scores were expressed as a percentage of the nonoperative limb (ie, limb symmetry index scores,
Tab. 2), the ANOVAs also indicated a significant main effect for time (P �.001) for each of the hop tests and for the combination of tests (overall limb symmetry index). For all tests, post hoc comparisons indi- cated that the limb symmetry index on the final test occasion was signif- icantly different from those on all other test occasions (P �.005), but there were no significant differences among the first, second, and third test occasions (P�.40).
Table 3. Mean � Standard Deviation (Minimum–Maximum) for Female Subjects for Hop Test Absolute Scores on the Operative and Nonoperative Limbs, the Limb Symmetry Index (Operative Limb Expressed as a Percentage of Nonoperative Limb), and the Lower Extremity Functional Scale Scores on 4 Separate Test Occasions
Test Day 1 (16 wk Postoperatively)
Day 2 (�24–48 hr) Day 3 (�24–48 hr) Day 4 (22 wk Postoperatively)
n 19 19 18 18
Single hop
Operative limb (cm) 105.9�26.2 (39.0–139.0) 116.4�29.5 (41.5–154.5) 121.6�28.4 (61.5–164.0) 133.2�25.9 (74.0–170.5)
Nonoperative limb (cm)
129.8�23.0 (78.5–166.0) 141.6�29.1 (77.0–188.0) 146.4�24.8 (92.5–182.0) 151.6�25.0 (100.5–188.0)
Limb symmetry index (%)
81.4�13.8 (46.4–98.7) 82.2�13.9 (47.1–103.2) 82.8�12.5 (53.7–103.7) 88.0�10.4 (63.8–103.2)
6-m timed hop
Operative limb (s) 3.7�2.4 (2.1–12.8) 3.2�1.3 (1.9–7.7) 3.0�1.1 (2.0–6.4) 2.8�0.9 (1.7–5.9)
Nonoperative limb (s) 2.7�0.7 (1.8–5.1) 2.4�0.5 (1.8–3.5) 2.5�0.6 (1.7–3.8) 2.5�0.6 (1.7–3.9)
Limb symmetry index (%)
79.9�16.2 (39.8–109.5) 81.1�14.7 (45.5–100.0) 84.4�11.2 (59.4–99.8) 89.8�10.1 (66.0–102.1)
Triple hop
Operative limb (cm) 307.7�76.2 (124.0–411.5) 329.8�82.6 (159.0–488.0) 343.7�87.7 (173.0–489.0) 362.2�82.1 (193.5–493.0)
Nonoperative limb (cm)
388.6�74.9 (247.0–538.0) 408.1�68.2 (271.5–518.5) 411.0�79.4 (249.0–559.0) 412.3�88.2 (239.0–552.0)
Limb symmetry index (%)
79.0�13.2 (49.2–94.5) 80.4�12.6 (48.4–94.1) 83.6�13.9 (54.4–102.7) 88.2�10.4 (69.6–102.3)
Crossover hop
Operative limb (cm) 265.7�81.3 (68.5–378.5) 301.4�85.3 (128.5–416.5) 305.1�87.7 (136.0–431.5) 336.9�87.9 (152.0–479.5)
Nonoperative limb (cm)
328.7�82.3 (140.0–469.5) 360.9�67.6 (237.0–461.0) 362.0�75.7 (220.5–472.0) 376.1�83.2 (194.0–500.0)
Limb symmetry index (%)
79.8�13.6 (48.9–97.7) 82.7�15.6 (46.0–99.4) 83.4�14.1 (47.5–103.4) 89.1�9.7 (68.2–105.7)
Overall combination of hops: limb symmetry index (%)
80.0�12.8 (46.1–99.6) 81.6�13.5 (47.3–99.0) 83.5�12.1 (55.9–102.1) 88.7�9.3 (70.0–101.7)
Lower Extremity Functional Scale
64.2�8.0 (45–76) 64.6�6.9 (53–76) 66.0�5.9 (55–77) 68.8�5.1 (61–78)
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In general, comparison of hop scores over the 4 test occasions indicated that substantial motor learning took place on both the operative and non- operative limbs between the first and second test occasions, which then leveled off by the third test oc- casion. The significant increases in hop scores on the fourth test occa- sion on the operative limb, but not on the nonoperative limb, suggested that hop performance improved over the 6-week period.
Reliability Reliability statistics for the hop test limb symmetry index scores are pre- sented in Table 5. The ICCs ranged from .82 to .93 and can be described as indicating excellent relative reli- ability.43 The single hop test and overall limb symmetry index scores demonstrated the highest relative re- liability. The SEM was lowest for the single hop test and overall limb sym- metry index scores, suggesting that these measures also demonstrated
the highest absolute reliability. The error in an individual’s limb symme- try index scores at one point in time and the minimal detectable changes upon reassessment, both at the 90% confidence level, also are presented in Table 5. An example of their in- terpretation is provided in the “Dis- cussion” section.
Longitudinal Validity Limb symmetry index change scores were 6.5% (95% CI�4.5–8.5) for the
Table 4. Mean � Standard Deviation (Minimum–Maximum) for Male Subjects for Hop Test Absolute Scores on the Operative and Nonoperative Limbs, the Limb Symmetry Index (Operative Limb Expressed as a Percentage of Nonoperative Limb), and the Lower Extremity Functional Scale Scores on 4 Separate Test Occasions
Test Day 1 (16 wk Postoperatively)
Day 2 (�24–48 hr) Day 3 (�24–48 hr) Day 4 (22 wk Postoperatively)
n 23 23 17 21
Single hop
Operative limb (cm) 117.0�36.8 (44.0–179.5) 136.4�32.4 (70.0–187.5) 136.7�35.4 (70.5–192.5) 148.5�28.5 (96.5–187.5)
Nonoperative limb (cm)
139.8�35.9 (71.5–204.0) 165.1�26.9 (115.5–213.5) 171.1�26.9 (123.0–215.0) 167.3�25.3 (122.0–212.0)
Limb symmetry index (%)
84.1�16.8 (33.8–110.1) 82.1�11.0 (50.5–99.7) 79.1�11.8 (51.6–92.9) 88.5�8.8 (71.5–102.7)
6-m timed hop
Operative limb (s) 3.1�1.9 (1.7–9.1) 2.7�1.1 (1.8–6.4) 2.7�1.3 (1.7–6.0) 2.4�0.6 (1.6–4.0)
Nonoperative limb (s) 2.3�0.6 (1.6–4.5) 2.2�0.4 (1.5–3.5) 2.1�0.5 (1.5–3.1) 2.1�0.4 (1.5–2.9)
Limb symmetry index (%)
83.1�16.7 (33.8–99.6) 82.4�12.5 (47.5–102.8) 81.8�14.4 (50.2–100.3) 89.5�9.2 (70.4–100.7)
Triple hop
Operative limb (cm) 375.4�93.1 (183.0–532.5) 391.3�86.0 (255.0–570.0) 401.3�99.0 (231.5–553.5) 419.8�87.7 (279.0–618.0)
Nonoperative limb (cm)
438.8�86.1 (265.5–576.5) 466.5�83.2 (317.5–606.5) 496.0�85.4 (302.5–633.5) 483.4�98.6 (310.5–666.5)
Limb symmetry index (%)
85.6�12.9 (45.1–99.6) 84.0�11.0 (55.5–99.7) 80.6�12.7 (57.2–96.2) 87.4�10.2 (68.0–101.3)
Crossover hop
Operative limb (cm) 334.3�87.8 (157.0–514.0) 349.9�94.0 (216.5–552.5) 358.2�104.9 (206.5–544.5) 377.2�88.3 (238.0–589.0)
Nonoperative limb (cm)
390.6�91.1 (195.5–534.0) 409.0�92.5 (204.5–602.0) 438.3�88.1 (240.0–604.5) 431.0�89.4 (240.5–618.5)
Limb symmetry index (%)
85.8�12.1 (54.2–106.1) 85.8�12.9 (58.2–112.5) 80.9�12.6 (48.4–93.2) 87.7�9.7 (69.2–99.0)
Overall combination of hops: limb symmetry index (%)
84.7�13.1 (41.8–98.9) 83.6�10.6 (52.9–100.8) 80.6�11.3 (55.4–92.1) 88.2�7.9 (72.1–98.1)
Lower Extremity Functional Scale
67.4�11.2 (24–79) 67.1�10.6 (28–79) 64.9�11.4 (26–78) 69.6�10.4 (30–80)
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single hop test, 7.9% (95% CI�5.3– 10.5) for the 6-m timed hop test, 5.3% (95% CI�2.8–7.8) for the triple hop test, 4.8% (95% CI�2.2–7.4) for the crossover hop test, and 6.1% (95% CI�4.2–8.0) for the overall combination of hop tests. The changes in absolute scores for hop tests on the operative limb were sta- tistically greater than the changes on the nonoperative limb for the single hop test (paired t38�6.4, P �.001), the 6-m timed hop test (paired t38�4.5, P �.001), the triple hop test (paired t38�3.3, P�.002), and the crossover hop test (paired t38�3.1, P�.004). Correlations among hop test change scores, the global rating of change, and LEFS change scores are reported in Table 6. Correlations (r) between performance-based and self-report measures ranged from .26
to .58. The global rating of change was most highly correlated to the overall limb symmetry index.
Discussion This study provides comparative hop scores in both absolute and limb symmetry index values for male and female subjects at the time during postoperative rehabilitation where training dynamic knee stability is em- phasized (Tabs. 2, 3, and 4). Al- though we are unaware of previ- ously published data describing the entire series of hop tests in patients undergoing rehabilitation after ACL reconstruction, the present values are similar to those previously re- ported for individual hop tests eval- uated in these types of pa- tients.11,17,29–31 In general, comparison of hop scores over the 4 test occasions
indicated that substantial motor learn- ing took place on both the operative and nonoperative limbs from the first to second test occasions, which tended to level off by the third test occasion. There were substantial in- creases in hop scores on the fourth test occasion on the operative limb, but not on the nonoperative limb, sug- gesting that the functional status of the operative limb improved over the 6-week period.
Limb symmetry index values provide important measures of performance on the operative limb in relation to the nonoperative limb. The fact that limb symmetry index values were relatively stable over the first 3 test occasions (ie, the limb symmetry in- dex accounted for learning that oc- curred in both limbs) and were sim- ilar for male and female subjects also supports their use. However, exam- ining absolute scores also is impor- tant. For example, although limb symmetry index values were similar for test occasions 1 and 2, the abso- lute scores were very different. Ex- amining limb symmetry index in iso- lation would mask this change in performance.
The ICCs observed in the present study for limb symmetry index scores suggest excellent relative reli- ability43 and indicate that these tests are appropriate for distinguishing
Table 5. Reliability of Hop Test Limb Symmetry Index Scores (n�35): Intraclass Correlation Coefficients (ICC) With Lower One-Sided 95% Confidence Intervals (CI) in Parentheses; Standard Errors of Measurement (SEM) With Upper One-Sided 95% CIs in Parentheses; and Corresponding Estimates of the Error in an Individual’s Score at One Point in Time and Minimal Detectable Change, Both Estimated Using the z Value for 90% Confidence (1.64)
Limb Symmetry Index ICC (Lower 95% CI) SEM (%) (Upper 95% CI)
Error in an Individual’s Score (%)
Minimal Detectable Change (%)
Single hop test .92 (0.87) �3.49 (4.37) �5.72 �8.09
6-m timed hop test .82 (0.70) �5.59 (7.01) �9.17 �12.96
Triple hop test .88 (0.80) �4.32 (5.41) �7.08 �10.02
Crossover hop test .84 (0.74) �5.28 (6.62) �8.66 �12.25
Overall combination of hop tests .93 (0.89) �3.04 (3.81) �4.99 �7.05
Table 6. Longitudinal Validity: Pearson r Values With Lower One-Sided 95% Confidence Intervals in Parentheses for Correlations Between Hop Test Limb Symmetry Index Change (Scores From Day 4 Versus the Averaged Score From Days 2 and 3), the Global Rating of Change, and Lower Extremity Functional Scale Change Scores (n�39)
Limb Symmetry Index Change
Global Rating of Change
Lower Extremity Functional Scale Change
Single hop test .48 (.24) .37 (.11)
6-m timed hop test .46 (.22) .28 (.01)
Triple hop test .44 (.20) .26 (.00)
Crossover hop test .45 (.21) .41 (.16)
Overall combination of hop tests .58 (.37) .41 (.16)
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among patients, such as is done when comparing groups of patients participating in randomized clinical trials of different postoperative pro- tocols. The relative reliability of the single hop for distance test in pa- tients 1 to 2 years following ACL re- construction has previously been re- ported.11,29,31 Intraclass correlation coefficients for the single hop for distance test reported in Table 5 were similar to those previously re- ported by Kramer et al29 (ICC�.76– .96). The ICCs for limb symmetry scores on the 6-m timed hop and crossover hop tests (Tab. 5) were slightly lower than those reported by Hopper et al31 (6-m timed hop test, ICC�.93–.96; crossover hop test, ICC�.94–.98). To our knowledge, the ICC for the triple hop for dis- tance test has not been previously reported in patients following ACL reconstruction.
We are unaware of previous reports of the SEM for hop test scores in patients following ACL reconstruc- tion. The present findings facilitate the clinical use of hop tests by pro- viding estimates of measurement er- ror and minimal detectable change (Tab. 5) that enable clinicians to de- termine how much confidence they can place in their assessment of an individual’s hop test limb symmetry index. For example, based on an in- dividual’s performance on the over- all combination of hops assessed at one point in time (Tab. 5), the limb symmetry index could vary �4.99% simply due to measurement error (ie, �SEM � z value for 90% con- fidence��3.04�1.64��4.99%). Ad- ditionally, based on the observed min- imal detectable change for the overall limb symmetry index (Tab. 5), 90% of stable patients would change by less than 7.05% on repeated mea- sures (ie, �SEM � z value for 90% confidence�√2��3.04�1.64�√2� �7.05%).
The following description provides an example of how a physical thera- pist might use these values in clinical practice. Following adequate prac- tice with hop testing, a patient 16 weeks after ACL reconstruction scores a limb symmetry index of 80% for the overall combination of hops, and the score improves to 90% fol- lowing 6 weeks of treatment. Upon initial assessment, the clinician can be 90% confident that the true limb symmetry index value could vary from 75% to 85% simply due to mea- surement error (ie, 80% � approxi- mately 5%). When tested 6 weeks later, the clinician can be confident that this patient has truly improved because the observed change of 10% (ie, an increase from 80% to 90%) exceeds the minimal detectable change of approximately 7%. Also note that the minimal detectable change could represent deteriora- tion in performance. For example, if the patient’s score dropped to 70% upon reassessment, the clinician can be confident that this patient has truly deteriorated because the ob- served change of 10% (ie, a decrease from 80% to 70%) also exceeds the minimal detectable change of ap- proximately 7%.
The present findings are consistent with our constructs for change and provide evidence of longitudinal va- lidity. When investigating known- groups validity, each of the hop tests demonstrated significantly greater changes on the operative limb than on the nonoperative limb over the 6-week period. When investigating convergent validity, the observed correlations between the change in limb symmetry index and change in both self-report measures, the single hop test, the crossover hop test, and the overall combination of hops met our criteria for at least moderate evidence of convergent validity. In- terestingly, only the correlation be- tween the change in the limb sym- metry index for the overall com-
bination of tests and the global rating of change exceeded .5 (Tab. 6). We speculate that this is because the change in combination of tests pro- vided a more global representation of change in motor performance than any one test alone.
We decided to keep the order of the individual hop tests that make up the full test consistent with its original description.18 In our experience, the 4 hop tests progress logically from less difficult to more difficult, and the initial tests may help to improve performance on the later, more dif- ficult tests. Although reliability would not likely differ from the present findings if a clinician de- cided to administer just the single hop for distance test (indeed, the present ICC is similar to those re- ported by Kramer et al29 on just the single hop test), reliability is more likely to change if a clinician decided to administer just one of the more difficult hop tests without adequate practice. Similarly, our experience with these tests suggests that consid- erable motor learning is likely when first performing them. It is advisable, therefore, to incorporate consider- able practice before stable values can be recorded (eg, we used a “practice day” in the present study to ensure that our subjects’ perfor- mances were stable). The limitation in the generalizability of the present findings to the described order of testing and the use of a practice ses- sion should be recognized.
Although no subject reported pain during a test session, it is important to note that 2 subjects experienced thigh pain after 2 consecutive days of testing and subsequently withdrew from this study. The 2 subjects were the only subjects to report pain fol- lowing testing. They were reviewed by the operating surgeon 6 months postoperatively and had fully recov- ered with no adverse effects. Al- though guidelines for the postopera-
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tive rehabilitation protocol used in the care of our subjects suggested that hopping activities should be in- corporated by the 16th week follow- ing surgery, this was not the case for the 2 subjects who experienced thigh pain. Considering the repeated eccentric muscle contractions re- quired for the landing portions of hop tests, we believe these 2 sub- jects experienced delayed onset muscle soreness. Clinicians should be aware of this possibility, clearly question patients about activities that they are accustomed to perform- ing before deciding to use the hop tests, and clearly state the risk to patients undergoing testing.
Conclusion The described series of 4 hop tests provide reliableandvalidperformance- based outcome measures for patients undergoing rehabilitation after ACL re- construction. These findings support the use and facilitate the interpretation of hop tests for research and clinical practice.
All authors provided concept/idea/research design. Ms Reid, Dr Birmingham, and Mr Stratford provided writing and data analysis. Ms Reid provided data collection and project management. Ms Reid and Dr Birmingham provided fund procurement. Dr Birming- ham, Mr Stratford, and Dr Griffin provided consultation (including review of manuscript before submission).
The authors acknowledge the assistance of Michael Hunt and Jennifer Symmes in the completion of this project.
This study was approved by the University of Western Ontario Research Ethics Board for Healthy Sciences Research Involving Human Subjects, which is organized and operates according to the Tri-Council Policy State- ment and the Health Canada/ICH Good Clinical Practice Practices: Consolidated Guidelines.
This research was undertaken, in part, thanks to funding from the Canadian Orthopaedic Foundation and the Canada Research Chairs Program.
This article was received May 21, 2006, and was accepted November 6, 2006.
10.2522/ptj.20060143
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doi: 10.2522/ptj.20060143 Originally published online January 23, 2007
2007; 87:337-349.PHYS THER. Greg K Alcock and J Robert Giffin Andrea Reid, Trevor B Birmingham, Paul W Stratford, Cruciate Ligament Reconstruction Measure During Rehabilitation After Anterior Hop Testing Provides a Reliable and Valid Outcome
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