This cross-sectional study was based on data extracted from a rehabilitation outcome registry, Project ACL, on 8 February 2019. Project ACL was established in 2014 and aims both to improve the care of patients with an ACL injury through the use of regular assessments as well as to provide patients and clinicians with treatment feedback. Data are collected prospectively at predefined follow-ups with ACL injury or ACL reconstruction as baseline [19-21]. The follow-up data consist of validated tests of MF and patient-reported outcomes (PROs). The patients undergo individualized rehabilitation under supervision of a registered physical therapist. Ethical approval has been obtained from the Regional Ethical Review Board (registration numbers: 265-13, T023-17).
In the present study, data from the 1-year follow-up were extracted for analysis. Patients included in the registry were eligible if: aged 18-65 years, had undergone a unilateral ACL reconstruction and attended Project ACL´s 1-year follow-up. Patients were excluded if any of the following criteria was met; registered with a second ACL injury, had not performed 1 or more of the 5 tests in the battery of MF tests, or had not responded to the Knee injury and Osteoarthritis Outcome Score, subscale Quality of Life (KOOS QoL) or the ACL Return to Sport after Injury scale (ACL-RSI).
Muscle function
The tests of MF comprised of 2 strength and 3 hop tests. Patients are required to go through a detailed familiarization procedure with their responsible physical therapist before they are tested in Project ACL. Before testing, patients performed a standardized warm up of 10 minutes on a stationary bike and sub maximum trials on each test (Table 1) [22].
TABLE 1. Tests of muscle function.
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Degrees of movement
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Practice trials
n (% of 1RM)
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Test trials
(n)
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Rest between test trials (seconds)
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Units
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Knee extension
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90°-0°
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10 (50%);
10 (75%);
1-2 (90%)
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3-4
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40
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Newton meters
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Knee flexion
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0°-90°
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10 (50%);
10 (75%);
1-2 (90%)
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3-4
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40
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Newton meters
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Vertical hop
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-
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2
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3
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20
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Centimeters
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Hop for distance
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-
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2
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3-5
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20
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Centimeters
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Side hop
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-
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-
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30 seconds
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180
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Number of hops
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n = number; 1RM = one repetition maximum.
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Maximum concentric knee muscle strength was tested in unilateral knee extension and knee flexion at 90°/second using an isokinetic dynamometer (Biodex System 4; Biodex Medical System, Shirley, NY, USA). The Biodex dynamometer is reliable for testing muscle strength [23]. Peak torque in Newton meters (Nm) is used for analysis in this study.
Hop performance is measured with 3 single-leg hops: vertical hop (Muscle lab, Ergotest Technology, Oslo, Norway), hop for distance and a 30-second side-hop test. Each hop test is performed with the patients holding their hands behind their back. For the vertical hop, the time from take-off to landing is converted into hop height in centimeters. In the hop for distance test, the distance between top of the toes at take-off to heel at landing is measured in centimeters. For the 30 second side hop test, one trial per leg is allowed, where the patient is instructed to hop as many times as possible over 2 lines 40 centimeters apart. The number of hops is recorded. The hop tests have good validity and reliability for measuring hop performance in patients with an ACL injury or reconstruction [22].
The results of the tests are presented as the Limb Symmetry Index (LSI), which is the result for the injured leg, divided by the result for the uninjured leg, multiplied by 100 and expressed as a percentage.
Psychological patient-reported outcome
The KOOS is valid and reliable for patients with an ACL injury [24]. The KOOS comprises 5 subscales: Pain, Symptoms, Activity of daily living, Function in sports and recreation, and QoL. Each item is rated from 0 to 4 on a 5-point Likert scale. In this study, the subscale of QoL was used.
The ACL-RSI has been developed to measure an athlete’s psychological readiness to return to sport. The ACL-RSI is reliable, valid, and widely used to predict return to sport [25, 26]. Each item is graded from 0 to 10, where 10 indicates the greatest readiness to return to sport. In this study, the 12-item version was used [26].
The Tegner Activity Scale (Tegner) is meant to reflect how strenuous a physical activity is for the knee [27]. The scale ranges from 0 to 10, where 10 indicates the most knee strenuous physical activity. The scale has good validity for patients with an ACL reconstruction [28]. In the present study, a modified version was used [20]. The modified version does not contain any “0” value, which represents “sick leave or disability pension because of knee problems” in the original version of the Tegner, and has recreation sports as a choice up to level 9.
The PROs were chosen as the ACL-RSI is specifically developed for patients with ACL injuries, and has been reported with the highest methodological quality to assess patients with ACL reconstruction [29]. The QoL is a subscale of the KOOS which reflects the impact of the knee injury on patient´s life and commonly used to assess patients after primary ACL injury [30].
Test batteries
In this study, 4 different test batteries were evaluated. The names of the test batteries subsequently used in this paper are presented in Table 2.
TABLE 2. Test batteries used in the present study.
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Type of test
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Strength tests
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Hop tests
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PROs
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2 MF tests
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· vertical hop
· hop for distance
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2 MF tests and 2 PROs
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· vertical hop
· hop for distance
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· KOOS QoL
· ACL-RSI
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5 MF tests
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· knee extension
· knee flexion
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· vertical hop
· hop for distance
· side hop
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5 MF tests and 2 PROs
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· knee extension
· knee flexion
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· vertical hop
· hop for distance
· side hop
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· KOOS QoL
· ACL-RSI
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ACL-RSI = Anterior Cruciate Ligament Return to Sport after Injury; KOOS QoL = Knee injury and Osteoarthritis Outcome Score, Quality of Life subscale; MF = Muscle Function; PROs = patient-reported outcomes.
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For the 2 MF tests, the vertical hop and the hop for distance were chosen as Abrahams et al. [31] reported these tests as the most commonly used functional tests following ACL reconstruction. Furthermore, the 2 hop tests require minimal equipment, cost or training compared to isokinetic testing and were chosen as clinician friendly. The battery of 5 MF tests was chosen as current consensus criteria for assessment of patients after ACL reconstruction include testing of both muscle strength and hop performance [8].
Definition of passing
Passing the tests of MF was defined as achieving an LSI value of ≥90% [8]. When 2 or 5 tests of MF were taken into account, passing was achieved when the LSI was ≥90% in all tests taken into account.
For the psychological PROs, Muller et al. [32] suggested a score of 62.5 points for the KOOS QoL as a threshold for the state of “feeling well”. With regard to the ACL-RSI, McPherson et al. [33] presented that a cut-off of 76.6 in young patients had maximal sensitivity (78%, with 39% specificity) for discriminating between patients who sustain a second ACL injury and patients who do not within 2 years from the index ACL reconstruction [34]. These 2 cut-offs for the KOOS QoL and the ACL-RSI were applied in this study and scores above the cut-offs were considered as passing.
Statistics
Statistical analysis was performed with the Statistical Package for Social Sciences (SPSS) (version 24, SPSS Inc., Chicago, IL, USA). Mean values, standard deviations, counts and percentages were calculated and presented for demographic data. To compare passing rates between the different test batteries, the sign test was used. Alpha was set at <0.05. To test correlations, the Phi coefficient was used for binary variables. Reference values used for the Phi coefficient were: >0.05 = weak; >0.10 = moderate; >0.15 = strong; >0.25 = very strong [35].