Introduction
nterior Cruciate Ligament (ACL) rupture is one of the most common knee joint injuries, especially in active and young people. It accounts for about 50% of knee injuries. This ligament is usually damaged during recreational or competitive sports activities [1]. ACL reconstruction surgery is usually recommended following an ACL rupture to restore the knee mechanical stability and achieve normal knee function [1, 2]. Studies have shown that many people still do not resume normal knee function after ACL reconstruction and report disorders such as knee instability, pain, decreased Range of Motion (ROM), defects in quadriceps and hamstrings, and neuromuscular dysfunction, which can lead to very different consequences among these people [4, 3]. Therefore, rehabilitation interventions are essential for patients with ACL reconstruction to reduce pain, improve ROM, muscle strength, dynamic and functional stability of the reconstructed knee, balance, and finally help return the patient to functional activities [5]. According to systematic review studies, the most common disorder-based outcome measures used to evaluate the effect of treatment and determine the success of treatment in randomized clinical trials on patients with ACL reconstruction are knee pain, ROM, hamstring, and quadriceps muscle strength, and dynamic balance using Star Excursion Balance Test (SEBT) [6, 7, 8]. 
In this regard, therapists need repetitive, valid, and responsive tests (sensitive to change) to detect significant changes during the treatment process [9, 10]. Responsiveness is an essential feature of a measurement tool and can detect the least clinically significant change in a patient’s health status over time. Responsiveness includes internal and external types [11, 12]. Internal responsiveness refers to the ability of a tool to detect changes over time, regardless of whether these changes are clinically significant or not [13, 14]. External responsiveness indicates how much of a change in the instrument has occurred in a given period in accordance with the change in the reference instrument. The reference measurement is based on the retrospective 7-point Likert scale [15]. Although clinical trial studies have repeatedly used pain, ROM, quadriceps, and hamstring muscle strength, and dynamic balance control to assess patients’ recovery from ACL reconstruction and the effect of treatment, no study has yet examined the responsiveness of these measures. Therefore, this study aims to investigate the responsiveness of tools measuring pain, ROM, quadriceps and hamstring muscle strength, and dynamic balance control in patients with ACL reconstruction following a rehabilitation treatment.
Materials and Methods
This research is a prospective cohort study. The study participants were 54 patients with an ACL injury who had undergone reconstructive surgery. The inclusion criteria were being 18-45 years old and passing 2 weeks from the ACL reconstruction surgery. The exclusion criteria were having a history of diabetes, neurological disorder, inflammatory arthritis, malignancy, musculoskeletal pain, musculoskeletal injury other than ACL rupture, and bilateral ACL rupture. 
Study measures
The Visual Analog Scale (VAS) was used to assess pain outcomes. It consists of a 10-cm horizontal line, with the left-hand side labeled “no pain” and the right-hand side labeled “most intense pain imaginable”. The VAS score was determined by measuring in centimeters from the left-hand side to the point that the patient marks [16]. 
A goniometer was used to measure the passive ROM of the knee in the supine position during flexion and extension. Thus, the amplitude of knee flexion and extension was recorded [16]. 
To evaluate the isometric strength of quadriceps and hamstrings, a manual tension/compression dynamometer (Danesh Salar Iranian Company, Iran) was used. The dynamometer was placed just above the malleolus in the anterior and posterior parts of the leg, and the person was asked to press the involved leg against the strap as much as possible. The primary test was performed by taking maximum contraction of quadriceps and hamstring muscles with 3 repetitions during maximum effort from each person. There was a 30-second rest interval. The strength values obtained from the three maximum contractions of each muscle group were averaged [17]. 
To evaluate the dynamic balance using the SEBT, a circle was assumed on the ground, and 8 lines were drawn at a 45-degree angle (like a star). The patient was asked to stand on the affected leg in the center of the circle and move the other leg as far as possible on the lines (in the selected direction), touch the ground with the toe at maximum reach without maintaining weight, and then return to the starting position (standing on two legs). The distance between the center of the star and the point of contact of the contralateral leg was considered the reach distance, an indicator of dynamic balance. This test was repeated three times, and their average was calculated and determined as the final score. The errors that cause exclusion from the test were displacement of the leg placed in the star center, loss of balance during the test, and putting weight on the reaching leg when the toe was in contact with the ground [18]. In previous studies, the reproducibility and high validity of these scales have been reported to evaluate the outcomes in patients with ACL reconstruction [19, 20, 21, 22, 23].
All participants were evaluated in the first stage of evaluation (2 weeks after ACL reconstruction surgery) and again in the second stage of evaluation (after 4 weeks of physiotherapy) using the mentioned instruments. In the last treatment session, the patients were asked to report changes in their health status from the beginning to the last session of treatment on a 7-point retrospective Likert scale [15, 24]. This scale was divided into two general subscales of improved (score 5-7) and not-improved (score 1-4) to create a two-part outcome variable [15]. Since the purpose of responsiveness studies is to evaluate the instrument’s specificity rather than to evaluate the effectiveness of the intervention, the control of physiotherapy intervention was not necessary for the present study [24]. However, all patients received similar rehabilitation interventions after the surgery.
Data analysis
The obtained data were analyzed in SPSS v. 21 software. First, we used the Kolmogorov-Smirnov test to investigate the normal distribution of variables. The paired t-test was used to examine the relationship between pre- and post-treatment scores. The Receiver Operating Characteristics (ROC) curve (with 95% confidence interval) was used to evaluate the responsiveness [25]. To do this, after two stages of evaluation, a change score (difference between the scores of two evaluation stages) was first obtained using the instruments for each patient. One by one, the change scores were determined as the cut-off point and compared with the score obtained from the 7-point Likert scale as a standard external criterion. Thus, sensitivity and specificity were calculated at each stage [10, 26, 27]. Then, the ROC curve was plotted, with the vertical axis corresponding to the sensitivity and the horizontal axis corresponding to the specificity [26]. In the ROC curve analysis, a point with the highest sensitivity and specificity was selected on the left and top of the curve, representing the minimum clinical change score of the outcome [14]. Besides, the correlation between the change score obtained from each instrument (as a quantitative variable) and the score obtained from the 7-point Likert scale (as a qualitative variable) were examined. The Spearman correlation coefficient in the range of 0.250-0.50 indicates a weak correlation, 0.50-0.70 moderate correlation, and >0.70 indicates a strong correlation [10]. Similarly, the area below the ROC curve in the range of 0.25-0.50 indicates poor responsiveness, 0.50-0.70 shows moderate responsiveness, and >0.70 indicates good responsiveness [26].
Results
The demographic and clinical characteristics of participates are presented in Table 1, and the results of the paired t-test are presented in Table 2. 




As seen, 34 patients (63%) were classified as an improved group, and 20 patients (37%) as a not-improved group. The results obtained from the area below the ROC curve showed that the knee extension ROM, quadriceps and hamstring strength, and dynamic balance at medial and posteromedial directions had a good response (<0.70). The Spearman correlation coefficient for these four outcomes between the change score and the score of 7-point global change rating Likert scale ranged from 0.36 to 0.51. Significant clinical changes were reported for all study tools (Table 3).


Discussion and Conclusion
One of the most critical problems and complications after ACL reconstruction is the loss of knee extension end ROM (5-10 degrees) [28]. This ROM loss can lead to significant pain and functional impairment, especially during walking and running in young and active patients, because the loss of more than 10 degrees prevents normal gait and leads to increased loads on the patellofemoral joint and, as a result, anterior knee pain [28]. Loss of knee flexion end ROM is usually not as debilitating as loss of knee extension end ROM [28]. Therefore, changes in ROM in the direction of extension rather than in the direction of flexion are more consistent with changes perceived by the young patients, and this factor can be a reason for greater responsiveness of goniometer for assessing knee extension ROM. Another major problem after ACL reconstruction is the weakness of the quadriceps and hamstring muscles [29]. Several studies have shown a greater reduction in the quadriceps strength than in hamstring strength in these patients [29]. It has also been shown that the correlation between reduced quadriceps strength and knee function is higher than the correlation between reduced hamstring strength and knee function [30]. The results of our study also showed that the hand-held dynamometer’s ability to respond to the knee extension strength (quadriceps muscle strength) was greater than the ability to respond to the knee flexion strength (hamstring muscle strength). However, it was reasonably responsive to both knee extension and flexion and can monitor the actual recovery of young and active ACL patients in clinical and research settings.
The results of the ROC curve and correlation analysis showed that VAS was not so much responsive to diagnose the ACL patient’s real and significant recovery. In a systematic review and meta-analysis by Collins et al. on the measurement characteristics of the knee injury and osteoarthritis outcome score instrument in young patients with an ACL injury or knee osteoarthritis, it was shown that the pain subscale of the instrument had poor content validity and therefore had no good capacity to show improvement [31]. Consistent with this study, VAS in our study had the poor potential to show the changes in patients’ real recovery after treatment, maybe, due to the participation of young and active patients. Another possible reason can be that the assessments were performed after the initial acute phase and in the intermediate phase of rehabilitation (2-6 weeks after surgery). During this period, patients typically have minimal pain and symptoms, and exercise during this period tends to be more difficult; hence, no significant change in pain is expected for ACL patients.
SEBT in medial and posteromedial directions showed acceptable responsiveness. The greater responsiveness in these two directions compared to other directions can be because these two directions exert more rotational force on the reconstructed knee during the test [32]. Thus, these two directions can more accurately identify important clinically significant changes in the dynamic balance of ACL patients. This result is somehow consistent with the findings of Herrington et al., who showed that the medial and posteromedial directions were more capable of differentiating between patients with ACL injury and controls [32]. The optimal cut-off point obtained from the ROC curve, which is considered as the least clinically significant change in the patient’s health status, can provide researchers and therapists with practical and valuable information to make decisions about the actual change in ACL patient’s health status.
The present study results provide evidence for the responsiveness of goniometer for knee extension ROM, hand-held dynamometer for quadriceps and hamstring strength, and SEBT for dynamic balance at medial and posteromedial directions in the assessment of clinical changes in patients with ACL reconstruction. The minimum clinically significant change score obtained for each tool in this study can help therapists and researchers to decide on determining the actual significant change in the ACL patient’s clinical conditions after physiotherapy.

Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them. was approved by the Ethics Committee of Ahvaz Judishapur University of Medical Sciences (Code: IR.AJUMS.REC.1394.275).

Funding
This study was extracted from a PhD. dissertation of the first author at the Musculoskeletal Rehabilitation Research Center, Ahvaz Judishapur University of Medical Sciences, Ahvaz. Also, this study was supported by the Ahvaz Judishapur University of Medical Sciences.

Authors' contributions
Conceptualization: Mohammad Jafar Shaterzadeh Yazdi, Hossein Negahban, Neda Mostafaee; Supervision: Mohammad Jafar Shaterzadeh Yazdi, Hossein Negahban, Shahin Goharpey; Methodology: Hossein Negahban, Shahin Goharpey, Neda Mostafaee; Writing – original draft: Neda Mostafaee; Writing – review & editing: All authors; Data collection: Neda Mostafaee, Nahid Pirayeh; Data analysis: Mohammad Mehravar, Neda Mostafaee.

Conflict of interest
The authors declared no conflict of interest.

Acknowledgments
The authors would like to thank study participants and Ahwaz University of Medical Sciences for their cooperation and support.


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