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Introduction
One of the most common knee injuries is anterior cruciate ligament (ACL) injury, which occurs among patients aged 16-39 years [1-3]. ACL injuries can have short- and long-term consequences that encompass several factors. These consequences may include muscle weakness, physical deficits, decreased engagement in sports activities, an elevated risk of re-injury to the knee, and the potential development of osteoarthritis in the affected knee. These outcomes can significantly impact an individual’s overall physical function and quality of life. It is essential to consider these factors when evaluating the effects of ACL injuries and determining appropriate interventions for rehabilitation and return to sports (RTS) [4-7]. Deciding when an athlete is ready to RTS can be difficult as a physician or clinician. Various elements must be considered, such as muscle strength, cardiovascular fitness, balance, and psychological readiness. By tracking progress in these areas, practitioners can make more informed decisions and increase the likelihood that the athlete will RTS at an optimal performance level [8]. Owing to the latest technological developments, medical, rehabilitation, and performance professionals can now easily gather data that can be utilized to enhance the care and development of athletes as they prepare for RTS. Nevertheless, understanding the significance and application of collected data remains challenging [8]. The success of RTS after ACL injuries often relies on a comprehensive biopsychosocial approach to rehabilitation. This approach considers the biological aspects of injury and recovery, as well as the psychological and social factors that can influence an individual’s ability to RTS successfully. By addressing all these dimensions, including physical rehabilitation, psychological support, and social factors, the chances of a successful RTS can be significantly enhanced [9], and several factors likely affect its success. As a result, RTS assessments have become a vital and indispensable clinical and functional tool for determining readiness to RTS [10]. The selected papers and their results can be clinically crucial in advancing the RTS procedure and assessing the risk of reinjury after ACL reconstruction (ACLR). The findings from these studies can provide valuable insights and inform clinical decision-making to enhance the RTS process and minimize the chances of reinjury following ACLR [11]. The RTS process consists of several phases that pave the way and open the door for clinicians, practitioners, and trainers to focus on functional tests that play a crucial role in helping athletes after ACLR. Both clinical and functional tests are available. The term “sensitivity” is commonly used in medical testing and refers to the ability of a test to accurately identify individuals with the disease or condition being tested. In other words, a test’s sensitivity measures its ability to correctly recognize individuals with errors [12]. An assessment with 100% sensitivity accurately identifies all individuals with the errors, whereas a test with 80% sensitivity can identify 80% of those with the error, leaving 20% of cases unnoticed [12]. To pinpoint a significant yet avoidable mistake, a high level of sensitivity is particularly crucial [12]. Conversely, the term “specificity” pertains to the test’s capability to accurately exclude individuals who do not possess the disease or condition under examination [12]. Hence, a test with 100% sensitivity precisely identifies all individuals without errors [12]. An 80% specificity test correctly classifies 80% of individuals without errors as test negatives (true negatives), yet it erroneously categorizes 20% of error-free individuals as test positives (false positives) [12]. Herbst et al. [13] and Hildebrandt et al. as an example, [14] conducted seven functional assessments, which included the two-leg stability test, one-leg stability test, two-leg counter-movement jump, one-leg counter-movement jump, plyometric jumps, speedy test, and quick feet test. The results indicated that all the tests demonstrated moderate to substantial levels of reliability [15]. Although these functional tests’ validity and test, re-test reliability have been investigated and verified, they have limitations when used as a standardized set of measurements. This is primarily due to their time-consuming and intricate testing procedures [14, 16, 17]. When assessing sports-related performance, it is crucial to consider factors beyond the reliability and validity of functional tests. Considerations, such as cost, user-friendliness, and portability, are also significant. The single-leg hop test, which has consistently been suggested, serves as a valuable tool for evaluating athletic outcomes after ACLR [18, 19]. Hence, this study aimed to explore functional tests and assess their sensitivity and specificity in RTS after ACLR.

Materials and Methods 
This scoping review was based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines.

Search strategy
A thorough analysis of functional tests, encompassing their sensitivity and specificity for RTS after ACLR by searching the Web of Science, PEDRO, Google Scholar, PubMed, ScienceDirect, SCOPUS, CINAHL databases with the keywords using three groups of search terms: (group 1) “ACL RTS” OR “return to play” OR “return to competition” OR “return to activities” OR “return to participation;” AND (group 2) “lower extremity” OR “lower body” OR “lower limb” OR “ACL complex;” AND (group 3) “ACL functional tests” OR “ACL functional battery tests” OR “ACL functional assessment” OR “ACL functional evaluation.” These combinations (involving three groups) were investigated in the titles, abstracts, and keywords of studies published in indexed journals from 2000 to January 2023. Manual searches were conducted to identify articles that might not have appeared in online searches, and a thorough review of article sources was performed.

Eligibility criteria
The inclusion criteria included athletes or people who underwent rehabilitation programs, suffered ACL injuries, used functional tests to RTS; English-language articles, full-text studies were published, and also studies which were randomized control trial (RCT), cross-sectional, and cohort designs; other knee injuries, such as fractures of the proximal arm and articles that were published briefly in congresses and seminars. The exclusion criteria included non-English articles and tests that were not used for RTS.

Study selection
English was used for this search. The researchers screened all the texts. Finally, 127 articles were received from databases using relevant keywords. At first, the titles of the articles were examined, and 12 duplicate articles were removed. In the next step, the abstracts of 115 articles were examined, and, 78 articles were excluded from the survey because they did not meet the inclusion criteria. Next, 37 articles were thoroughly examined in their entirety. Subsequently, 23 of these articles were eliminated from consideration because they did not meet the inclusion criteria. For example, articles in Chinese (9 articles) and Hindi (5 articles) that did not use performance tests for returning to sports after ACLR were deleted. Finally, 14 articles were included in the present review and their results were fully reported. The final compilation of the acquired articles was reviewed by another prominent researcher to ensure that they aligned with the study objectives. The process of obtaining study 2 is illustrated in Figure 1. The structure adheres to the population, intervention, comparison, outcomes, and study (PICOS) framework (Table 1), which is connected to the survey question below.









By searching databases, such as Web of Science, PEDRO, Google Scholar, PubMed, ScienceDirect, SCOPUS, and CINAHL, 127 papers were acquired. After removing duplicate titles, 115 abstracts were chosen for assessment. After reviewing these abstracts, 78 papers were excluded. Subsequently, 37 full-text papers were selected for further evaluation. After scrutinizing these full-text papers, 23 articles were disqualified, and 14 articles that met the criteria were included in the study. Table 2 shows the process of evaluating the quality of this study, and Figure 1 shows the process of evaluating and selecting research articles.







Results
The study included ACL functional tests, and the results were compared with those of existing studies. Approximately, 31 tests, including balance and postural control, agility, power, screening, and movement pattern tests, were included, each explained separately. Tables 3, 4, 5 and 6 present the RTS functional tests and their specificities and sensitivities.







Discussion
This review article was conducted to report the common functional tests and their sensitivity and specificity for RTS after ACLR. A significant finding of this review was that 60% of the articles showed that hop group tests are the most common functional tests after ACLR. Four tables were included: Balance and postural control, agility, power, screening, and movement pattern tests. The related explanation for each is discussed separately below.
Table 3 presents 20 tests of balance and postural control. Fifteen of 20 crucial tests were related to the hop group tests. The highest sensitivity was for the single-leg agility hop test and hopping obstacle course test (100% each) and the lowest sensitivity was for the balance error scoring system (BESS) test (34%). Also, the forward hop test and the hopping obstacle course test had the highest specificity (97% each), and the multiple single-leg hop stabilization test had the lowest specificity (63%). Finally, the sensitivity and specificity of the dynamic leap and balance (DLBT) test were unknown.
Table 4 presents the three agility tests. The shuttle run test had the highest sensitivity (80%), and the Illinois test had the lowest sensitivity (34%). Also, the shuttle run test had the highest specificity (68%), and the Illinois test had the lowest specificity (35%).
Table 5 presents three tests of power. The vertical jump test and the single-leg vertical jump test had the highest sensitivity (95% each), and the Carioca test had the lowest sensitivity (75%). Also, the Carioca test had the highest specificity (80%), while the vertical jump and single-leg vertical jump tests had the lowest specificity (46% each).




Table 6 presents six screening and movement pattern tests; one was a self-report questionnaire. The highest sensitivity was for the lower extremity functional test (LEFT) protocol (95%), and the lowest sensitivity was for the tuck jump test (52%). Also, the highest specificity was for the LEFT (95%), and the lowest specificity was for the tuck jump test (50%). Finally, the sensitivity and specificity of the self-report questionnaire adapted from de Bie et al. were unknown [20].




According to the above findings of each Table, it can be mentioned that the hop tests (Table 3), the shuttle run test (Table 4), the vertical jump tests and the Carioca test (Table 5), and the LEFT test may be more appropriate functional tests for RTS after ACLR in terms of sensitivity and specificity which were the main focus of this scoping review. Additionally, 10 of the 14 articles in this review assessed football players using hop tests, highlighting the importance of dynamic and static stability in football. However, others have focused on other components that are also crucial in sports. Considering the body as a whole and focusing on training principles, such as performing a task unilaterally in an open or closed kinetic chain with perturbation, along with dual tasks and focusing on movement pattern tests in terms of skill and movement quality, may effectively involve these components when assessing athletes [8]. As previously mentioned, these functional tests could offer a more time and cost-effective alternative to clinical tests. For instance, given the significance of the hop tests, they enable us to efficiently assess performance factors, such as speed, explosive force, acceleration, stability, and balance within a short timeframe [15]. Hop tests are vital in ACL injury prevention and RTSs [11]. Different hop tests include the single-leg hop test for distance, the stair-hop test, the 6-meter single-leg hop test for time, and the triple jump test, of which three are measured based on the distance and one is measured based on time [41, 42]. 
Due to the complexity of identifying specific subjective and objective criteria for RTS after ACLR, a wide range of assessments were conducted to determine if individuals were prepared for a functional and quantifiable return to their previous levels of physical activity [14, 43, 44]. Most injury prevention protocols, such as 11+ and the Prevent Injury and Enhance Performance Program (PEP), include hop tests. Furthermore, hop tests are crucial and occupy a distinct position in the context of RTS after ACLR [11]. RTS outcomes following ACLR are disappointing [11]. Athletes who frequently experience ACL tears are often advised to undergo ACLR to ensure their fitness for sports [45]. RTS tests may include muscular strength and power, cardiovascular fitness, postural control, dynamic balance, movement quality, plyometric tests [8], and psychological readiness assessments using questionnaires [46, 47]. According to the studies, the RTS time has been reported as 6, 9, 12 months [48-50] and even more. Nevertheless, a significant disagreement is observed regarding the appropriate timeframe for safe return to competitive sports.
We hope that upcoming studies will focus on more up-to-date and effective functional tests that provide insights to clinicians, practitioners, and trainers so that they can better assess athletes for RTS after ACLR in the future.

Conclusion
According to the results of this study, the most common functional tests were the shuttle run, vertical jump, Carioca, and LEFT tests, especially the hop tests. These tests may be more appropriate functional tests for RTS after ACLR in terms of sensitivity and specificity, which were the main focus of this scoping review. We hope these tests will be useful for personal trainers, athletic trainers, practitioners, and clinicians when evaluating injured athletes.

Limitations
This review article’s limitation included studies in non-English languages, such as Chinese and Hindi, unpublished studies, and studies in other motor searches. The authors hope that future research will address these gaps.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflict of interest
The authors declared no conflict of interest.

Acknowledgments
The authors are grateful to those who helped them prepare and write this manuscript.