**3. Tuck Jump Assessment**

TJA was carried out according to previously described protocols [27,28]. Jumping efforts were recorded with the resolution 736 × 352 and 125 fps frame rate using the NiNOX 125 camcorder (NiNOX 125, Noraxon USA, Scottsdale, AZ, USA) from the sagittal and frontal plane view. Technique flaws were scored according to previously published form [28]. The reported TJA intra-tester mean percentage of exact agreement ranged between 87.2% and 100%, with kappa values of k = 0.86–1.0 [29].

### *3.1. Composite Score of Readiness (CSR)*

The CSR was calculated from the 3 performed tests (FMS, YBT, and TJA) [10,11]. The CSR was the sum of z-scores, which represented the number of standard deviations by which the value of a raw score was above or below the mean of the measured variables. Raw scores above the mean have positive z-scores, while for those below the mean, the z-scores are negative. The z-scores were then summed to form a single score. Because z-scores and SD are unitless, the results can be summed across all tests. The CSR allows highlighting an athlete's functional deficits across tests relative to the evaluated group. The interpretation of the CSR is based on the methodology described by Thurner et al. [10,11]. If zero represents the group average, any value above zero means that the athlete is better than average, while values below 0 indicate worse performance.


Two types of CSR were calculated. One type is the CSR, in which the z-score for individual athletes was calculated relative to their own group.

CSRA—calculated for athletes after ACL reconstruction relative to the group of athletes also after ACL reconstruction;

CSRM—calculated for athletes after mild lower limb injuries relative to the group of athletes with similar mild lower limb injuries;

CSRH—calculated for athletes without injuries relative to the group of similar athletes without injuries.

Then, the value of the CSR index for each athlete was converted relative to the remaining 2 groups. In this way, the relative CSR index was created, showing the size of the functional deficits of a given athlete in relation to different reference groups.

CSRA-M—calculated for athletes after ACL reconstruction relative to the group of athletes with mild lower limb injuries;

CSRA-H—calculated for athletes after ACL reconstruction relative to the group of athletes without injuries;

CSRM-H—calculated for athletes after mild lower limb injuries relative to the group of athletes without injuries.

#### *3.2. Statistical Analysis*

Statistical analysis was performed using STATISTICA 12.0 Pl software. All evaluated variables were reported as the arithmetic mean (x) and standard deviation (SD). The data were evaluated for normality with Shapiro–Wilk test. The *t*-test was used to determine the differences between groups. Additionally, Pearson's linear correlation coefficient (r) was calculated (below 0.50—poor; between 0.50 and 0.75—moderate; between 0.75 and 0.90—good; above 0.90—excellent). Statistical significance was set at the level of (*p* < 0.05).

#### **4. Results**

All CSR indices were normally distributed (*p* > 0.05). They are graphically presented in Figure 1, highlighting particular athletes' performance relative to their group. Zero represents the team average in terms of CSR (sum of performed tests). Bars above the zero

line represent athletes better than average, while bars below the line indicate worse than average athletes.

**Figure 1.** The values of CSRA (**A**), CSRM (**B**), and CSRH (**C**) in athletes relative to the own group. Zero represents the group average CSR value; green bars indicate positive value (it means that the particular athlete is better than average); red bars indicate negative value (it means that the particular athlete is worse than average).

It has been shown that the CSRA in 15 athletes was below zero (Figure 1A). The CSRM was below zero in 11 athletes (Figure 1B), while the CSRH in 10 athletes did not exceed zero (Figure 1C). This means that relative to the own group, athletes represented similar functional deficits. The CSRA, CSRM, and CSRH indices allow demonstrating who, within the group, exhibits better and who exhibits worse performance.

Comparing athletes after ACL reconstruction to both the MI and C groups, it can be seen that in the majority of subjects, the CSRA-M (Figure 2A) and CSRA-H indices (Figure 2B) were below zero. It has been demonstrated that athletes with severe injury, similar to ACL reconstruction, despite passing the RTS, are in a functionally worse state compared to athletes with mild injuries or healthy ones. On the other hand, when comparing athletes with mild injuries to those healthy, they did not differ significantly from each other because the number of positive and negative CSRM-H indices was similar (Figure 2C).

**Figure 2.** The values of CSRA-M (**A**), CSRA-H (**B**), and CSRM-H (**C**) when comparing athletes after ACL reconstruction to both the MI and C groups. Zero represents the group average CSR value; green bars indicate positive value (it means that the particular athlete is better than average); red bars indicate negative value (it means that the particular athlete is worse than average).

The analysis of differences between the presented CSR indices showed similar results. The values of the CSRM-H were significantly higher than CSRA-M (Figure 3A). This difference was more visible when the CSRA-H was compared to CSRM-H and it was significant (Figure 3B). However, when CSRA-M and CSRA-H were compared, the values were similar, while the difference was non-significant (*p* > 0.05) (Figure 3C).

**Figure 3.** The difference between (**A**) CSRA-M and CSRM-H; (**B**) between CSRA-H and CSRM-H; (**C**) between CSRA-M and CSRA-H.

The correlation between CSR indices and raw test results indicated that the CSR is determined most strongly by YBT (Table 2). There was a strong and significant relationship between the composite score of the YBT test and each of the three CSR indices (r = 0.9–0.93) (Table 2). Interestingly, analyzing the correlation between the value of each CSR and the YBT components, it was noted that the relationship was good and significant for all reaching directions of both limbs but was stronger in the posterolateral and posteromedial directions (r = 0.79–0.85). The anterior direction demonstrated a moderate relationship (r = 0.57–0.64). Both the FMS and TJA tests presented a much weaker relationship with the CSR indices. The relationship between the FMS composite score and CSR was poor (r = 0.35–0.38). Between the TJA composite score and the CSR, the correlation was also poor and non-significant (r = 0.30–0.35) (Table 2).

**Table 2.** The relationship between CSR and raw tests results.


(I) Involved side; (U) uninvolved side. CSRA—index calculated for athletes after ACL reconstruction relative to the group of athletes also after ACL reconstruction. CSRA-M—index calculated for athletes after ACL reconstruction relative to the group of athletes with mild lower limbs injuries. CSRA-H—index calculated for athletes after ACL reconstruction relative to the group of athletes without injuries. (TJA) Tuck Jump Assessment; (FMS) Functional Movement Screen; (YBT) Y-balance test. Values are expressed as Pearson correlation coefficient (r). \* *p* < 0.05.

#### **5. Discussion**

The most important information from this study is that the CSR is a simple way to differentiate athletes with severe functional deficits following serious injuries such as ACL rupture and reconstruction from those without injuries or with only small functional deficits. This difference may be expressed as one index, which is easy to interpret. We have also noted that athletes after ACL reconstruction are in a functionally worse state than athletes following mild injuries or those who are healthy.

Unfortunately, as indicated by some authors, passing the RTS criteria did not result in a decreased risk of ACL re-injury, suggesting that some functional deficits are still present and may not be diagnosed during evaluation [30,31]. The RTS may include many tests, and there is no consensus as to which are the most sensitive and valid. Moreover, many of them may be difficult to rate unambiguously, especially when a number of tests are performed by the athlete and provide non-homogenous results. Very often, coaches and athletes are not interested in the raw score of the test, which may be difficult for interpretation, but they require a simple indication of an athlete's readiness-to-play after ACL reconstruction.

This manner of assessment, through a one-number index, has been discussed in the literature on the subject of gait pattern disorder assessment [14–17] or in the evaluation of athletes' sports level [10,11]. The single number reflects the value by which a subject's gait deviates from the average for normal gait [14–17]. Additionally, the TSA is a method providing a single score of holistic athleticism, from fitness tests and informing how well someone performed relative to the others taking part in the test [10,11]. Therefore, we suggest that the CSR may be a good tool for properly discriminating athletes with high functional deficits after serious injuries such as ACL rupture and reconstruction from athletes with mild injuries or from healthy ones. The CSR may allow to easily rate the severity of post-injury functional deficits in relation to the reference value (the value close to zero). According to the methodology described by Thurner et al. [10,11], a score of one indicates that an athlete has obtained a better score than 84% of his/her group, while a score of two indicates a score better than 97%.

In the present study, the CSRA, CSRM, and CSRH indices allowed to show who, within the group, demonstrates better and who demonstrates worse performance. As was presented, when compared athletes with similar functional deficiencies, those with greater and those with smaller deficits can be distinguished. However, based on this type of CSR, we do not know the size of these deficits in relation to the normal values, e.g., in healthy controls without injuries. Only by comparing athletes after ACL reconstruction to other reference groups (healthy or mildly injured) can the size of these deficits be observed. In our study, by comparing athletes post-ACL reconstruction to both the MI and C groups, it could be seen that the majority exhibited CSR indices below zero. It was shown that athletes with severe injuries, such as ACL reconstruction, despite passing the RTS, were in a functionally worse state compared to those with mild injuries or healthy individuals. On the other hand, comparing athletes with mild injuries to healthy subjects, they did not differ significantly from each other because the amount of positive and negative indices was similar.

It has been recommended that RTS testing after ACL reconstruction should incorporate several tests, but it is unclear which are most associated with a successful RTS [8,9]. The TJA was reported to identify players at risk of ACL injuries by identifying side-toside asymmetries, neuromuscular imbalances, as well as jumping and landing technique flaws [7]. The FMS has been suggested as an effective tool during RTS evaluation [24], while its limited usage has been underlined by others [32,33]. Chorba et al. [34] have reported that the FMS was unable to differentiate between subjects who had not experienced any ACL rupture and those who had. Moreover, it was shown that ACL injuries alter movement patterns, but the FMS test is unable to detect them, and these deficits may increase the risk of re-injury [35]. Additionally, the application of YBT in RTS screening provides equivocal results. Some researchers have indicated the usefulness of YBT [36–38], while others have not found any differences in YBT scores among athletes following ACL reconstruction who were or were not cleared for return to unrestricted activity [39]. In the current study, it has been reported that CRS calculated from FMS, YBT, and TJA tests indicated that athletes after ACL reconstruction are in a functionally worse state than those following mild injuries or without injuries.

A crucial issue, which was also underlined by Thurner et al. [10,11], is that the diagnostic value of such an index may be influenced by the kind of applied test as well as by the amount (number) of the tests from which z-scores are summed. They have reported that the validity of the TSA index was largely determined by the relevance of the implemented fitness tests [10,11]. This problem may also be a weakness of the CSR index. Therefore, we have analyzed which test influenced the final index the most; thus, in other words, which test is most indicative of functional deficits related to ACL reconstruction. Our

results allowed us to suggest that CSR is determined most strongly by YBT. Interestingly, analyzing the correlation between the value of each CSR and the YBT components (not only a composite score), it was noted that the relationship was good and significant for all reaching directions of both limbs but was stronger in the posterolateral and posteromedial directions. The anterior direction (considered by many authors to be the most deficient in people after ACL) showed a moderate relationship. On the other hand, both the FMS and TJA tests presented a much weaker association with the CSR indices. This means that these tests are less indicative for assessing the size of functional deficits in athletes after ACL reconstruction. However, this issue requires further research to see which tests are the most sensitive in detecting specific deficiencies post-ACL reconstruction. Another critical aspect that requires further research is the analysis of individual test components because, as has been shown in our research, the composite score of a given test does not always provide the same information as its individual elements. Perhaps, most optimal would be an index created from parts of the tests and not only from their composite results.

This study also has some limitations which should be addressed. We calculated the CSR only from three tests (FMS, YBT, TJA). Perhaps, if more tests would be included in CSR calculations, its diagnostic value would be more substantial, and the CSR itself would be more comprehensive. Additionally, because psychological factors have emerged as important in the RTS process, it seems required to include psychological tests as a component of CSR. Therefore, there is a need for future research, including a broader selection of tests. Moreover, the utility of CSR should be assessed in future studies concerning its reliability, sensitivity to rehabilitation interventions, and predictability of RTS outcomes.

## **6. Conclusions**

The CSR is a simple way to differentiate individuals following serious injuries (with large functional deficits) from those without injuries or with only small deficits. Because the CSR is a single number, it allows easier interpretation of the functional deficit size in athletes than if rating those deficits from raw tests results.

**Author Contributions:** Ł.O.: conceptualization, methodology, investigation, resources, data curation, formal analysis, writing—original draft, supervision, and funding acquisition. A.M.: methodology, investigation, resources, data curation, formal analysis, and writing—original draft. A.K.: methodology, resources, and writing—review and editing. M.K.: methodology, investigation, resources, data curation, and writing—review and editing. M.S.: methodology and writing—review and editing. R.K.: methodology and writing—review and editing. H.R.: methodology and writing—review and editing. J.S.: methodology and writing—review and editing. E.Ł.: methodology and writing—review and editing. A.S.: methodology, writing review and editing, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Approval of the Ethical Committee of Regional Medical Chamber in Kraków was obtained for this study (protocol number 16/KBL/OIL/2016; date of approval 10/02/2016). All procedures were performed in accordance with the 1964 Declaration of Helsinki and its later amendments.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All data generated or analyzed during this study are included in this published article.

**Conflicts of Interest:** The authors declare no conflict of interest.

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