Background

Most athletes with an anterior cruciate ligament (ACL) injury undergo ACL reconstruction (ACLR) with the goal to return to sport (RTS) at the same level of competition as before the injury [1] but only 63% of athletes are able to achieve this [2]. Multiple factors are associated with RTS after ACLR, including injury site and surgical technique, physical functioning, and psychology [3]. The influence of psychological factors is particularly large in athletes in the RTS phase [2, 4]. Compared to before the injury, athletes RTS after ACLR have the following psychological characteristics: weak kinesiophobia (fear of re-injury and movement) [5,6,7], high self-efficacy [6], high self-esteem [8], and high psychological readiness to RTS [6, 9, 10].

Recently, athletic identity and sport commitment have been recognized as important psychological variables that could be related to RTS status post-ACLR [4, 8, 11, 12]. Athletic identity is the sport-specific component of an individual’s self-concept and is the extent to which an individual identifies with the athletic role [13]. Post-ACLR athletes with a higher degree of athletic identity show greater adherence to rehabilitation [14]. Sport commitment is defined as a psychological state representing the desire and resolve to continue participating in a particular athletic program, specific sport, or sports in general [15]. Athletes who have suffered severe injuries, including ACL injuries, can continue being committed to RTS through sport commitment [11].

Based on these studies, it is expected that athletic identity and sport commitment would be associated with RTS status in post-ACLR athletes. However, no previous study has quantified the relationship between athletic identity and sport commitment and RTS status following ACLR. Therefore, the purpose of this study was to determine the relationships between athletic identity and sport commitment, and RTS status in athletes after ACLR. We hypothesized that post-ACLR athletes who have returned to sports at their pre-injury competition level have higher athletic identity and sport commitment scores compared to athletes who have not returned to sports.

Methods

Participants

Participants who had undergone primary ACLR between August 2015 and May 2019 were included if they met the following inclusion criteria: (1) they were 16 to 45 years old at the time of measurement [10, 16]; (2) their sport participation estimated with a modified Tegner activity scale [17] was ≥5 before ACL injury; (3) it had been 8–24 months since the surgery [18]; and (4) they had indicated an intention to RTS before surgery. Participants were excluded if they had an ACL injury to the contralateral knee or ACL reinjury to the reconstructed knee; had a complication that affects RTS; had not participated in sports for social reasons such as pregnancy or employment; had a cartilage injury requiring surgery; and had difficulty in follow-up until RTS. The sample size was analyzed using G*Power software [19]. The minimum sample size was calculated to be 38 patients in total, referring to the effect size determined from previous studies [6, 9, 10, 20, 21] analyzing group differences in ACL-Return to sport after injury scale (ACL-RSI) and Tampa Scale for Kinesiophobia (TSK) scores (effect size = 0.96, alpha = 0.05, power = 0.80, two-tailed). All surgeries were performed by orthopedic surgeons specialized in knee joint. The autograft sources were hamstrings or bone-patellar tendon-bone. The surgery technique and postoperative rehabilitation protocol were based on previous research [22]. Jogging started 3 months post-ACLR, and the running speed was gradually increased. Sports participation was allowed by a doctor when the following were achieved: it was at least 6 months after surgery; the limb symmetry index (LSI) on the single-leg hop for distance was > 90%; and the LSI of isokinetic knee flexion and extension strength was > 85%, as measured with an Isokinetic Dynamometer (BIODEX System 4, BIODEX Medical Inc., Shirley, NY) at 60°/s and 180°/s).

Procedures

This was a cross-sectional study completed in a single center. Demographic, injury, and surgical information were collected from medical records. Sport type, participation level, and psychological variables were collected using a questionnaire. Sport type was categorized as collision, contact, limited contact, and noncontact based on a previous study [23]. Participation level was categorized as recreation, competitive, and elite based on a previous study [8]. Ethical approval was obtained from the Ethics Committee (approval number: M2016–197). All subjects provided written informed consent before participation. The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement was used as guidance when reporting the design of this study [24].

Psychological variables

This study measured athletic identity and sport commitment as psychological variables. In addition, we measured kinesiophobia and psychological readiness using standard psychological measures that have already been found to be associated with RTS status after ACLR [5, 6, 10].

Athletic identity was assessed with the Athletic Identity Measurement Scale (AIMS) [13]. The AIMS is a 10-item questionnaire where responses are on a seven-point Likert scale that ranges from 1 (strongly disagree) to 7 (strongly agree). Total scores range from 7 to 49, with higher scores indicating stronger athletic identity. The Japanese version of the AIMS was used, which has good internal consistency and good criterion-related and construct validity [25].

Sport commitment was measured using the Sport Commitment Scale (SCS) [15]. The Japanese version of the SCS [26] was used. This scale is a self-report inventory measuring an athlete’s psychological desire to continue sport participation. SCS is a six-item questionnaire where responses are provided on a five-point Likert scale. Total scores range from 6 to 30, with higher scores indicating greater sport commitment. The SCS has good reliability (internal consistency, reproducibility) and validity (construct and criterion-related validity) [26].

Kinesiophobia was measured using the TSK [27]. The Japanese version of the TSK was used [28]. The TSK is a 17-item questionnaire with a four-point Likert scale. Total scores range from 17 to 68, with higher scores indicating greater kinesiophobia. The TSK has good internal consistency [27].

The ACL-RSI is designed to measure comprehensive psychological readiness to RTS after ACL injury or reconstruction surgery [29]. It is a 12-item questionnaire and includes three domains: emotions, confidence in performance, and risk appraisal. Scores for each domain are summed and averaged for a total score between 0 and 100. Higher scores indicate greater psychological readiness to RTS. The Japanese version of the ACL-RSI was used; it has good internal consistency, construct validity, and reliability [30].

RTS status

To determine RTS status, all subjects responded to two questions, one of which was a continuous variable and the other was dichotomous. The continuous variable question was assessed using post-operative subjective athletic performance (POSAP) on a scale of 0–100% [22]. In the dichotomous question, the participants were asked to answer “Yes” or “No” if they were returning to the same level of sports as before the injury [5, 21, 31,32,33]. The subjects who answered > 80% for the PoSAP and “Yes” to the dichotomous question were included in the Yes-RTS (YRTS) group. The No-RTS (NRTS) group included those who met none or only one of the criteria.

Statistical analysis

The normality of the distribution of each variable was determined by a histogram and the Shapiro–Wilk normality test. The differences between the NRTS and YRTS groups in demographic data and psychological variables were analyzed using the chi-squared test, Fisher’s exact test, unpaired t-test, and Mann-Whitney’s U test. The effect sizes (chi-squared test or Fisher’s exact test = φ coefficient, Cramer’s V, t-test = Cohen’s d, Mann-Whitney’s U test = r) were also calculated for each variable. Psychological variables may be influenced by activity levels and the months since the surgery. Thus, Spearman’s rank correlation coefficient (ρ) was calculated between the modified Tegner activity scale and the months since the surgery, and the psychological variables. The a priori α level was 0.05. Data were analyzed using SPSS Ver. 21.0 (IBM Corp, Armonk, NY).

Results

Forty-one participants met the criteria for this study, but two refused to participate (Fig. 1). Therefore, 39 participants were included in the analysis. The demographic information of these athletes is presented in Table 1. In total, 16 athletes (41%) were assigned to the NRTS group, and 23 athletes (59%) were assigned to the YRTS group. Four subjects responded “Yes” to the dichotomous question but had a PoSAP ≤80%. The lowest PoSAP for the YRTS group was 85%.

Fig. 1
figure 1

Participant flow chart. ACLR, anterior cruciate ligament reconstruction

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Table 1 Demographic variable distributions
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There were no significant differences in demographic information, surgical information, or modified Tegner activity scale scores, sports type, and participation level other than age (P = 0.044). The data for the psychological variables are shown in Table 2. The YRTS group had a higher AIMS score (P = 0.023), SCS score (P = 0.027), and ACL-RSI score (P = 0.002) than the NRTS group. The YRTS group had a lower TSK score (P = 0.014) than the NRTS group.

Table 2 Group differences in psychological variables
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The correlations between the modified Tegner activity scale and months since the surgery and the psychological variables are shown in Tables 3 and 4. No significant correlations were found for each variable.

Table 3 Correlation between activity level, months from surgery and psychological variables in “NO” return to sports
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Table 4 Correlation between activity level, months from surgery and psychological variables in “YES” return to sports
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Discussion

The results of this study showed that the YRTS group had significantly higher AIMS, SCS, and ACL-RSI scores and significantly lower TSK scores compared to the NRTS group. Consequently, the results of the present study supported our hypothesis.

A meta-analysis examining the RTS rates of post-ACLR athletes reported that approximately 63% of athletes were able to RTS at the same level of competition as before the injury [2]. The YRTS group in this study included approximately 59% of the participants so the RTS rate for the study population did not differ from previous meta-analyses.

The dichotomous question evaluating RTS as Yes/No alone may overestimate the RTS [22]. Therefore, in the present study, a matrix of the dichotomous question and the PoSAP was used as a measure of RTS. Thereby, four of the 27 participants (15%) who answered ‘Yes’ to the dichotomous question had PoSAP scores under 80%, and they were included in the NRTS. In this study, those who returned to sport closer to their pre-injury status were selected as YRTS.

The demographic data showed that those included in the NRTS group were significantly older than those included in the YRTS group. This supports the findings of previous studies [5, 10]. The wide range of age distribution in the NRTS group in this study may have affected the results. Notwithstanding, several studies have reported that age is not related to RTS status [20, 34] but no consensus has been reached regarding this. Future studies may be necessary to gather more detailed evidence.

The results of the present study showed that those with YRTS had significantly higher AIMS scores than those with NRTS. Athletic identity has been identified as one of the psychological factors associated with RTS after ACLR [4, 8]. However, no previous studies have examined these associations quantitatively. In a cohort study, Brewer et al. [14] reported a positive correlation between preoperative AIMS scores and adherence to rehabilitation (home exercise and self-care) after ACLR. Brewer et al. [35] showed in another cohort study that AIMS scores were reduced in those who did not progress sufficiently in rehabilitation between 6 and 12 months after ACLR. These studies suggest that athletic identity in post-ACLR athletes may influence rehabilitation progression toward RTS. These characteristics may be reflected in the results of this study.

The findings of this study showed that those with YRTS had significantly higher SCS scores than those with NRTS. Until now, no previous studies had quantified the association between RTS status and SCS after ACLR. In semi-structured interviews with post-ACLR athletes, Mahood et al. [12] demonstrated the importance of commitment as one of the driving reasons to RTS. Inigo et al. [11] provided interesting information in response to the question regarding why injured athletes, including post-ACLR athletes, continue to gravitate towards RTS despite the increased potential for future relapses and complications. This interview study showed that a commitment to sport (enjoyment of sport, valuable opportunities, personal investment, social constraints, and social support) enables severely injured athletes to continue to commit to RTS. Thus, it is considered that athletes after ACLR can engage in rehabilitation toward RTS through sports commitment.

The AIMS and SCS show different characteristics depending on the activity level and postoperative period [26, 35, 36]. To consider the confounding effects of these variables on outcomes, the present study analyzed the correlations between each psychological measure and the modified Tegner activity scale and months from surgery but there were no significant correlations. The results of this study show an association between RTS status and athletic identity and sport commitment, regardless of the activity level and postoperative period.

The results of this study showed that the YRTS group had lower TSK scores and higher ACL-RSI scores than the NRTS group. Excessive kinesiophobia and lack of psychological readiness for RTS are the major psychological factors affecting RTS status in post-ACLR athletes [6, 9, 10, 21]. The results of the present study support these findings and provide evidence that kinesiophobia and psychological readiness are associated with RTS status.

Clinical implications

The minimum time from post-ACLR to RTS is 6 months and, in recent years, it has been recommended to extend the duration of RTS to reduce the risk of re-injury [18, 37]. During such a long rehabilitation period, patients may experience a loss of sport commitment and athletic identity. Additionally, those with significant declines in these variables may need to consider collaborating with a psychologist [8]. In the rehabilitation of post-injury athletes, it is important to set suitable goals and to explain the reasons for exercising to maintain patient and athlete adherence and commitment to rehabilitation [38, 39]. Rehabilitation milestones after ACLR are jogging, running, partial participation in competition, and RTS [40, 41]. We should try to prevent the loss of the patient’s sport commitment and athletic identity by always explaining to them the rationale and purpose of these milestones and what treatment is needed to achieve them.

Limitations of this study

There are several limitations to this study. First, it was conducted in a single center with a small sample size, so results should be generalized with caution. Second, this study only showed a cross-sectional association between RTS status and psychological scales; the causal relationship between the results is unknown. Although no statistical association was found between the psychological variables and months since surgery in this study, we included athletes whose psychological scores could vary (8–24 months) in the analysis. Third, the present study analyzed subjects who met the criteria for sport participation but did not analyze physical factors that may affect RTS. Future research is necessary to clarify the relationship between athletic identity and sport commitment and RTS status during rehabilitation in a cohort study and to clarify the degree of influence of each factor using a multivariate analysis, including physical functioning.

Conclusion

Athletes who were able to RTS at their pre-injury level of competition showed higher scores on the athletic identity and sport commitment questionnaires than those who did not. These self-beliefs regarding sport participation may play an important role in post-ACLR athletes’ RTS.