Confidence to Return to Play After Anterior Cruciate Ligament Reconstruction Is Influenced by Quadriceps Strength Symmetry and Injury Mechanism

Abstract

Background:

Although the restoration of quadriceps strength symmetry is a primary rehabilitation goal after anterior cruciate ligament reconstruction (ACLR), little is known about the potential relationship between quadriceps strength symmetry and psychological readiness to return to play (RTP).

Hypothesis:

Quadriceps strength symmetry will be associated with psychological readiness to RTP after ACLR. Secondarily, injury mechanism will influence the association between quadriceps strength and psychological readiness to RTP.

Study Design:

Retrospective cohort.

Level of Evidence:

Level 3 (cohort study).

Methods:

A total of 78 female patients completed strength testing and the Injury–Psychological Readiness to Return to Sport (I-PRRS) scale at an outpatient clinical facility as part of return to sport testing after ACLR. Linear regression analysis was used to assess the relationship between the I-PRRS and the independent variables of interest (quadriceps strength symmetry and injury mechanism).

Results:

For all patients combined, a significant symmetry × mechanism interaction was found. When split by injury mechanism, a significant linear relationship was found between quadriceps strength symmetry and the I-PRRS score in patients who experienced a noncontact injury (n = 55; P = 0.01; R² = 0.24). No such relationship was found for those who experienced a contact injury (n = 23; P = 0.97; R² = 0.01).

Conclusion:

Greater quadriceps strength symmetry was associated with greater psychological readiness to RTP in female athletes after ACLR. This relationship, however, was present only in those who experienced a noncontact injury.

Clinical Relevance:

Clinicians should consider both the physical and the psychological factors in assessing a patient’s readiness to RTP. This may be particularly important for those who have experienced an ACL tear through a noncontact mechanism.

Anterior cruciate ligament (ACL) injury is one of the most common and devastating knee injuries in pivoting and contact sports.^8,20 With an annual incidence of 68.6 injuries per 100,000 person-years,²⁰ more than 200,000 ACL tears occur in the United States annually.¹² ACL reconstruction (ACLR) is considered the standard of care for ACL tears, especially for athletes who wish to return to pivoting, jumping, and cutting sports.¹⁴ Despite the fact that return to previous level of activity is a primary goal after ACLR, the reported rate of return to play (RTP) is less than optimal, with only 55% of patients returning to competitive sports and 65% returning to preinjury participation.^1,7,9

Reasons for failing to RTP after ACLR include both psychological and physical factors. With regard to psychological factors, self-confidence, optimism, and self-motivation are predictive of outcomes and RTP after ACLR.^1,7,9 Interestingly, younger patients and patients who have a higher frequency of preinjury participation in sport are more likely to exhibit a higher level of psychological readiness to RTP.²³ On the other hand, female athletes have been reported to have a lower psychological readiness to RTP after ACLR compared with male athletes as measured with the ACL–Return to Sport after Injury scale.²³

With regard to physical factors, greater quadriceps strength⁷ and quadriceps strength symmetry¹⁵ have been reported to be associated with a higher rate of RTP after ACLR. In addition, hop test limb symmetry also has been reported to be associated with a higher rate of RTP after ACLR.¹⁵ Given that the restoration of quadriceps muscle strength is considered a primary goal of the rehabilitation process post-ACLR, it is possible that greater quadriceps strength may be associated with greater confidence in loading the knee during sport-specific tasks, thereby increasing a patient’s confidence to RTP. Evidence in support of this concept is provided by Webster et al²³ and Zarzycki et al²⁴ who reported that greater hop test symmetry and knee kinematic symmetry during gait were associated with a higher psychological readiness to RTP, respectively.

To date, only 1 study has evaluated the relationship between quadriceps strength and psychological readiness to RTP. Lepley et al¹³ reported that injured limb isometric quadriceps strength was positively correlated with psychological readiness to RTP in a small sample of patients (N = 20). Interestingly, these same authors reported that quadriceps strength symmetry (between limb comparison) was not associated with psychological readiness to RTP.¹³ This result is somewhat surprising given the above-noted finding that quadriceps strength symmetry has been reported to be associated with a higher rate of RTP.¹⁵

A potential confounding factor with regard to assessing the relationship between quadriceps strength symmetry and psychological readiness to RTP may be the mechanism of injury. Chmielewski and George⁶ reported that individuals who sustained a contact ACL injury were 3 times more likely to exhibit a higher readiness for advanced rehabilitation compared with those who experienced a noncontact injury. This suggests that the mechanism of ACL injury also may influence one’s psychological readiness to RTP after rehabilitation. A potential explanation of this phenomenon may be found in the Bandura self-efficacy theory, specifically the performance accomplishment tenant. According to this philosophy, success raises expectations while failure lowers expectations.³ Interpreting this theory within the context of an ACL injury in which there is not a clear cause (ie, noncontact), an individual may feel that the injury was the result of one’s poor performance, thereby affecting future self-efficacy (confidence).

The purpose of the current study was to evaluate the association between quadriceps strength symmetry and psychological readiness to RTP after ACLR. We also explored whether injury mechanism influences the association between quadriceps strength and psychological readiness to RTP. We hypothesized that quadriceps strength symmetry would be associated with psychological readiness to RTP after ACLR. We also hypothesized that injury mechanism would influence the association between quadriceps strength and psychological readiness to RTP.

Methods

Patients

We performed a retrospective analysis of 78 female patients who completed strength testing and a psychological readiness questionnaire within the same outpatient facility. All assessments were performed as part of return to sport testing after ACLR between June 2016 and March 2019 and were referred for evaluation by different orthopaedic surgeons. Only female patients were selected for this analysis as sex has been shown to influence psychological readiness to RTP.²²

Patients were between 13 and 30 years of age and previously participated in a level 1 sport (ie, sport with frequent pivoting or cutting such as football, soccer, volleyball, lacrosse) or level 2 sport (ie, sport with less frequent pivoting or cutting such as softball, tennis, dance). Only patients who had undergone RTP testing within 12 months of ACLR were included. Patients who met the above criteria were included in the study regardless of mechanism of injury, graft type, or presence of any coexisting pathologies (ie, meniscal tears, etc). Descriptive statistics for the patient sample are presented in Table 1. The health sciences institutional review board of the University of Southern California approved this retrospective analysis.

Table 1.

Descriptive statistics for the study sample ^a

Variable	Total Sample (N = 78)	Noncontact (n = 55)	Contact (n = 23)	P b
Age, y	17.0 ± 2.5 (13-26)	16.7 ± 2.4 (13-26)	17.7 ± 2.7 (13-25)	0.15
Time surgery to testing, mo	7.1 ± 2.1 (3-12)	6.9 ± 1.9 (4-12)	8.0 ± 2.4 (3-12)	0.10
Noncontact	55 (71)	—	—
Level 1 sport	72 (92)	51 (93)	21 (91)	>0.99
I-PRRS	48.1 ± 11.3 (15-60)	48.3 ± 11.5 (15.5-60)	48.0 ± 10.9 (15-59.5)	0.82
Quadriceps strength symmetry	0.85 ± 0.17 (0.56-1.17)	0.85 ± 0.17 (0.56-1.17)	1.0 ± 0.17 (0.56-1.08)	0.80
Graft type
Allograft	4 (5)	3 (5)	1 (4)	>0.99
Autograft	74 (95)	52 (95)	22 (96)

I-PRRS, Injury–Psychological Readiness to Return to Sport scale. —, not assessed.

^aValues are presented as mean ± SD (range) for continuous variables and number (percentage) for categorical variables.

^bP values are based on independent t tests for continuous variables and Fisher exact tests for categorical variables.

Procedures

Patients underwent quadriceps strength testing and psychological readiness to RTP assessment as part of normal clinical care. Rehabilitation was not standardized, as patients were treated at various clinical facilities. Graft type and the timing of the testing post-ACLR also differed among the patients (Table 1).

Quadriceps Strength Assessment

A motor-driven dynamometer (BTE Primus; BTE) was used to assess isometric quadriceps strength. Patients were seated on a chair and straps were used to secure the thigh of the testing limb. The rotational axis of the unit was aligned with the lateral femoral condyle, and the resistance pad of the lever arm was positioned above the malleoli. Bilateral quadriceps strength measures were performed at 60° of knee flexion. Three maximal contractions (approximately 5 seconds in duration) were performed for each limb with verbal encouragement. Thirty seconds of rest was provided between trials. The peak torque value from each of the 3 trials was averaged for each limb. Quadriceps strength symmetry was calculated by dividing the average peak torque of the involved limb by the average peak torque of the uninvolved limb.

Psychological Readiness Assessment

The Injury–Psychological Readiness to Return to Sport (I-PRRS) scale was used to assess psychological readiness to RTP. The I-PRRS is a valid and reliable questionnaire used to assess psychological readiness to return to sports after ACL injury.^21,22 This 6-item questionnaire is designed to measure confidence (1 aspect of psychological readiness) in various domains (Table 2). Scores on the I-PRRS can range from 0 to 60 with a maximum score of 60 indicating a high psychological readiness to return to sport.

Table 2.

Injury–Psychological Readiness to Return to Sport scale ^a

My overall confidence to play is _____

My confidence to play without pain is _____

My confidence to give 100% effort is _____

My confidence to not concentrate on the injury is_____

My confidence in my knee to handle the demands of sport is_____

My confidence in my skill level/ability is______

^aAll questions are answered on a 0 to 100 scale, with 0 equating to no confidence, 50 moderate confidence, and 100 complete confidence. The total score is calculated by summing all 6 metrics and then dividing by 10. Score range is 0 (minimum) to 60 (maximum).

Statistical Analysis

Multiple linear regression analysis was used to assess the relationship between the I-PRRS (dependent variable), quadriceps strength symmetry (independent variable), and injury mechanism (independent variable). First, we tested for the presence of a symmetry × mechanism interaction. If significant, we divided the patients in 2 groups based on the injury mechanism (noncontact injury group and contact injury group) and reran our regression analyses. To account for the varied time frames in which the return to sport assessments were completed, all regression models were adjusted for the time between surgery and testing.

The assumptions of normality, homoscedasticity, and linearity were verified for all regression models. We examined all models for outliers using the standardized residuals from the regression model. Values greater than the absolute value of 3 were excluded. Using this criterion, 1 patient was eliminated from the analysis. Statistical significance was defined as P < 0.05. All analyses were performed using SPSS statistical software, Version 24 (IBM Corp).

Results

Descriptive statistics for quadriceps strength symmetry and the I-PRRS scores are presented in Table 1. For all patients combined, a significant interaction between quadriceps strength and injury mechanism (symmetry × mechanism) was found after adjusting for the time from surgery to testing (P = 0.02) (Table 3). Therefore, we divided the sample into 2 groups by mechanism of injury. For the noncontact injury group (n = 55), a significant positive linear relationship was found between quadriceps strength symmetry and the I-PRRS score after adjusting for time from surgery to testing (P = 0.011; R² = 0.24) (Figure 1, Table 3). For the contact injury group (n = 23), no linear relationship was found between quadriceps strength symmetry and the I-PRRS score after adjusting for time from surgery to testing (P = 0.97; R² = 0.01) (Figure 2, Table 3).

Table 3.

Regression models to predict psychological readiness to return to sport

Model	Predictors	Unstandardized β (95% CI)	R 2	Adjusted R2	F	P a
1	Constant	48.4	0.21	0.17	4.8	0.0
	Quadriceps strength symmetry	−11.9				0.36
	Mechanism	29.6				0.03
	Quadriceps strength symmetry × Mechanism	35.2				0.02
	Time surgery to testing, mo	1.4				0.01
2 b	Constant	43.4	0.01	−0.01	0.12	0.006
	Quadriceps strength symmetry	0.5				0.97
	Time surgery to testing, mo	0.5				0.64
3 c	Constant	17.6	0.24	0.21	8.1	0.03
	Quadriceps strength symmetry	22.4				0.01
	Time surgery to testing, mo	1.7				0.03

^aP values are independent contribution of predictor to model. P values <0.05 are statistically significant.

^bModel 2. Contact mechanism.

^cModel 3. Noncontact mechanism.

Figure 1.

Scatterplot of predicted values for the Injury–Psychological Readiness to Return to Sport (I-PRRS) scale relative to the observed I-PRRS in patients who experienced a noncontact injury (n = 55). Predicted values calculated from a linear regression equation using quadriceps symmetry index and time. ACL, anterior cruciate ligament.

Figure 2.

Scatterplot of predicted values for the Injury–Psychological Readiness to Return to Sport (I-PRRS) scale relative to the observed I-PRRS in patients who experienced a contact injury (n = 23). Predicted values calculated from a linear regression equation using quadriceps symmetry index and time. ACL, anterior cruciate ligament.

Discussion

The purpose of this study was to determine the association between quadriceps strength symmetry and psychological readiness to RTP in female patients post-ACLR and to determine whether this association is influenced by injury mechanism. Injury mechanism was found to influence the association between quadriceps strength and psychological readiness to RTP. After controlling for the time from surgery to testing, we observed that greater quadriceps strength symmetry was associated with a higher score on the I-PRRS, but only in patients who sustained an ACL injury through a noncontact mechanism.

A particular construct that may explain our findings is one’s belief system, which known as the locus of control (LOC).^11,19 LOC is composed of 2 dimensions: internal (perception that one has a strong control over life events or his or her own health) and external (perception that life events or one’s health are more based on chance, luck, or the influence of others).^11,19 Higher levels of external LOC (or lower levels of internal LOC) are associated with a range of adverse health outcomes, such as depression,⁵ worry,¹⁸ and lower life expectancy.⁴ Higher levels of internal LOC (or lower levels of external LOC) have been reported to be associated with higher perceived sports involvement,¹⁷ and satisfaction with knee function post-ACLR.¹⁶

Critical life events may result in a shift of one’s LOC.¹¹ For example, negative life events that are not clearly understood or out of one’s control could lead to a compensatory shift in one’s belief system in order to regain control.¹¹ In line with this reasoning, individuals who experience an ACL tear through a noncontact mechanism may experience a shift in their LOC toward being more internally focused, as there is not a clear understanding of what went wrong (eg, “I just planted my foot and my ACL tore”). Conversely, individuals who experience an ACL tear through a contact mechanism may have little or no shift in their LOC due to a clear understanding of the injury event (eg, “A player hit my knee and my ACL tore”). Although speculative at this point, those injured through a noncontact mechanism may develop a shift in their belief system that is more dependent on internal factors such as quadriceps strength. Further research is needed to fully understand how psychological factors modulate the relationship between injury mechanism and psychological readiness to RTP.

Previous authors have reported a relationship between higher physical performance, specifically hop test symmetry²³ and injured limb isometric quadriceps strength,¹³ and a higher psychological readiness to RTP. Our results extend these findings to quadriceps symmetry, a variable that not only has been linked to higher rates of RTP¹⁵ but also has been shown to protect against a secondary ACL injury.¹⁰ The findings of the current study are in contrast to the results of Lepley et al,¹³ who reported that quadriceps strength symmetry was not associated with psychological readiness to RTP. It should be noted, however, these authors evaluated both male and female patients and did not take into consideration the mechanism of injury. The former is particularly important, as previous research has shown that sex may have an influence on psychological readiness to RTP after ACLR.⁸

Although we found a significant relationship between quadriceps strength and the I-PRRS, only 24% of the variance in I-PRRS score could be explained by quadriceps strength symmetry. Psychological readiness is an all-encompassing term of multiple constructs, including injury-related fear, risk appraisal, confidence, and so on.² An argument could be made that the I-PRRS questionnaire only evaluates 1 aspect of psychological readiness (ie, confidence). It is possible that use of a more robust tool that measures multiple aspects of psychological readiness to RTP would have revealed a stronger relationship with quadriceps strength symmetry.

The findings of our study have potential clinical implications in the management of the ACL-injured patient. Based on our results, clinicians should consider both the physical and the psychological factors in assessing a patient’s readiness to RTP. This may be particularly important for those who have experienced an ACL tear through a noncontact mechanism. For example, low confidence or low psychological readiness to RTP may be indicative of quadriceps strength asymmetry or limited leg strength in general. Longitudinal studies evaluating how changes in strength (or other physical factors) influence changes in psychological readiness to RTP over time are needed to better understand such relationships.

The results of this study should be interpreted in light of several limitations. First, the cross-sectional nature of the study design does not permit an understanding of cause and effect. Second, we analyzed only a cohort of female patients who wished to return to high level sport. As such, our findings cannot be generalized to male patients and those returning to less demanding sport or activity. Third, the study sample consisted of patients who completed return to sport testing over a wide time frame (3-12 months). Although we controlled for the varied RTS assessment times in our statistical model, future studies should consider evaluating patients at a single time point. patients Last, we only analyzed 1 physical attribute in the study (quadriceps strength symmetry). It is possible that evaluating other physical factors such as movement speed, agility, balance, and so forth, may provide greater insight into the psychological factors that limit one’s ability to RTP after ACLR.

In summary, greater quadriceps strength symmetry was associated with higher psychological readiness to RTP in female patients after ACLR. However, this relationship only was present in persons who experienced a noncontact injury. As such, interventions that address psychological and physical factors to improve RTP outcomes also should consider the potential influence of injury mechanism.