FUNCTIONAL MEASURES DO NOT DIFFER IN LATE STAGE REHABILITATION AFTER ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION ACCORDING TO MECHANISM OF INJURY

Abstract

Background:

Anterior cruciate ligament injuries are among the most common knee injuries. Mechanism of injury is classified as contact or non-contact. The majority of anterior cruciate ligament ruptures occur through a non-contact mechanism of injury. Non-contact anterior cruciate ligament ruptures are associated with biomechanical and neuromuscular risk factors that can predispose athletes to injuries and may impact future function. Non-contact mechanism of injury may be preceded by poor dynamic knee stability and therefore those with a non-contact mechanism of injury may be prone to poor dynamic knee stability post-operatively. Understanding how mechanism of injury affects post-operative functional recovery may have clinical implications on rehabilitation.

Purpose:

The purpose of this study was to determine if mechanism of injury influenced strength, functional performance, patient-reported outcome measures, and psychological outlook in athletes at four time points in the first two years following anterior cruciate ligament reconstruction.

Study Design:

Secondary analysis of a clinical trial.

Methods:

Seventy-nine athletes underwent functional testing at enrollment after impairment resolution. Quadriceps strength, hop testing, and patient-reported outcome measures were evaluated post-operatively at enrollment, following return-to-sport training and one year and two years after anterior cruciate ligament reconstruction. Participants were dichotomized by mechanism of injury (29 contact, 50 noncontact). Independent t-tests were used to compare differences between groups.

Results:

There were no meaningful differences between contact and non-contact mechanism of injury in any variables at enrollment, post-training, one year, or two years after anterior cruciate ligament reconstruction.

Conclusion:

Function did not differ according to mechanism of injury during late stage rehabilitation or one or two years after anterior cruciate ligament reconstruction.

Level of Evidence:

III

INTRODUCTION

Anterior cruciate ligament (ACL) injuries are among the most common traumatic knee injuries.¹ Mechanisms of ACL injuries are classified as direct contact to the knee or body, or non-contact.^2-5 ACL injuries from direct contact occur when an external load (e.g., athlete or an object) precedes the ACL injury. Non-contact injuries occur without physical contact between athletes.⁴ The majority of ACL ruptures occur through a non-contact mechanism of injury (MOI), as opposed to a contact MOI.^4,6-9 Non-contact ACL ruptures are primarily related to biomechanical and neuromuscular risk factors^10-13 that can predispose athletes to initial injuries and have the potential to impact future function.^14,15 After ACL rupture, reconstruction and rehabilitation, clinicians recommend athletes pass return to sport (RTS) criteria to reduce reinjury risk.^16-19 Considering the neuromuscular deficits that often precede non-contact ACL injury, individuals who sustained a non-contact MOI may be predisposed to poor dynamic knee stability and neuromuscular deficits post-operatively. Further understanding how MOI might affect post-operative rehabilitation may have important clinical implications on functional recovery.

MOI may influence knee stability after acute ACL injury, with those sustaining non-contact ACL injuries, compared to contact ACL injuries, being more likely to be classified as noncopers.¹⁵ A noncoper is characterized by the inability to dynamically stabilize the knee after ACL injury, resulting in episodes of knee giving way.²⁰ Those categorized as noncopers after ACL rupture demonstrate worse overall clinical presentation including decreased movement pattern quality, quadriceps strength, and poorer odds of long-term success.^21-27 Although the exact etiology of non-contact injury remains unknown, video analyses during time of injury show most injuries occur with aberrant mechanics during change in direction, sudden deceleration, valgus collapse, or pivoting and cutting with the knee in a more extended position and a planted foot.^4,8,13,28 Similarly, risk factors in non-contact injuries are often associated with poor neuromuscular control and coordination of the limb during an intended movement, for example, dynamic knee valgus and decreased muscle stiffness around the knee joint.^12,29-32 Biomechanical deficits and poor neuromuscular control are prevalent in non-contact MOI. Athletes who demonstrate neuromuscular deficits preceding their injuries may potentially be predisposed to poor dynamic knee stability and function post-operatively.

It is unclear if athletes who sustained a non-contact mechanism of ACL injury, compared to athletes who sustain a contact mechanism of injury, differ in their functional outcomes following anterior cruciate ligament reconstruction (ACLR). The purpose of this study was to investigate the strength, functional performance, patient-reported outcome measures, and psychological outlook of athletes who sustained non-contact versus contact ACL injuries at four key time points within the first two years following ACLR. These time points were: after rehabilitation (enrollment), following RTS training, and at one and two years after ACLR. Considering the potential neuromuscular and biomechanical differences in those who sustain non-contact MOI, the hypothesis was that there would be differences in clinical and patient reported outcomes during rehabilitation based on MOI.

METHODS

This is a secondary analysis of prospectively collected data from a clinical trial (NCT01773317) approved by the University of Delaware Institutional Review Board. Athletes participated in this study between November 2011 and August 2018 at the University of Delaware. Participants provided written informed consent; additional consent and assent were obtained from parents/guardians and minors, and rights of the participants were protected.

Participants

Male and female athletes who sustained a primary ACL injury and underwent ACLR were recruited for the study. To enroll in the study, participants must have previously been level I or II athletes³³ who participated in at least 50 hours of sport per year prior to their injury. Additionally, participants must have been three to nine months after unilateral ACLR and have achieved ≥ 80% quadriceps strength limb symmetry index (QI, assessed via electromechanical dynamometry, as described below), minimal to no knee effusion,³⁴ full knee range of motion (equivalent to the uninvolved knee in flexion and extension), the ability to hop on each leg without pain, and successful initiation of a running progression in their post-operative rehabilitation.^35,36 Exclusion criteria to control for the severity of injury were: concomitant grade III ligamentous injury, > 1 cm² full thickness chondral defects assessed via MRI or arthroscopy, or a history of previous serious injury or surgery to either lower extremity.³⁵ Participants were also excluded from analysis if they sustained a second ACL injury prior to each time point tested.

ACL-SPORTS Trial

The methods of the larger randomized control trial, the Anterior Cruciate Ligament-Specialized Post-Operative Return to Sports (ACL-SPORTS), have been previously published.³⁵ Participants received 10 physical therapy sessions of either strengthening, agility, plyometric, and secondary prevention exercises (SAP) or strengthening, agility, plyometric, and secondary prevention exercises plus perturbation training (SAP + PERT).²¹ Previous analyses of ACL-SPORTS found no difference between SAP and SAP+PERT in gait biomechanics, knee function, or patient-reported outcomes at any timepoint up to and including two years after ACLR.^37-42 For the purposes of this analysis, participants were collapsed across the SAP and SAP+PERT groups.

Present Study

All 79 participants who were included in the parent ACL-SPORTS trial were included in the present study; readers may consult Arundale et al.³⁷ for details of participant recruitment, selection, and enrollment, including a flow diagram.³⁷ The present secondary analysis reports the functional performance of participants according to MOI. Participants were dichotomized by MOI, with 29 contact and 50 non-contact ACL injuries (Table 1). MOI was determined by participant self-report and verified verbally by the treating, licensed physical therapist. A contact injury was defined as contact to the knee or body directly preceding the ACL injury. Non-contact injury was defined as no contact made to the individual being injured (e.g., cutting, pivoting, landing) prior to the injury.

TABLE 1.

Participant Demographics

		Non-Contact (n = 50)	Contact (n = 29)	p-value
Age at Surgery (years ± SD)		20.6 ± 7.2	22.1 ± 8.3	0.41
BMI ( ± SD)		26.4 ± 3.5	25.4 ± 2.9	0.17
Sex		28 Female, 22 Male	11 Female, 18 Male	0.12
Pre-Injury Level		44 Level I, 6 Level II	28 Level I, 1 Level II	0.20
Time to Meet Enrollment Criteria (weeks)		21.8 ± 7.3	24.5 ± 8.3	0.148
Second Injury by 2 Years (% population) †		5 (10%)	5 (17%)	0.35
Graft Type	Allograft	10	8	0.05
	BPTB	20	4
	Hamstring	20	17
Medial Meniscus Pathology*	None	24	21	0.24
	Partial Meniscectomy	11	3
	Repair	7	5
Lateral Meniscus Pathology*	None	18	19	0.15
	Partial Meniscectomy	19	7
	Repair	5	3

n  =  number of participants; SD  =  standard deviation; BMI  =  body mass index; BPTB  =  bone patellar tendon bone

^*8 participants did not have operative reports, therefore no meniscal data were available

^†Participants who sustained a second injury were excluded from all analyses that occurred after the second injury.

Functional Testing

Participants were evaluated post-operatively at enrollment (pre-training), after return-to-sport training (post-training), and at one and two years post-operatively on strength, hop testing, and patient-reported outcomes measures (Figure 1, Appendix B). Quadriceps strength was tested using an electromechanical dynamometer (Kin-com; DJO Global, Chula Vista, CA, USA; or System 3; Biodex, Shirley, NY, USA) during a maximal voluntary isometric contraction (MVIC). Participants were seated with their hips and knees positioned at 90 degrees and the machine's axis of rotation aligned with the axis of rotation of the knee. The participant's leg was strapped in at the pelvis, thigh, and shank during the MVIC testing. The uninvolved limb was tested first, followed by the involved limb, for three trials per limb. Quadriceps strength was reported as a quadriceps strength limb symmetry index. QI was calculated by dividing the involved limb maximum torque by the uninvolved limb's maximum torque and multiplying by 100.

Figure 1.

Athlete progression through enrollment, RTS, and follow-up time points. Abbreviations: n  =  number of participants; QI  =  quadriceps strength limb symmetry index; PROM  =  patient reported outcome measures; ACLR  =  anterior cruciate ligament reconstruction; RTS  =  return to sport

A series of four hop tests (single, crossover, triple hops for distance, 6-meter timed) tests were performed.⁴³ Participants performed two practice trials of each hop test, then two recorded trials. The tests were always performed on the uninvolved limb prior to the involved limb and given in the same order (single, crossover, triple hops for distance, 6-meter timed). Limb symmetry indexes (LSI) of the first three single-leg hop tests were calculated by dividing the average of the two recorded trials on the involved limb by the average of the two trials on the uninvolved limb and multiplying by 100. The 6-meter timed hop was calculated by dividing the average of the two recorded trials on the uninvolved limb by the average of the two recorded trials on the involved limb and multiplying by 100 to account for a shorter time indicating a better score.

Clinical Measures

Patient-reported outcome measures (PROMs) included the IKDC subjective knee form, the Knee Injury and Osteoarthritis Outcome Score (KOOS) sports and recreation and quality-of-life subscales, the ACL Return to Sport after Injury (ACL-RSI), the Tampa Scale of Kinesiophobia (TSK-11). The IKDC quantifies knee symptoms, function, and sports activity on a scale of 0 to 100, with a minimal clinically important difference of 11.5.^44-46 The KOOS-sports and recreation subscale asks participants about the degree of difficulty with activities involving jumping, kneeling, and running.⁵⁰ The KOOS-quality-of-life subscale asks participants about their daily awareness of knee problems including modifications made to their lifestyle and overall difficulty with their knee. These KOOS subscales are both calculated as a percentage from 0 to 100 with 100 indicating no impairment.⁴⁷ Both the KOOS subscales have a minimal clinically important difference of 8.⁴⁷ We also used the Knee Outcomes Survey-Activities of Daily Living Scale (KOS-ADLS)⁴⁸ and the global rating of perceived knee function (GRS).⁴⁹ The KOS-ADLS is a valid, reliable, and responsive 14 item questionnaire which assesses knee related disability by asking how well one is able to perform functional tasks, with lower scores representing greater functional limitation.⁴⁸ The GRS is a single question asking athletes to rate their perceived knee function level from 0-100% with lower scores representing poorer function and higher indicating ability to perform all previous activities and sports without limitation. To pass the RTS criteria, we require ≥90% QI, ≥ 90% LSI on all four hop tests, ≥90% Knee Outcome Survey- Activities of Daily Living Scale (KOS-ADLS), and ≥90% Global Rating of Perceived Function.

To measure psychological fear of movement, we used the ACL-RSI and the TSK-11. The ACL-RSI⁵⁰ is a valid and reliable questionnaire targeting psychological readiness to RTS.⁵¹ The ACL-RSI asks 12 questions about the athlete's confidence, emotions and risk appraisal, and is scored on a scale from 0 to 100. A score of 0 indicates an extremely negative psychological response.⁵⁰ Similarly, the TSK-11 assesses pain-related fear of reinjury and ranges from 11 to 44, with higher scores indicating more fear of reinjury.⁵² The TSK-11 has been used previously in patients after ACLR⁵³ and can be used to divide patients into high fear and low fear groups.

Statistical Analyses

All statistical analyses were performed using SPSS Version 25 (IBM Corporation, Armonk, New York, USA). A significance level of p<0.05 was set a priori. Independent t-tests and chi-squared analyses were performed to compare demographic characteristics and determine difference between groups. Participants were dichotomized into two groups, contact and non-contact, based on MOI. As all previous literature from this trial showed study participants improved across time up to and including 2 years after ACLR,^37-39,41,42 data were analyzed at each time point using independent t-tests. Independent t-tests were run at every time point (enrollment, post-training, one year, two years) to determine if there was a difference between groups in QI, single-leg hop test LSIs, and scores on the KOS-ADLS, KOOS-sports and recreation and quality of life subsets, GRS, TSK-11 and ACL-RSI at 4 time points after ACLR. Secondary repeated measures ANOVAs corroborated these findings.

RESULTS

There were no statistically significant differences between groups in demographic or surgical characteristics, including graft type and meniscal status, or in time to meet enrollment criteria (Table 1). Both independent t-tests and secondary repeated measures ANOVA demonstrated no statistically significant or clinically meaningful differences between contact and non-contact groups in any variables at enrollment, post-training, one year, and two years after ACLR (p≥.05), with the exception of the timed hop at one year where both group's limb symmetry indexes were > 100% (p = 0.03) (Figures 2-5, Appendix A).

Figure 2.

There were no differences in quadriceps index in contact vs. non-contact MOI at any time-point.

Figure 5.

There were no differences between groups on the ACL-RSI (shown here) or TSK-11 at any time-point. Abbreviations: ACL-RSI  =  Anterior Cruciate Ligament-Return to Sport after Injury scale

DISCUSSION

The purpose of this study was to determine if MOI influenced strength, functional performance, patient-reported outcome measures, or psychological outlook among athletes at 4 time points within the first two years following ACLR. Our findings refute our hypothesis and suggest that MOI does not influence performance on strength, functional performance, patient-reported outcome measures, or psychological outlook among athletes during the late post-operative rehabilitation phases or one or two years after ACLR in patients who are well-rehabilitated^37,38,41,42 after ACLR.

This study compared group differences before and after late phase rehabilitation training and one and two years after ACLR. Previous analyses of this cohort indicate that functional and patient-reported outcomes improved throughout the duration of rehabilitation.^37,38,42 This study adds to these results by analyzing the effect of MOI on functional outcomes. Measurements of quadriceps strength did not differ between contact and non-contact MOI at any time point during late rehabilitation or at follow-up time points (Figure 2, Appendix A). These data suggest that athletes may progress through rehabilitation milestones similarly regardless of MOI. Functional performance was also similar at each time point, regardless of MOI (Figure 3, Appendix A). Although timed hop was statistically different between groups at one year (p = 0.03), the difference was not clinically meaningful and both groups achieved over 100% limb symmetry index (non-contact 101.1%, contact 104.6%).

Figure 3.

There were no differences in single hop test results in contact vs. non-contact MOI at any time-point.

Functional performance after ACLR may influence an athlete's ability to return to preinjury sport level.⁵⁴ Demonstrating poorer hop symmetry and self-reports of knee function is prospectively associated with not returning to preinjury level of sport.⁵⁴ These data also indicated that patient reported outcome measures of knee function and psychological factors were not different between groups (Figure 4, Figure 5). Psychological factors, including fear of movement, fear of reinjury, confidence, and readiness to RTS may influence functional performance and outcomes after ACLR.^55-58 Lentz et al. showed patients with fear of reinjury who did not RTS at one year after ACLR, compared to those who did RTS, showed significantly lower quadriceps strength at 6 and 12 months after ACLR.⁵⁹ These data suggest that MOI does not impact clinical milestones in late rehabilitation and up to and including two years after ACLR.

Figure 4.

There were no differences between groups on the IKDC (shown here) or any other patient reported outcome measure at any time-point Abbreviations: IKDC  =  International Knee Documentation Committee Subjective Knee Evaluation Form.

MOI is often described by patients and referring physicians within the initial evaluation, and could influence an athlete's rehabilitation process. Clinically, there are often biases about how MOI may affect a patient's progression through rehabilitation. Biomechanical influences contributing to MOI, specifically non-contact MOI, are well described in the literature.^{7,12,30,60-62} These mechanisms are significant to our understanding of how initial ACL rupture may occur and influence prevention programs. MOI itself, however, may not inform clinicians how to treat a patient after ACLR. These data suggest late phase rehabilitation may not need to be tailored according to MOI during functional testing. These data, however, do not examine outcomes early in rehabilitation time points, and are specific to the functional outcomes we measured. There may be biomechanical differences in movement quality, and future studies should investigate biomechanical differences based on MOI. While the MOI may not be relevant to successful progression through rehabilitation, how clinicians structure both post-operative rehabilitation and RTS training is critical to safe RTS. Following criterion-based post-operative rehabilitation programs is recommended,^36,63-66 which are typically divided into three phases of rehabilitation (early post-operative, middle, and late) followed by RTS training.^36,63-67 The late phase of postoperative rehabilitation focuses on continued activity progression, quadriceps strength, and sport-specific interventions. Finally, the RTS phase of training consists of 10 training sessions focused on quadriceps strengthening, prevention exercises, balance, and agility and plyometric training.³⁵ During RTS testing, participants went through a battery of functional performance tests and patient-reported outcome measures that are used to guide clinical decision making. RTS criteria in our study included ≥ 90% limb symmetry index (LSI) on 4 hop tests,⁶⁸ ≥ 90% QI, and ≥90% on the KOS-ADLS and GRS.^35,36 Even after athletes passed RTS criteria, immediate unrestricted RTS was withheld and athletes continued with a progression into full, unopposed sport participation.

The findings of this study suggest there is no difference in performance on functional outcomes according to contact or non-contact MOI during the late phases of post-operative rehabilitation or at one or two years after ACLR. Clinicians may ask, “Should I treat an athlete differently based on their MOI?” These data suggest that when considering functional outcomes (QI, hop testing, PROMs), late phase rehabilitation may not need to be tailored according to MOI.

Limitations

MOI can be defined in different ways, with the definition varying from study to study. Some studies report direct contact, indirect contact, and non-contact.⁵ Olsen et al. define a contact injury as a direct blow specifically to the knee.²⁸ This definition, however, excludes injuries that occur with physical contact to other parts of the body. In this study, MOI was self-reported by the athlete, and confirmed verbally by a physical therapist. Contact injury was defined as contact to any part of the body immediately preceding the injury, and non-contact injury as no contact made to the athlete immediately preceding the injury. The inclusion criteria for this study are stringent, ensuring participants were ready to initiate a progressive RTS training program, and therefore we may be missing athletes who may initially perform poorly due to their MOI. The study had strict exclusion criteria controlling for concomitant injury, which excluded those with significant concomitant injury that happen with contact MOI. Data were analyzed using independent t-tests to maximize the amount of participants at each time point and detect differences in between groups. Secondary repeated measures ANOVA analyses corroborated the findings that there are no differences between groups according to MOI and that participants improve over time regardless of MOI. Although the group sample sizes represent similar frequencies of contact vs. noncontact injury similar to the literature, having more equally sized groups may have been beneficial for statistical analyses. Finally, the time points of the larger study were during the late phases of rehabilitation after clinical impairment resolution. These data do not include pre-operative or early post-operative rehabilitation time points or hip strength assessment, where differences based on MOI could exist.

CONCLUSION

The results of the current study indicate that there were no differences in strength, functional performance, patient-reported outcome measures, or psychological outlook between athletes with either contact or non-contact mechanism of injury up to and including two years after ACLR. At each time point including pre and post-training, one year, and two years, both groups had similar performance on all functional outcome measures, indicating MOI may not influence rehabilitation outcomes. Patients following ACLR may benefit from additional strengthening, agility, and injury prevention exercises beyond traditional physical therapy regardless of MOI. MOI may not need to be considered when implementing late phase rehabilitation and RTS.

APPENDIX A. MEAN DATA OF ALL FUNCTIONAL OUTCOME MEASURES. P-VALUE REPRESENTS THE UNADJUSTED P-VALUE FOR THE INDEPENDENT T-TEST COMPARISONS BETWEEN GROUPS FOR EACH VARIABLE, AT EACH TIME-POINT.

1 N 1 Non-Contact Mean (:±SD) I Contact Mean (±SD) I F-Value
Enrollment
Qi	79	89.7 (8.1)	93.5 (9.4)	0.06
Single Hop LSI	79	77.2 (15.1)	82.6 (14.4)	0.13
Crossover Hop LSI	79	83.7 (16.4)	89.5 (12.0)	0.10
Triple Hop LSI	79	85.1(11.9)	88.7 (12.0)	0.19
Timed Hop LSI	79	91.1 (9.5)	92.7 (8.5)	0.44
KOS-ADLS	79	92.3 (6.6)	93.9 (5.6)	0.26
GRS	79	79.6 (8.5)	78.5 (10.1)	0.62
nCDC	79	78.5 (8.2)	79.8 (8.6)	0.52
KOOS Sports and Recreation Subset	79	78.3 (14.4)	81.0(14.8)	0.42
KOOS Quality of Life Subset	79	58.5 (15.5)	58.4 (17.2)	0.98
ACL-RSI	79	60.7 (21.9)	64.9(21.3)	0.41
TSK-11	79	19.3(4.1)	18.2 (4.4)	0.29
Post-Training
QI	79	91.9(12.3)	95.6(11.3)	0.19
Single Hop LSI	64	91.9 (9.9)	95.8 (13.4)	0.19
Crossover Hop LSI	64	95.5 (7.2)	98.7 (7.3)	0.09
Triple Hop LSI	64	95.3 (5.9)	98.0 (6.9)	0.10
Timed Hop LSI	64	99.1 (7.6)	100.4 (6.5)	0.49
KOS-ADLS	79	94.6 (5.7)	95.9 (4.8)	0.33
GRS	79	87.1 (7.2)	86.4 (10.0)	0.74
IKDC	79	85.6 (8.7)	88.0 (9.0)	0.24
KOOS Sports and Recreation Subset	79	88.2 (12.8)	89.8 (10.9)	0.57
KOOS Quality of Life Subset	79	65.9 (16.4)	69.6 (22.5)	0.39
ACL-RSI	79	70.4 (21.5)	76.2 (19.7)	0.23
TSK-11	79	18.1 (4.5)	17.7 (5.0)	0.69
1 Year
QI	72	98.2 (12.8)	99.3 (12.9)	0.74
Single Hop LSI	65	98.0 (8.7)	97.5 (6.6)	0.85
Crossover Hop LSI	65	98.8 (7.3)	100.8 (8.2)	0.34
Triple Hop LSI	65	98.1 (6.7)	99.9 (5.7)	0.29
Timed Hop LSI	65	101.1 (6.0)	104.6 (5.7)	0.03t
KOS-ADLS	73	96.6 (4.6)	97.0 (5.0)	0.69
GRS	73	93.3 (11.0)	95.5 (5.9)	0.36
IKDC	73	92.3 (9.2)	94.8 (7.4)	0.22
KOOS Sports and Reereation Subset	73	93.4 (9.4)	93.3 (10.0)	0.95
KOOS (Quality of Life Subset	73	79.5 (15.8)	83.7(15.3)	0.28
ACL-RSI	73	82.3 (21.9)	88.0(15.0)	0.24
TSK-11	73	16.6(4.8)	15.3 (4.0)	0.25
2 Years
QI	61	103.2(15.0)	97.2 (13.2)	0.12
Single Hop LSI	55	101.0 (6.9)	97.6 (8.8)	0.12
Crossover Hop LSI	55	100.3 (7.0)	100.1 (6.3)	0.91
Triple Hop LSI	55	101.5 (6.1)	99.1 (5.2)	0.16
Timed Hop LSI	55	99.7 (6.2)	99.8 (7.5)	0.94
KOS-ADLS	63	96.4 (5.5)	96.3 (5.2)	0.93
GRS	63	95.0 (7.9)	92.0 (14.8)	0.31
IKDC	63	96.0 (6.8)	95.7 (7.6)	0.85
KOOS Sports and Recreation Subset	63	96.0 (6.2)	93.9 (13.2)	0.48
KOOS Quality of Life Subset	63	87.3 (14.3)	89.9 (13.4)	0.48
ACL-RSI	63	88.8 (14.8)	90.8 (15.0)	0.61
TSK-11	63	15.9(4.1)	15.7 (3.9)	0.80

n  =  number of participants; QI  =  quadriceps strengdi limb symmetry index; KOS-ADLS  =  Knee Outcome Survey-Activities of Daily Living Subscale; GRS  =  Global Rating Score of Perceived Function; IKDC  =  International Knee Documentation Committee; KOOS  =  Knee Injury and Osteoarthritis Outcome Score; ACL-RSI  =  Anterior Cruciate Ligament-Return to Sport after Injury scale; TSK-11  =  Tanq)a Scale for Kinesiophobia ^Values are mean (Standard deviation) t Significant t-test (p<0.05)

APPENDIX B. ATHLETES WHO DID NOT UNDERGO HOP TESTING AT EACH TIME POINT. REASON FOR NOT HOPPING (N).

	Non-Contact Ininrv (n)	Contact Ininrv (n)
Post-Training	QI < 80% (4), Low CAR (2), Effusion (3),	QI < 80% (3), Low CAR
(15)	Pain (2)	(1)
1 Year (8)	QI < 80% (1), Effusion (2), Pain (I)	QI < 80% (2), Low CAR (I), Not reported (I)
2 Years (8)	QI < 80% (I), Did not return for testing/Not reported (4)	Effusion (I), Not reported (I), Low back nain/stifOiess (1)

n  =  number of participants; QI  =  quadriceps strength limb symmetry index; CAR  =  central activation ratio