Quadriceps Strength and Hamstring-to-Quadriceps Ratios in Pediatric Patients After ACL Reconstruction: Revisiting Norms from Adult Data

Abstract

Background:

Pediatric patients who undergo anterior cruciate ligament reconstruction (ACLR) experience higher rates of graft failure and secondary knee injuries compared with adults. Neuromuscular recovery differences may contribute to these disparities.

Purpose/Hypothesis:

This study aimed to assess the effect of age on functional limb testing (FLT) outcomes at 9 months after ACLR. It was hypothesized that pediatric patients would demonstrate greater deficits compared with adults, which may help explain higher reinjury rates.

Study Design:

Cohort study; Level of evidence, 3.

Methods:

All pediatric patients (≤16 years) who underwent hamstring autograft ACLR between 2017 and 2022 and completed FLT at 9 months after surgery were included. An adult cohort (>16 years) was matched for sex ratio, body mass index, meniscal pathology, preoperative Tegner activity score, and rate of double tendon graft constructs. FLT outcomes included quadriceps strength limb symmetry index (LSI), hamstring strength LSI, hamstring-to-quadriceps (H:Q) ratio, Y-balance test LSI, hop testing LSI, and ACL-Return to Sport after Injury (ACL-RSI) score.

Results:

The final analysis included 120 pediatric and 210 adult patients. Pediatric patients demonstrated significantly higher quadriceps strength LSI (94% vs 88%; P < .001), hop distance LSI (97% vs 92%; P < .05), and hop height LSI (92% vs 84%; P < .001) compared with adults. However, adults exhibited a significantly higher H:Q ratio (48.90 vs 41.50; P < .001). No significant differences were observed in hamstring strength, LSI, or ACL-RSI scores.

Conclusion:

At 9 months after ACLR, pediatric patients demonstrated a faster rate of recovery for quadriceps strength and hop function compared with adults. Both groups exhibited similar hamstring strength deficits. The lower H:Q ratio observed in pediatric patients may indicate a higher risk of reinjury, highlighting the need for targeted rehabilitation strategies to address hamstring deficits in this population.

Children and adolescents are participating in more organized sports at a junior level. As a result, the number of anterior cruciate ligament (ACL) injuries in this demographic has increased significantly in the last 15 years in Australia. ⁴ ACL injuries represent 6.30% of sports injuries in children aged between 5 and 12 years and 10.30% of sports injuries in children aged between 13 and 17 years. Although this combined cohort still comprises <7% of all ACL injuries, they are forming a larger proportion of patients undergoing ACL reconstruction (ACLR).^4,5

The treatment of ACL injuries in this cohort presents a complex clinical scenario because of the rate of ACL graft reinjuries versus the ongoing risk of persistent instability and subsequent cartilage loss in those managed nonoperatively.^12,42 In their meta-analysis, Ramski et al ⁴² showed ongoing clinical instability in 75% of nonoperatively managed patients and a 12-fold increase in the likelihood of meniscal injury compared with those undergoing operative treatment, supporting the trend of higher ACLR in the pediatric cohort. ¹⁶ Conversely, rates of reinjury leading to ACL graft rupture in the pediatric cohort are higher than in adults (up to 33%) and provide a dilemma and opportunity for seeking secondary injury prevention strategies.^7,38,47 A meta-analysis by Wiggins et al ⁵⁰ reported a >30% secondary injury risk in young athletes.

Inadequate neuromuscular rehabilitation, strengthening, and prolonged graft healing time have all been hypothesized as potential factors for the increased prevalence of ACL injury. ¹⁷ Given that younger age and return to sports (RTS) have been proven to be risk factors for secondary ACL injury, attention must be paid to the criteria for safe RTS.^39,49 Current ACL rehabilitation guidelines incorporate both time-based and criterion-based approaches to determine RTS readiness. Time remains a key variable, with consensus guidelines recommending a conservative approach that respects the biological healing process and neuromuscular recovery. The 9-month timeframe is widely recognized as an important milestone, allowing for adequate graft revascularization and ligamentization, while aligning with evidence recommending delayed RTS to reduce reinjury risk. ⁶ A scoping review of 209 studies for current RTS criteria after ACLR found that strength tests were included in 41% of the criteria. In contrast, hop tests and patient-reported outcome measures (PROMs) were included in 15% and 12% of protocols, respectively.^{6,14,17,28,51} At the time of this study, there was no defined and widely accepted criterion for safe RTS that could demonstrate acceptability in terms of application, reduction in further injury, or patient satisfaction.

Previous literature has shown a wide variation in strength and functional recovery after ACLR; however, limited studies have analyzed this in the pediatric population.^{4, 24, 25,31} A meta-analysis revealed up to a 44% quadriceps strength deficit, measured as the symmetry index, at 6 months after surgery, although this had significantly reduced at the 1-year assessment. ⁸ Hamstring weakness was <10% weaker at these respective points. Conversely, various smaller studies found a persistent hamstring weakness at 1 year after surgery in the pediatric cohort.^4,18,25 A study of male professional athletes found that a decreased hamstring-to-quadriceps (H:Q) strength ratio was linked with increased graft rupture.^28,29 Multiple studies have revealed that an accepted criterion for RTS is a limb symmetry index of 85% using isokinetic and isometric testing measures, although meeting this criterion is no guarantee for return to preinjury levels of sport.^6,24,36

The focus of this study was to assess the effect of age on functional limb testing (FLT) results at 9 months after ACLR. We hypothesized that a pediatric population would have significantly greater limb symmetry deficits than a matched adult population.

Methods

Ethical Considerations

All data utilized in this study were collected as part of the orthopaedic research institution's database of pre, peri-, and postoperative information for patients undergoing ACLR. Printed information was provided to all participants on the testing format, requirements, and data collection for research use; participation was voluntary, and informed consent was obtained before participation. Patients were able to choose to “opt out” from the study at any time.

Study Design

Inclusion Criteria

A retrospective review of consecutive patients who underwent primary ACLR by 5 surgeons and completed 9-month RTS testing at a single orthopaedic institute between 2017 and 2022.

Exclusion Criteria

Patients were excluded if they had any of the following: use of any graft other than the hamstring tendon; incomplete functional limb testing data at 9 months after surgery; or revision surgery.

Descriptive Data

Descriptive data were collected at patients’ initial presentation to the clinic. These included age at the time of surgery, sex, weight, and height.

Age Standardization

The age cutoff of ≤16 years was selected based on established clinical guidelines and the practical availability of chronological age data. The existing literature supports the use of age as a practical and standardized classification metric in ACLR research, enabling comparability across studies and reducing the need for additional imaging assessments.^12,47 Furthermore, age-based classification aligns with developmental milestones critical to neuromuscular recovery and clinical decision-making, while avoiding the ethical and financial burdens associated with routine skeletal maturity evaluations. ²⁰

Timeline

Participants were assessed at 9 months after ACLR, a time point selected based on current rehabilitation guidelines that promote biological healing with functional progression. The 9-month mark has been widely implemented in clinical practice as a conservative RTS threshold, allowing sufficient time for graft revascularization and neuromuscular adaptation. ⁶ Evidence recommends that RTS before this period may increase the risk of reinjury due to residual deficits in strength, balance, and proprioception that persist beyond the initial 6 months of rehabilitation. ⁶ Furthermore, studies have indicated that >90% of RTS decisions utilizing time-based criteria recommend a minimum of 9 months before athletes return to unrestricted activity. This timeframe was chosen to standardize assessments and provide meaningful comparisons between pediatric and adult populations in terms of neuromuscular recovery and psychological readiness for sport.

Patient-Reported Outcome Measures

PROMs included the Tegner activity scale (TAS), scored on an 11-point scale (0-10), and Anterior Cruciate Ligament Return to Sport after Injury scale (ACL-RSI), which were collected preoperatively and at 9 months postoperatively.

Postoperative Rehabilitation

All patients followed a structured rehabilitation protocol guided by established postoperative guidelines. The rehabilitation program was divided into 5 key phases, focusing on progressive strength and functional restoration over 9 months:

1. Phase 1 – Protection and mobilization (0-6 weeks): Focused on pain and swelling management, achieved full knee extension, and initiated muscle activation.

2. Phase 2 – Strength and neuromuscular control (6-12 weeks): Emphasized progressive strength building, gait normalization, and early proprioception training.

3. Phase 3 – Advanced strengthening and agility preparation (3-6 months): Introduced more complex strength exercises, balance training, and early plyometrics.

4. Phase 4 – Running and change of direction training (6-9 months): Focused on sport-specific drills, agility, and dynamic movement with gradual load progression.

5. Phase 5 – RTS preparation (9+ months): Clearance for full sport participation based on functional testing, strength benchmarks, and psychological readiness.

Patients attended supervised physical therapy sessions 2 to 3 times per week during the early phases and then weekly after 3 months, with progression criteria based on functional milestones. The full rehabilitation protocol is available in Appendix A1.

Functional Limb Testing

Measurements included quads strength limb symmetry index (LSI) (%), hamstring strength LSI (%), H:Q ratio, Y-balance test (YBT) LSI (%), and hop testing LSI (%). Registered and experienced physical therapists supervised the muscle function tests, balance tests, and hop tests in a systematic order. Knee range of motion was measured, followed by quadriceps and hamstring isokinetic concentric strength testing (N·m) using the Biodex System 4 Pro (Biodex Medical Systems).^11,41 Testing was performed under standardized conditions across all assessments-concentric-concentric mode with 60°/sec angular velocity (peak torque) for 4 to 5 repetitions. Specific testing parameters were applied in accordance with established clinical protocols. Hop testing (cm) in vertical, forward, and side orientation was tested within well-published guidelines to yield each patient's best results.^3,17 FLT was conducted as part of a private practice RTS testing protocol. As this was a nonessential service, financial considerations may have acted as a barrier to participation. Some patients elected not to participate in RTS testing, while others commenced but did not complete the full testing protocol.

The YBT is a validated assessment tool for measuring dynamic balance and injury risk in both upper and lower body quarters. ²⁷ Dynamic stability, which is crucial in sports, involves maintaining stability while performing tasks to prevent injuries. The YBT demonstrates high inter- and intrarater reliability and involves testing for anterior, posterolateral, and posteromedial planes dynamic stability. ¹⁵

All FLT assessments were conducted by a team of registered physical therapists and accredited exercise physiologists, with extensive experience in ACL rehabilitation. Physical therapists held a minimum qualification of a bachelor's or master's degree in physical therapy, with a mean of 7 years of clinical experience. They were all certified in advanced musculoskeletal assessment. Exercise physiologists hold degrees in exercise science or sports science, with expertise in movement retraining and neuromuscular re-education for patients with ACL injury. A standardized protocol was used across all assessments to ensure consistency, with routine equipment calibration and interrater reliability checks implemented.

Statistical Analysis

The mean, standard deviations, and data spread were calculated and presented for continuous data, while count (n) and proportion (%) were discussed for categorical variables. These included predictor variables such as age at surgery, sex, and graft construct type. Between-group comparisons were performed using the Fisher exact test for dichotomous variables. Continuous variables were compared using a 2-tailed, unpaired t test in Microsoft Excel.

We also performed a subgroup analysis according to sex, comparing male and female pediatric patients, female pediatric and female adult patients, and male pediatric and male adult patients. This was based on established literature that shows women are at an increased risk of ACL injury secondary to developmental, anatomic, neuromuscular, and hormonal factors.^19,32,45

A 5% significance level was applied for all statistical analyses. Cohen's d value was 0.504 for H:Q data, and the achieved power was 0.99, indicating a well-powered sample size, with a moderate difference between groups.

Results

Patient Selection

Within the 6-year timeframe, 1377 patients underwent a primary ACLR, and 707 patients had completed FLT. Of these, 625 patients underwent ACLR with the hamstring autograft. A total of 120 pediatric patients formed the case group. We then selected a nested control group of adult patients who were matched to the pediatric case cohort for sex, body mass index (BMI), meniscal pathology rate, meniscal repair rate, preoperative TAS, and double-tendon graft construct rate. This was done to control for confounding variables on the functional limb testing results and including patients with complete FLT datasets.

Descriptive Data

Final analysis included 120 “pediatric” (mean age, 14.6 ± 1.6 years) patients and 210 “adult” patients (mean age, 29.70 ± 10.20 years) who were selected as the nested adult control group (Figure 1). Also, 46% of the pediatric cohort were female compared with 42% of the adult cohort. Table 1 demonstrates that the nested control group was matched to the pediatric case cohort for sex ratio, BMI, preoperative TAS, meniscal pathology rate, meniscal repair rate, and graft construct type. Depending on the number of strands of hamstring tendon utilized to attain the desired diameter, graft constructs were grouped into 2 groups: single or double hamstring grafts. The rate of lateral extra-articular tenodesis (LEAT) was the only variable that the nested cohort was unable to match, which was higher in the pediatric cohort (20%) compared with the adult cohort (3%).

Figure 1.

A flow chart of patients in the database and filtering those meeting the inclusion criteria into 2 cohorts. ACLR, anterior cruciate ligament reconstruction; FLT, functional limb testing.

Table 1.

Pre- and Intraoperative Patient Characteristics ^a

Cohort	Pediatric, ≤16 Years		Adult, >16 Years
	Mean	SD	Mean	SD	P
Number	120		210
Age, years	14.60	1.30	29.70	10.20
Sex, % F	0.46		0.42		.54
BMI, kg/m2	23	3.90	24	2.40	.06
TAS before ACLR	8.56	1.10	8.32	1.10	.06
Double HS graft construct, %	0.39		0.40		.81
Meniscal pathology, %	35		34		.83
Meniscal repair, %	18		21		.39
LEAT	20		3		<.001

^aACLR, anterior cruciate ligament reconstruction; BMI, body mass index; HS, hamstring; LEAT, lateral extra-articular tenodesis; Pre, preoperative; TAS, Tegner activity scale.

Functional Outcomes

Compared with the adult cohort, the pediatric cohort had a significantly higher hop distance LSI (97% vs 92%; P < .05), higher hop height LSI (92% vs 84%; P < .001), and greater quadriceps strength LSI (94% vs 88%; P < .001). The hamstring strength LSI was not significantly different between the pediatric and adult cohorts (P = .03). Together, this combination resulted in a lower H:Q ratio in the ACLR limb for the pediatric cohort compared with the adult cohort (41.49 vs 48.87; P < .001) (Table 2). The measured means of the hop distance (114 vs 102 cm), hop height (15 vs 13 cm), quadriceps strength (292 vs 277 N.m.), and hamstring strength (106 vs 116 N.m.) are provided for comparison.

Table 2.

Pediatric and Adult Cohorts’ Muscle Function Testing and ACL-RSI PROMs at 9 Months After ACLR ^a

Cohort	Pediatric, ≤16 Years		Adult, >16 Years
	Mean	SD	Mean	SD	P
LSI hop distance, %	97	13	92	16	<.05
LSI hop height, %	92	19	84	20	<.001
Quads strength LSI, %	94	13	88	16	<.001
HS strength LSI, %	90	16	92	19	.30
H:Q ratio healthy	44.23	16.15	45.37	11.01	.50
H:Q ratio ACLR	41.49	12.39	48.87	16.51	<.001
RSI change after ACL, %	103	20	104	24	.70
Post-ACL RSI total	74	18.96	63.81	22.48	<.001
Delta RSI	1.80	9.06	2.45	14.95	.60
Post-ACL RSI confidence subcategory	8	1.74	6.96	2.13	<.001

^aACL, anterior cruciate ligament; ACLR, anterior cruciate ligament reconstruction; H:Q, hamstring-to-quadriceps ratio; HS, hamstring; LSI, limb symmetry index (compared with contralateral limb score); PROMS, patient-reported outcome measures; RSI, return to sport index.

The postoperative ACL-RSI was broken down into 3 components: emotions, confidence, and risk appraisal. The pediatric cohort had a higher absolute score after ACLR (74 vs 64; P < .001) and confidence component (8 vs 6.90; P < .001) than the adult cohort.

Neuromuscular balance testing results showed higher distances in the posterolateral and posteromedial directions for the adults compared with the pediatric cohort; nonetheless, this was displayed in both affected and contralateral limbs, with both limbs carrying significant differences over their junior counterparts. Interestingly, the score in the anterior direction was not significant across the affected and contralateral limbs between the 2 cohorts. The LSI was nearly 100% for both limbs and both cohorts (Table 3).

Table 3.

Neuromuscular Balance Results for Both Cohorts Showing Only Posterior Direction Superiority Between Adults and Pediatric Groups ^a

	Pediatric, ≤16 Years		Adult, >16 Years
	Mean	SD	Mean	SD	P
YBT PM, ACLR	96.72	10.31	101.23	10.10	<.001
YBT PL, ACLR	94.87	10.08	98.03	10.22	<.05
YBT Ant, ACLR	59.83	8.97	60.65	7.99	.40
YBT PM, healthy	96.68	10.78	101.17	9.75	<.05
YBT PL, healthy	94.75	10.61	97.98	10.61	<.05
YBT anterior, healthy	60.32	7.48	61.37	7.28	.20
LSI PM, %	100	5	100	5	.80
LSI PL, %	101	10	100	7	.70
LSI anterior, %	99	9	99	9	.80

^aACLR, anterior cruciate ligament reconstruction; Ant, anterior; H:Q, hamstring-to-quadriceps ratio; LSI, limb symmetry index (compared with contralateral limb score); PL, posterolateral; PM, posteromedial; YBT, neuromuscular Y-balance test.

Sex Subgroup Analysis

We performed a subanalysis to compare pediatric females to their male counterparts. The only significant demographic difference between the 2 groups was BMI (24 ± 4.10 vs 22 ± 3.50 kg/m² ; P < .05) (Table 4). There were no significant differences between pediatric boys and girls in FLT (Table 5).

Table 4.

Descriptive Data for Subgroup Analysis of Pediatric Girls and Boys ^a

	Pediatric Girls		Pediatric Boys
	Mean	SD	Mean	SD	P
Number	55		65
LEAT, %	20		18
BMI, kg/m2	24	4.10	22	3.50	<.05
% double tendon construct	42		37
Meniscal pathology, %	33		37
Meniscal repair, %	18		17
TAS before ACLR	8.58	1.07	8.49	1.43	.70
TAS after ACLR	8.53	1.05	8.63	0.99	.60

^aACLR, anterior cruciate ligament reconstruction; BMI, body mass index; LEAT, lateral extra-articular tenodesis; TAS, Tegner activity scale.

Table 5.

Strength, Balance, and ACL-RSI of Pediatric Girls and Boys ^a

Cohort	Pediatric Girls		Pediatric Boys
	Mean	SD	Mean	SD	P
YTB PM LSI, %	98	13	100	4	.26
YBT PL LSI, %	99	12	102	9	.23
YBT Ant LSI, %	99	13	99	5	.99
LSI hop distance, %	98	7	97	16	.75
LSI hop height, %	90	17	94	20	.16
Quads strength LSI, %	94	13	94	13	.97
HS strength LSI, %	90	17	89	16	.67
H:Q ratio, healthy	44.29	20	44.17	12.14	.97
H:Q ratio, ACLR	41.18	13.16	41.75	11.79	.80
Post-ACL RSI total	69	20	78	18	<.05

^aACLR, anterior cruciate ligament reconstruction; Ant, anterior; H:Q, hamstring-to-quadriceps ratio;

HS, hamstring; LSI, limb symmetry index (compared with contralateral limb score); PL, posterolateral; PM, posteromedial; RSI, return to sport index; YBT, neuromuscular Y-balance test.

Neuromuscular balance tests, hop tests, and limb strength indices were comparable and showed no statistically significant difference between female and male pediatric patients. The PROM ACL RTS index score showed a significant difference in favor of the male pediatric cohort (69 vs 78; P < .05). This indicated that male pediatric patients are more confident than female patients after surgery.

Subgroup Analysis—Pediatric Female Versus Adult Female Patients

The second subanalysis, between female patients within the pediatric and adult cohorts, reflected the main analysis. Demographically, only the LEAT percentage and the preoperative TAS had higher scores in the pediatric groups with statistical significance (Table 6). The RTS data comparison showed that the pediatric cohort had a higher LSI hop distance (98% vs 92%; P < .05), LSI hop height (90% vs 83%; P < .05), quadriceps strength LSI (94% vs 86%; P < .05), and H:Q strength ratio of ACLR limb (41.18 vs 48.50; P < .05) compared with the adult cohort (Table 7). Girls demonstrated higher ACL-RSI scores than women, indicating they were more confident; however, this was not statistically significant.

Table 6.

Descriptive Data of Pediatric and Adult Female Patients ^a

Cohort	Girls		Women
	Mean	SD	Mean	SD	P
Number	55		89
LEAT, %	20		2
BMI, kg/m2	24	4.10	23	2.20	<.05
% double tendon construct	42		46
Meniscal pathology, %	33		38
Meniscal repair, %	18		26
TAS before ACLR	8.58	1.07	8.16	1.25	<.05
TAS after ACLR	8.53	1.05	8.29	1.09	.20

^aACLR, anterior cruciate ligament reconstruction; BMI, body mass index; LEAT, lateral extra-articular tenodesis; TAS, Tegner activity scale.

Table 7.

Strength, Balance, and ACL-RSI of Female Pediatric and Adult Patients ^a

Cohort	Girls		Women
	Mean	SD	Mean	SD	P
YTB PM LSI, %	98	13	100	4	.20
YBT PL LSI, %	99	12	100	6	.80
YBT Ant LSI, %	99	13	98	7	.50
LSI hop distance, %	98	7	92	16	<.05
LSI hop height, %	90	17	83	17	<.05
Quads strength LSI, %	94	13	86	16	<.001
HS strength LSI, %	90	17	92	21	.60
H:Q ratio, healthy	44.29	20	45.23	11.11	.70
H:Q ratio ACLR	41.18	13.16	48.57	14.61	<.05
Post-ACL RSI total	71	20	62	22	.06

^aACLR, anterior cruciate ligament reconstruction; Ant, anterior; H:Q, hamstring-to-quadriceps ratio; HS, hamstring; LSI, limb symmetry index (compared with contralateral limb score); PL, posterolateral; PM, posteromedial; RSI, return to sport index; YBT, neuromuscular Y-balance test.

Subgroup Analysis—Pediatric Versus Adult Male Patients

The final subanalysis conducted in this study compared men from each cohort. This reflected the female findings. The BMI was found to be statistically different (22 vs 24 kg/m²; P < .001). Pre- ACLR TAS scores were similar; however, post-ACLR TAS scores showed a larger disparity (8.63 vs 8.27; P < .05) (Table 8). Functionally, the pediatric male cohort had a higher LSI hop distance (97% vs 92%; P < .05), LSI hop height (94% vs 84%; P < .05), quadriceps strength LSI (94% vs 89%; P < .05), and H:Q ratio (41.75 vs 49.09; P < .05) (Table 9). In addition. ACL-RSI scores after ACLR were 78 versus 65 (P < .001), indicating that boys were more confident than the adult male cohort.

Table 8.

Descriptive Data for Pediatric Males Compared With Adult Males ^a

Cohort	Boys		Men
	Mean	SD	Mean	SD	P
Number	65		121
LEAT, %	18		3
BMI, kg/m2	22	3.50	24	2.30	<.001
% double tendon construct	37		39
Meniscal pathology	37		31
Meniscal repair	17		18
TAS before ACLR	8.49	1.43	8.36	1.02	.52
TAS after ACLR	8.63	0.99	8.27	1.02	<.05

^aACLR, anterior cruciate ligament reconstruction; BMI, body mass index; LEAT, lateral extra-articular tenodesis; TAS, Tegner activity scale.

Table 9.

Strength, Balance, and ACL-RSI of Pediatric Males Versus Adult Males ^a

Cohort	Boys		Men
	Mean	SD	Mean	SD	P
YTB PM LSI, %	100	4	100	5	.82
YBT PL LSI, %	102	9	101	7	.42
YBT Ant LSI, %	99	5	100	10	.65
LSI hop, % distance	97	16	92	17	<.05
LSI hop height, %	94	20	84	22	<.05
Quads strength LSI, %	94	13	89	16	<.05
HS strength LSI, %	89	16	91	17	.36
H:Q ratio healthy	44.17	12.14	45.46	10.99	.48
H:Q ratio ACLR	41.75	11.79	49.09	17.82	<.001
Post-ACL RSI total	78	18	65	23	<.001

^aACLR, anterior cruciate ligament reconstruction; Ant, anterior; H:Q, hamstring-to-quadriceps ratio; HS, Hamstring; LSI, limb symmetry index (compared with contralateral limb score); PL, posterolateral; PM, posteromedial; RSI, return to sport index; YBT, neuromuscular Y-balance test.

Discussion

This study aimed to look for differences in neuromuscular recovery, donor site morbidity, and psychological confidence after ACLR between adults and children. We hypothesized that functional recovery in children would be slower than that in adults, and perhaps this might be 1 reason behind their increased risk of reinjury. Through our analyses, we tracked functional recovery prospectively to find points of disparity between adults and pediatrics in what is a nested case-control cohort study.

A significant finding of our study revolved around quadriceps strength, where the pediatric population demonstrated higher rates of limb symmetry compared with adults. Specifically, indices such as hop height LSI, hop distance LSI, and quadriceps strength LSI were notably superior in pediatric patients. Olson et al ³⁶ found that satisfactory strength scores do not always translate to consistent RTS rates. Despite this enhanced quadriceps LSI in pediatric patients, hamstring LSI was not found to be significantly different between adults and children. Both adults and children demonstrate a significant level of hamstring weakness at a similar severity. This finding of accelerated quadriceps recovery in children was reflected in a lower H:Q ratio, a factor previously associated with increased risks of rerupture and contralateral injury. ²⁸ This was shown in comparisons between girls and women, boys and men, and children and adults. The cause of this discrepancy in quadriceps strength LSI is unknown.

The H:Q strength ratio is 1 parameter in assessing the risk of ACL rupture and postsurgical outcomes. Several studies have highlighted its importance in understanding the biomechanical factors influencing ACL integrity.^18,25,31 Our study found that in the pediatric cohort, the H:Q ratio was lower compared with the uninjured leg (44.23 to 41.49) and was higher in the adult cohort (45.37 to 48.87), primarily because of stronger quadriceps and relatively weaker hamstrings in the pediatric group. Research by Hewett et al ²⁰ demonstrated that a lower H:Q ratio is associated with an increased risk of ACL injury, particularly in female athletes. This imbalance in muscle strength may predispose individuals to dynamic knee instability, potentially leading to noncontact ACL injuries during activities involving rapid deceleration, cutting, and pivoting motions. Moreover, a systematic review by Sugimoto et al^40,45 found that athletes with ACL injuries often exhibit a reduced H:Q ratio compared with uninjured counterparts. This imbalance not only affects ACL injury risk but also influences postoperative outcomes after ACLR. A study by Léger-St-Jean et al ³⁰ observed that patients with a lower preoperative H:Q ratio had a higher likelihood of ACL graft failure after surgery.^10,26,28 Furthermore, rehabilitation strategies aimed at restoring a balanced H:Q ratio have been shown to improve functional outcomes and reduce the risk of ACL reinjury.^10,44 Studies by Myer et al ³⁴ and Iga et al ²³ emphasized the importance of incorporating hamstring strengthening exercises into rehabilitation protocols to enhance dynamic knee stability and mitigate the risk of ACL graft rupture. The relative weakness of the pediatric hamstring compared with the quadriceps may contribute to excessive anterior tibial translation, placing additional strain on the ACL graft. In the context of higher reinjury rates in this cohort, the imbalance underscores a need for targeted rehabilitation strategies that prioritize hamstring conditioning to gain dynamic knee stability.

While this study does not directly assess reinjury rates, the findings suggest that addressing H:Q imbalances should be a key focus in RTS protocols to minimize long-term injury risk.

Our investigation delved into sex-based differences within the pediatric cohort, revealing no significant disparities in hop height, distance, quadriceps strength, limb strength index, or neuromuscular balance between men and women. However, a noteworthy finding was the higher postsurgical ACL-RSI scores observed in the pediatric cohort for boys versus men, suggesting potentially faster psychological recovery and greater readiness for athletic activities. Girls did not show a statistically significant difference in this aspect. ⁹ Notably, while previous literature has delineated differences in quadriceps strength among postmenarche women, our study found no such distinctions in premenarche women, underscoring the importance of age and physiological maturity in assessing rehabilitation outcomes.^19,45

Another point of discussion was the difference between neuromuscular testing results between pediatric and adult cohorts for the posterolateral and posteromedial directions, but also the similarities between the anterior direction results. It has been hypothesized that the posterolateral and posteromedial aspects rely more on the hip abductors to stabilize the body as the trunk is furthest away from the centre of mass in the sagittal plane. ³⁵ Development of a strong core, gluteal muscle bulk through adolescence, and a lower center of gravity may be supportive of better performance by the adult cohort here.

With regard to the PROMS, the pediatric group improved their TAS after ACLR and rehabilitation, while the adult cohort recorded a minor negative trend. The difference was only statistically significant with the “post-ACLR” TAS score. The ACL-RSI scores for both groups differed by comparable amounts between pre-ACLR and postoperative RTS testing. Moreover, our findings align with a related study by Thorolfsson et al, ⁴⁶ which suggests increased psychological readiness and self-efficacy among pediatric patients.^33,46 This heightened confidence may contribute to the observed higher RTS index in our pediatric cohort. However, it also implies that pediatric patients may exhibit reduced caution in their RTS decision-making, warranting careful consideration in postoperative management strategies. Randsborg et al ⁴³ also support the view that increased self-reported confidence and a lack of fear are major drivers in RTS, a view supported by Cronström et al. ⁹

Despite these significant findings, our study is not without limitations. First, we did not analyze reinjury rates, a crucial aspect in evaluating the long-term efficacy of ACLR. Second, our classification of pediatric patients was based solely on age and not skeletal maturity, potentially overlooking variances in recovery trajectories based on physiological developmental stage. LEAT rates in the cohort groups were not matched, with a sizeable increase in utility in the younger cohort. This additional procedure is receiving more interest and usage in patients at high risk of graft rupture, as there is high-quality evidence of increased ACL graft survivorship due to the restrained tibial internal rotatory forces.^{13,27, 37} As there has been recent evidence that a LEAT procedure, when done with an ACLR, has led to reduced quadriceps strength at 6 to 9 months postoperatively, it is likely that LEAT would have the effect of increasing extensor mechanism donor site morbidity. ⁴⁸ Our study may be underpowered; however, the decision to compare closely matched nested control groups meant limiting our data to yield better results. Another limitation of this study is the potential for selection bias because of the voluntary nature of RTS testing, which required financial investment from patients. This may have led to an overrepresentation of highly motivated individuals with greater access to resources, potentially skewing the results. This may also explain the low RTS completion rate (707/1377). Moreover, some patients who commenced the RTS protocol did not complete all required follow-ups, further limiting data completeness. While efforts were made to incentivize continued participation and improve physical therapy engagement, selection bias remains an inherent challenge in this study design. Last, our study population consisted solely of private patients, potentially limiting the generalizability of our findings to the broader population, particularly those with differing access to physical therapy resources in public healthcare settings. Addressing these limitations in future research endeavors would provide a more comprehensive understanding of the nuances in neuromuscular recovery and morbidity outcomes after ACLR across diverse demographics.

In terms of future directions, the potential of prolonged donor site morbidity and the possible role it plays in weaker hamstring functional tests may demand a reconsideration of graft choices in the pediatric population undergoing ACLR. The generation of personalized rehabilitation programs dedicated to developing hamstring strength and hamstring-quadriceps balance in pediatric patients is well supported.^1,2 Dekker et al ¹² have linked an earlier RTS with a high risk of second ACL injury. Hence, the role for prolonged rehabilitation before RTS in the pediatric population would be worth discussing—although controversial and difficult to implement.^{9,16,21,22,51} Future studies could consider alternative strategies to incentivize completion of RTS testing—such as subsidized testing or more structured follow-up protocols—to enhance data completeness and minimize bias.

Conclusion

This study aimed to seek neuromuscular, functional, psychological, and sex differences between pediatric and adult cohorts undergoing RTS testing 9 months after ACLR. Male pediatric patients were significantly more confident than pediatric female patients with ACL-RSI scores (mean, 78 vs 69; P < .05). Children had higher ACL-RSI scores than adults (mean, 74 vs 63.80; P < .001), which was mainly because of the increased scores among pediatric males.

We identified that children outperformed adults with greater hop distance LSI (97% vs 92%; P < .05), greater hop height LSI (92% vs 84%; P < .001), and greater quadriceps LSI in all (94% vs 88%; P < .001). These 3 findings had no relationship to sex. Both adults and children demonstrated similar levels of reduced hamstring LSI (donor site morbidity). As a result, the pediatric cohort had a reduced H:Q ratio compared with the adult cohort (41.50 vs 48.90; P < .001).

Increased confidence in male pediatric patients and reduced H:Q ratio in all pediatric patients may represent additional risk factors for subsequent ACL reinjury in those returning to sports. Further studies are required to explore these plausible relationships.

Appendix

Appendix A1. Detailed ACL Rehabilitation Protocol

Phase 1—Protection and mobilisation (0-6 weeks):

• Goals:

  • Reduce pain and swelling through cryotherapy and compression.
  • Achieve full knee extension and at least 90° of flexion.
  • Initiate quadriceps activation and protected weightbearing.
• Key exercises:

  • Isometric quadriceps exercises, passive range of motion (heel slides), and patellar mobilisations.
  • Weightbearing progression with crutches as tolerated.
  • Closed kinetic chain exercises (eg, mini squats, step-ups).

Phase 2—Strength and Nneuromuscular control (6-12 weeks):

• Goals:

  • Normalize gait without assistive devices.
  • Improve quadriceps and hamstring strength.
  • Introduce proprioceptive exercises and functional movements.
• Key exercises:

  • Progressive resistance training (leg press, step-downs, squats).
  • Proprioceptive exercises such as balance boards and single-leg stance.
  • Stationary cycling and controlled step-ups/downs.

Phase 3—Advanced strengthening and agility preparation (3-6 months):

• Goals:

  • Build muscle endurance and dynamic stability.
  • Initiate controlled plyometrics.
  • Prepare for the gradual reintroduction of impact activities.
• Key exercises:

  • Resistance training with increased loads (deadlifts, lunges).
  • Controlled jumping and landing drills.
  • Single-leg strengthening and proprioception drills.
  • Progressive resistance exercises with free weights and machines.

Phase 4—Running and change of direction training (6-9 months):

• Goals:

  • Progress to running with controlled changes in speed and direction.
  • Develop sport-specific skills with gradual intensity.
  • Enhance neuromuscular control to prevent reinjury.
• Key exercises:

  • Treadmill running progressing to outdoor terrain.
  • Sprinting, agility drills, and sport-specific movement patterns.
  • Lateral movement and deceleration training.
  • Increasing plyometric loading with multidirectional jumps.

Phase 5—Return-to-sports preparation (9+ months):

• Goals:

  • Achieve full strength and functional symmetry.
  • Pass return-to-sports criteria based on functional testing.
  • Ensure psychological readiness for competitive activity.
• Key exercises:

  • Maximal effort plyometrics and reaction drills.
  • Simulated game movements under supervision.
  • Sport-specific conditioning with full intensity.
• Return-to-sport criteria:

  • Limb symmetry index of ≥90% for strength and hop tests.
  • Completion of psychological readiness assessments.
  • Clearance by medical professionals and physiotherapists.