Association of Preoperative Asymmetry in Vertical Ground-Reaction Force With Knee Extensor Symmetry 1 Year After Anterior Cruciate Ligament Reconstruction

Abstract

Background:

Preoperative and postoperative asymmetries in knee kinematics and kinetics, particularly in vertical ground-reaction force (vGRF), have been linked to long-term functional outcomes after anterior cruciate ligament (ACL) reconstruction. Gait asymmetry may indicate altered neuromuscular control and compensatory cocontraction strategies, which can compromise joint integrity and delay recovery.

Purpose:

To determine whether preoperative vGRF asymmetry could predict postoperative knee extensor strength symmetry and neuromuscular recovery 1 year after ACL reconstruction.

Study Design:

Cohort study; Level of evidence, 2

Methods:

A total of 55 patients undergoing ACL reconstruction were prospectively evaluated before surgery and at 1 year postoperatively. Assessments included vGRF, gait speed, surface electromyography, isokinetic knee extensor/flexor strength, and the Knee injury and Osteoarthritis Outcome Score (KOOS) questionnaire. Based on preoperative vGRF data, participants were categorized into lesser-loading and greater-loading groups. Three-dimensional motion capture and synchronized force plates were used to analyze gait and cocontraction indices. Generalized estimating equations were applied to evaluate group × time effects, with statistical significance set at P < .05.

Results:

Both groups demonstrated pain reduction and improved KOOS scores postoperatively, accompanied by partial restoration of vGRF symmetry. The greater-loading group (n = 28) exhibited significant improvement in knee extensor strength symmetry (from 66% to 87%; P = .001), whereas the lesser-loading group (n = 27) showed persistent muscle cocontraction (0.12-0.12; P = .94) and less improvement in knee extensor strength symmetry (from 67% to 75%; P = .11). These results indicate distinct neuromuscular adaptation patterns depending on preoperative loading asymmetry.

Conclusion:

Keywords

vertical ground-reaction force walking cocontraction health education

Residual quadriceps weakness and functional deficits remain major problems for many patients after anterior cruciate ligament (ACL) reconstruction.⁵² Previous studies have shown that age, sex, knee pain, and preoperative quadriceps strength can predict postoperative quadriceps recovery.⁵⁴ In addition to these individual factors, kinematic and kinetic asymmetries are often observed between the injured and uninjured limbs after ACL rupture or reconstruction.³⁹ In particular, after knee surgery, kinetic asymmetry has been identified as an important predictor of poor knee function.^2,19 These asymmetries often reflect compensatory gait strategies that have been related to lower limb muscle strength.⁶⁰ However, whether preoperative gait adaptations influence postoperative muscle and neuromuscular recovery remains unclear.

Previous studies have shown that patients may continue to demonstrate asymmetry in vertical ground-reaction force (vGRF) during everyday activities such as stair climbing, sitting, drop jumps, and bilateral squats, even up to 7 years after reconstruction.^4,30,44 More importantly, greater limb symmetry in peak vGRF at 6 months after surgery has been associated with better knee function at 12 months.⁴⁰ Therefore, vGRF asymmetry may not only serve as a surrogate for knee kinetic asymmetry but also reveal underlying movement disorders.^4,58 Moreover, restoring gait symmetry is a long-standing therapeutic goal in physical therapy and has recognized clinical value across various diseases and functional impairments.³⁴ In this context, vGRF is a viable target for modifying gait biomechanics.⁷

On the other hand, vGRF asymmetry may reflect altered neuromuscular control associated with thigh muscle cocontraction. High cocontraction in the injured limb may represent a protective strategy to stabilize the knee in the absence of adequate passive restraint following ACL injury.¹² Such neuromuscular adaptations may lead to a cautious gait pattern and reduced limb loading during walking, thereby contributing to lower vGRF on the involved side. A similar pattern has been observed during early postoperative stages,⁵¹ and elevated cocontraction can persist during single-leg landing tasks for up to 3 years after ACL reconstruction.⁵³ Persistently high cocontraction may compromise joint health and increase the risk of posttraumatic osteoarthritis after ACL reconstruction.⁹ However, few studies have directly examined the relationship between gait-related vGRF asymmetry and postoperative quadriceps strength and neuromuscular activity.

Therefore, the purpose of this study was to investigate postoperative muscle recovery in patients stratified according to preoperative injured-limb vGRF during walking. We hypothesized that patients exhibiting lower vGRF on the injured limb preoperatively would demonstrate poorer recovery of quadriceps strength and persistent thigh muscle cocontraction, as assessed by electromyography, 1 year after ACL reconstruction.

Methods

Study Cohort

Between November 2013 and July 2020, we prospectively enrolled 118 patients (aged 18-60 years) who underwent ACL reconstruction. The surgery was performed using a single-bundle technique with quadruple hamstring grafts. All patients were monitored for >1 year. Patients were excluded from consideration in the current study if they (1) had a history of surgery involving either knee, (2) had a history of partial ACL tear, (3) had concurrent injury with other ligaments, (4) had participated in regular exercise training within the 6 months preceding surgery, or (5) were long-term athletes prior to injury. We measured anthropometric parameters, vGRF, walking speed, muscle strength, and muscle activity and administered the Knee injury and Osteoarthritis Outcome Score (KOOS) questionnaire and a visual analog scale (VAS) both before and 1 year after ACL reconstruction. All biomechanical and neuromuscular assessments were conducted in a motion laboratory. All participants were of Taiwanese descent, and they resided in the southern region of the country. The study protocol was approved by the institutional review board of Chang Gung Memorial Hospital.

Measurement of vGRF and Walking Speed

Three-dimensional kinematic data were recorded at 100 Hz by using an optoelectronic 8-camera motion capture system (MX-T20; Vicon) during 3e walking tests. Bilateral gait evaluation was performed using a modified Plug-in Gait marker set, with markers affixed to the skin of the lower body by using adhesive surgical tape.²⁴ Patients were instructed to walk barefoot at their preferred speed. Two floor-embedded force plates (OR6; AMTI) synchronized with the motion capture system recorded vGRF during walking at a sampling rate of 1000 Hz. Three commonly used vGRF parameters (F1, F2, and F3) were evaluated from the total force–time curve and normalized to body weight. F1 represents the first maximal vertical force generated during heel contact (weight acceptance peak force), and F3 represents the second maximal vertical force required to lift the forefoot from the ground (push-off peak force). F2 represents the minimal force between F1 and F3 (Figure 1).⁵⁶ The vGRF loading rate was calculated from the initial half of the stance phase and normalized to body weight.⁴⁰ All data were processed using the Nexus motion analysis system, which was integrated with a data recording software (Version 2.5; Oxford Metrics). Each walking test was conducted 3 times, and the mean value was used for subsequent analysis. The patients warmed up by walking back and forth for 5 minutes before each test and then walked barefoot at a preferred speed across the embedded force plates. On the basis of preoperative vGRF data, we divided the patients into 2 groups: lesser and greater loading groups. In the lesser loading group, vGRF F1 on the injured side was less than that on the uninjured side. By contrast, in the greater loading group, vGRF F1 on the injured side was more than or equal to that on the uninjured side.⁴⁰

Figure 1.

Vertical ground-reaction force parameters measured during the stance phase of walking. F1, F2, and F3 represent the first maximal force (impact force), minimal force between the impact force and propulsive force, and second maximal force (propulsive force), respectively.

Measurement of Muscle Activity

Original surface electromyography signals of the 2 dominant knee muscles (vastus lateralis [VL] and lateral hamstring [LH]) were recorded during the stance phase of walking.³⁵ Electromyography amplitudes for VL and LH were recorded using surface electrodes (Medi-Trace 200; Covidien/Kendall) and sampled at 1000 Hz (BioNomadix; BIOPAC Systems). All electromyography data were processed using the Nexus motion analysis system integrated with the data recording software. Surface electromyography parameters (root mean square [RMS] areas and RMS amplitudes) were normalized to the mean RMS amplitude recorded during quiet stance for each muscle and limb, providing a consistent baseline reference for gait-related neuromuscular activity rather than a measure of maximal activation. Normalized RMS amplitude was calculated as the mean RMS amplitude during each walking phase divided by the mean RMS amplitude obtained during quiet stance for the same muscle.²⁷ The muscle cocontraction index (CCI), defined as the simultaneous activation of the agonist and antagonist muscles (VL–LH), was calculated from the linear envelopes of corresponding muscles by using the following formula⁴²: CCI value = (less active muscle/more active muscle) × (less active muscle + more active muscle). The CCI is a dimensionless variable, as it represents a ratio derived from normalized electromyographic signals.

Measurement of Muscle Strength

The muscle strength of the lower extremity including the knee flexor and extensor was tested using the HUMAC NORM system (CSMi Solutions) in the concentric/concentric contraction mode at an angular velocity of 60 deg/s.⁵² Participants were securely positioned according to the manufacturer's standardized protocol, with stabilization straps applied across the trunk, pelvis, and thigh, and were allowed to hold the dynamometer handles for postural stability. Each participant completed 5 isokinetic trials, with verbal encouragement provided to promote maximal torque generation. Muscle strength was normalized to body weight for statistical analysis. The knee extensor symmetry index was calculated using the following formula³² and compared between the groups: (maximal torque on the injured side/maximal torque on the uninjured side) × 100%.

Study Tools

The patients completed the 42-item self-administered KOOS questionnaire. This knee-specific questionnaire includes 5 subscales that assess Pain (9 items), Symptoms (7 items), Activities of Daily Living (17 items), Sport and Recreational activities (Sport/Rec, 5 items), and knee-related Quality of Life (4 items). Each item is scored from 0 to 4, and subscale scores are transformed to a 0 to 100 scale, where 0 indicates severe knee problems and 100 indicates no knee problems.⁴¹ Furthermore, the patients rated their pain on a VAS ranging from 0 cm (no pain) to 10 cm (maximal pain).⁵

Statistical Analysis

All data were analyzed using SPSS (Version 26.0; SPSS Inc). Continuous data are presented as mean and standard deviation values. No abnormal distributions were noticed in pretests according to Shapiro-Wilk results for both groups. The relative effect size for the performance data was calculated using Cohen d. It is defined as the difference between 2 means divided by a standard deviation for the data. Intergroup differences in patient characteristics were determined using the independent-samples t test. A paired-samples t test was used to compare differences between the injured and uninjured sides. The Pearson chi-square test was used to analyze nominal categorical variables. Generalized estimating equations were used to determine intergroup and intragroup differences (group × time).²¹ A P value of < .05 was considered to indicate significance.

Results

A total of 55 patients completed the 1-year follow-up assessment. Among them, 27 patients were in the lesser loading group, and 28 were in the greater loading group. Some patients were lost to follow-up because of personal reasons, such as relocation, perceived full recovery, reinjury, and return to training. The mean age was 32 ± 12 years in the lesser loading group and 31 ± 11 years in the greater loading group. The 2 groups were similar in terms of height (P = .88), weight (P = .19), body mass index (P = .07), and interval between injury and reconstruction. In the lesser loading group, 15 patients underwent reconstruction in the acute phase (14-90 days after injury), whereas 12 patients underwent reconstruction in the nonacute phase (>90 days after injury). In the greater loading group, 9 patients underwent reconstruction in the acute phase, and 19 did in the nonacute phase. The Pearson chi-square test revealed no significant intergroup difference in reconstruction timing (P = .08), sex (P = .93), affected side (P = .35), or meniscal injury (P = .49) (Appendix Table A1).

In the lesser loading group, the mean VAS scores decreased significantly from 3.1 ± 0.5 before surgery to 0.9 ± 0.3 (P = .001) at the 1-year follow-up. A similar trend was observed in the greater loading group, with the VAS score decreasing from 2.9 ± 0.4 to 1.2 ± 0.3 (P = .001). No significant intergroup difference was noted.

Preoperatively, all patients exhibited vGRF asymmetry between the injured and uninjured sides while walking, and significant intergroup differences were observed (Table 1). Notably, vGRF on the uninjured side was higher in the lesser loading group than in the greater loading group, but the opposite trend was observed for the injured side. In the lesser loading group, vGRF on the injured side increased significantly by the 1-year follow-up, whereas that (F2) on the uninjured side decreased. An opposite trend was noted in the greater loading group (Figure 2, A-C).

Table 1

Gait Loading and Walking Speed in the Lesser and Greater Loading Groups ^a

	Injured Side				Uninjured Side
	Preoperatively		1-y Follow-up		Preoperatively		1-y Follow-up
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
	F1 (%BW)
Lesser loading	81	2	94	3 ^b	100	2 ^d	96	3
Greater loading	96	2 ^c	90	3	80	2 ^{c d}	94	3 ^b
P between groups	.001		.33		.001		.65
Cohen d	7.50		1.33		10.00		0.67
	F2 (%BW)
Lesser loading	61	2	68	3 ^b	75	2 ^d	66	3 ^b
Greater loading	78	2 ^c	68	3 ^b	62	2 ^c	72	3 ^b
P between groups	.001		.96		.001		.09
Cohen d	8.50		0.00		6.50		2.00
	F3 (%BW)
Lesser loading	84	2	95	3 ^b	104	2 ^d	98	3
Greater loading	97	2 ^c	93	3	84	2 ^{c d}	99	3 ^b
P between groups	.001		.58		.001		.90
Cohen d	6.50		0.67		10.00		0.33
	Loading rate (%BW/s)
Lesser loading	34	1	41	2 ^b	41	2 ^d	42	2
Greater loading	35	1	41	2b	38	2	41	2
P between groups	.63		.96		.24		.61
Cohen d	–1.00		0.00		1.50		050
	Walking speed (m/s)
Lesser loading	1.15	0.03	1.21	0.03	1.13	0.03	1.22	0.03 ^b
Greater loading	1.05	0.03 ^c	1.14	0.02 ^{b c}	1.03	0.03 ^c	1.13	0.02 ^{b c}
P between groups	.03		.03		.04		.01
Cohen d	3.33		2.76		3.33		3.54

BW, body weight.

P < .05, between preoperatively and 1-year follow-up.

P < .05, between lesser loading and greater loading.

P < .05, between uninjured side and injured side.

Figure 2.

(A) Impact force on the injured side (F1). (B) Minimal force between F1 and propulsive force (F3) on the injured side (F2). (C) Propulsive force on the injured side (F3). (D) Loading rate on the injured side (F2). ^*P < .05, between lesser and greater loading group; ^†P < .05, between preoperatively and 1-year follow-up.

During the follow-up period, loading rate increased significantly on the injured side in both groups, whereas no significant temporal changes were observed on the uninjured side. Preoperatively, significant between-limb differences in loading rate were present only in the lesser loading group, whereas no such asymmetry was observed in the greater loading group (Figure 2D, Table 1).

Walking speed was significantly lower bilaterally in the greater loading group compared with the lesser loading group prior to surgery and remained lower at the 1-year follow-up. Across all participants, walking speed increased significantly over time on both the injured and the uninjured sides. However, in the lesser loading group, this temporal improvement was not observed on the injured side, whereas the uninjured side demonstrated a significant increase (Figure 3, Table 1).

Figure 3.

Walking speed for the injured side. Significance: ^*P < .05, between lesser and greater loading group; ^†P < .05, between preoperatively and 1-year follow-up.

Significant postoperative changes were noted in the CCI values, with the exception of that corresponding to the injured side in the lesser loading group. At the 1-year follow-up, both groups exhibited significant differences between the injured and uninjured sides. The lesser loading group exhibited higher CCI values than did the greater loading group (0.12 vs 0.08; P = .049), while comparable reductions over time were observed on the uninjured side in both groups (Figure 4, Table 2).

Figure 4.

Cocontraction index (vastus lateralis–lateral hamstrings) during walking with stance phase. ^*P < .05, between lesser and greater loading group; ^†P < .05, between preoperatively and 1-year follow-up.

Table 2

Neuromuscular Function and Knee Muscle Strength in the Lesser and Greater Loading Groups ^a

	Injured Side				Uninjured Side
	Preoperatively		1-y Follow-up		Preoperatively		1-y Follow-up
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
	Cocontraction Index
Lesser loading	0.12	0.02	0.12	0.02	0.11	0.02	0.07	0.01 ^{b c}
Greater loading	0.10	0.01	0.08	0.01 ^{b d}	0.10	0.01	0.06	0.01 ^{b c}
P between groups	.57		.049		.66		.64
Cohen d	1.27		2.54		0.14		1.00
	Knee extensor (%BW)
Lesser loading	84	7	121	8 ^b	127	9 ^c	162	9 ^{b c}
Greater loading	85	8	137	10 ^b	131	8 ^c	160	9 ^b
P between groups	.90		.22		.74		.90
Cohen d	0.13		1.76		0.47		0.22
	Knee flexor (%BW)
Lesser loading	68	4	93	5 ^b	86	5 ^c	105	5 ^b
Greater loading	69	5	107	5 ^{b d}	87	5 ^c	118	5 ^b
P between groups	.95		.047		.95		.08
Cohen d	0.22		2.80		0.20		2.60

BW, body weight.

P < .05, between preoperatively and 1-year follow-up.

P < .05, between un-injured side and injured side.

P < .05, between lesser loading and greater loading.

Knee extensor strength increased significantly from preoperative testing to the 1-year follow-up on both the injured and the uninjured sides in both groups (Table 2). On the injured side, improvements were observed in both the lesser loading (84%-121% body weight) and greater loading groups (85%-137% body weight), with no significant between-group differences at either time point. On the uninjured side, knee extensor strength also increased significantly over time in both groups and remained higher than that of the injured side at both assessments.

Knee flexor strength similarly increased significantly on both sides in both groups (Table 2). At the 1-year follow-up, the greater loading group demonstrated significantly higher injured-side knee flexor strength than the lesser loading group (107% vs. 93% body weight; P = .047), whereas no between-group differences were observed on the uninjured side.

The knee extensor symmetry index increased significantly in the greater loading group, from 66% ± 6% to 87% ± 4% body weight (P = .001). No significant improvement was observed in the lesser loading group (from 67% ± 4% to 75% ± 3% body weight; P = .11). At the 1-year follow-up, the knee extensor symmetry index was significantly higher in the greater loading group than in the lesser loading group (P = .03; Cohen d = 3.39) (Figure 5).

Figure 5.

Knee extensor symmetry index. ^*P < .05, between lesser and greater loading group; ^†P < .05, between preoperatively and 1-year follow-up.

Significant improvements were noted in all KOOS subscale scores, except for the KOOS Symptoms score in the greater loading group (Table 3). No intergroup difference in the KOOS was observed preoperatively or 1 year postoperatively.

Table 3

Knee Injury and Osteoarthritis Outcome Scores of the Study Cohort ^a

	Lesser Loading				Greater Loading
	Preoperatively		1-y Follow-up		Preoperatively		1-y Follow-up
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
Symptoms	64	3	72	3 ^b	71	3	77	3
Pain	75	3	84	3 ^b	73	3	86	3 ^b
ADL	80	3	90	3 ^b	83	3	90	2 ^b
Sport/Rec	50	5	70	4 ^b	42	4	77	2 ^b
QOL	46	4	62	4 ^b	43	3	65	3 ^b

ADL, Activities of Daily Living; QOL, Quality of Life; Sport/Rec, Sport and Recreation.

P < .05, between preoperatively and 1-year follow-up.

Discussion

In this study, patients were stratified based on preoperative vGRF asymmetry, resulting in predefined differences in limb loading at baseline by study design. The major findings of our study showed that vGRF converged between groups and between limbs 1 year after ACL reconstruction. However, despite this convergence in gait kinetics, differences in knee muscle strength, neuromuscular activity, and walking speed persisted at follow-up. At 1 year, the greater loading group demonstrated a significantly higher knee extensor symmetry index than the lesser loading group (87% ± 4% vs 75% ± 3%; P = .03). In contrast, the lesser loading group exhibited higher CCI values than the greater loading group (0.12 vs 0.08; P = .049). Walking speed also remained lower in the greater loading group on both the injured side (1.14 ± 0.02 vs 1.21 ± 0.03 m/s; P = .03) and the uninjured side (1.13 ± 0.02 vs 1.22 ± 0.03 m/s; P = .01). Preoperatively, the lesser loading group demonstrated lower vGRF on the injured limb relative to the uninjured limb, whereas the opposite pattern was observed in the greater loading group, consistent with prior work.⁴⁰ The lesser loading group also walked faster preoperatively, which may have magnified between-limb loading differences, as walking speed is known to influence gait biomechanics.¹⁸ Following ACL reconstruction, vGRF and walking speed improved and converged between limbs and groups, a finding consistent with previous reports.⁸

Although preoperative vGRF differed between the 2 groups, their CCI values were comparable. This finding is consistent with prior work by Khandha et al,²⁵ who reported that greater muscle cocontraction does not necessarily translate into increased joint loading, which may explain the absence of significant between-limb differences in CCI. Our results further align with existing evidence indicating that ACL injury and subsequent reconstruction alter knee cocontraction patterns, typically characterized by increased hamstring activation and reduced vastus lateralis activity during dynamic tasks such as landing.^9,33,46

Previous studies have demonstrated that cocontraction ratios of the injured knee generally decline within the first few months following ACL reconstruction.⁵¹ In the present study, a similar reduction was observed only in the greater loading group at the 1-year follow-up, whereas elevated CCI values persisted across participants. This sustained elevation in cocontraction may be attributed to the loss of ligament mechanoreceptors following ACL injury and reconstruction, which disrupts afferent feedback and compromises neuromuscular control.³³ In addition, persistent quadriceps weakness and restricted knee flexion may shift energy absorption during gait and functional tasks from active muscular control to passive structures, potentially increasing the risk of reinjury and cartilage degeneration.¹

Strength deficits following ACL reconstruction are likely multifactorial, involving both central and peripheral mechanisms. Central adaptations may include altered cortical excitability and reorganization within somatosensory and visual processing regions,³⁶ whereas peripheral mechanisms may involve suppressed Ia afferent input, reduced motor neuron recruitment, and abnormal gain modulation.^23,28 These neural and mechanical adaptations, together with altered variability in knee extensor and flexor force output,^48,50 may lead individuals to rely on increased hamstring neural drive during tasks requiring greater dynamic stability. Although such a strategy may be adaptive in preserving joint stability, it may also represent a potentially maladaptive compensation following ACL reconstruction.⁴⁷

In the present study, knee flexor strength on the injured side improved significantly in all participants at the 1-year follow-up. Prior investigations have shown that harvested hamstring tendons regenerate in a substantial proportion of patients following ACL reconstruction¹⁴ and that knee flexor strength often increases postoperatively even in the absence of targeted rehabilitation interventions.^31,45 Collectively, these findings suggest that recovery of knee flexor strength is a common and expected postoperative outcome.

Improved knee flexor strength and endurance are essential for functional recovery after ACL injury and reconstruction, as they contribute to enhanced joint stability and increased knee flexor torque.^6,55,59 In other knee-related pathologies, gains in knee muscle strength, particularly in the knee flexors, have been associated with reduced activity limitations.⁴³ Consequently, when quadriceps strength recovery is incomplete, the knee flexors may assume a compensatory role in maintaining joint stability.

Quadriceps strength recovery following ACL reconstruction using autologous hamstring grafts is frequently incomplete, with reported values reaching approximately 80% of the uninjured limb at 1 year postoperatively.³¹ A similar pattern was observed in the present study: the greater loading group demonstrated improvement in knee extensor symmetry to approximately 87%, whereas recovery in the lesser loading group remained more limited, at approximately 73%. Psychological factors may partially account for these between-group differences, as fear of reinjury has been consistently identified as a major barrier to full functional recovery following ACL reconstruction.¹⁷ Individuals in the lesser loading group may have continued to adopt compensatory gait strategies that limited loading of the injured limb, thereby constraining the mechanical stimuli necessary for optimal quadriceps recovery. In contrast, sustained utilization of the injured limb during gait in the greater loading group may have facilitated more symmetrical restoration of quadriceps strength over time.

The walking speed of the injured side in the lesser loading group remained unchanged throughout the 1-year follow-up but was consistently faster compared with the greater loading group. In healthy young individuals, faster walking is associated with a higher CCI value.²² This observation is consistent with our findings and may explain the lesser loading group's elevated CCI value for the injured side. Individuals who walk faster after ACL reconstruction exhibit greater asymmetry, which is associated with early development of osteoarthritis and increased susceptibility to a second ACL injury.^26,38

Reduced knee extensor torque in individuals with a history of ACL reconstruction may be related to an elevated CCI value.^16,25,37 We noted that the knee extensor symmetry index in the lesser loading group did not significantly improve during the follow-up period: it remained lower than that in the greater loading group. This may be attributable to the high CCI values persisting till follow-up in the lesser loading group. By contrast, the greater loading group exhibited relatively efficient recovery of knee extensor strength on the injured side. Subconscious self-organization of the sensorimotor system may occur after surgery; if the knee joint functions well, structural adaptations may emerge, and compensatory strategies may diminish or disappear.¹³ Psychological factors, such as fear of reinjury, often prevent patients with ACL injury from returning to playing sports.^29,57 These factors may explain the persistently elevated CCI value in the lesser loading group, even after postoperative normalization of vGRF. Thus, the lesser loading group exhibited poor recovery outcomes at the 1-year follow-up.

Azus et al³ demonstrated that peak vGRF during the second half of the stance phase was positively associated with considerable improvement in the KOOS Symptoms score at 1 year after surgery. In our lesser loading group, but not our greater loading group, vGRF's F1 and F3 values on the injured side exhibited significant improvements at the 1-year follow-up. This finding may explain why the KOOS Symptoms score exhibited no significant improvement in the greater loading group. Additionally, a previous study reported no significant association between the peak vGRF limb symmetry index (injured side/uninjured side) and KOOS at 1 year after ACL reconstruction.⁴⁰ These findings suggest that variations in vGRF symmetry may not directly influence patient-reported functional outcomes. Since this study used vGRF as the basis for group classification, the present investigation revealed no significant differences between the 2 groups in terms of KOOS outcomes.

Limitations and Future Directions

Several limitations should be acknowledged. Lower limb vGRF asymmetry may persist for years after ACL reconstruction across functional tasks, although some gait parameters may normalize by approximately 17 months postoperatively.^4,10,20 Because participants were followed for only 1 year, gait stabilization may not have been fully achieved. The sample size was relatively limited, and psychological factors influencing movement behavior after ACL reconstruction were not assessed.

In addition, concomitant intra-articular injuries such as meniscal or chondral lesions were not analyzed as independent factors. Recent studies have shown that meniscal injuries may impair postoperative neuromuscular control and muscle strength recovery after ACL reconstruction.¹¹ Although meniscal injury prevalence was comparable between groups in our cohort, their potential influence on recovery cannot be fully excluded.

Furthermore, although analyses focused on knee-related outcomes, ACL injury was associated with early and persistent neuromuscular deficits in distal musculature, including suppressed tibialis anterior activation and prolonged ankle strength asymmetries,^15,49 which may have contributed to residual gait asymmetry.

Future studies should include longer follow-up periods, integrate psychological and biomechanical factors, and establish clinically meaningful thresholds for interlimb vGRF differences. Incorporating assessments of distal musculature may further improve identification of abnormal force transmission during gait and support more personalized rehabilitation strategies following ACL reconstruction.

Conclusion

Our study showed that preoperative vGRF asymmetry is a clinically feasible indicator of neuromuscular recovery potential after ACL reconstruction. Patients with reduced injured-limb loading showed persistent cocontraction and weaker quadriceps strength, underscoring the value of early gait assessment for targeted rehabilitation. These findings provide clinically relevant insight for identifying patients at risk of suboptimal recovery and support the use of preoperative gait assessment to guide individualized rehabilitation strategies.

Footnotes

Appendix

Table A1

Cohort Characteristics ^a

	Greater Loading	Lesser Loading	P (between group)
Age, y	32 ± 12	31 ± 11	.96
Height, cm	170 ± 9	170 ± 10	.88
Weight, kg	78 ± 17	73 ± 14	.19
Body mass index, kg/m²	27 ± 4	25 ± 4	.07
Time from injury to reconstruction, mo	13 ± 27	19 ± 30	.44
Injury, acute:nonacute	15:12	9:19	.08
Sex, female:male	8:19	8:20	.93
Injured side, right:left	12:15	16:12	.35
Meniscal injury, no:yes	9:18	10:18	.49

Data are presented as mean ± SD or n. vGRF, vertical ground reaction force.

Final revision submitted March 12, 2026; accepted March 20, 2026.

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

This study was approved by the Chang Gung Medical Foundation Institutional Review Board (IRB Nos. 201300977B0, 201700284B0, and 201901827B0).

ORCID iD

Wei-Hsiu Hsu

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