Velocity-dependent quadriceps neuromuscular activation asymmetry after anterior cruciate ligament reconstruction: A pilot secondary analysis of a public electromyographic dataset

INTRODUCTION

Anterior cruciate ligament (ACL) rupture is among the most common serious injuries affecting the knee joint, particularly in physically active individuals and athletes. Epidemiological studies estimate that more than 200,000 ACL injuries occur annually in the United States alone, with a substantial proportion requiring surgical reconstruction to restore knee stability and functional capacity.^[1] Although ACL reconstruction (ACLR) has become the standard surgical treatment for symptomatic instability, postoperative recovery is often complicated by persistent deficits in quadriceps strength, neuromuscular control, and functional performance.^[2,3] These impairments may persist for months or even years after surgery and have been implicated as important contributors to delayed return to sport and increased risk of reinjury.^[4]

Quadriceps neuromuscular dysfunction following ACL injury and ACLR is multifactorial. In addition to structural ligament disruption, altered afferent signaling from mechanoreceptors within the injured ligament can lead to arthrogenic muscle inhibition (AMI), characterized by reflexive suppression of quadriceps motor neuron excitability.^[5,6] This neuromuscular inhibition may impair voluntary muscle activation despite adequate muscle strength and structural healing of the reconstructed ligament. Consequently, traditional clinical assessments based solely on strength measurements or functional testing may underestimate persistent neuromuscular deficits that can compromise dynamic knee stability during athletic activities.^[7]

Electromyography (EMG) provides a valuable tool for quantifying neuromuscular activation patterns during dynamic movements and rehabilitation exercises. Surface EMG recordings enable objective evaluation of muscle recruitment, activation amplitude, and temporal contraction patterns, offering insights into neuromuscular control strategies that cannot be captured through conventional strength testing alone.^[8] In the context of ACL rehabilitation, EMG analysis has been used to investigate quadriceps activation deficits, altered co-contraction patterns, and asymmetries between reconstructed and contralateral limbs during functional tasks.^[9,10] These neuromuscular abnormalities may persist even in individuals who demonstrate apparently normal clinical recovery.

Limb symmetry has emerged as a widely used metric for evaluating functional recovery following ACLR. The limb symmetry index (LSI), calculated as the ratio of performance between the reconstructed and contralateral limb, is commonly employed in return-to-sport assessment protocols.^[11] Although symmetry thresholds of 90% or higher are often considered indicative of satisfactory recovery, recent investigations suggest that neuromuscular activation patterns may remain abnormal even when strength or functional symmetry appears acceptable.^[12] Persistent neuromuscular asymmetry may therefore represent an underrecognized factor contributing to recurrent injury and long-term functional impairment after ACLR.

Recent advances in open science and data sharing have made publicly accessible biomechanical datasets available, enabling secondary analyses that may yield additional insights into musculoskeletal rehabilitation. Public repositories such as Dryad provide curated datasets that enable researchers to investigate neuromuscular and biomechanical variables using standardized experimental protocols, thereby promoting scientific transparency and reproducibility while circumventing logistical and financial barriers associated with primary data collection.^[13]

Despite growing interest in neuromuscular recovery following ACLR, relatively few studies have examined EMG-derived activation symmetry using publicly available datasets acquired during controlled isokinetic exercise. Evaluating activation patterns at different contraction velocities may reveal velocity-dependent neuromuscular deficits not apparent during conventional strength assessments. The present pilot study was therefore designed to provide preliminary data on quadriceps neuromuscular activation asymmetry following ACLR, assess the feasibility of this analytical approach using a publicly available EMG dataset, and generate effect size estimates to power a future larger-scale investigation. Specifically, we aimed to compare EMG activation characteristics between reconstructed and contralateral limbs and to examine differences in neuromuscular symmetry between individuals with ACLR and healthy controls. We hypothesized that individuals with prior ACLR would demonstrate greater variability in quadriceps activation patterns and reduced neuromuscular symmetry compared with healthy individuals, particularly during higher-velocity contractions.

MATERIALS AND METHODS

This investigation was designed as a pilot secondary observational analysis to evaluate quadriceps neuromuscular activation asymmetry following ACLR using EMG recordings obtained during controlled isokinetic knee extension exercises. The study was conducted and reported in accordance with the strengthening the reporting of observational studies in epidemiology guidelines. Because the investigation involved analysis of anonymized, publicly available biomechanical data without identifiable patient information, institutional review board approval and informed consent were not required.^[13]

Signal processing and feature extraction

Raw EMG signals were provided as time-series recordings and processed using standardized signal processing techniques commonly employed in neuromuscular biomechanics research. Each recording was inspected for signal integrity and artifacts before quantitative analysis. For each participant and contraction condition, peak EMG amplitude and RMS activation values were calculated to quantify quadriceps muscle activation during the isokinetic knee extension task. Peak EMG amplitude was defined as the maximum recorded activation value within the contraction interval. In contrast, RMS amplitude was calculated to represent the average magnitude of muscle activation across the entire contraction cycle. Statistical analyses were performed using IBM Statistical Package for Social Sciences Statistics version 26 (IBM Corp., Armonk, New York, USA) [Figure 1].

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Figure 1: Analytical workflow for pilot secondary analysis of the electromyographic (EMG) dataset. Raw surface EMG recordings from the vastus lateralis during isokinetic knee extension at 90°/s and 180°/s underwent preprocessing and feature extraction to derive peak EMG amplitude and root-mean-square activation values. Limb symmetry indices were calculated for anterior cruciate ligament reconstruction participants; statistical comparisons were performed between groups. EMG: Electromyography; ACLR: Anterior cruciate ligament reconstruction; RMS: Root-mean-square; LSI: Limb symmetry index.

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RESULTS

Twenty-two participants were included in this pilot analysis: 12 with a history of ACLR and 10 healthy controls. EMG recordings from the VL were successfully obtained for all participants during isokinetic knee extension tasks at angular velocities of 90°/s and 180°/s. No recordings were excluded because of signal artifacts or incomplete data. Extracted neuromuscular variables are summarized in Tables 1 and 2.

Participants with ACLR demonstrated greater variability in quadriceps EMG activation patterns compared with healthy controls. Although peak activation values were generally comparable between groups during lower-velocity contractions, greater dispersion of activation amplitudes was observed in the ACLR group, particularly during higher-velocity contractions [Figure 2].

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Figure 2: Distribution of limb symmetry indices (LSIs) for quadriceps electromyographic activation at 90°/s and 180°/s. Box plots illustrate median, interquartile range, and distribution of LSI values in anterior cruciate ligament reconstruction (ACLR) and control groups. Greater dispersion was observed in the ACLR group, particularly at higher velocity. EMG: Electromyography.

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Analysis of LSIs revealed notable differences between groups. In healthy participants, neuromuscular activation symmetry was consistently high, with LSI values approaching or exceeding 95% across both velocities. In contrast, the ACLR group demonstrated lower and more variable symmetry values. Several ACLR participants demonstrated LSI values below the commonly used 90% clinical threshold for functional recovery [Figure 3].

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Figure 3: Comparison of electromyographic (EMG) activation patterns between reconstructed and contralateral limbs. Box plots demonstrate peak EMG amplitude and root-mean-square activation for anterior cruciate ligament reconstruction (ACLR) reconstructed limb, ACLR contralateral limb, control left limb, and control right limb. EMG: Electromyography.

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Velocity-dependent differences were also observed. During contractions at 180°/s, the ACLR group demonstrated greater variability in EMG activation amplitude and reduced symmetry between limbs compared with 90°/s. Peak EMG activation at 180°/s tended to be lower in reconstructed limbs compared with contralateral limbs, with a medium effect size (Cohen’s d = 0.62) [Figure 4].

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Figure 4: Velocity-dependent differences in quadriceps neuromuscular activation symmetry. Bar graphs compare mean Limb symmetry indices (LSIs) at 90°/s and 180°/s for anterior cruciate ligament reconstruction (ACLR) and control participants. Error bars represent standard errors of the mean. Lower and more variable LSI values were observed in the ACLR group at both velocities, more pronounced at 180°/s.

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DISCUSSION

The present pilot study investigated quadriceps neuromuscular activation asymmetry following ACLR through secondary analysis of a publicly available EMG dataset. The principal finding is that the ACLR group demonstrates greater variability in quadriceps activation patterns and reduced neuromuscular symmetry compared with healthy controls, particularly during higher-velocity contractions. Given the exploratory nature and modest sample size, these findings should be interpreted as hypothesis-generating rather than confirmatory.

Persistent quadriceps dysfunction following ACL injury and ACLR has been widely reported. Quadriceps weakness and impaired voluntary activation are thought to result from AMI, which suppresses quadriceps motor neuron excitability even after structural stabilization of the knee has been achieved.^[5,6]

Mechanistic Pathway of AMI: Following ACL rupture and reconstruction, mechanoreceptors within the native ligament are disrupted, reducing afferent input to spinal quadriceps alpha motor neurons and thereby impairing voluntary activation. Post-operative joint effusion further activates inhibitory interneurons through Group II and IV afferents, amplifying motor neuron suppression. This inhibitory cascade is disproportionately pronounced during high-velocity contractions, which depend on the rapid recruitment of type II motor units that are most susceptible to AMI. This neurophysiological pathway provides a mechanistic basis for the velocity-dependent activation asymmetry observed in the present study.^[5,6]

The VMO is recognized as the most functionally vulnerable quadriceps component to AMI following knee surgery, as it is primarily responsible for terminal knee extension and medial patellar stabilization.^[5] The exclusive use of VL EMG in the present analysis, dictated by the source dataset, may have underestimated the true magnitude of quadriceps neuromuscular deficits in the ACLR population, as VMO inhibition may be disproportionately greater. Future prospective investigations should incorporate multi-channel quadriceps EMG to capture the full profile of neuromuscular dysfunction following ACLR.

Limb symmetry has emerged as a widely used metric for evaluating functional recovery following ACLR, with thresholds of approximately 90% or higher considered indicative of adequate recovery.^[11] The present study demonstrates that EMG-derived LSI values may serve as a complementary metric to conventional strength-based assessments, as some participants demonstrated clinical-threshold LSI values yet showed persistent variability in activation patterns.

The velocity-dependent pattern observed, greater asymmetry at 180°/s than 90°/s, is consistent with the hypothesis that higher-velocity contractions impose greater demands on neuromuscular coordination and rapid motor unit recruitment, thereby exposing deficits in neuromuscular control that remain undetected during slower or isolated strength assessments [Table 3].^[8]

Regarding athletic level: The available dataset did not include sport classification data. However, as noted by one reviewer, AMI may be less prevalent or less persistent in elite athletes due to superior baseline neuromuscular conditioning before injury. Future prospective studies should stratify participants by athletic level to evaluate this hypothesis.

The findings also highlight the potential value of open biomechanical datasets such as Dryad in advancing musculoskeletal research, enabling secondary analyses that generate new insights without the logistical constraints of primary data collection.^[13]

CONCLUSION

In this pilot secondary analysis of a publicly available EMG dataset, individuals with ACLR demonstrated greater variability in quadriceps neuromuscular activation and reduced limb symmetry during isokinetic knee extension tasks compared with healthy controls. Neuromuscular asymmetry was more pronounced during higher-velocity contractions, suggesting that dynamic testing conditions reveal deficits not captured through conventional strength-based assessments. The velocity-dependent pattern is consistent with the neurophysiological mechanism of AMI, which disproportionately affects rapid motor unit recruitment during high-velocity tasks. The effect sizes observed in this pilot analysis provide a preliminary basis for the design of a larger-scale prospective investigation, including comparative studies evaluating EMG profiles across different knee surgical procedures.