Single Leg Hop Performance After Anterior Cruciate Ligament Reconstruction: Ready for Landing but Cleared for Take-Off?

CONTEXT: While the landing phases of the single-leg hop for distance (SLHD) are commonly assessed, limited work reflects how the take-off phase influences hop performance in patients with anterior cruciate ligament reconstruction (ACLR). OBJECTIVE: To compare trunk and lower extremity biomechanics between individuals with ACLR and matched uninjured controls during take-off of the SLHD. DESIGN: Cross-sectional study design. SETTING: Laboratory setting. PATIENTS OR OTHER PARTICIPANTS: 16 individuals with ACLR and 18 uninjured controls. MAIN OUTCOME MEASURES: Normalized quadriceps isokinetic torque, hop distance, and respective limb symmetry indices (LSI) were collected for each participant. Sagittal and frontal kinematics and kinetics of the trunk, hip, knee, and ankle, as well as vertical and horizontal ground reaction forces (GRF) were recorded for loading and propulsion of the take-off phase of the SLHD. RESULTS: Those with ACLR had weaker quadriceps peak torque in the involved limb (p=0.001) and greater strength asymmetry (p<0.001) compared to controls. Normalized hop distance was not statistically different between limbs or between groups (p>0.05) and hop distance symmetry was not different between groups (p>0.05). During loading, the involved limb demonstrated lesser knee flexion angles (p=0.030) and knee power (p=0.007) compared to the uninvolved limb, and lesser knee extension moments compared to the uninvolved limb (p=0.001) and controls (p=0.005). During propulsion, the involved limb demonstrated lesser knee extension moment (p=0.027), knee power (p=0.010), knee (p=0.032) and ankle work (p=0.032), anterior- posterior GRF (p=0.047), and greater knee (p=0.016) abduction excursions compared to the uninvolved limb. CONCLUSIONS: Between-limb differences in SLHD take-off suggest a knee underloading strategy in the involved limb. These results provide further evidence that distance covered during SLHD assessment can overestimate function and fail to identify compensatory biomechanical strategies.

Cognitive-Motor Dual-Task Performance of the Landing Error Scoring System.

CONTEXT: The Landing Error Scoring System (LESS) is a common assessment used to determine biomechanical landing errors. However, this assessment is completed as a single motor task, which does not require additional attentional resources. It is unclear if the LESS can be used to detect cognitive-motor interference (i.e., dual-task cost) in biomechanical errors associated with lower extremity injury. OBJECTIVE: To determine if the LESS is a suitable clinical assessment of dual-task performance in uninjured females and to evaluate whether specific landing criteria are more affected by an additional cognitive load than others. DESIGN: Observational Cross-Sectional Design. Setting: University research laboratory. PATIENTS OR OTHER PARTICIPANTS: Twenty uninjured, physically active females participated. Intervention(s): Participants performed the LESS under three different conditions: baseline landing with no cognitive distraction (Single), a visual-based dual-task (Visual), and a number-based dual-task (Number). MAIN OUTCOME MEASURES(S): Mean sagittal, frontal, and total LESS scores were compared between conditions using a one-way repeated measures analysis of variance with Tukey post hoc corrections. Cohen's d effect sizes with 95% confidence intervals were used to determine the magnitude of differences. The frequency of errors for each individual LESS item under the three conditions were compared using Chi-Squared analyses. RESULTS: Participants exhibited greater sagittal plane (P=.013, d=0.91 [0.26, 1.56]) and total (P=.008, d=1.03 [0.37, 1.69]) errors during the Visual condition compared to the Single condition. Frontal plane errors were significantly greater during the Number condition compared to the Single condition (P=.008, d=1.03 [0.37, 1.69]). The frequency of errors observed for each LESS item did not statistically differ between conditions (all P>.05). CONCLUSIONS: The LESS was able to detect a dual-task cost in landing errors during both the Visual and Number conditions. We recommend developing clinically-oriented solutions to incorporate similar dual- task paradigms in traditional injury risk reduction programs.

Corticomuscular cross-recurrence analysis reveals between-limb differences in motor control among individuals with ACL reconstruction.

Surgical reconstruction of the anterior cruciate ligament (ACL) and subsequent physical therapy can help athletes return to competition; however, re-injury rates remain disproportionately high due, in part, to lingering biomechanical and neurological factors that are not fully addressed during rehabilitation. Prior reports indicate that individuals exhibit altered electrical activity in both brain and muscle after ACL reconstruction (ACLR). In this investigation, we aimed to extend existing approaches by introducing a novel non-linear analysis of corticomuscular dynamics, which does not assume oscillatory coupling between brain and muscle: Corticomuscular cross-recurrence analysis (CM-cRQA). Our findings indicate that corticomuscular dynamics vary significantly between involved (injured) and uninvolved legs of participants with ACLR during voluntary isometric contractions between the brain and both the vastus medialis and lateralis. This finding points to a potential lingering neural deficit underlying re-injury for athletes after surgical reconstruction, namely the dynamical structure of neuromuscular (brain to quad muscle) coordination, which is significantly asymmetric, between limbs, in those who have ACLR.

Neural drive and motor unit characteristics after anterior cruciate ligament reconstruction: implications for quadriceps weakness.

Purpose: The purpose of this investigation was to compare the quality of neural drive and recruited quadriceps motor units' (MU) action potential amplitude (MUAPAMP) and discharge rate (mean firing rate (MFR)) relative to recruitment threshold (RT) between individuals with anterior cruciate ligament reconstruction (ACLR) and controls. Methods: Fourteen individuals with ACLR and 13 matched controls performed trapezoidal knee extensor contractions at 30%, 50%, 70%, and 100% of their maximal voluntary isometric contraction (MVIC). Decomposition electromyography (dEMG) and torque were recorded concurrently. The Hoffmann reflex (H-reflex) and central activation ratio (CAR) were acquired bilaterally to detail the proportion of MU pool available and volitionally activated. We examined MUAPAMP-RT and MFR-RT relationships with linear regression and extracted the regression line slope, y-intercept, and RT range for each contraction. Linear mixed effect modelling used to analyze the effect of group and limb on regression line slope and RT range. Results: Individuals with ACLR demonstrated lower MVIC torque in the involved limb compared to uninvolved limb. There were no differences in H-reflex or CAR between groups or limbs. The ACLR involved limb demonstrated smaller mass-normalized RT range and slower MU firing rates at high contraction intensities (70% and 100% MVIC) compared to uninvolved and control limbs. The ACLR involved limb also demonstrated larger MU action potentials in the VM compared to the contralateral limb. These differences were largely attenuated with relative RT normalization. Conclusions: These results suggest that persistent strength deficits following ACLR may be attributable to a diminished quadriceps motor neuron pool and inability to upregulate the firing rate of recruited MUs.

Intersession reliability of quadriceps corticospinal excitability: A functional transcranial magnetic stimulation study.

Recording transcranial magnetic stimulation-derived measures during a closed kinetic chain task can serve as a functional technique to assess corticomotor function, which may have implications for activities of daily living or lower extremity injury in physically active individuals. Given the novelty of TMS use in this way, our purpose was to first determine the intersession reliability of quadriceps corticospinal excitability during a single-leg squat. We used a descriptive laboratory study to assess 20 physically active females (22.1 ± 2.5 years, 1.7 ± 0.7 m, 66.3 ± 13.6 kg, Tegner Activity Scale: 5.90 ± 1.12) over a 14-day period. Two-way mixed effects Intraclass Correlation Coefficients (3,1) (ICC) for absolute agreement were used to assess intersession reliability. The active motor threshold (AMT) and normalized motor evoked potential (MEP) amplitudes were assessed in the vastus medialis of each limb. The dominant limb AMTs demonstrated moderate-to-good reliability (ICC = 0.771, 95% CI = 0.51-0.90; p < 0.001). The non-dominant limb AMTs (ICC = 0.364, 95% CI = 0.00-0.68, p = 0.047), dominant limb MEPs (ICC = 0.192, 95% CI = 0.00-0.71; p = 0.340), and non-dominant limb MEPs (ICC = 0.272, 95% CI = 0.00-0.71; p = 0.235) demonstrated poor-to-moderate reliability. These findings may provide insight into corticomotor function during activities requiring weight-bearing, single-leg movement. However, variability in agreement suggests further work is warranted to improve the standardization of this technique prior to incorporating in clinical outcomes research.

Hamstrings fatigue does not improve quadriceps function in individuals with anterior cruciate ligament reconstruction.

OBJECTIVES: Our purpose was to investigate the immediate and prolonged effects of hamstrings fatigue on quadriceps neuromuscular function in individuals with anterior cruciate ligament reconstruction (ACLR) and matched uninjured controls. DESIGN: Cross-Sectional. SETTING: Laboratory. PARTICIPANTS: 16 participants with a history of ACLR and 16 uninjured controls. MAIN OUTCOME MEASURES: Quadriceps peak torque (PT), central activation ratio (CAR), early (RTD100) and late (RTD200) rate of torque development, vastus medialis and lateralis electromyographic (EMG) activity, and hamstrings-to-quadriceps co-activation assessed at baseline. Outcomes were evaluated pre-fatigue (PRE), immediately post-fatigue (POST), and 30min post-fatigue (POST30). The involved limbs of individuals with ACLR were assessed and control limbs were matched based on limb dominance. RESULTS: Individuals with ACLR demonstrated lesser quadriceps PT (p = 0.004), CAR (p < 0.001), RTD100 (p = 0.042), RTD200 (p = 0.028), and vastus medialis EMG (p = 0.040) than controls, regardless of time. Quadriceps CAR (p < 0.001) and RTD200 (p < 0.001) decreased at POST and POST30, whereas RTD100 (p < 0.001) decreased at POST, regardless of group. CONCLUSIONS: The observed reductions in quadriceps neuromuscular function may suggest involvement of central fatigue mechanisms, which should be explored prior to recommending hamstrings fatigue as a therapeutic intervention.

No Association Between Processing Speed and Risk of Sport-Related Concussion in Youth Soccer.

OBJECTIVE: To determine whether slow processing speed is associated with risk of sport-related concussion. DESIGN: We conducted a retrospective cohort study using computerized neurocognitive assessments (Immediate Post-concussion Assessment and Cognitive Testing [ImPACT]) from the Massachusetts Concussion Management Coalition. Slow processing speed was defined as 2 SD below the sample mean (n = 131) and fast processing speed as 2 SD above the sample mean (n = 259). We used a binary logistic regression model to determine the odds of sustaining a concussion with our main predictor being processing speed (high or low) adjusted for the effects of age, sex, and prior number of concussions. SETTING: Massachusetts Concussion Management Coalition, Institutional care. PARTICIPANTS: Three hundred ninety junior high soccer players ages 10 to 15 with a baseline score for ImPACT. INDEPENDENT VARIABLES: Processing Speed. MAIN OUTCOME MEASURES: Risk of sustaining a concussion. RESULTS: Those with slow processing speed had a visual motor composite score of ≤19.92, those with fast-processing speed had a score of ≥46.20. Athletes with slow processing speed were younger (13 vs 14 years; P < 0.001) and more likely to be male (57% vs 49%; P = 0.014). After adjusting for the effects of age, sex, and prior concussions, there was no significant difference in the odds of sustaining a concussion between groups (aOR 1.01; 95% CI, 0.99-1.04). CONCLUSIONS: Despite previous research showing that slow processing speed is a risk factor for musculoskeletal injuries during sports, our study suggests that processing speed is not associated with the risk of sustaining a concussion among junior high school soccer players.

Understanding Athletic Trainers' Knowledge, Intervention, and Barriers Toward Arthrogenic Muscle Inhibition.

CONTEXT: Arthrogenic muscle inhibition (AMI) is a common neurophysiological response to joint injury. While athletic trainers (ATs) are constantly treating patients with AMI, it is unclear how clinicians are using the available evidence to treat the condition. OBJECTIVE: To investigate ATs' general knowledge, clinical practice, and barriers for treating AMI. METHODS: A cross-sectional web-based survey was utilized. The survey was distributed to a random sample of 3000 ATs from the National Athletic Trainers' Association and through social media. 143 board certified ATs (age: 34.6 [10.3] y; experience: 11.7 [9.8] y) from various clinical settings and educational backgrounds were included in the analysis. RESULTS: One hundred one respondents were able to correctly identify the definition of AMI. The majority of these respondents correctly reported that joint effusion (n = 95, 94.1%) and abnormal activity from joint receptors (n = 91, 90.1%) resulted in AMI. Of the 101 respondents, only 58 (57.4%) reported using disinhibitory interventions to treat AMI. The most frequently used evidence supported interventions were transcutaneous electrical nerve stimulation (n = 38, 65.5%), neuromuscular electrical stimulation (n = 33, 56.9%), and focal joint cooling (n = 25, 43.1%). The interventions used correctly most often based on current evidence were neuromuscular electrical stimulation (n = 29/33, 87.9%) and transcutaneous electrical nerve stimulation (n = 26/38, 68.4%). Overall, difficulty quantifying AMI (n = 62, 61.24%) and lack of education (n = 71, 76.2%) were most frequently perceived as barriers. Respondents that did not use disinhibitory interventions perceived lack of experience treating AMI, understanding the terminology, and access to therapeutic modalities more often than the respondents that reported using disinhibitory interventions. CONCLUSION: Further education about concepts and treatment about AMI is warranted for ATs. Continued understanding of ATs' clinical practice in regard to AMI may help identify gaps in athletic training clinical education.

Arthrogenic Muscle Inhibition: Best Evidence, Mechanisms, and Theory for Treating the Unseen in Clinical Rehabilitation.

CONTEXT: Arthrogenic muscle inhibition (AMI) impedes the recovery of muscle function following joint injury, and in a broader sense, acts as a limiting factor in rehabilitation if left untreated. Despite a call to treat the underlying pathophysiology of muscle dysfunction more than three decades ago, the continued widespread observations of post-traumatic muscular impairments are concerning, and suggest that interventions for AMI are not being successfully integrated into clinical practice. OBJECTIVES: To highlight the clinical relevance of AMI, provide updated evidence for the use of clinically accessible therapeutic adjuncts to treat AMI, and discuss the known or theoretical mechanisms for these interventions. EVIDENCE ACQUISITION: PubMed and Web of Science electronic databases were searched for articles that investigated the effectiveness or efficacy of interventions to treat outcomes relevant to AMI. EVIDENCE SYNTHESIS: 122 articles that investigated an intervention used to treat AMI among individuals with pathology or simulated pathology were retrieved from 1986 to 2021. Additional articles among uninjured individuals were considered when discussing mechanisms of effect. CONCLUSION: AMI contributes to the characteristic muscular impairments observed in patients recovering from joint injuries. If left unresolved, AMI impedes short-term recovery and threatens patients' long-term joint health and well-being. Growing evidence supports the use of neuromodulatory strategies to facilitate muscle recovery over the course of rehabilitation. Interventions should be individualized to meet the needs of the patient through shared clinician-patient decision-making. At a minimum, we propose to keep the treatment approach simple by attempting to resolve inflammation, pain, and effusion early following injury.

Sex influences the relationship between hamstrings-to-quadriceps strength imbalance and co-activation during walking gait.

BACKGROUND: While traditionally viewed as a beneficial adaptation to preserve stability in the presence of knee pathology, excessive muscle co-activation may be detrimental for joint health when extrapolated to repetitive movement patterns over time. Lesser hamstrings strength relative to the quadriceps (low H:Q strength ratio) may influence neuromuscular patterns about the knee, as it is reported to increase risk for lower extremity injury among healthy females. RESEARCH QUESTION: Does the relationship between H:Q strength ratio and H:Q co-activation differ between sexes during walking and jogging? METHODS: We used a descriptive laboratory study to assess hamstrings and quadriceps strength and muscle activity patterns during the loading response of treadmill walking gait (1.34 m/s) and jogging (2.68 m/s) in healthy males (n = 11) and females (n = 12). Concentric-concentric peak isokinetic torque (60°/s) was used to derive the H:Q strength ratio, which was treated as an explanatory variable for H:Q co-activation indices (medial, lateral, composite) and constituent EMG amplitudes. Bivariate correlations (Pearson r or Spearman ρ) were used for analysis. RESULTS: In females, lesser H:Q strength ratios were associated with greater lateral co-activation (r=-.715, P = .007) and biceps femoris EMG amplitude (ρ=-.532, P = .046) during the loading response of walking gait. When controlling for sex differences in knee flexion, the relationship between lesser H:Q strength ratios and greater lateral co-activation was preserved (partial r=-.699, P = .012); yet, biceps femoris EMG was no longer correlated (partial r=-.331, P = .175). Significant relationships were not observed among male participants during walking or in either sex during jogging (all P > .05). SIGNIFICANCE: Collectively, these data provide evidence of a sex-specific neuromuscular pattern with implications for joint health. Excessive lateral co-activation may consequently promote a greater valgus moment and ligamentous strain. Future investigations would benefit from understanding the influence of hamstrings-dominant exercise programs on the neuromuscular patterns of the knee.

Hamstrings Muscle Morphology After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-Analysis.

BACKGROUND: Hamstrings muscle morphology is determinant of muscle function (i.e. strength). Among individuals with ACL reconstruction (ACLR), less cross-sectional area (CSA) and volume in the ACLR-limb are associated with muscle weakness, and may contribute to lower rates of return to preinjury activity level and an increased risk for long-term sequelae. OBJECTIVES: To effectively treat muscular impairments, an accurate understanding of differences in hamstrings morphology following ACLR is needed. A systematic review and meta-analysis were undertaken to describe the morphology of the hamstring muscle complex after ACLR. METHODS: We searched five databases for studies evaluating the difference between hamstrings size and architecture in individuals with ACLR. Two independent reviewers assessed each paper for inclusion and quality. Means and standard deviations were extracted from each included study to allow fixed-effect size meta-analysis calculations for comparison of results. RESULTS: Twenty-four studies were included for final review. Eight categories of morphological outcomes were identified, and studies were grouped accordingly: (1) volume, (2) cross-sectional area (CSA), (3) muscle length, (4) muscle thickness, (5) fascicle length, (6) pennation angle, (7) fiber area, and (8) fiber type. Meta-analysis demonstrated lower hamstrings volume in the ACLR-limb in both contralateral and control group comparisons, and lower CSA, length, and thickness in the ACLR-limb in contralateral comparisons. The semitendinosus and gracilis were most profoundly impacted. Limited moderate evidence demonstrated greater biceps femoris pennation angle in the ACLR-limb. CONCLUSIONS: Individuals with ACLR demonstrated large deficits in semitendinosus and gracilis muscle CSA and volume in the ACLR-limb compared contralaterally, with no differences observed in the biceps femoris or semimembranosus. Clinical implications regarding assessment and treatment of individuals with ACLR are discussed.

Assessment of Quadriceps Corticomotor and Spinal-Reflexive Excitability in Individuals with a History of Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis.

BACKGROUND: Differences in the excitability of motor generating neural pathways are reported following anterior cruciate ligament reconstruction (ACLR) that is associated with quadriceps dysfunction and theorized to prevent the full recovery of muscle function. OBJECTIVE: The aims of this systematic review and meta-analysis were to compare quadriceps neural excitability between the involved ACLR limb, the uninvolved limb, and uninjured controls, and to determine at what time intervals these differences are present after surgery. METHODS: We conducted a search of PubMed, SPORTDiscus, Embase, and Web of Science, and extracted measures assessing difference of quadriceps spinal-reflexive, corticospinal, and intracortical excitability from studies that compared (1) involved limb to the uninvolved limb, (2) involved limb to a control limb, or (3) uninvolved limb to a control limb. We stratified time at 24 months, since this represents a period of heightened risk for reinjury. A modified Downs and Black checklist and Egger's test were used to determine the methodological quality of individual studies and risk of bias between studies. RESULTS: Fourteen studies comprising 611 participants (371 individuals with a history of ACLR; median time from surgery: 31.5 months; range 0.5-221.1 months) were included in the review. Overall, the involved (g = 0.60, 95% CI [0.24, 0.96]) and uninvolved (g = 0.49, 95% CI [0.00, 0.98]) limbs exhibited greater motor threshold (MT) in comparison to uninjured controls. Motor-evoked potential (MEP) amplitudes were greater in the uninvolved limb in comparison to uninjured controls (g = 0.31, 95% CI [0.03, 0.59]). Lesser intracortical inhibition was exhibited in the uninvolved limb compared to uninjured controls (g = 0.54, 95% CI [0.14, 0.93]). When stratified by time from surgery, MEP amplitudes were greater in the uninvolved limb compared to uninjured controls (g = 0.33, 95% CI [0.03, 0.63]) within the first 24 months after surgery. When evaluated more than 24 months after surgery, the involved limb exhibited greater Hoffmann reflex (H-reflex) compared to uninjured controls (g = 0.38, 95% CI [0.00, 0.77]). MT were greater in the involved limb (g = 0.93, 95% CI [- 0.01, 1.88]) and uninvolved limb (g = 0.57, 95% CI [0.13, 1.02]) compared to uninjured controls. MEP amplitudes in the involved limb were lesser compared to uninjured controls when evaluated more than 24 months after ACLR (g = -1.11, 95% CI [- 2.03, - 0.20]). CONCLUSIONS: The available evidence supports that there are neural excitability differences within the corticospinal tract in individuals with ACLR when compared to uninjured controls. Future research should focus further on longitudinal assessments of neural excitability prior to and following ACLR. Identifying interventions aimed to facilitate corticospinal excitability after ACLR appears to be warranted to improve quadriceps function. TRIAL REGISTRATION: Registered through PROSPERO CRD42020158714.

Alterations in Quadriceps Neurologic Complexity After Anterior Cruciate Ligament Reconstruction.

CONTEXT: Traditionally, quadriceps activation failure after anterior cruciate ligament reconstruction (ACLR) is estimated using discrete isometric torque values, providing only a snapshot of neuromuscular function. Sample entropy (SampEn) is a mathematical technique that can measure neurologic complexity during the entirety of contraction, elucidating qualities of neuromuscular control not previously captured. OBJECTIVE: To apply SampEn analyses to quadriceps electromyographic activity in order to more comprehensively characterize neuromuscular deficits after ACLR. DESIGN: Cross-sectional. SETTING: Laboratory. PARTICIPANTS: ACLR: n = 18; controls: n = 24. INTERVENTIONS: All participants underwent synchronized unilateral quadriceps isometric strength, activation, and electromyography testing during a superimposed electrical stimulus. MAIN OUTCOME MEASURES: Group differences in strength, activation, and SampEn were evaluated with t tests. Associations between SampEn and quadriceps function were evaluated with Pearson product-moment correlations and hierarchical linear regressions. RESULTS: Vastus medialis SampEn was significantly reduced after ACLR compared with controls (P = .032). Vastus medialis and vastus lateralis SampEn predicted significant variance in activation after ACLR (r2 = .444; P = .003). CONCLUSIONS: Loss of neurologic complexity correlates with worse activation after ACLR, particularly in the vastus medialis. Electromyographic SampEn is capable of detecting underlying patterns of variability that are associated with the loss of complexity between key neurophysiologic events after ACLR.

Limb differences in hamstring muscle function and morphology after anterior cruciate ligament reconstruction.

OBJECTIVE: To compare inter-limb differences in hamstring strength, muscle volume, and neural activity between individuals with anterior cruciate ligament reconstruction (ACLR) and healthy controls, and to identify associations between hamstring neuromuscular function and subjective knee function. DESIGN: Cross-sectional. SETTING: Laboratory. PARTICIPANTS: Eleven participants with a history of ACLR (69.45 ± 22.48 months from surgery) and eleven healthy matched controls. MAIN OUTCOME MEASURE(S): Hamstring isokinetic torque at 60 and 240°â€¢s-1, muscle volume, and medial and lateral hamstring electromyographic (EMG) activity during a single leg hop task were all evaluated during one study visit. Percent limb-differences (%LD) were calculated between each outcome measure; greater inter-limb differences indicated greater asymmetries between limbs. The International Knee Documentation Committee (IKDC) scores were used to determine associations between subjective knee function and each outcome measure. RESULTS: Individuals with ACLR had large magnitude inter-limb differences in medial hamstring EMG (p = .04; d = 0.94), biceps femoris short head volume (p = .02; d = 1.07) and semitendinosus (p = .03; d = 1.07) volume. Lower semimembranosus volume was associated with lower IKDC scores (r = 0.754; p = .012). CONCLUSIONS: We observed greater inter-limb differences in medial hamstring EMG, and semitendinosus and long head of the biceps femoris volume, which may negatively influence knee function years after ACLR.

The Immediate Effects of Transcranial Direct Current Stimulation on Quadriceps Muscle Function in Individuals With a History of Anterior Cruciate Ligament Reconstruction: A Preliminary Investigation.

CONTEXT: Altered quadriceps activation is common following anterior cruciate ligament reconstruction (ACLR), and can persist for years after surgery. These neural deficits are due, in part, to chronic central nervous system alterations. Transcranial direct current stimulation (tDCS) is a noninvasive modality, that is, believed to immediately increase motor neuron activity by stimulating the primary motor cortex, making it a promising modality to use improve outcomes in the ACLR population. OBJECTIVE: To determine if a single treatment of tDCS would result in increased quadriceps activity and decreased levels of self-reported pain and dysfunction during exercise. DESIGN: Randomized crossover design. SETTING: Controlled laboratory. PATIENTS: Ten participants with a history of ACLR (5 males/5 females, 22.9 [4.23] y, 176.57 [12.01] cm, 80.87 [16.86] kg, 68.1 [39.37] mo since ACLR). INTERVENTIONS: Active tDCS and Sham tDCS. MAIN OUTCOME MEASURES: Percentage of maximum electromyographic data of vastus medialis and lateralis, voluntary isometric strength, percentage of voluntary activation, and self-reported pain and symptom scores were measured. The 2 × 2 repeated-measures analysis of variance by limb were performed to explain the differences between time points (pre and post) and condition (tDCS and sham). RESULTS: There was a significant time main effect for quadriceps percentage of maximum electromyographic of vastus medialis (F9,1 = 11.931, P = .01) and vastus lateralis (F9,1 = 9.132, P = .01), isometric strength (F9,1 = 5.343, P = .046), and subjective scores for pain (F9,1 = 15.499, P = .04) and symptoms (F9,1 = 15.499, P = .04). Quadriceps percentage of maximum electromyographic, isometric strength, and voluntary activation showed an immediate decline from pre to post regardless of tDCS condition. Subjective scores improved slightly after each condition. CONCLUSIONS: One session of active tDCS did not have an immediate effect on quadriceps activity and subjective scores of pain and symptoms. To determine if tDCS is a valid modality for this patient population, a larger scale investigation with multiple treatments of active tDCS is warranted.