Quadriceps cortical adaptations in individuals with an anterior cruciate ligament injury

Sex differences in corticospinal excitability and quadriceps performance after anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) ruptures result in lasting quadriceps dysfunction that contributes to secondary injury risk and development of osteoarthritis. There is evidence of persistent reduced nervous system drive (corticospinal excitability [CSE]) to the quadriceps and sex differences in both quadriceps performance and CSE post-ACL reconstruction (ACLR). The purposes of this study were to investigate the differences in CSE and quadriceps dysfunction after ACLR between sexes and relative to controls. Twenty subjects 4-9 months post-ACLR and 20 age, sex, and activity matched controls participated in this study. Quadriceps performance (peak torque, PT; rate of torque development from onset to 100 ms, RTD100; and RTD from 100 to 200ms, RTD200) and CSE (active motor threshold; slope of the stimulus response (SR) curve, SR curve slope) were measured using an isokinetic dynamometer (HUMAC NORM) and transcranial magnetic stimulation respectively. Significant group differences were found for SR curve slope, PT, RTD100, and RTD200 on the surgical limb. Males after ACLR had higher slopes (higher CSE) than females. Females after ACLR had worse surgical limb quadriceps PT than control males and slower RTD100 and RTD200 than control males and control females. Higher CSE in males after ACLR may point to a potentially adaptive neurological change in males post-ACLR and indicate greater need for novel interventions to address cortical drive in females after ACLR.

Determining intracortical, corticospinal and alpha motoneurone excitability in athletes with patellar tendinopathy compared to asymptomatic controls

BACKGROUND

Lower capacity to generate knee extension maximal voluntary force (MVF) has been observed in individuals affected with patellar tendinopathy (PT) compared to asymptomatic controls. This MVF deficit is hypothesized to emanate from alterations in corticospinal excitability (CSE). The modulation of CSE is intricately linked to the excitability levels at multiple sites, encompassing neurones within the corticospinal tract (CST), intracortical neurones within the primary motor cortex (M1), and the alpha motoneurone. The aim of this investigation was to examine the excitability of intracortical neurones, CST neurones, and the alpha motoneurone, and compare these between volleyball and basketball athletes with PT and matched asymptomatic controls.

METHOD

Nineteen athletes with PT and 18 asymptomatic controls participated in this cross-sectional study. Transcranial magnetic stimulation was utilized to assess CST excitability, corticospinal inhibition (silent period, and short-interval cortical inhibition). Peripheral nerve stimulation was used to evaluate lumbar spine and alpha motoneurone excitability, including the evocation of lumbar-evoked potentials and maximal compound muscle action potential (MMAX ), and CSE with central activation ratio (CAR). Knee extension MVF was also assessed.

RESULTS

Athletes with PT exhibited longer silent period duration and greater electrical stimulator output for MMAX , as well as lower MVF, compared to asymptomatic controls (p < 0.05).

CONCLUSION

These findings indicate volleyball and basketball athletes with PT exhibit reduced excitability of the alpha motoneurone or the neuromuscular junction, which may be linked to lower MVF. Subtle alterations at specific sites may represent compensatory changes to excitability aiming to maintain efferent drive to the knee extensors.

Differences in psychological readiness for return to sport after anterior cruciate ligament injury is evident in thigh musculature motor unit characteristics

Background

Following anterior cruciate ligament (ACL) injury, many athletes that undergo surgery and 6-9 months of rehabilitation struggle to return to sport. Evidence suggests that psychological factors contribute to this failure to return-to-sport.

Objective

Determine the motor control relationship between thigh musculature motor unit characteristics and psychological readiness to return to sport between ACL-injured and healthy controls.

Study design

A longitudinal cohort study.

Methods

Athletes longitudinally completed the ACL Return to Sport after Injury (ACL-RSI) survey and isometric strength measures with a measurement of electromyography (EMG) of the vastus lateralis, vastus medialis, biceps femoris, and semitendinosus. A score cut-off of 61 on the ACL-RSI was used to divide ACL-injured groups. EMG was decomposed to provide each identified motor unit's characteristics (amplitude, average firing rate, etc).

Results

Data demonstrated increased average firing rate for hamstrings (p<0.001), decreased average firing rate for vastus lateralis (p<0.001) and decreased motor unit size for both the quadriceps and hamstrings at return-to-sport post-ACL reconstruction compared with sex-matched and age-matched healthy controls (p<0.001). Furthermore, there were marked differences in disparate ACL-RSI scores between ACL-injured athletes.

Conclusions

At return to sport, ACL-injured athletes have major alterations of thigh musculature motor control, with smaller motor units used by those with low ACL-RSI scores. This study uniquely demonstrates objective thigh muscle motor unit characteristics that coincide with subjective reports of psychological readiness. This information will be important to address psychomotor complexes of injury for future rehabilitation protocols.

Quadriceps Weakness is Associated with Neuroplastic Changes Within Specific Corticospinal Pathways and Brain Areas After Anterior Cruciate Ligament Reconstruction: Theoretical Utility of Motor Imagery-Based Brain-Computer Interface Technology for Rehabilitation

Persistent quadriceps weakness is a problematic sequela of anterior cruciate ligament reconstruction (ACLR). The purposes of this review are to summarize neuroplastic changes after ACL reconstruction; provide an overview of a promising interventions, motor imagery (MI), and its utility in muscle activation; and propose a framework using a brain-computer interface (BCI) to augment quadriceps activation. A literature review of neuroplastic changes, MI training, and BCI-MI technology in postoperative neuromuscular rehabilitation was conducted in PubMed, Embase, and Scopus. Combinations of the following search terms were used to identify articles: "quadriceps muscle," "neurofeedback," "biofeedback," "muscle activation," "motor learning," "anterior cruciate ligament," and "cortical plasticity." We found that ACLR disrupts sensory input from the quadriceps, which results in reduced sensitivity to electrochemical neuronal signals, an increase in central inhibition of neurons regulating quadriceps control and dampening of reflexive motor activity. MI training consists of visualizing an action, without physically engaging in muscle activity. Imagined motor output during MI training increases the sensitivity and conductivity of corticospinal tracts emerging from the primary motor cortex, which helps "exercise" the connections between the brain and target muscle tissues. Motor rehabilitation studies using BCI-MI technology have demonstrated increased excitability of the motor cortex, corticospinal tract, spinal motor neurons, and disinhibition of inhibitory interneurons. This technology has been validated and successfully applied in the recovery of atrophied neuromuscular pathways in stroke patients but has yet to be investigated in peripheral neuromuscular insults, such as ACL injury and reconstruction. Well-designed clinical studies may assess the impact of BCI on clinical outcomes and recovery time. Quadriceps weakness is associated with neuroplastic changes within specific corticospinal pathways and brain areas. BCI-MI shows strong potential for facilitating recovery of atrophied neuromuscular pathways after ACLR and may offer an innovative, multidisciplinary approach to orthopaedic care.

Level of Evidence

V, expert opinion.

Altered corticospinal excitability of scapular muscles in individuals with shoulder impingement syndrome

The purpose of this study is to assess and compare corticospinal excitability in the upper and lower trapezius and serratus anterior muscles in participants with and without shoulder impingement syndrome (SIS). Fourteen participants with SIS, and 14 without SIS were recruited through convenient sampling in this study. Transcranial magnetic stimulation assessment of the scapular muscles was performed while the participants were holding their arm at 90 degrees scaption. The motor-evoked potential (MEP), active motor threshold (AMT), latency of MEP, cortical silent period (CSP), activated area and center of gravity (COG) of cortical mapping were compared between groups using the Mann-Whitney U tests. The SIS group demonstrated following significances, higher AMTs of the lower trapezius (SIS: 0.60 ± 0.06; Comparison: 0.54 ± 0.07, p = 0.028) and the serratus anterior (SIS: 0.59 ± 0.04; Comparison: 0.54 ± 0.06, p = 0.022), longer CSP of the lower trapezius (SIS: 62.23 ± 22.87 ms; Comparison: 45.22 ± 14.64 ms, p = 0.019), and posteriorly shifted COG in the upper trapezius (SIS: 1.88 ± 1.06; Comparison: 2.76 ± 1.55, p = 0.048) and the serratus anterior (SIS: 2.13 ± 1.02; Comparison: 3.12 ± 1.88, p = 0.043), than the control group. In conclusion, participants with SIS demonstrated different organization of the corticospinal system, including decreased excitability, increased inhibition, and shift in motor representation of the scapular muscles.

Increased short interval intracortical inhibition in participants with previous hamstring strain injury

PURPOSE

Cortical mechanisms may contribute to weakness in participants with previous hamstring strain injury. This study aims to examine intra-cortical inhibition (SICI) and corticospinal excitability in previously injured participants.

METHODS

In this cross-sectional study, TMS was used to examine SICI, silent period, silent period: MEP ratios and area under the stimulus response curve in the biceps femoris and medial hamstrings. Comparisons were made between participants with (n = 10) and without (n = 10) previous hamstring strain injury. Motor threshold and isometric knee flexor strength were also compared between participants and the relationship between strength and SICI in control and previously injured participants was examined.

RESULTS

Isometric knee flexor strength was lower in previously injured limbs compared with control limbs (mean difference = - 41 Nm (- 26%) [95% CI = - 80 to - 2 Nm], p = 0.04, Cohen's d = - 1.27) and contralateral uninjured limbs (mean difference = - 23 Nm (- 17%), [95% CI = - 40 to - 6 Nm], p = 0.01, Cohen's d = - 0.57). Previously injured limbs exhibited smaller responses to paired pulse stimulation (i.e. greater levels of SICI) in the biceps femoris compared with control limbs (mean difference = - 19%, [95% CI = - 34 to - 5%], p = 0.007, Cohen's d = - 1.33). Isometric knee flexor strength was associated with the level of SICI recorded in the biceps femoris in previously injured participants (coefficient = 23 Nm [95% CI = 7-40 Nm], adjusted R² = 0.31, p = 0.01). There were no differences in markers of corticospinal excitability between previously injured and control limbs (all p > 0.24, all Cohen's d < 0.40).

CONCLUSION

Athletes with previous injury in the biceps femoris exhibit increased SICI in this muscle compared with control participants. Increased SICI is related to lower levels of hamstring strength, and rehabilitation programs targeting the removal of intra-cortical inhibition should be considered.

Influence of different transcranial magnetic stimulation current directions on the corticomotor control of lumbar erector spinae muscles during a static task

Different directions of transcranial magnetic stimulation (TMS) can activate different neuronal circuits. Whereas posteroanterior current (PA-TMS) depolarizes mainly interneurons in primary motor cortex (M1), an anteroposterior current (AP-TMS) has been suggested to activate different M1 circuits and perhaps axons from the premotor regions. Although M1 is also involved in the control of axial muscles, no study has explored whether different current directions activate different M1 circuits that may have distinct functional roles. The aim of the study was to compare the effect of different current directions (PA- and AP-TMS) on the corticomotor control and spatial cortical organization of the lumbar erector spinae muscle (LES). Thirty-four healthy participants were recruited for two independent experiments, and LES motor-evoked potentials (MEPs) were recorded. In experiment 1 (n = 17), active motor threshold (AMT), MEP latencies, recruitment curve (90% to 160% AMT), and excitatory and inhibitory intracortical mechanisms by paired-pulse TMS (80% followed by 120% AMT stimuli at 2-, 3-, 10-, and 15-ms interstimulus intervals) were tested with a double-cone (n = 12) and a figure-of-eight (n = 5) coil. In experiment 2 (n = 17), LES cortical representations were tested with PA- and AP-TMS. AMT was higher for AP- compared with PA-TMS (P = 0.002). Longer latencies with AP-TMS were present compared with PA-TMS (P = 0.017). AP-TMS produced more inhibition compared with PA-TMS at 2 ms and 3 ms (P = 0.010), but no difference was observed for longer intervals. No difference was found for recruitment curve and mapping. These findings suggest that PA- and AP-TMS may activate different cortical circuits controlling low back muscles, as proposed for hand muscles.NEW & NOTEWORTHY For the first time, anteroposterior and posteroanterior induced electric currents in the brain were compared when targeting back muscle representation with transcranial magnetic stimulation. The use of the anteroposterior current resulted in later response latency, larger inhibition probed by paired-pulse stimulation, and higher motor threshold. These important differences between current directions suggest that each of the current directions may recruit specific cortical circuits involved in the control of back muscles, similar to that for hand muscles.

Rupture, reconstruction, and rehabilitation: A multi-disciplinary review of mechanisms for central nervous system adaptations following anterior cruciate ligament injury

BACKGROUND

Despite surgical reconstruction and extensive rehabilitation, persistent quadriceps inhibition, gait asymmetry, and functional impairment remain prevalent in patients after anterior cruciate ligament (ACL) injury. A combination of reports have suggested underlying central nervous system adaptations in those after injury govern long-term neuromuscular impairments. The classic assumption has been to attribute neurophysiologic deficits to components of injury, but other factors across the continuum of care (e.g. surgery, perioperative analgesia, and rehabilitative strategies) have been largely overlooked.

OBJECTIVE

This review provides a multidisciplinary perspective to 1) provide a narrative review of studies reporting neuroplasticity following ACL injury in order to inform clinicians of the current state of literature and 2) provide a mechanistic framework of neurophysiologic deficits with potential clinical implications across all phases of injury and recovery (injury, surgery, and rehabilitation) RESULTS: Studies using a variety of neurophysiologic modalities have demonstrated peripheral and central nervous system adaptations in those with prior ACL injury. Longitudinal investigations suggest neurophysiologic changes at spinal-reflexive and corticospinal pathways follow a unique timecourse across injury, surgery, and rehabilitation.

CONCLUSION

Clinicians should consider the unique injury, surgery, anesthesia, and rehabilitation on central nervous system adaptations. Therapeutic strategies across the continuum of care may be beneficial to mitigate maladaptive neuroplasticity in those after ACL injury.

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.

Gamma Loop Dysfunction as a Possible Neurophysiological Mechanism of Arthrogenic Muscle Inhibition: A Narrative Review of the Literature

Context: Quadriceps activation failure has been observed following various pathological conditions in a knee joint such as knee surgery, pain, effusion in knee, and osteoarthritis also could be aging matter. Those patients are unable to attain maximal quadriceps strength for a long period of time although their quadriceps itself is not damaged. This impairment is termed arthrogenic muscle inhibition (AMI). AMI has been of concern to clinicians because this weakness hinders the rehabilitation process considerably and delays recovery because strengthening protocols for the AMI could be largely ineffective. Clinically, it is important to understand neurophysiological mechanisms of the AMI to treat patients with the impairment. Objectives: This is a narrative review of the literature. The purpose of this review is to understand the following: (1) Why investigations of only peripheral spinal reflexive pathways are not enough for elucidation of the mechanisms of the AMI? (2) What we know about the role of the gamma spindle system in AMI so far? (3) Could a dysfunctional gamma spindle system contribute to AMI lead neural changes in upper central nervous system? and (4) Concerns that a clinician should take into consideration when deciding whether to apply therapeutic interventions for AMI. Data Sources: The databases PubMed, MEDLINE, SPORTDiscus, and CINAHL were searched with the terms arthrogenic muscle inhibition (AMI), reflex inhibition, joint mechanoreceptor, gamma loop, corticospinal pathway, spinal reflex, effusion, and joint injury. The remaining citations were collected from references of similar papers. Conclusions: AMI is a limiting factor in the rehabilitation of joint injury. Motor unit recruitment could be hindered in patients with AMI as a result of a dysfunctional gamma spindle system. Clinicians should understand the mechanism of AMI well in order to establish effective rehabilitation programs for AMI. Indeed, AMI is not caused by a single factor, but rather, multiple neural factors can change over time following the appearance of AMI. Therefore, multiple interventions targeting different neural pathways should be combined to achieve the ideal therapeutic goal for the treatment of AMI.

Athletes after anterior cruciate ligament reconstruction demonstrate asymmetric intracortical facilitation early after surgery

Quadriceps dysfunction persists after anterior cruciate ligament reconstruction (ACLR), yet the etiology remains elusive. Inhibitory and facilitatory intracortical networks (ie, intracortical excitability) may be involved in quadriceps dysfunction, yet the investigation of these networks early after ACLR is sparse. The purposes of this study were to examine (a) changes in intracortical excitability in athletes after ACLR compared to uninjured athletes during the course of postoperative rehabilitation, (b) the association between intracortical excitability and quadriceps strength in athletes after ACLR. Eighteen level I/II athletes after ACLR between the ages of 18 to 30 years and eighteen healthy sex, age, and activity matched athletes were tested at three-time points: (a) 2 weeks after surgery, (b) achievement of a "quiet knee" defined as full range of motion and minimal effusion, (c) return to running time point defined as achievement of a quadriceps index ≥80% and at least 12 weeks post-ACLR. Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), measured via transcranial magnetic stimulation and isometric quadriceps strength were examined bilaterally at each time point. There was a significant group × limb interaction (P = .017) for ICF. The ACLR group demonstrated asymmetric ICF (greater in the nonsurgical limb) compared to controls and a significant relationship between SICI and quadriceps strength of the surgical limb at the quiet knee time point (P = .018). ACLR individuals demonstrate differential effects on ICF between limbs. Also, SICI is associated with isometric quadriceps strength after ACLR, suggesting increased inhibition of the motor cortex may contribute to impaired quadriceps strength following ACLR.

TMS coil orientation and muscle activation influence lower limb intracortical excitability

INTRODUCTION

Previous research with transcranial magnetic stimulation (TMS) indicates that coil orientation (TMS current direction) and muscle activation state (rest or active) modify corticospinal and intracortical excitability of upper limb muscles. However, the extent to which these factors influence corticospinal and intracortical excitability of lower limb muscles is unknown. This study aimed to examine how variations in coil orientation and muscle activation affect corticospinal and intracortical excitability of tibialis anterior (TA), a lower leg muscle.

METHODS

In 21 young (21.6 ± 3.3 years, 11 female) adults, TMS was administered to the motor cortical representation of TA in posterior-anterior (PA) and mediolateral (ML) orientations at rest and during muscle activation. Single-pulse TMS measures of motor evoked potential amplitude, in addition to resting and active motor thresholds, were used to index corticospinal excitability, whereas paired-pulse TMS measures of short-interval intracortical inhibition (SICI) and facilitation (SICF), and long-interval intracortical inhibition (LICI), were used to assess excitability of intracortical circuits.

RESULTS

For single-pulse TMS, motor thresholds and test TMS intensity were lower for ML stimulation (all P < 0.05). In a resting muscle, ML TMS produced greater SICI (P < 0.001) and less SICF (both P < 0.05) when compared with PA TMS. In contrast, ML TMS in an active muscle resulted in reduced SICI but increased SICF (both P ≤ 0.001) when compared with PA TMS.

CONCLUSION

TMS coil orientation and muscle activation influence measurements of intracortical excitability recorded in the tibialis anterior, and are therefore important considerations in TMS studies of lower limb muscles.

Using transcranial magnetic stimulation to map the cortical representation of lower-limb muscles

Objective

To evaluate the extent to which transcranial magnetic stimulation (TMS) can identify discrete cortical representation of lower-limb muscles in healthy individuals.

Methods

Motor evoked potentials were recorded from resting vastus medialis, rectus femoris, vastus lateralis, medial and lateral hamstring, and medial and lateral gastrocnemius muscles on the right leg of 16 young healthy adults using bipolar surface electrodes. TMS was delivered through a 110-mm double-cone coil at 63 sites over the left hemisphere. Location and size of cortical representation and number of discrete peaks were quantified.

Results

Within the quadriceps group there was a main effect of muscle on anterior-posterior centre of gravity (p = 0.010), but the magnitude of the difference was small. There was also a main effect of muscle on medial-lateral hotspot (p = 0.027) and map volume (p = 0.047), but no post-hoc tests were significant. The topography of each lower-limb muscle was complex and variable across individuals.

Conclusions

TMS delivered with a 110-mm double-cone coil could not reliably identify discrete cortical representations of resting lower-limb muscles when responses were measured using bipolar surface electromyography.

Significance

The characteristics of the cortical representation provide a basis against which to evaluate cortical reorganisation in clinical populations.

Effect of paired-pulse stimulus parameters on the two phases of short interval intracortical inhibition in the quadriceps muscle group

BACKGROUND

Short interval intracortical inhibition (SICI) is commonly used to assess inhibition in the motor cortex and is known to be affected by the paired-pulse stimulus parameters (i.e., interstimulus interval [ISI], conditioning stimulus intensity [CSI] and test stimulus intensity [TSI]) used during testing. While the effects of stimulus parameters are well-studied in the upper-extremity, evidence in the lower-extremity is lacking.

OBJECTIVE

To comprehensively examine the effects of alterations in paired-pulse stimulus parameters on the two phases of SICI in the quadriceps muscle group.

METHODS

Seventeen adults (8 males, 9 females) volunteered to participate in this study. SICI was examined over a range of CSIs (70-90% active motor threshold [AMT]), TSIs (100-140% AMT), and ISIs (1.0-3.0 ms) using both EMG and torque responses elicited by transcranial magnetic stimulation (TMS).

RESULTS

The results indicated that SICI at 1.0 ms ISI was best revealed with a CSI of 70% and TSI ≥110% AMT, whereas SICI at 2.5 ms ISI was best revealed with a CSI of 80-90% and a TSI of ≥130% AMT. Unlike upper-extremity muscles, evaluating SICI with a CSI of 70% AMT and an ISI of 1.0 ms produced the greatest inhibition for all TSIs. In general, inhibitory effects were contaminated by facilitatory effects when using a TSI of 100% AMT.

CONCLUSIONS

The amount of inhibition was dependent on the stimulation parameters used during testing. A CSI of 70% AMT, ISI of 1.0 ms, and TSI of ≥110% AMT appear to be optimal for measuring SICI in the quadriceps muscle; however, other parameters can be used if careful consideration is given to the described interaction between the parameters.

Functional Brain Plasticity Associated with ACL Injury: A Scoping Review of Current Evidence

Anterior cruciate ligament (ACL) injury is a common problem with consequences ranging from chronic joint instability to early development of osteoarthritis. Recent studies suggest that changes in brain activity (i.e., functional neuroplasticity) may be related to ACL injury. The purpose of this article is to summarize the available evidence of functional brain plasticity after an ACL injury. A scoping review was conducted following the guidelines of the Joanna Briggs Institute and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. The terms “brain,” “activity,” “neuroplasticity,” “ACL,” “injury,” and “reconstruction” were used in an electronic search of articles in PubMed, PEDro, CINAHL, and SPORTDiscus databases. Eligible studies included the following criteria: (a) population with ACL injury, (b) a measure of brain activity, and (c) a comparison to the ACL-injured limb (contralateral leg or healthy controls). The search yielded 184 articles from which 24 were included in this review. The effect size of differences in brain activity ranged from small (0.05, ACL-injured vs. noninjured limbs) to large (4.07, ACL-injured vs. healthy control). Moreover, heterogeneity was observed in the methods used to measure brain activity and in the characteristics of the participants included. In conclusion, the evidence summarized in this scoping review supports the notion of functional neuroplastic changes in people with ACL injury. The techniques used to measure brain activity and the presence of possible confounders, as identified and reported in this review, should be considered in future research to increase the level of evidence for functional neuroplasticity following ACL injury.

Deficits in Quadriceps Force Control After Anterior Cruciate Ligament Injury: Potential Central Mechanisms

CONTEXT

Poor quadriceps force control has been observed after anterior cruciate ligament (ACL) reconstruction but has not been examined after ACL injury. Whether adaptations within the central nervous system are contributing to these impairments is unknown.

OBJECTIVE

To examine quadriceps force control in individuals who had sustained a recent ACL injury and determine the associations between cortical excitability and quadriceps force control in these individuals.

DESIGN

Cross-sectional study.

SETTING

Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Eighteen individuals with a recent unilateral ACL injury (6 women, 12 men; age = 29.6 ± 8.4 years, height = 1.74 ± 0.07 m, mass = 76.0 ± 10.4 kg, time postinjury = 69.5 ± 42.5 days) and 18 uninjured individuals (6 women, 12 men; age = 29.2 ± 6.8 years, height = 1.79 ± 0.07 m, mass = 79.0 ± 8.4 kg) serving as controls participated.

MAIN OUTCOME MEASURE(S)

Quadriceps force control was quantified as the root mean square error between the quadriceps force and target force during a cyclical force-matching task. Cortical excitability was measured as the active motor threshold and cortical silent period. Outcome measures were determined bilaterally in a single testing session. Group and limb differences in quadriceps force control were assessed using mixed analyses of variance (2 × 2). Pearson product moment correlations were performed between quadriceps force control and cortical excitability in individuals with an ACL injury.

RESULTS

Individuals with an ACL injury exhibited greater total force-matching error with their involved (standardized mean difference [SMD] = 0.8) and uninvolved (SMD = 0.9) limbs than did controls (F_1,27 = 11.347, P = .03). During the period of descending force, individuals with an ACL injury demonstrated greater error using their involved (SMD = 0.8) and uninvolved (SMD = 0.8) limbs than uninjured individuals (F_1,27 = 4.941, P = .04). Greater force-matching error was not associated with any cortical excitability measures (P > .05).

CONCLUSIONS

Quadriceps force control was impaired bilaterally after recent ACL injury but was not associated with selected measures of cortical excitability. The findings highlight a need to incorporate submaximal-force control tasks into rehabilitation and "prehabilitation," as the deficits were present before surgery.

Corticospinal and intracortical excitability differ between athletes early after ACLR and matched controls

Neuromuscular impairments, such as quadriceps weakness and activation deficits, persist after anterior cruciate ligament reconstruction (ACLR). Recent research demonstrating changes in the function of the primary motor cortex after ACLR posits that quadriceps impairments may be influenced by reduced corticospinal excitability. The purpose of this study was to investigate whether the integrity of the neuromotor axis of the vastus medialis is altered in subjects 2 weeks post-ACLR compared to uninjured control subjects. Eighteen athletes 2 weeks post-ACLR and 18 age and sex matched uninjured control subjects participated in this cross-sectional study. We quantified corticospinal (resting motor threshold, RMT; motor evoked potential amplitudes at 120% RMT, MEP₁₂₀ ) and intracortical (inhibition and facilitation) excitability using single and paired pulse transcranial magnetic stimulation (TMS), respectively. We assessed spinal-reflex excitability (H-reflex amplitude normalized to maximal M-wave, H/M ratio) using peripheral stimulation. Subjects post-ACLR had higher RMTs (p = 0.001), greater MEP₁₂₀ amplitudes (p = 0.001), and more asymmetric facilitation (p = 0.041) than the uninjured control subjects. No significant group differences were found for intracortical inhibition (p = 0.289) and H/M ratio (p = 0.332). Our findings indicate that both intracortical and corticospinal excitability of vastus medialis are bilaterally altered in subjects 2 weeks after ACLR. Given persistent neuromuscular deficits seen after ACLR, rehabilitation strategies targeting intracortical and corticospinal deficits may potentially improve clinical outcomes. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2941-2948, 2018.

Effect of high body mass index on knee muscle strength and function after anterior cruciate ligament reconstruction using hamstring tendon autografts

BACKGROUND

Increased body mass index (BMI) has been associated with poorer function in patients who have undergone anterior cruciate ligament (ACL) reconstruction. However, the effect of high BMI on muscle strength in these patients remained unclear. The current study aimed to compare knee muscle strength and Knee injury and Osteoarthritis Outcome Score (KOOS) in ACL-reconstructed patients with a variety of different BMIs.

METHODS

From November 2013 to March 2016, we prospectively enrolled 30 patients who underwent ACL reconstruction (18-60 years of age). Anthropometric parameters, body compositions, isokinetic muscle strength and KOOS were assessed preoperatively, and at post-operative 16th week and 28th week. The patients were stratified into two groups by BMI, i.e. normal BMI (18.5-24.9 kg/m²) and high BMI (≥25.0 kg/m²).

RESULTS

Twelve patients in the normal BMI group completed the follow-up, while sixteen patients did so in the high BMI group. In comparison of muscle strength between baseline and 28th week follow-up, the normal BMI group had significant increases in overall knee muscle strength, while the high BMI group only had increases in extensors of uninjured knee and flexors of the injured knee. However, there were significant increases in all KOOS subscales for the high BMI group. The high BMI patients reported increased KOOS, which may reflect the contribution of ligament stability in the presence of inadequate muscle strength.

CONCLUSIONS

The normal BMI patients had improvement in all knee muscle strength following ACL reconstruction, while high BMI patients only had increases in certain knee muscles. High BMI patients had a decreased quadriceps muscle symmetry index, as compared to their normal BMI counterparts. Increases in quadriceps muscle strength of the uninjured knee and ACL reconstruction were associated with improvements in KOOS in high BMI patients.

Eccentric Exercise to Enhance Neuromuscular Control

Context:

Neuromuscular alterations are a major causal factor of primary and secondary injuries. Though injury prevention programs have experienced some success, rates of injuries have not declined, and after injury, individuals often return to activity with functionality below clinical recommendations. Considering alternative therapies to the conventional concentric exercise approach, such as one that can target neuromuscular injury risk and postinjury alterations, may provide for more effective injury prevention and rehabilitation protocols.

Evidence Acquisition:

Peer-reviewed sources available on the Web of Science and MEDLINE databases from 2000 through 2016 were gathered using searches associated with the keywords eccentric exercise, injury prevention, and neuromuscular control.

Hypothesis:

Eccentric exercise will reduce injury risk by targeting specific neural and morphologic alterations that precipitate neuromuscular dysfunction.

Study Design:

Clinical review.

Level of Evidence:

Level 4.

Results:

Neuromuscular control is influenced by alterations in muscle morphology and neural activity. Eccentric exercise beneficially modifies several underlying factors of muscle morphology (fiber typing, cross-sectional area, working range, and pennation angle), and emerging evidence indicates that eccentric exercise is also beneficial to peripheral and central neural activity (alpha motorneuron recruitment/firing, sarcolemma activity, corticospinal excitability, and brain activation).

Conclusion:

There is mounting evidence that eccentric exercise is not only a therapeutic intervention influencing muscle morphology but also targets unique alterations in neuromuscular control, influencing injury risk.

Permanent knee sensorimotor system changes following ACL injury and surgery

The cruciate ligaments are components of the knee capsuloligamentous system providing vital neurosensory and biomechanical function. Since most historical primary ACL repair attempts were unsuccessful, reconstruction has become the preferred surgery. However, an increased understanding of the efficacy of lesion-site scaffolding, innovative suturing methods and materials, and evolving use of biological healing mediators such as platelet-rich plasma and stem cells has prompted reconsideration of what was once believed to be impossible. A growing number of in vivo animal studies and prospective clinical studies are providing increasing support for this intervention. The significance of ACL repair rather than reconstruction is that it more likely preserves the native neurosensory system, entheses, and ACL footprints. Tissue preservation combined with restored biomechanical function increases the likelihood for premorbid neuromuscular control system and dynamic knee stability recovery. This recovery should increase the potential for more patients to safely return to sports at their desired intensity and frequency. This current concepts paper revisits cruciate ligament neurosensory and neurovascular anatomy from the perspective of knee capsuloligamentous system function. Peripheral and central nerve pathways and central cortical representation mapping are also discussed. Surgical restoration of a more physiologically sound knee joint may be essential to solving the osteoarthritis dilemma. Innovative rehabilitative strategies and outcome measurement methodologies using more holistic and clinically relevant measurements that closely link biomechanical and neurosensory characteristics of physiological ACL function are discussed. Greater consideration of task-specific patient physical function and psychobehavioral links should better delineate the true efficacy of all ACL surgical and non-surgical interventions. Level of evidence IV.