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

No Evidence of Hysteresis in Quadriceps or Hamstring Active Motor Evoked Potentials

BackgroundThe excitability of the corticospinal tract (i.e., corticospinal excitability) is a valuable tool for assessing neurophysiology and the effectiveness of interventions in individuals with and without neurological and/or orthopaedic injuries. Corticospinal excitability is often measured with an input-output recruitment curve, which is produced by stimulating the motor cortex via transcranial magnetic stimulation (TMS) at several intensities and measuring the changes in the evoked responses. However, it is currently unclear if hysteresis in motor evoked potentials (MEPs) (i.e., changes in MEP amplitude due to the order of stimulus intensities) affects the resulting measure of excitability, particularly in lower extremity muscles.ObjectiveTo evaluate whether the order of stimulus intensity (ascending, descending, randomized) affects input-output recruitment curves measured in the lower extremity muscles.MethodsRecruitment curves were produced in neurologically intact individuals by stimulating the primary motor cortex at 70% to 140% of active motor threshold in 10% increments. We examined three stimulus intensity ordering paradigms: ascending (70→ 140), descending (140→ 70), and randomized. We measured MEPs of the quadriceps and the antagonistic hamstring muscles using surface electromyography in addition to quadriceps motor evoked torque. We computed the area under the recruitment curve (AUC) of the raw and normalized motor evoked responses and used classical and Bayesian inference methods to comprehensively evaluate hysteresis in MEPs.ResultsClassical hypothesis testing revealed no significant main effects of stimulus order. Bayesian analyses also confirmed that the null model was more favored than the main effects model.ConclusionsCorticospinal excitability of the quadriceps and antagonistic hamstring muscles were not influenced by stimulus intensity order. Any of the three approaches (ascending, descending, randomized) may be used when measuring recruitment curves for the quadriceps and hamstring muscles.

Neuromuscular characteristics of eccentric, concentric and isometric contractions of the knee extensors

PURPOSE

We compared voluntary drive and corticospinal responses during eccentric (ECC), isometric (ISOM) and concentric (CON) muscle contractions to shed light on neurophysiological mechanisms underpinning the lower voluntary drive in a greater force production in ECC than other contractions.

METHODS

Sixteen participants (20-33 years) performed ISOM and isokinetic (30°/s) CON and ECC knee extensor contractions (110°-40° knee flexion) in which electromyographic activity (EMG) was recorded from vastus lateralis. Voluntary activation (VA) was measured during ISOM, CON and ECC maximal voluntary contractions (MVCs). Transcranial magnetic stimulation elicited motor-evoked potentials (MEPs) and corticospinal silent periods (CSP) during MVCs and submaximal (30%) contractions, and short-interval intracortical inhibition (SICI) in submaximal contractions.

RESULTS

MVC torque was greater (P < 0.01) during ECC (302.6 ± 90.0 Nm) than ISOM (269.8 ± 81.5 Nm) and CON (235.4 ± 78.6 Nm), but VA was lower (P < 0.01) for ECC (68.4 ± 14.9%) than ISOM (78.3 ± 13.1%) and CON (80.7 ± 15.4%). In addition, EMG/torque was lower (P < 0.02) for ECC (1.9 ± 1.1 μV.Nm-1) than ISOM (2.2 ± 1.2 μV.Nm-1) and CON (2.7 ± 1.6 μV.Nm-1), CSP was shorter (p < 0.04) for ECC (0.097 ± 0.03 s) than ISOM (0.109 ± 0.02 s) and CON (0.109 ± 0.03 s), and MEP amplitude was lower (P < 0.01) for ECC (3.46 ± 1.67 mV) than ISOM (4.21 ± 2.33 mV) and CON (4.01 ± 2.06 mV). Similar results were found for EMG/torque and CSP during 30% contractions, but MEP and SICI showed no differences among contractions (p > 0.05).

CONCLUSIONS

The lower voluntary drive indicated by reduced VA during ECC may be partly explained by lower corticospinal excitability, while the shorter CSP may reflect extra muscle spindle excitation of the motoneurons from vastus lateralis muscle lengthening.

Altered Corticospinal and Intracortical Excitability After Stroke: A Systematic Review With Meta-Analysis

BACKGROUND

Intracortical inhibitory/faciliatory measures are affected after stroke; however, the evidence is conflicting.

OBJECTIVE

This meta-analysis aimed to investigate the changes in motor threshold (MT), motor evoked potential (MEP), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF), and identify sources of study variability using a machine learning approach.

METHODS

We identified studies that objectively evaluated corticospinal excitability and intracortical inhibition/facilitation after stroke using transcranial magnetic stimulation. Pooled within- (ie, affected hemisphere [AH] vs unaffected hemisphere [UH]) and between-subjects (ie, AH and UH vs Control) standardized mean differences were computed. Decision trees determined which factors accurately predicted studies that showed alterations in corticospinal excitability and intracortical inhibition/facilitation.

RESULTS

A total of 35 studies (625 stroke patients and 328 healthy controls) were included. MT was significantly increased and MEP was significantly decreased (ie, reduced excitability) in the AH when compared with the UH and Control (P < .01). SICI was increased (ie, reduced inhibition) for the AH when compared with the UH, and for the AH and UH when compared with Control (P < .001). ICF was significantly increased (ie, increased facilitation) in the AH when compared with UH (P = .016) and decreased in UH when compared with Control (P < 0.001). Decision trees indicated that demographic and methodological factors accurately predicted (73%-86%) studies that showed alterations in corticospinal and intracortical excitability measures.

CONCLUSIONS

The findings indicate that stroke alters corticospinal and intracortical excitability measures. Alterations in SICI and ICF may reflect disinhibition of the motor cortex after stroke, which is contrary to the notion that stroke increases inhibition of the affected side.

Shaping corticospinal pathways in virtual reality: effects of task complexity and sensory feedback during mirror therapy in neurologically intact individuals

BACKGROUND

Restoration of limb function for individuals with unilateral weakness typically requires volitional muscle control, which is often not present for individuals with severe impairment. Mirror therapy-interventions using a mirror box to reflect the less-impaired limb onto the more-impaired limb-can facilitate corticospinal excitability, leading to enhanced recovery in severely impaired clinical populations. However, the mirror box applies limitations on mirror therapy, namely that all movements appear bilateral and are confined to a small area, impeding integration of complex activities and multisensory feedback (e.g., visuo-tactile stimulation). These limitations can be addressed with virtual reality, but the resulting effect on corticospinal excitability is unclear.

OBJECTIVE

Examine how virtual reality-based unilateral mirroring, complex activities during mirroring, and visuo-tactile stimulation prior to mirroring affect corticospinal excitability.

MATERIALS AND METHODS

Participants with no known neurological conditions (n = 17) donned a virtual reality system (NeuRRoVR) that displayed a first-person perspective of a virtual avatar that matched their motions. Transcranial magnetic stimulation-induced motor evoked potentials in the nondominant hand muscles were used to evaluate corticospinal excitability in four conditions: resting, mirroring, mirroring with prior visuo-tactile stimulation (mirroring + TACT), and control. During mirroring, the movements of each participant's dominant limb were reflected onto the nondominant limb of the virtual avatar, and the avatar's dominant limb was kept immobile (i.e., unilateral mirroring). The mirroring + TACT condition was the same as the mirroring condition, except that mirroring was preceded by visuo-tactile stimulation of the nondominant limb. During the control condition, unilateral mirroring was disabled. During all conditions, participants performed simple (flex/extend fingers) and complex (stack virtual blocks) activities.

RESULTS

We found that unilateral mirroring increased corticospinal excitability compared to no mirroring (p < 0.001), complex activities increased excitability compared to simple activities during mirroring (p < 0.001), and visuo-tactile stimulation prior to mirroring decreased excitability (p = 0.032). We also found that these features did not interact with each other.

DISCUSSIONS

The findings of this study shed light onto the neurological mechanisms of mirror therapy and demonstrate the unique ways in which virtual reality can augment mirror therapy. The findings have important implications for rehabilitation for design of virtual reality systems for clinical populations.

How Does Ankle Mechanical Stiffness Change as a Function of Muscle Activation in Standing and During the Late Stance of Walking?

OBJECTIVE

Ankle joint stiffness is known to be modulated by co-contraction of the ankle muscles; however, it is unclear to what extent changes in agonist muscle activation alone affect ankle joint stiffness. This study tested the effects of varying levels of ankle muscle activation on ankle joint mechanical stiffness in standing and during the late stance phase of walking.

METHODS

Dorsiflexion perturbations were applied at various levels of ankle muscle activation via a robotic platform in standing and walking conditions. In standing, muscle activation was modulated by having participants perform an EMG target matching task that required varying levels of plantarflexor activation. In walking, muscle activation was modulated by changing walking speeds through metronome-based auditory feedback. Ankle stiffness was evaluated by performing a Least-squares system identification using a parametric model consisting of stiffness, damping, and inertia. The association between ankle muscle activation and joint stiffness was evaluated using correlation analyses. Linear regression models were used to determine the extent to which muscle activation contributed to ankle stiffness. An inclusive statistical approach (both classical and Bayesian analyses) was adopted to measure the statistical significance (p-value) and Bayes Factor (BF₁₀).

RESULTS

Results indicate that plantarflexor activity was positively correlated with ankle stiffness in both standing and walking (p<0.001, BF₁₀>900), whereas dorsiflexor activity was negatively correlated with ankle stiffness in walking (p = 0.014, BF₁₀ = 3.9) but not in standing (p = 0.725). Regression analyses indicated that ankle muscle activation predicted about 84% of the variation in ankle stiffness in standing and 45% in walking (p<0.001, BF₁₀>100).

CONCLUSION

Ankle muscle activation significantly contributes to ankle stiffness during standing and walking.

SIGNIFICANCE

The results highlight the role of muscle activation on maintaining joint stiffness and underscore the importance of accounting for muscle activation when measuring ankle stiffness in healthy as well as patient populations.

Effect of conventional transcranial direct current stimulation devices and electrode sizes on motor cortical excitability of the quadriceps muscle

BACKGROUND

There is a growing concern among the scientific community that the effects of transcranial direct current stimulation (tDCS) are highly variable across studies. The use of different tDCS devices and electrode sizes may contribute to this variability; however, this issue has not been verified experimentally.

OBJECTIVE

To evaluate the effects of tDCS device and electrode size on quadriceps motor cortical excitability.

METHODS

The effect of tDCS device and electrode size on quadriceps motor cortical excitability was quantified across a range of TMS intensities using a novel evoked torque approach that has been previously shown to be highly reliable. In experiment 1, anodal tDCS-induced excitability changes were measured in twenty individuals using two devices (Empi and Soterix) on two separate days. In experiment 2, anodal tDCS-induced excitability changes were measured in thirty individuals divided into three groups based on the electrode size. A novel Bayesian approach was used in addition to the classical hypothesis testing during data analyses.

RESULTS

There were no significant main or interaction effects, indicating that cortical excitability did not differ between different tDCS devices or electrode sizes. The lack of pre-post time effect in both experiments indicated that cortical excitability was minimally affected by anodal tDCS. Bayesian analyses indicated that the null model was more favored than the main or the interaction effects model.

CONCLUSIONS

Motor cortical excitability was not altered by anodal tDCS and did not differ by devices or electrode sizes used in the study. Future studies should examine if behavioral outcomes are different based on tDCS device or electrode size.

Functional Resistance Training to Improve Knee Strength and Function After Acute Anterior Cruciate Ligament Reconstruction: A Case Study

BACKGROUND

Thigh muscle weakness after anterior cruciate ligament reconstruction (ACLR) can persist after returning to activity. While resistance training can improve muscle function, "nonfunctional" training methods are not optimal for inducing transfer of benefits to activities such as walking. Here, we tested the feasibility of a novel functional resistance training (FRT) approach to restore strength and function in an individual with ACLR.

HYPOTHESIS

FRT would improve knee strength and function after ACLR.

STUDY DESIGN

Case report.

LEVEL OF EVIDENCE

Level 5.

METHODS

A 15-year-old male patient volunteered for an 8-week intervention where he performed 30 minutes of treadmill walking, 3 times per week, while wearing a custom-designed knee brace that provided resistance to the thigh muscles of his ACLR leg. Thigh strength, gait mechanics, and corticospinal and spinal excitability were assessed before and immediately after the 8-week intervention. Voluntary muscle activation was evaluated immediately after the intervention.

RESULTS

Knee extensor and flexor strength increased in the ACLR leg from pre- to posttraining (130 to 225 N·m [+74%] and 44 to 88 N·m [+99%], respectively) and increases in between-limb extensor and flexor strength symmetry (45% to 92% [+74%] and 47% to 72% [+65%], respectively) were also noted. After the intervention, voluntary muscle activation in the ACLR leg was 72%, compared with the non-ACLR leg at 75%. Knee angle and moment during late stance phase decreased (ie, improved) in the ACLR leg and appeared more similar to the non-ACLR leg after FRT training (18° to 14° [-23.4] and 0.07 to -0.02 N·m·kg^-1·m^-1 [-122.8%], respectively). Corticospinal and spinal excitability in the ACLR leg decreased (3511 to 2511 [-28.5%] and 0.42 to 0.24 [-43.7%], respectively) from pre- to posttraining.

CONCLUSION

A full 8 weeks of FRT that targeted both quadriceps and hamstring muscles lead to improvements in strength and gait, suggesting that FRT may constitute a promising and practical alternative to traditional methods of resistance training.

CLINICAL RELEVANCE

FRT may serve as a viable approach to improve knee strength and function after ACL reconstruction.

Evaluation of motor cortical excitability using evoked torque responses: A new tool with high reliability

BACKGROUND

Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) are typically recorded via surface electromyography (EMG). However, another suitable alternative may be recording torque output associated with MEPs, especially when studying multiheaded muscles (e.g. quadriceps) for which EMG may not be ideal.

METHODS

We recorded the motor evoked torque elicited by TMS along with conventional EMG-based MEPs (MEP_EMG) over a range of TMS intensities (100-140 % of active motor threshold [AMT]) from twenty healthy young adults on two different days. MEPs were normalized using different normalization procedures (raw, normalized to maximum voluntary isometric contraction [MVIC], and peak MEP). Additionally, motor evoked torque was normalized to TMS-evoked peripheral resting twitch torque. Intraclass correlation coefficients (ICCs) were determined for each of these variables to compute reliability.

RESULTS

Motor evoked torque showed good to excellent reliability (ICC: 0.65-0.90) at TMS intensities ≥ 110 % AMT, except when normalized by peak MEP. The reliability of raw MEP_EMG and MVIC normalized MEP_EMG was fair to excellent only at ≥ 130 % AMT (ICC: 0.42-0.82) and at ≥ 120 % AMT (ICC: 0.41-0.83), respectively. The reliability of both MEP_EMG and motor evoked torque generally increased with increasing TMS intensities, with motor evoked torque normalized to the resting twitch torque yielding the best ICC scores.

COMPARISON WITH EXISTING METHODS

When compared with conventional MEP_EMG, motor evoked torque offers superior and reliable estimates of corticospinal excitability, particularly when normalized to resting twitch torque.

CONCLUSIONS

TMS-induced motor evoked torque can reliably be used to measure corticospinal excitability in the quadriceps muscles.

Use of transcranial magnetic stimulation to assess relaxation rates in unfatigued and fatigued knee-extensor muscles

We examined whether transcranial magnetic stimulation (TMS) delivered to the motor cortex allows assessment of muscle relaxation rates in unfatigued and fatigued knee extensors (KE). We assessed the ability of this technique to measure time course of fatigue-induced changes in muscle relaxation rate and compared relaxation rate from resting twitches evoked by femoral nerve stimulation. Twelve healthy men performed maximal voluntary isometric contractions (MVC) twice before (PRE) and once at the end of a 2-min KE MVC and five more times within 8 min during recovery. Relative (intraclass correlation coefficient; ICC_2,1) and absolute (repeatability coefficient) reliability and variability (coefficient of variation) were assessed. Time course of fatigue-induced changes in muscle relaxation rate was tested with generalized estimating equations. In unfatigued KE, peak relaxation rate coefficient of variation and repeatability coefficient were similar for both techniques. Mean (95% CI) ICC_2,1 for peak relaxation rates were 0.933 (0.724–0.982) and 0.889 (0.603–0.968) for TMS and femoral nerve stimulation, respectively. TMS-induced normalized muscle relaxation rate was − 11.5 ± 2.5 s⁻¹ at PRE, decreased to − 6.9 ± 1.2 s⁻¹ (− 37 ± 17%, P < 0.001), and recovered by 2 min post-exercise. Normalized peak relaxation rate for resting twitch did not show a fatigue-induced change. During fatiguing KE exercise, the change in muscle relaxation rate as determined by the two techniques was different. TMS provides reliable values of muscle relaxation rates. Furthermore, it is sufficiently sensitive and more appropriate than the resting twitch evoked by femoral nerve stimulation to reveal fatigue-induced changes in KE.

A time-efficient method to determine parameters for measurement of short-interval intracortical inhibition for quadriceps

Short-interval intracortical inhibition (SICI) is often assessed to investigate inhibitory responses in the primary motor cortex representation of the quadriceps. However, determining appropriate paired-pulse transcranial magnetic stimulation (TMS) parameters to optimise SICI measurement can be impractical and time-consuming. This study investigated the intensity required to elicit maximal and 50% of maximum inhibition, and the test-retest reliability of a time-efficient approach for SICI measurement in quadriceps. Nine men and six women (26.6 ± 4.4 years) underwent single and paired-pulse (3-ms interval) TMS during 10% maximal voluntary isometric contraction on two days. Responses were recorded from vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM). Test stimulus intensity was 140% of active motor threshold (AMT), and conditioning stimulus intensities (CSIs) ranged from 55% to 90% (eight intensities) of AMT (five test and five paired responses for each intensity). With CSI of 55% AMT, SICI was minimal (conditioned:test motor evoked potential [MEP]; 1.00, 0.96 and 0.95 for VL, RF and VM, respectively, <1.00 indicates inhibition). Inhibition was greater at 70%-90% AMT for VL (0.67-0.85), at 75%-90% AMT for RF (0.70-0.78) and at 80%-90% AMT for VM (0.59-0.68) when compared to 55% AMT. The CSIs that elicited maximal and 50% maximal inhibition were ~84% and ~75% AMT, respectively. Reliability for individual CSIs ranged from "poor-to-good" for all muscles. SICI averaged across all CSIs demonstrated "moderate" reliability for VL and VM, but "poor" reliability for RF. This method may offer a practical approach to individualise and select CSIs to investigate quadriceps inhibitory networks in neurophysiological studies.