Motor Unit Recruitment Strategies Research Articles

Task-related differences in peroneus longus muscle fiber conduction velocity.

It has been identified that the peroneus longus presents a regional activity. Specifically, a greater activation of the anterior and posterior compartments has been observed during eversion, whereas a lower activation of the posterior compartment has been reported during plantarflexion. In addition to myoelectrical amplitude, motor unit recruitment can be inferred indirectly from muscle fiber conduction velocity (MFCV). However, there are few reports of MFCV of the regions that make up a muscle, and even less, MFCV of the peroneus longus compartments. This study aimed to analyze the MFCV of peroneus longus compartments during eversion and plantarflexion. Twenty-one healthy individuals were assessed. High-density surface electromyography was recorded from the peroneus longus during eversion and plantarflexion at 10%, 30%, 50%, and 70% of maximal voluntary isometric contraction. The posterior compartment presented a lower MFCV than the anterior compartment during plantarflexion, and both compartments did not show differences in MFCV during eversion; however, the posterior compartment showed an increase in MFCV during eversion compared to plantarflexion. Differences observed in the MFCV of the peroneus longus compartments could support a regional activation strategy and, to some extent, explain different motor unit recruitment strategies of the peroneus longus during ankle movements.

Motor unit placement in a realistic muscle cross section model: performance of a new algorithm and effects of muscle architecture on surface EMG power spectral components

Objetive. Surface electromyogram (sEMG) characteristics are strongly dependent on the spatial localization of motor units (MU). Thus, the definition of the muscle architecture is a crucial step in the simulation of sEMG. Muscles cross section or even the shape of the MU territories is not a regular geometric figure. Moreover, MU may have different fibre densities, and its distribution may not be random but follow a spatial regionalization by type. Despite these prior conditions, the method for MU placement in a muscle model should be able to achieve constant muscle fibre density, which is a generally accepted property of any muscle. Approach. An algorithm was developed that meets the previously established requirements for the distribution of MU in a muscle model. The algorithm considers the muscle cross section and MU territories as bitmaps and encodes the muscle fibre density distribution by pixel brightness. This allowed the use of digital image processing techniques to optimally distribute the MU. The impact of the different muscle architectures on sEMG was investigated using one of the most accepted models of motor unit pool recruitment combined with a model of MU action potential simulation. element-citationitalic Main results. The algorithm produced a completely regionalized architecture with constant muscle fibre density in a muscle with both an irregular cross-section and elliptic MU territory. Significance. In addition to the improvement in the realism of the muscle simulations, the algorithm allowed the investigation of the influence of the spatial distribution of MU on the sEMG power spectrum, thus helping to explain the existing inability to detect changes in motor unit recruitment strategies through the spectral analysis of interference patterns.

Simultaneous monitoring of mouse grip strength, force profile, and cumulative force profile distinguishes muscle physiology following surgical, pharmacologic and diet interventions

Grip strength is a valuable preclinical assay to study muscle physiology in disease and aging by directly determining changes in muscle force generation in active laboratory mice. Existing methods to statistically evaluate grip strength, however, have limitations in the power and scope of the physiological features that are assessed. We therefore designed a microcontroller whose serial measure of resistance-based force enables the simultaneous readout of (1) peak grip strength, (2) force profile (the non-linear progress of force exerted throughout a standard grip strength trial), and (3) cumulative force profile (the integral of force with respect to time of a single grip strength trial). We hypothesized that muscle pathologies of different etiologies have distinct effects on these parameters. To test this, we used our apparatus to assess the three muscle parameters in mice with impaired muscle function resulting from surgically induced peripheral pain, genetic peripheral neuropathy, adverse muscle effects induced by statin drug, and metabolic alterations induced by a high-fat diet. Both surgically induced peripheral nerve injury and statin-associated muscle damage diminished grip strength and force profile, without affecting cumulative force profile. Conversely, genetic peripheral neuropathy resulting from lipin 1 deficiency led to a marked reduction to all three parameters. A chronic high-fat diet led to reduced grip strength and force profile when normalized to body weight. In high-fat fed mice that were exerted aerobically and allowed to recover for 30 min, male mice exhibited impaired force profile parameters, which female mice were more resilient. Thus, simultaneous analysis of peak grip strength, force profile and cumulative force profile distinguishes the muscle impairments that result from distinct perturbations and may reflect distinct motor unit recruitment strategies.

Surface Electromyography Based Core Muscle Fatigue Analysis During Repetitive Plank Using Multivariate Dimensionality Reduction Methods in Boys Aged 12-14

The aims of the study were: 1. To analyse the discriminative power of neuromuscular components for classifying the pre and post muscle fatigued states. 2. To examine whether the modification of neural recruitment strategies become more/less heterogeneous due to fatigue. 3. To research the effect of Erector Spinae (ES) muscle activity collectively with Rectus Abdominis (RA) and External Oblique (EO) muscle activity to identify the reduced spine stability during fatiguing Plank. 
 Material and methods. Twelve boys (age – 12-14 years, height 148.75 ± 10 cm, body mass 38.9 ± 7.9 kg) participated in the study. Multivariate Discriminant Analysis (DA) and Principal Component Analysis (PCA) were applied to identify the changes in the pattern of the electromyographic signals during muscle fatigue. In DA the Wilks’ lambda, p-value, canonical correlation, classification percentage and structure matrix were used. To evaluate the component validity the standard limit for Kaiser-Meyer-Olkin (KMO) was set at ≥0.529 and the p-value of Bartlett’s test was ≤0.001. The eigenvalues ≥1 were used to determine the number of Principal Components (PCs). The satisfactory percentage of non-redundant residuals were set at ≤50% with standard value >0.05. The absolute value of average communality (x̄ h2) and component loadings were set at ≥0.6, ≥0.4 respectively. 
 Results. Standardized canonical discriminant analysis showed that pre and post fatigued conditions were significantly different (p = 0.000, Wilks’ lambda = 0.297, χ2 = 24.914, df=3). The structure matrix showed that the parameter that correlated highly with the discriminant function was ES ARV (0.514). The results showed that the classification accuracy was 95.8% between fatigued conditions. In PCA the KMO values were reduced [0.547Pre fatigue vs. 0.264Post fatigue]; the value of Bartlett’s sphericity test was in pre χ2 = 90.72 (p = 0.000) and post fatigue χ2 = 85.32 (p = 0.000); The Promax criterion with Kaiser Normalization was applied because the component rotation was non-orthogonal [Component Correlation Matrix (rCCM) = 0.520 Pre fatigue >0.3Absolute<0.357Post fatigue]. In pre fatigue two PCs (cumulative s2 – 80.159%) and post fatigue three PCs (cumulative s2 – 83.845%) had eigenvalues ≥1. The x̄ h2 increased [0.802 Pre fatigue vs. 0.838 Post fatigue] and the percentage of nonredundant residuals reduced [50% Pre fatigue vs. 44% Post fatigue] from pre to post fatigue. 
 Conclusions. The variability and heterogeneity increase in the myoelectric signals due to fatigue. The co-activity of antagonist ES muscle is significantly sensitive to identify the deteriorating spine stability during the fatiguing Plank. Highly correlated motor unit recruitment strategies between ES and RA, providing supportive evidence to the concept of shared agonist-antagonist motoneuron pool or “Common Drive” phenomenon during fatigue.

Tracking of Individual Motor Units Following Concentric and Eccentric Exercise‐induced Fatigue Reveals Contraction‐type Specific Changes in Discharge Properties

Muscle fatigue is characterised by a reduction in force output, with increases in force production partly mediated by adjustments in motor unit (MU) firing properties. Conflicting research has found MU firing rate (FR) to both increase and decrease following fatiguing exercise and this disparity may be influenced by contraction type. Using high density surface electromyography (HD-EMG) we have previously reported a decrease in FR following concentric (CON), and an increase following eccentric (ECC) contractions (1). However, HD-EMG also enables the tracking of individual MUs across tasks (2), potentially revealing more detailed MU-specific adaptations to fatigue. The aims of this study were to track the FR of individual MUs following CON and ECC contractions of the vastus lateralis (VL) muscle. HD-EMG was used to identify individual MU potentials (MUPs) from the VLs of 8 young volunteers (4 females; 21 ± 0.5 years). All MUPs were recorded during 4 sustained isometric contractions held at a target line of 25 and 40% of the participants maximum voluntary contraction (MVC) before and after completing CON and ECC fatiguing exercise (CON: ‘stepping up’ with one leg, ECC: ‘stepping down’ on the other leg, wearing a weighted vest +25-40% body weight). The coefficient of variation of force was calculated to assess force steadiness (FS). A total of 43 MUs were tracked pre and post fatigue. Paired t-tests were used for comparisons of tracked MUs. Significance was assumed when p<0.05. MVC decreased in both CON and ECC legs post exercise (-15.3% and -21.1% respectively; p<0.01) and EMG root-mean square amplitude (RMS) significantly decreased in both legs after fatigue (both p≤0.001). FS deteriorated in ECC only at both 25% and 40% MVC (p=0.004 and p=0.04 respectively), with no change in CON. Data from individual tracked MUs supported our previous finding with a decreased FR following CON (-4.7%; p=0.037) but no difference in FR with ECC (+6.9%; p=0.168). These data demonstrate that motor control and MU FR following exercise-induced fatigue differ according to exercise modality. HD-EMG enables specific changes in single motor units to be studied across sessions in addition to the overall MU population effects, and here the FR did not change following ECC exercise even with a notably reduced absolute force (21%). The possibility of differential MU recruitment strategies compensating for discharge characteristics and impacting motor control requires further investigation as this may have translational relevance in ageing, neuromuscular conditions and for post-exercise recovery from fatigue. 1. Jones EJ, Martinez-Valdes E, Negro F, McCormick D, Atherton PJ, Phillips BE, et al. Motor unit discharge properties following concentric and eccentric exercise-induced fatigue are dependent upon contraction type. American Physiological Society: Integrative Physiology of Exercise. 2020. 2. Martinez-Valdes E, Negro F, Falla D, Dideriksen JL, Heckman C, Farina D. Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions. J Neurophysiol. 2020.

Muscle fiber conduction velocity in the vastus lateralis and medialis muscles of soccer players after ACL reconstruction.

The neural factors underlying the persistency of quadriceps weakness after anterior cruciate ligament reconstruction (ACLR) have been only partially explained. This study examined muscle fiber conduction velocity (MFCV) as an indirect parameter of motor unit recruitment strategies in the vastus lateralis (VL) and medialis (VM) muscles of soccer players with ACLR. High-density surface electromyography (HDsEMG) was acquired from VL and VM in nine soccer players (22.7±2.9years; BMI: 22.08±1.72kg·m-2 ; 7.7±2.2months post-surgery). Voluntary muscle force and the relative myoelectrical activity from the reconstructed and contralateral sides were recorded during linearly increasing isometric knee extension contractions up to 70% of maximal voluntary isometric force (MVIF). The relation of MFCV and force was examined by linear regression analysis at the individual subject level. The initial (intercept), peak (MFCV70 ), and rate of change (slope) of MFCV related to force were compared between limbs and muscles. The MVIF was lower in the reconstructed side than in the contralateral side (-%20.5; P<.05). MFCV intercept was similar among limbs and muscles (P>.05). MFCV70 and MFCV slope were lower in the reconstructed side compared to the contralateral for both VL (-28.5% and -10.1%, respectively; P<.001) and VM (-22.6% and -8.1%, respectively; P<.001). The slope of MFCV was lower in the VL than VM, but only in the reconstructed side (-12.4%; P<.001). These results suggest possible impairments in recruitment strategies of high-threshold motor units (HTMUs) as well as deficits in sarcolemmal excitability, fiber diameter, and discharge rate of knee extensor muscles following ACLR.

Understanding the Consequences of Repetitive Subconcussive Head Impacts in Sport: Brain Changes and Dampened Motor Control Are Seen After Boxing Practice.

ObjectivesThe potential effects of exposure to repetitive subconcussive head impacts through routine participation in sport are not understood. To investigate the effects of repetitive subconcussive head impacts we studied boxers following customary training (sparring) using transcranial magnetic stimulation (TMS), decomposition electromyographic (EMG) and tests of memory.MethodsTwenty amateur boxers performed three 3-min sparring bouts. Parameters of brain function and motor control were assessed prior to sparring and again immediately, 1 h and 24 h post-sparring. Twenty control participants were assessed following mock-sparring.ResultsOne hour after sparring boxers showed increased corticomotor inhibition, altered motor unit recruitment strategies, and decreased memory performance relative to controls, with values returning to baseline by the 24 h follow up.ConclusionRepetitive subconcussive head impacts associated with sparring resulted in acute and transient brain changes similar to those previously reported in soccer heading, providing convergent evidence that sport-related head impacts produce a GABAergic response. These acute changes in brain health are reminiscent of effects seen following brain injury, and suggest a potential mechanism underlying the damaging long-term effects of routine repetitive head impacts in sport.

Wavelet analyses of electromyographic signals derived from lower extremity muscles while walking or running: A systematic review.

Surface electromyography is often used to assess muscle activity and muscle function. A wavelet approach provides information about the intensity of muscle activity and motor unit recruitment strategies at every time point of the gait cycle. The aim was to review papers that employed wavelet analyses to investigate electromyograms of lower extremity muscles during walking and running. Eleven databases were searched up until June 1st 2017. The composition was based on the PICO model and the PRISMA checklist. First author, year, subject characteristics, intervention, outcome measures & variables, results and wavelet specification were extracted. Eighteen studies included the use of wavelets to investigate electromyograms of lower extremity muscles. Three main topics were discussed: 1.) The capability of the method to correctly assign participants to a specific group (recognition rate) varied between 68.4%-100%. 2.) Patients with ankle osteoarthritis or total knee arthroplasty presented a delayed muscle activation in the early stance phase but a prolonged activation in mid stance. 3.) Atrophic muscles did not contain type II muscle fiber components but more energy in their lower frequencies. The simultaneous information of time, frequency and intensity is of high clinical relevance because it offers valuable information about pre-and reflex activation behavior on different walking and running speeds as well as spectral changes towards high or low frequencies at every time point of the gait cycle.

Does vibration superimposed on low-level isometric contraction alter motor unit recruitment strategy?

Objective. Beneficial effects, including improved muscle strength and power performance, have been observed during vibration exercise (VE) and partially ascribed to a specific reflex mechanism referred to as Tonic vibration reflex (TVR). TVR involves motor unit (MU) activation synchronized and un-synchronized with the vibration cycle; this suggests VE to alter the temporal MU recruitment strategy. However, the effects of VE on MU recruitment remain poorly understood. This study aims to elucidate the influence of VE on MU recruitment indirectly, by investigating the effects of low-intensity VE on muscle activation. Approach. Twenty volunteers performed isometric contractions on the biceps brachii of the right arm at a baseline (low) force equal to 30% of the maximum voluntary contraction without vibration (control) and with vibration at 20, 30, 40, and 55 Hz. Three vibration amplitudes were employed at 12.5%, 25%, and 50% of the baseline. Mean muscle-fiber conduction velocity (mCV), mean frequency (MF), and root mean square (RMS) value were estimated from surface electromyography as indicators of the alteration in MU recruitment strategies. Main results. The mCV estimates during VE were significantly (p < 0.05) higher compared to the control condition. Furthermore, six VE conditions produced significantly larger RMS values compared to control condition. The estimated MF did not show any consistent trend. Significance. These results suggest that vibration superimposed on low-level isometric contraction alters the MU recruitment strategy, activating larger and faster MUs.

Examination of muscle morphology and neuromuscular function in normal weight and overfat children aged 7-10 years.

This study examined isometric and isokinetic leg extensor strength, muscle size (mCSA) and tissue composition as measured via echo intensity (mEI), and motor unit (MU) firing rates in normal weight (NW) and OF children aged 7-10years. Fourteen NW (eight girls and six boys, BMI: 15.8±1.4kg/m2 ) and 15 OF (10 girls and five boys, BMI: 20.8±2.3kg/m2 ) children volunteered to perform this study. Percentage body fat (%BF) was measured, and mCSA, mEI, and subcutaneous fat (sFAT) of the vastus lateralis (VL) was assessed. MU mean firing rates (MFRs) in relation to recruitment threshold (RT) of the VL were assessed during submaximal isometric contractions. Maximal isokinetic contractions were performed at 1.05 and 4.20rad/s. The 95% confidence intervals (CI) from the statistical tests are presented. The OF children had greater %BF (95% CI=-15.1 to -7.2), mCSA (95% CI=-4.1 to -1.2), mEI (95% CI=-22.3 to -5.9), and sFAT (95% CI=-0.5 to -0.1), greater rate of strength loss with increasing isokinetic velocities (95% CI=0.4 to 5.4), and a smaller range of MFRs (95% CI=0.007 to 7.136) at the steady torque than the NW children. The OF children had poorer muscle tissue composition, greater velocity-related impairments in muscle strength, and a smaller range of MFRs at the targeted torque that may suggest altered MU recruitment strategies. Interventions in OF children should include exercises and recruit higher-threshold MUs, such as high-intensity resistance exercises.

Higher torque and muscle fibre conduction velocity of the Biceps Brachii in karate practitioners during isokinetic contractions

PurposeAlthough upper limb techniques are largely utilized during karate combat competitions scarce information regarding their NM control is available. This study aims at investigating the effect of karate practice on the NM control of Biceps and Triceps Brachii during isokinetic contractions to enhance current knowledge on neuromuscular control adaptations and training methodologies in combat sports. MethodsTorque and surface electromyograms (sEMG) of Biceps Brachii Caput Longum (BB) and Triceps Brachii Lateral Head (TB) were recorded in eight karate practitioners (KA) and eight age-matched sedentary individuals (CO) during isokinetic elbow flexion-extensions (0–240°/s−1). BB and TB sEMG amplitude (Root Mean Square - RMS) and frequency (Median Frequency – MDF) were computed during agonist and antagonist activity. Moreover, muscle fibre conduction velocity (MFCV) of the BB was computed. ResultsDuring the isokinetic contractions, KA group demonstrated higher peak torque and higher MFCV in the BB with respect to CO. KA and CO presented comparable activation of agonist and antagonist muscles and comparable frequency content in both BB and TB. ConclusionsThe greater torque observed in KA should be interpreted in the light of a different motor unit recruitment strategy as suggested by the higher MFCV. Karate and combat sport practitioners should consider including in their training programmes methodologies emphasising neural rather than morphological adaptations.

Adaptations in antagonist co-activation: Role in the repeated-bout effect.

Eccentric exercise results in an adaptation which attenuates muscle damage from subsequent exercise—termed the “repeated-bout effect (RBE).” Purpose: Study examined antagonist co-activation and motor-unit recruitment strategy, assessed via dEMG, concomitant to the RBE. Methods: Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and its recruitment threshold (RT) was examined using linear regression. Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. Surface EMG of the biceps and triceps were collected during eccentric exercise. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. Results: Changes in MVC (-32±14% vs -25±10%; p = 0.034), ROM (-11% vs 6%; p = 0.01), and DOMS (31.0±19mm vs 19±12mm; p = 0.015) were attenuated following the second bout of exercise. Triceps EMG was reduced (16.8±9.5% vs. 12.6±7.2%; p = 0.03) during the second bout of eccentric exercise. The slope (-0.60±0.13 vs -0.70±0.18; p = 0.029) and y-intercept (46.5±8.3 vs 53.3±8.8; p = 0.020) of the MFR vs. RT relationship was altered during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% of MVC. Conclusion: A reduction in antagonist co-activation during the second bout of eccentric exercise suggests less total force was required to move an identical external load. This finding is supported by the increased negative slope coefficient and an increased y-intercept of the linear relationship between RT and MFR.

Alterations in Spectral Attributes of Surface Electromyograms after Utilization of a Foot Drop Stimulator during Post-Stroke Gait.

A foot drop stimulator (FDS) is a rehabilitation intervention that stimulates the common peroneal nerve to facilitate ankle dorsiflexion at the appropriate time during post-stroke hemiplegic gait. Time-frequency analysis (TFA) of non-stationary surface electromyograms (EMG) and spectral variables such as instantaneous mean frequency (IMNF) can provide valuable information on the long-term effects of FDS intervention in terms of changes in the motor unit (MU) recruitment during gait, secondary to improved dorsiflexion. The aim of this study was to apply a wavelet-based TFA approach to assess the changes in neuromuscular activation of the tibialis anterior (TA), soleus (SOL), and gastrocnemius (GA) muscles after utilization of an FDS during gait post-stroke. Surface EMG were collected bilaterally from the TA, SOL, and GA muscles from six participants (142.9 ± 103.3 months post-stroke) while walking without the FDS at baseline and 6 months post-FDS utilization. Continuous wavelet transform was performed to get the averaged time-frequency distribution of band pass filtered (20-300 Hz) EMGs during multiple walking trials. IMNFs were computed during normalized gait and were averaged during the stance and swing phases. Percent changes in the energies associated with each frequency band of 25 Hz between 25 and 300 Hz were computed and compared between visits. Averaged time-frequency representations of the affected TA, SOL, and GA EMG show altered spectral attributes post-FDS utilization during normalized gait. The mean IMNF values for the affected TA were significantly lower than the unaffected TA at baseline (p = 0.026) and follow-up (p = 0.038) during normalized stance. The mean IMNF values significantly increased (p = 0.017) for the affected GA at follow-up during normalized swing. The frequency band of 250-275 Hz significantly increased in the energies post-FDS utilization for all muscles. The application of wavelet-based TFA of EMG and outcome measures (IMNF, energy) extracted from the time-frequency distributions suggest alterations in MU recruitment strategies after the use of FDS in individuals with chronic stroke. This further establishes the efficacy of FDS as a rehabilitation intervention that may promote motor recovery in addition to treating the secondary complications of foot drop due to post-stroke hemiplegia.

Associations between motor unit action potential parameters and surface EMG features.

The surface interference EMG signal provides some information on the neural drive to muscles. However, the association between neural drive to muscle and muscle activation has long been debated with controversial indications due to the unavailability of motor unit population data. In this study, we clarify the potential and limitations of interference EMG analysis to infer motor unit recruitment strategies with an experimental investigation of several concurrently active motor units and of the associated features of the surface EMG. For this purpose, we recorded high-density surface EMG signals during linearly increasing force contractions of the tibialis anterior muscle, up to 70% of maximal force. The recruitment threshold (RT), conduction velocity (MUCV), median frequency (MDFMU), and amplitude (RMSMU) of action potentials of 587 motor units from 13 individuals were assessed and associated with features of the interference EMG. MUCV was positively associated with RT (R2 = 0.64 ± 0.14), whereas MDFMU and RMSMU showed a weaker relation with RT (R2 = 0.11 ± 0.11 and 0.39 ± 0.24, respectively). Moreover, the changes in average conduction velocity estimated from the interference EMG predicted well the changes in MUCV (R2 = 0.71), with a strong association to ankle dorsiflexion force (R2 = 0.81 ± 0.12). Conversely, both the average EMG MDF and RMS were poorly associated with motor unit recruitment. These results clarify the limitations of EMG spectral and amplitude analysis in inferring the neural strategies of muscle control and indicate that, conversely, the average conduction velocity could provide relevant information on these strategies.NEW & NOTEWORTHY The surface EMG provides information on the neural drive to muscles. However, the associations between EMG features and neural drive have been long debated due to unavailability of motor unit population data. Here, by using novel highly accurate decomposition of the EMG, we related motor unit population behavior to a wide range of voluntary forces. The results fully clarify the potential and limitation of the surface EMG to provide estimates of the neural drive to muscles.

Regionalization of the stretch reflex in the human vastus medialis

Regionalization of the stretch reflex, i.e. the notion that the activation of 1a afferents from a muscle region influences only the activation of motor units in the same region, has been demonstrated previously in animals but not in humans. Mechanical stretches applied to regions of vastus medialis as close as 10mm apart resulted in recruitment of motor units localized topographically with respect to the location of the mechanical stretch. Stretch reflexes are regionalized in the human vastus medialis. The human spinal cord has the neuromuscular circuitry to preferentially activate motoneurones innervating muscle fibres located in different regions of the vastus medialis. The localization of motor unit territories provides an anatomical basis to suggest that the CNS may have more independence in motor unit recruitment and control strategies than what was previously thought. In this study, we investigated whether the human spinal cord has the neuromuscular circuitry to independently activate motor units located in different regions of the vastus medialis. Mechanical taps were applied to multiple locations in the vastus medialis (VM) in nine healthy individuals. Regional responses within the muscle were observed using a grid of 5×13 surface EMG electrodes. The EMG amplitude was quantified for each channel, and a cluster of channels showing the largest activation was identified. The spatial location of the EMG response was quantified as the position of the channels in the cluster. In a subset of three participants, intramuscular recordings were performed simultaneously with the surface EMG recordings. Mechanical taps resulted in localized, discrete responses for each participant. The spatial location of the elicited responses was dependent on the location of the tap (P<0.001). Recordings with intramuscular electrodes confirmed the regional activation of the VM for different tap locations. Selective stimulation of 1a afferents localized in a region of the VM results in reflex recruitment of motor units in the same region. These findings suggest that the human spinal cord has the neuromuscular circuitry to modulate spatially the motoneuronal output to vastus medialis regions, which is a neuroanatomical prerequisite for regional activation.

Changes In Motor Unit Recruitment And De-recruitment Strategies Are Not Associated With The Repeated-bout Effect

The “repeated bout effect” (RBE) is an adaption that attenuates muscle damage following eccentric exercise. Several neural adaptations have been proposed to underlie the RBE. PURPOSE: This study used decomposition of surface EMG signals (dEMG) to examine the relationship between recruitment (RT) and de-recruitment thresholds (DRT) and changes in firing rates or motor units during recruitment and de-recruitment prior to and following eccentric exercise resulting in the RBE. METHODS: Nine participants performed 5 sub-maximal isometric trapezoid contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between RT and DT was examined using linear regression. The slope of the change in mean firing rate (MFR) during the ramp-up and ramp-down phase of the trapezoid contraction was also examined. RESULTS: No changes were found in the slope of the RT vs DT relationship for 50% MVC (1.13±0.17 vs 1.29±0.43; p=0.42) and 80% MVC (1.09±0.18 vs 1.25±0.41; p=0.25). There were also no changes in the y-intercept of the RT vs DT relationship at 50% (-13.79±6.77 vs -15.37±17.07; p=0.80) and at 80% (-9.29±10.71 vs -23.07±20.28; p=0.06) of MVC. The mean slope of the increase in firing rate during recruitment did not change between bouts 10.2±1.8 vs 10.5±2.2 pps/s (p=0.77) and 8.4±0.7 vs 9.0±1.6 pps/s (p=0.28) for 50% and 80% of MVC, respectively. The slope of the decrease in firing rate during de-recruitment did not differ at 50% of MVC -9.7±1.5 vs -10.2±1.8 pps/s (p=0.48), but became steeper during contractions at 80% of MVC -7.3±0.9 vs -8.7±1.7 pps/s (p=0.04). However, no relationship was observed between the change in DT slope and the magnitude of the RBE. CONCLUSION: A bout of eccentric exercise conferred protection from a subsequent identical bout. Few changes in motor-unit recruitment and de-recruitment behavior were observed suggesting changes in these parameters are not responsible for the RBE.

RELATIONSHIP BETWEEN MOTOR CONTROL ASYMMETRIES AND MOTOR PERFORMANCE ASYMMETRIES

BackgroundThe in-line lunge (ILL) is used to determine muscle sling behavior and possible unilateral motor control asymmetries (UMCA). Though, its validity remains unclear.ObjectiveThe aim of this analysis was to explore...

Changes In Motor-unit Recruitment Strategy And The Repeated-bout Effect

Changes In Motor-unit Recruitment Strategy And The Repeated-bout Effect

Myoelectric Manifestations of Fatigue after ACL Reconstruction: A Cross-Sectional Study after Postoperative Rehabilitation

An increased fatigability associated with anterior cruciate ligament (ACL) injury may persist several months after surgery. The purpose of this study was to investigate the behavior of muscle fiber conduction velocity (CV) as a descriptor of myoelectric fatigue, at different stages after rehabilitation, post-ACL reconstruction. Nineteen subjects acted as control group (CG), 10 patients had undergone surgery within 12 months (R12), and 23 patients were more than 24 months post-surgery (R24+). Surface electromyography (EMG) signals were detected from the quadriceps femoris using bidimensional arrays during isometric contractions at 20% and 60% maximal voluntary contraction (MVC). We observed a lower fatigability in the R24+ group during the 60% MVC contraction, with respect to the other groups. Lower fatigability of quadriceps muscle after ACL reconstruction in the long term may be linked to a recovery from a transitory altered motor unit recruitment strategy due to surgery, observed in the R12 group. Therefore, the findings of this study do not suggest an impaired fatigability of the quadriceps muscle during sustained isometric contractions in active patients in the long term.

Recurrence quantification analysis of surface electromyogram supports alterations in motor unit recruitment strategies by anodal transcranial direct current stimulation.

Recent evidence indicates that anodal transcranial direct current stimulation (tDCS) can selectively alter the EMG/force relationship of agonist arm muscles; however, the mechanisms mediating those changes are less clear. The purpose of this study was to evaluate the effect of anodal tDCS on motor unit synchronization by using a sophisticated non-linear EMG analysis called recurrence quantification analysis (RQA). Surface EMG signals were collected from the biceps brachii muscle of eighteen healthy young adults (9 tDCS and 9 control) at various force levels (12.5%, 25%, 37.5%, and 50% maximum) before and after the application of anodal tDCS over the primary motor cortex. RQA was employed to quantify the changes in percentage of determinism (% DET) and laminarity (% LAM) of the surface EMG signals, which are surrogate measures of motor unit synchronization. RQA analyses indicated that the changes in % DET and % LAM scores were significantly higher in the tDCS group than in the control group (p < 0.05) and this effect was particularly pronounced at higher force levels. The results of this study provide novel evidence supporting that anodal tDCS significantly alters motor unit firing strategies (i.e., the degree of synchronization) of the biceps brachii muscle.