Antagonist Activation Measurement in Triceps Surae Using High-Density and Bipolar Surface EMG in Chronic Hemiparesis.

After a stroke, antagonist muscle activation during agonist command impedes movement. This study compared measurements of antagonist muscle activation using surface bipolar EMG in the gastrocnemius medialis (GM) and high-density (HD) EMG in the GM and soleus (SO) during isometric submaximal and maximal dorsiflexion efforts, with knee flexed and extended, in 12 subjects with chronic hemiparesis. The coefficients of antagonist activation (CAN) of GM and SO were calculated according to the ratio of the RMS amplitude during dorsiflexion effort to the maximal agonist effort for the same muscle. Bipolar CAN (BipCAN) was compared to CAN from channel-specific (CsCAN) and overall (OvCAN) normalizations of HD-EMG. The location of the CAN centroid was explored in GM, and CAN was compared between the medial and lateral portions of SO. Between-EMG system differences in GM were observed in maximal efforts only, between BipCAN and CsCAN with lower values in BipCAN (p < 0.001), and between BipCAN and OvCAN with lower values in OvCAN (p < 0.05). The CAN centroid is located mid-height and medially in GM, while the CAN was similar in medial and lateral SO. In chronic hemiparesis, the estimates of GM hyperactivity differ between bipolar and HD-EMGs, with channel-specific and overall normalizations yielding, respectively, higher and lower CAN values than bipolar EMG. HD-EMG would be the way to develop personalized rehabilitation programs based on individual antagonist activations.

Lumbar muscle adaptations to external perturbations are modulated by trunk posture.

PURPOSE: To investigate if the recruitment of different regions within the lumbar extensor muscles in response to unexpected perturbations depends on trunk posture. METHODS: In a semi-seated posture, healthy adult participants experienced unexpected posterior-anterior trunk perturbations in three different postures: neutral, trunk flexion and left trunk rotation. High-density surface electromyography was used to identify the regional distribution of activation within the lumbar erector spinae muscles. The effect of posture and side (left vs right) on muscle activity and centroid coordinates was investigated at baseline and in response to perturbations. RESULTS: Higher muscle activity was observed in trunk flexion compared to neutral and rotation postures at baseline (multiple p < 0.001) and in response to the perturbation (multiple p < 0.01). At baseline, the centroid of the electromyographic amplitude distribution was localized more medially in trunk flexion compared to trunk neutral posture (p = 0.003), while activation was localized more laterally in response to the perturbation (multiple p < 0.05). When the trunk was rotated, the electromyographic amplitude distribution was localized more cranially on the left than the right side, both at baseline (p = 0.001) and in response to the perturbation (p = 0.001). Finally, a more lateral location of the centroid on the left side in rotation compared to neutral posture was observed in response to the perturbation (multiple p < 0.001). CONCLUSIONS: Regional differences in the distribution of electromyographic amplitude indicate that different muscle regions were recruited in different trunk postures and in response to perturbations, possibly based on regional mechanical advantage of the erector spinae muscle fibers.

A Novel Screen-Printed Textile Interface for High-Density Electromyography Recording.

Recording electrical muscle activity using a dense matrix of detection points (high-density electromyography, EMG) is of interest in a range of different applications, from human-machine interfacing to rehabilitation and clinical assessment. The wider application of high-density EMG is, however, limited as the clinical interfaces are not convenient for practical use (e.g., require conductive gel/cream). In the present study, we describe a novel dry electrode (TEX) in which the matrix of sensing pads is screen printed on textile and then coated with a soft polymer to ensure good skin-electrode contact. To benchmark the novel solution, an identical electrode was produced using state-of-the-art technology (polyethylene terephthalate with hydrogel, PET) and a process that ensured a high-quality sample. The two electrodes were then compared in terms of signal quality as well as functional application. The tests showed that the signals collected using PET and TEX were characterised by similar spectra, magnitude, spatial distribution and signal-to-noise ratio. The electrodes were used by seven healthy subjects and an amputee participant to recognise seven hand gestures, leading to similar performance during offline analysis and online control. The comprehensive assessment, therefore, demonstrated that the proposed textile interface is an attractive solution for practical applications.

Superficial lumbar muscle recruitment strategies to control the trunk with delayed-onset muscle soreness.

PURPOSE: The lumbar region offers various muscle recruitment strategies to achieve a task goal under varying conditions. For instance, trunk movement control can be reorganized under the influence of low back pain. How such task-modulation is obtained is not fully understood. The objective of this study was to characterize superficial lumbar muscle recruitment strategies under the influence of delayed-onset muscle soreness (DOMS) during unexpected trunk perturbations. METHODS: Healthy participants experienced a series of 15 sudden external perturbations with and without the influence of low back DOMS. During these perturbations, high-density surface electromyography was used to characterize recruitment strategies of superficial lumbar muscles, while kinematics sensors were used to characterize movements of the trunk. Lumbar muscle recruitment strategies, characterized by the amplitude of muscle activity amplitude, the latencies of the reflex activity and the spatial distribution of muscle activity, were compared across perturbations trials and with and without DOMS. RESULTS: An attenuation of lumbar muscle activity amplitude was observed across perturbation trials without DOMS, but not with DOMS. The spatial distribution of muscle activity was similar with and without DOMS. No significant changes in reflex activity latency and trunk flexion movement were observed. CONCLUSIONS: Following an unexpected trunk perturbation under DOMS effects, trunk movement are controlled using two different superficial lumbar muscles control strategies: keeping a constant level of their overall muscle activity and using a variable muscle recruitment pattern.

Force control during submaximal isometric contractions is associated with walking performance in persons with multiple sclerosis.

Individuals with multiple sclerosis (MS) experience progressive declines in movement capabilities, especially walking performance. The purpose of our study was to compare the amount of variance in walking performance that could be explained by the functional capabilities of lower leg muscles in persons with MS and a sex- and age-matched control group. Participants performed two walking tests (6-min walk and 25-ft walk), strength tests for the plantar flexor and dorsiflexor muscles, and steady submaximal (10% and 20% maximum) isometric contractions. High-density electromyography (EMG) was recorded during the steady contractions, and the signals were decomposed to identify the discharge times of concurrently active motor units. There were significant differences between the two groups in the force fluctuations during the steady contractions (force steadiness), the strength of the plantar flexor and dorsiflexor muscles, and the discharge characteristics during the steady contractions. Performance on the two walking tests by the MS group was moderately associated with force steadiness of the plantar flexor and dorsiflexor muscles; worse force steadiness was associated with poorer walking performance. In contrast, the performance of the control group was associated with muscle strength (25-ft test) and force steadiness of the dorsiflexors and variance in common input of motor units to the plantar flexors (6-min test). These findings indicate that a reduction in the ability to maintain a steady force during submaximal isometric contractions is moderately associated with walking performance of persons with MS.NEW & NOTEWORTHY The variance in walking endurance and walking speed was associated with force control of the lower leg muscles during submaximal isometric contractions in individuals with multiple sclerosis (MS). In contrast, the fast walking speed of a sex- and age-matched control group was associated with the strength of lower leg muscles. These findings indicate that moderate declines in the walking performance of persons with MS are more associated with impairments in force control rather than decreases in muscle strength.

Contraction level, but not force direction or wrist position, affects the spatial distribution of motor unit recruitment in the biceps brachii muscle.

PURPOSE: Different motor units (MUs) in the biceps brachii (BB) muscle have been shown to be preferentially recruited during either elbow flexion or supination. Whether these different units reside within different regions is an open issue. In this study, we tested wheter MUs recruited during submaximal isometric tasks of elbow flexion and supination for two contraction levels and with the wrist fixed at two different angles are spatially localized in different BB portions. METHODS: The MUs' firing instants were extracted by decomposing high-density surface electromyograms (EMG), detected from the BB muscle of 12 subjects with a grid of electrodes (4 rows along the BB longitudinal axis, 16 columns medio-laterally). The firing instants were then used to trigger and average single-differential EMGs. The average rectified value was computed separately for each signal and the maximal value along each column in the grid was retained. The center of mass, defined as the weighted mean of the maximal, average rectified value across columns, was then consdiered to assess the medio-lateral changes in the MU surface representation between conditions. RESULTS: Contraction level, but neither wrist position nor force direction (flexion vs. supination), affected the spatial distribution of BB MUs. In particular, higher forces were associated with the recruitment of BB MUs whose action potentials were represented more medially. CONCLUSION: Although the action potentials of BB MUs were represented locally across the muscle medio-lateral region, dicrimination between elbow flexion or supination seems unlikely from the surface representation of MUs action potentials.

Peripheral fatigue: new mechanistic insights from recent technologies.

Peripheral fatigue results from multiple electrochemical and mechanical events in the cell body and the muscle-tendon complex. Combined force and surface electromyographic signal analysis is among the most widely used approaches to describe the behaviour of a fatigued muscle. Advances in technologies and methodological procedures (e.g. laser diffraction, 31P magnetic resonance spectroscopy, shear-wave elastography, tensiomyography, myotonometry, mechanomyography, and high-density surface electromyography) have expanded our knowledge of muscle behaviour before, during, and after a fatiguing task. This review gives an update on recent developments in technologies for investigating the effects of peripheral fatigue linked to skeletal muscle contraction and on mechanistic insights into the electrochemical and mechanical aspects of fatigue. The salient points from the literature analysis are: (1) the electrochemical and mechanical events in the cell (alterations in cross-bridge formation and function and in depolarization of the tubular membrane) precede the events taking place at the muscle-tendon complex (decrease in muscle-tendon unit stiffness); (2) the changes in the fatigued muscle are not homogenous along its length and width but rather reflect a functional compartmentalisation that counteracts the decline in performance; (3) fatigue induces changes in load sharing among adjacent/synergistic muscles. A focus of future studies is to observe how these regional differences occur within single muscle fibres. To do this, a combination of different approaches may yield new insights into the mechanisms underlying muscle fatigue and how the muscle counteracts fatigue.

Variation in the spatial distribution of erector spinae activity during a lumbar endurance task in people with low back pain.

This study aimed to investigate the spatial distribution and redistribution of lumbar erector spinae (ES) activity during a lumbar extension endurance task in pain-free participants and how this is modified in people with low back pain (LBP). High density surface electromyography (HDEMG) was recorded using 13 × 5 electrode grids placed over the lumbar ES in 13 LBP and 13 control participants while completing an Ito test to task failure. The root mean square of the HDEMG signals was computed, a topographical map of the EMG amplitude generated and the centre of the activity (centroid) determined throughout the task. The centroid of the EMG amplitude map was systematically more cranial (F = 6.09, P = 0.022) for the LBP participants compared with the control subjects. Regression analysis showed that the extent of redistribution of ES activity was associated with longer endurance. These results show that LBP participants utilised a different motor strategy to perform the endurance task, characterised by greater activation of more cranial regions of the ES and less redistribution of ES activity throughout the task. This study provides new insight into the functional activation of the lumbar ES and how it is modified when people have pain.

Reciprocal inhibition between motor neurons of the tibialis anterior and triceps surae in humans.

Motor neurons innervating antagonist muscles receive reciprocal inhibitory afferent inputs to facilitate the joint movement in the two directions. The present study investigates the mutual transmission of reciprocal inhibitory afferent inputs between the tibialis anterior (TA) and triceps surae (soleus and medial gastrocnemius) motor units. We assessed this mutual mechanism in large populations of motor units for building a statistical distribution of the inhibition amplitudes during standardized input to the motor neuron pools to minimize the effect of modulatory pathways. Single motor unit activities were identified using high-density surface electromyography (HDsEMG) recorded from the TA, soleus (Sol), and medial gastrocnemius (GM) muscles during isometric dorsi- and plantarflexion. Reciprocal inhibition on the antagonist muscle was elicited by electrical stimulation of the tibial (TN) or common peroneal nerves (CPN). The probability density distributions of reflex strength for each muscle were estimated to examine the strength of mutual transmission of reciprocal inhibitory input. The results showed that the strength of reciprocal inhibition in the TA motor units was fourfold greater than for the GM and the Sol motor units. This suggests an asymmetric transmission of reciprocal inhibition between ankle extensor and flexor muscles. This asymmetry cannot be explained by differences in motor unit type composition between the investigated muscles since we sampled low-threshold motor units in all cases. Therefore, the differences observed for the strength of inhibition are presumably due to a differential reciprocal spindle afferent input and the relative contribution of nonreciprocal inhibitory pathways. NEW & NOTEWORTHY We investigated the mutual transmission of reciprocal inhibition in large samples of motor units using a standardized input (electrical stimulation) to the motor neurons. The results demonstrated that the disynaptic reciprocal inhibition exerted between ankle flexor and extensor muscles is asymmetric. The functional implication of asymmetric transmission may be associated with the neural strategies of postural control.

Later stages of diabetic neuropathy affect the complexity of the neuromuscular system at the knee during low-level isometric contractions.

INTRODUCTION: This study evaluates the complexity of force and surface electromyography (sEMG) during knee extension and flexion at low-level isometric contractions in individuals with different degrees of diabetic peripheral neuropathy (DPN). METHODS: Ten control and 38 diabetic participants performed isometric contractions at 10%, 20%, and 30% of maximal voluntary contraction. Knee force and multichannel sEMG from vastus lateralis (VL) and biceps femoris were acquired. The SD of force and sample entropy (SaEn) of both force and sEMG were computed. RESULTS: Participants with moderate DPN demonstrated high force-SD and low force-SaEn. Severely affected participants showed low SaEn in VL at all force levels. DISCUSSION: DPN affects the complexity of the neuromuscular system at the knee for the extension task during low-level isometric contractions, with participants in the later stages of the disease (moderate and severe) demonstrating most of the changes. Muscle Nerve 57: 112-121, 2018.

Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle.

Pain is associated with changes in the neural drive to muscles. For the upper trapezius muscle, surface electromyography (EMG) recordings have indicated that acute noxious stimulation in either the cranial or the caudal region of the muscle leads to a relative decrease in muscle activity in the cranial region. It is, however, not known if this adaption reflects different recruitment thresholds of the upper trapezius motor units in the cranial and caudal region or a nonuniform nociceptive input to the motor units of both regions. This study investigated these potential mechanisms by direct motor unit identification. Motor unit activity was investigated with high-density surface EMG signals recorded from the upper trapezius muscle of 12 healthy volunteers during baseline, control (intramuscular injection of isotonic saline), and painful (hypertonic saline) conditions. The EMG was decomposed into individual motor unit spike trains. Motor unit discharge rates decreased significantly from control to pain conditions by 4.0 ± 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 ± 2.8 pps; not significant). These changes were compatible with variations in the synaptic input to the motoneurons of the two regions. These adjustments were observed, irrespective of the location of noxious stimulation. These results strongly indicate that the nociceptive synaptic input is distributed in a nonuniform way across regions of the upper trapezius muscle.

EMG coherence of foot and ankle muscles increases with a postural challenge in men.

BACKGROUND: The functional role of intrinsic foot muscles in the control of standing balance is often overlooked in rehabilitation, partly because the interactions with ankle muscles are poorly understood. RESEARCH QUESTION: How does coactivation of Flexor Digitorum Brevis (FDB) and soleus (SOL) vary across standing tasks of increasing difficulty. METHODS: Postural sway (Centre of Pressure, CoP) and the electromyographic (EMG) activity of FDB, SOL, Medial Gastrocnemius (MG) and Tibialis Anterior (TA) were measured during bipedal standing, tandem stance, one-legged balance, and standing on toes. Coherence of the rectified EMG signals for SOL and FDB in two bandwidths (0-5 and 10-20 Hz) was calculated as a coactivation index. RESULTS AND SIGNIFICANCE: The CoP sway and the EMG activity of all muscles was greater (P<0.05) for the three difficult tasks. Significant coherence between the SOL and FDB EMG activity was found in both frequency regions: 0-5 and 10-20 Hz. The coherence integral increased with the difficulty of the postural task, especially in the 10-20 Hz band. The findings underscore the important role of FDB in the control of standing balance across tasks and its coactivation with SOL. Clinical recommendations to improve balance control need to consider the interaction between the plantar flexor and intrinsic-foot muscles.

The effect of trunk position and pain location on lumbar extensor muscle recruitment strategies.

The aim of this study was to investigate the effect of trunk position and experimental lumbar pain location on lumbar extensor muscle recruitment strategies. Nineteen healthy participants (10 men and 9 women), aged 25.3 ± 4.7 yr, performed isometric back extension contractions in three positions (neutral, 45°, and 90° trunk flexion) and under three conditions (no pain, caudal pain, and cranial pain). Lumbar muscle activation strategies were recorded using high-density surface electromyography. The effect of position and pain condition on muscle activity amplitude and spatial redistributions was assessed. Muscle activity amplitude was 43% higher in 45° trunk flexion than in neutral position on both sides (P < 0.05). In the 90° trunk flexion, participants showed a more lateral spatial distribution than in the 45° trunk flexion on the left side (P < 0.01, 5.4 mm difference) and the neutral position on both sides (P < 0.05, 8.2 mm difference). In the 45° trunk flexion, participants exhibited a more lateral spatial distribution compared with the neutral position on the right side (P < 0.05, 3.7 mm difference). A lateral spatial redistribution of muscle activity was observed in the caudal pain condition compared with the no pain condition on the right side (P < 0.05, 3.0 mm difference). Individual responses to pain varied across all variables. Different trunk positions result in different distributions of activation within the lumbar extensor muscles, possibly based on regional mechanical advantage. No clear indication of location-specific pain adaptation and no effect of task-dependent pain adaptation were found, whereas individual-specific adaptations were observed.NEW & NOTEWORTHY Changes in muscle activity amplitude and spatial redistribution of lumbar extensor muscles were observed in different trunk positions, potentially due to changes in their mechanical advantage. The results complement the current pain-adaptation theory by illustrating individual spatial redistributions of activation within lumbar extensor muscles during pain. The study found no clear indication of location-specific pain adaptation and no effect of task-dependent pain adaptation.

Lower limb suspension induces threshold-specific alterations of motor units properties that are reversed by active recovery.

PURPOSE: This study aimed to non-invasively test the hypothesis that (a) short-term lower limb unloading would induce changes in the neural control of force production (based on motor units (MUs) properties) in the vastus lateralis muscle and (b) possible changes are reversed by active recovery (AR). METHODS: Ten young males underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of AR. During ULLS, participants walked exclusively on crutches with the dominant leg suspended in a slightly flexed position (15°-20°) and with the contralateral foot raised by an elevated shoe. The AR was based on resistance exercise (leg press and leg extension) and executed at 70% of each participant's 1 repetition maximum, 3 times/week. Maximal voluntary isometric contraction (MVC) of knee extensors and MUs properties of the vastus lateralis muscle were measured at baseline, after ULLS, and after AR. MUs were identified using high-density electromyography during trapezoidal isometric contractions at 10%, 25%, and 50% of the current MVC, and individual MUs were tracked across the 3 data collection points. RESULTS: We identified 1428 unique MUs, and 270 of them (18.9%) were accurately tracked. After ULLS, MVC decreased by 29.77%, MUs absolute recruitment/derecruitment thresholds were reduced at all contraction intensities (with changes between the 2 variables strongly correlated), while discharge rate was reduced at 10% and 25% but not at 50% MVC. Impaired MVC and MUs properties fully recovered to baseline levels after AR. Similar changes were observed in the pool of total as well as tracked MUs. CONCLUSION: Our novel results demonstrate, non-invasively, that 10 days of ULLS affected neural control predominantly by altering the discharge rate of lower-threshold but not of higher-threshold MUs, suggesting a preferential impact of disuse on motoneurons with a lower depolarization threshold. However, after 21 days of AR, the impaired MUs properties were fully restored to baseline levels, highlighting the plasticity of the components involved in neural control.

Supraspinal, spinal, and motor unit adjustments to fatiguing isometric contractions of the knee extensors at low and high submaximal intensities in males.

Contraction intensity is a key factor determining the development of muscle fatigue, and it has been shown to induce distinct changes along the motor pathway. The role of cortical and spinal inputs that regulate motor unit (MU) behavior during fatiguing contractions is poorly understood. We studied the cortical, spinal, and neuromuscular response to sustained fatiguing isometric tasks performed at 20% and 70% of the maximum isometric voluntary contraction (MVC), together with MU behavior of knee extensors in healthy active males. Neuromuscular function was assessed before and after performance of both tasks. Cortical and spinal responses during exercise were measured via stimulation of the motor cortex and spinal cord. High-density electromyography was used to record individual MUs from the vastus lateralis (VL). Exercise at 70%MVC induced greater decline in MVC (P = 0.023) and potentiated twitch force compared with 20%MVC (P < 0.001), with no difference in voluntary activation (P = 0.514). Throughout exercise, corticospinal responses were greater during the 20%MVC task (P < 0.001), and spinal responses increased over time in both tasks (P ≤ 0.042). MU discharge rate increased similarly after both tasks (P ≤ 0.043), whereas recruitment and derecruitment thresholds were unaffected (P ≥ 0.295). These results suggest that increased excitability of cortical and spinal inputs might be responsible for the increase in MU discharge rate. The increase in evoked responses together with the higher MU discharge rate might be required to compensate for peripheral adjustments to sustain fatiguing contractions at different intensities.NEW & NOTEWORTHY Changes in central nervous system and muscle function occur in response to fatiguing exercise and are specific to exercise intensity. This study measured corticospinal, neuromuscular, and motor unit behavior to fatiguing isometric tasks performed at different intensities. Both tasks increased corticospinal excitability and motor unit discharge rate. Our findings suggest that these acute adjustments are required to compensate for the exercise-induced decrements in neuromuscular function caused by fatiguing tasks.

Task-dependent neuromuscular adaptations in low back pain: a controlled experimental study.

Introduction: This study investigated the variability in lumbar neuromuscular adaptations to pain, the task dependency of pain adaptations and the effect of these adaptations on motor performance. Methods: Twenty-four healthy participants performed isometric back extension contractions at 45° and 90° trunk flexion under pain-free and experimental low back pain conditions induced by electrical stimulation. High-density surface electromyography recorded lumbar muscle activation strategies, and force steadiness was measured using a load cell. Results: While considerable variability in neuromuscular adaptations to lumbar pain was observed among participants, consistent patterns were found between tasks. In the 90° trunk flexion position, both sides exhibited greater magnitudes of pain adaptations for muscle activity redistribution in the mediolateral axis (p < 0.05, 86% increase) and muscle activity amplitude (p < 0.001, 183% increase) compared to the 45° trunk flexion position. A significant negative correlation was found between the magnitude of the mediolateral spatial redistribution of muscle activity and force steadiness on the left side (p = 0.045). Discussion: These findings highlight the intricate and task-dependent nature of neuromuscular adaptations to pain within lumbar muscles, and points toward a potential trade-off between pain adaptations and performance.

NeuroMechanics: Electrophysiological and computational methods to accurately estimate the neural drive to muscles in humans in vivo.

The ultimate neural signal for muscle control is the neural drive sent from the spinal cord to muscles. This neural signal comprises the ensemble of action potentials discharged by the active spinal motoneurons, which is transmitted to the innervated muscle fibres to generate forces. Accurately estimating the neural drive to muscles in humans in vivo is challenging since it requires the identification of the activity of a sample of motor units (MUs) that is representative of the active MU population. Current electrophysiological recordings usually fail in this task by identifying small MU samples with over-representation of higher-threshold with respect to lower-threshold MUs. Here, we describe recent advances in electrophysiological methods that allow the identification of more representative samples of greater numbers of MUs than previously possible. This is obtained with large and very dense arrays of electromyographic electrodes. Moreover, recently developed computational methods of data augmentation further extend experimental MU samples to infer the activity of the full MU pool. In conclusion, the combination of new electrode technologies and computational modelling allows for an accurate estimate of the neural drive to muscles and opens new perspectives in the study of the neural control of movement and in neural interfacing.

Fatigue task-dependent effect on spatial distribution of lumbar muscles activity.

This study aims to identify how spatial distribution of lumbar muscle activity is modulated by different fatigue tasks. Twenty healthy adults performed two different isometric trunk extension endurance tasks (the modified Sorensen test and the inverted modified Sorensen test) until exhaustion. During these tasks, bilateral superficial lumbar muscle activity was recorded using high-density electromyography. The spatial distribution of activation within these muscles was obtained using the centroid coordinates in the medio-lateral and cranio-caudal directions. The effects of task and endurance time (left and right sides) were investigated using repeated measures ANOVA. Results revealed a significant lateral shift of the centroid throughout the fatigue tasks on both sides and no difference between tasks. Significant task × time interaction effects were found for the cranio-caudal direction on both sides showing a significantly more caudal location of the centroid in the modified Sorensen test compared to the inverted test at the beginning of the tasks. Our findings suggest that spatial distribution of lumbar muscle activity is task-dependent in a pre-fatigue stage while an alternative but similar muscle recruitment strategy is used in both tasks to maintain performance in the later stages of muscle fatigue.

Larger and Denser: An Optimal Design for Surface Grids of EMG Electrodes to Identify Greater and More Representative Samples of Motor Units.

The spinal motor neurons are the only neural cells whose individual activity can be noninvasively identified. This is usually done using grids of surface electromyographic (EMG) electrodes and source separation algorithms; an approach called EMG decomposition. In this study, we combined computational and experimental analyses to assess how the design parameters of grids of electrodes influence the number and the properties of the identified motor units. We first computed the percentage of motor units that could be theoretically discriminated within a pool of 200 simulated motor units when decomposing EMG signals recorded with grids of various sizes and interelectrode distances (IEDs). Increasing the density, the number of electrodes, and the size of the grids, increased the number of motor units that our decomposition algorithm could theoretically discriminate, i.e., up to 83.5% of the simulated pool (range across conditions: 30.5-83.5%). We then identified motor units from experimental EMG signals recorded in six participants with grids of various sizes (range: 2-36 cm2) and IED (range: 4-16 mm). The configuration with the largest number of electrodes and the shortest IED maximized the number of identified motor units (56 ± 14; range: 39-79) and the percentage of early recruited motor units within these samples (29 ± 14%). Finally, the number of identified motor units further increased with a prototyped grid of 256 electrodes and an IED of 2 mm. Taken together, our results showed that larger and denser surface grids of electrodes allow to identify a more representative pool of motor units than currently reported in experimental studies.

The length of tibialis anterior does not influence force steadiness during submaximal isometric contractions with the dorsiflexors.

The purpose of the study was to assess the influence of short, intermediate, and long muscle lengths on dorsiflexor force steadiness and the discharge characteristics of motor units in tibialis anterior during submaximal isometric contractions. Steady contractions were performed at 5 target forces (5, 10, 20, 40, and 60% maximal voluntary contraction, MVC) for 3 ankle angles (75°, 90°, and 105°). MVC force was less (p = 0.043) at the smallest joint angle compared with the other two angles. The absolute (standard deviation) and normalised amplitudes (coefficient of variation) of the force fluctuations were similar for all 3 ankle angles at each target force. The coefficient of variation for force decreased progressively from 5% to 20% MVC force and then it plateaued at 40% and 60% MVC force. At all target forces, the mean discharge rate (MDR) of the motor units at 75° was greater than at 90° (p = 0.006) and 105° (p = 0.034). Moreover, the MDR was similar for 5% and 10% MVC forces and then increased gradually until 60% MVC force (p < 0.005). The variability in discharge times (coefficient of variation for interspike interval) and variability in neural drive (coefficient of variation of filtered cumulative spike train) were similar at all ankle angles. Variability in neural drive had a greater influence on force steadiness than did the variability in discharge times. Changes in ankle-joint angle did not influence either the normalised amplitude force fluctuations during steady submaximal contractions or the underlying modulation of the discharge characteristics of motor units in tibialis anterior.