Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent

The physiological mechanisms determining the progressive decline in the maximal muscle torque production capacity during isometric contractions to task failure are known to depend on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR), however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee extension tasks to failure. Participants performed a sustained and an intermittent task at 20% and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface EMG signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During the 20% MVT sustained task, MUDR decreased until ∼50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until ∼40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30% and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until ∼80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after ∼70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are contraction modality- and intensity-dependent. KEY POINTS: During prolonged muscle contractions a constant motor output can be maintained by recruitment of additional motor units and adjustments in their discharge rate. Whilst contraction-induced decrements in neuromuscular function are known to depend on task demands, task-specificity of motor unit discharge behaviour adjustments is still unclear. In this study, we tracked and compared discharge activity of several concurrently active motor units in the vastii muscles during different submaximal isometric knee extension tasks to failure, including intermittent vs. sustained contraction modalities performed in the same intensity domain (Experiment 1), and two sustained contractions performed at different intensities (Experiment 2). During each task, motor units modulated their discharge rate in a distinct, biphasic manner, with the modulation pattern depending on contraction intensity and modality. These results provide insight into motoneuronal adjustments during contraction tasks posing different demands on the neuromuscular system.

Effects of repetition of a car-driving pedal maneuver and neural output in older adults

Understanding the ability of older adults to control pedal position angle and investigating whether this ability can be enhanced through practice may contribute to the prevention of traffic accidents. This study aimed to investigate repetitive effects on variability of the pedal position and neural drive during car-pedal operation in older adults. Thirteen older and 11 young adults performed 105 (21 sets × 5 repetitions) pedal angle control tasks with plantar flexor contraction. High-density surface electromyograms were recorded of triceps surae muscles. A cumulative spike train as a neural drive was calculated using continuously active motor unit activities. The coefficient of variation of the angle was higher in older (1.47 ± 1.06 %) than young (0.41 ± 0.21 %) adults in the first sets, and improved to 0.67 ± 0.51 % in the final sets in older adults only. There was no significant difference in neural drive variability between older and young adults. Our results suggest that repetition improves angular steadiness in older adults. However, this effect could not be explained by neural output which is estimated from lower threshold motor units that are continuously active.

Sex differences in laterality of motor unit firing behavior of the first dorsal interosseous muscle in strength-matched healthy young males and females

PURPOSE

The purpose of this study was to compare laterality in motor unit firing behavior between females and males.

METHODS

Twenty-seven subjects (14 females) were recruited for this study. The participants performed ramp up and hold isometric index finger abduction at 10, 30, and 60% of their maximum voluntary contraction (MVC). High-density surface electromyography (HD-sEMG) signals were recorded in the first dorsal interosseous (FDI) muscle and decomposed into individual motor unit (MU) firing behavior using a convolution blind source separation method.

RESULTS

In total, 769 MUs were detected (females, n = 318 and males, n = 451). Females had a significantly higher discharge rate than males at each relative torque level (10%: male dominant hand, 13.4 ± 2.7 pps vs. female dominant hand, 16.3 ± 3.4 pps; 30%: male dominant hand, 16.1 ± 3.9 pps vs. female dominant hand, 20.0 ± 5.0 pps; and 60%: male dominant hand, 19.3 ± 3.8 vs. female dominant hand, 25.3 ± 4.8 pps; p < 0.0001). The recruitment threshold was also significantly higher in females than in males at 30 and 60% MVC. Furthermore, males exhibited asymmetrical discharge rates at 30 and 60% MVC and recruitment thresholds at 30 and 60% MVC, whereas no asymmetry was observed in females.

CONCLUSION

In the FDI muscle, compared to males, females exhibited different neuromuscular strategies with higher discharge rates and recruitment thresholds and no asymmetrical MU firing behavior. Notably, the findings that sex differences in neuromuscular activity also occur in healthy individuals provide important information for understanding the pathogenesis of various diseases.

Acute effects of caffeine or quercetin ingestion on motor unit firing pattern before and after resistance exercise

The aim of the present study was to investigate the acute effect of caffeine or quercetin ingestion on motor unit firing patterns and muscle contractile properties before and after resistance exercise. High-density surface electromyography (HDs-EMG) during submaximal contractions and electrically elicited torque in knee extensor muscles were measured before (PRE) and 60 min after (POST1) ingestion of caffeine, quercetin glycosides, or placebo, and after resistance exercise (POST2) in ten young males. The Convolution Kernel Compensation technique was used to identify individual motor units of the vastus lateralis muscle for the recorded HDs-EMG. Ingestion of caffeine or quercetin induced significantly greater decreases in recruitment thresholds (RTs) from PRE to POST1 compared with placebo (placebo: 94.8 ± 9.7%, caffeine: 84.5 ± 16.2%, quercetin: 91.9 ± 36.7%), and there were significant negative correlations between the change in RTs (POST1-PRE) and RT at PRE for caffeine (rs = - 0.448, p < 0.001) and quercetin (rs =  - 0.415, p = 0.003), but not placebo (rs =  - 0.109, p = 0.440). Significant positive correlations between the change in firing rates (POST2-POST1) and RT at PRE were noted with placebo (rs = 0.380, p = 0.005) and quercetin (rs = 0.382, p = 0.007), but not caffeine (rs = 0.069, p = 0.606). No significant differences were observed in electrically elicited torque among the three conditions. These results suggest that caffeine or quercetin ingestion alters motor unit firing patterns after resistance exercise in different threshold-dependent manners in males.

The dose-response relationship of quercetin on the motor unit firing patterns and contractile properties of muscle in men and women

Quercetin is one type of ergogenic aid and its effects on the neuromuscular system have recently attracted interest, but its dose-effect is not yet fully understood. The aim of this study was to examine the effect of different doses of quercetin ingestion on motor unit firing patterns and muscle contractile properties in humans. Thirteen young males and females conducted neuromuscular performance tests before (PRE) and 60 min after (POST) ingestions of 500 or 200 mg of quercetin glycosides (Qg500/Qg200, respectively) or placebo (PLA) on three different days. At PRE and POST, motor unit firing rates were calculated from high-density surface electromyography of the vastus lateralis muscle during 120-s isometric contraction of knee extension at 10% of maximal voluntary contraction. Electrically elicited forces in knee extensor muscles were also measured. After 60 s of voluntary contraction, motor unit firing rates, normalized by the exerted muscle force at POST, were significantly lower at POST than PRE with Qg500 and Qg200 (p < 0.05), but not with PLA (p > 0.05). Changes in motor unit firing rates normalized by the exerted force from PRE to POST were significantly greater with Qg500 than Qg200 at the end of contraction (p < 0.05). Under all three conditions, the electrically elicited force did not significantly change from PRE to POST (p > 0.05). These results suggest that both 500 and 200-mg quercetin ingestions alter motor unit firing patterns, and that quercetin's effect is at least partially dose-dependent.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Motor Unit Discharge Characteristics and Conduction Velocity of the Vastii Muscles in Long-Term Resistance-Trained Men

PURPOSE

Adjustments in motor unit (MU) discharge properties have been shown after short-term resistance training; however, MU adaptations in long-term resistance-trained (RT) individuals are less clear. Here, we concurrently assessed MU discharge characteristics and MU conduction velocity in long-term RT and untrained (UT) men.

METHODS

Motor unit discharge characteristics (discharge rate, recruitment, and derecruitment threshold) and MU conduction velocity were assessed after the decomposition of high-density electromyograms recorded from vastus lateralis (VL) and vastus medialis (VM) of RT (>3 yr; n = 14) and UT ( n = 13) during submaximal and maximal isometric knee extension.

RESULTS

Resistance-trained men were on average 42% stronger (maximal voluntary force [MVF], 976.7 ± 85.4 N vs 685.5 ± 123.1 N; P < 0.0001), but exhibited similar relative MU recruitment (VL, 21.3% ± 4.3% vs 21.0% ± 2.3% MVF; VM, 24.5% ± 4.2% vs 22.7% ± 5.3% MVF) and derecruitment thresholds (VL, 20.3% ± 4.3% vs 19.8% ± 2.9% MVF; VM, 24.2% ± 4.8% vs 22.9% ± 3.7% MVF; P ≥ 0.4543). There were also no differences between groups in MU discharge rate at recruitment and derecruitment or at the plateau phase of submaximal contractions (VL, 10.6 ± 1.2 pps vs 10.3 ± 1.5 pps; VM, 10.7 ± 1.6 pps vs 10.8 ± 1.7 pps; P ≥ 0.3028). During maximal contractions of a subsample population (10 RT, 9 UT), MU discharge rate was also similar in RT compared with UT (VL, 21.1 ± 4.1 pps vs 14.0 ± 4.5 pps; VM, 19.5 ± 5.0 pps vs 17.0 ± 6.3 pps; P = 0.7173). Motor unit conduction velocity was greater in RT compared with UT individuals in both VL (4.9 ± 0.5 m·s -1 vs 4.5 ± 0.3 m·s -1 ; P < 0.0013) and VM (4.8 ± 0.5 m·s -1 vs 4.4 ± 0.3 m·s -1 ; P < 0.0073).

CONCLUSIONS

Resistance-trained and UT men display similar MU discharge characteristics in the knee extensor muscles during maximal and submaximal contractions. The between-group strength difference is likely explained by superior muscle morphology of RT as suggested by greater MU conduction velocity.

Decoding firings of a large population of human motor units from high-density surface electromyogram in response to transcranial magnetic stimulation

We describe a novel application of methodology for high-density surface electromyography (HDsEMG) decomposition to identify motor unit (MU) firings in response to transcranial magnetic stimulation (TMS). The method is based on the MU filter estimation from HDsEMG decomposition with convolution kernel compensation during voluntary isometric contractions and its application to contractions elicited by TMS. First, we simulated synthetic HDsEMG signals during voluntary contractions followed by simulated motor evoked potentials (MEPs) recruiting an increasing proportion of the motor pool. The estimation of MU filters from voluntary contractions and their application to elicited contractions resulted in high (>90%) precision and sensitivity of MU firings during MEPs. Subsequently, we conducted three experiments in humans. From HDsEMG recordings in first dorsal interosseous and tibialis anterior muscles, we demonstrated an increase in the number of identified MUs during MEPs evoked with increasing stimulation intensity, low variability in the MU firing latency and a proportion of MEP energy accounted for by decomposition similar to voluntary contractions. A negative relationship between the MU recruitment threshold and the number of identified MU firings was exhibited during the MEP recruitment curve, suggesting orderly MU recruitment. During isometric dorsiflexion we also showed a negative association between voluntary MU firing rate and the number of firings of the identified MUs during MEPs, suggesting a decrease in the probability of MU firing during MEPs with increased background MU firing rate. We demonstrate accurate identification of a large population of MU firings in a broad recruitment range in response to TMS via non-invasive HDsEMG recordings. KEY POINTS: Transcranial magnetic stimulation (TMS) of the scalp produces multiple descending volleys, exciting motor pools in a diffuse manner. The characteristics of a motor pool response to TMS have been previously investigated with intramuscular electromyography (EMG), but this is limited in its capacity to detect many motor units (MUs) that constitute a motor evoked potential (MEP) in response to TMS. By simulating synthetic signals with known MU firing patterns, and recording high-density EMG signals from two human muscles, we show the feasibility of identifying firings of many MUs that comprise a MEP. We demonstrate the identification of firings of a large population of MUs in the broad recruitment range, up to maximal MEP amplitude, with fewer required stimuli compared to intramuscular EMG recordings. The methodology demonstrates an emerging possibility to study responses to TMS on a level of individual MUs in a non-invasive manner.

Association between force fluctuation during isometric ankle abduction and variability of neural drive in peroneus muscles

Analyzing motor unit (MU) activities of peroneus muscles may reveal the causes of force control deficits of ankle eversion. This study aimed to examine peroneus muscles' MU discharge characteristics and associations between force fluctuation and variability of the neural drive in healthy participants. Thirty-one healthy males participated in this study. MU activities were identified from high-density surface electromyography of peroneus muscles during ankle eversion at 15 and 30% of maximal voluntary contraction (MVC). Participants increased the contraction level until reaching the target and held it for 15 s. The central 10 s of the hold phase were used for analysis. A cumulative spike train (CST) was calculated using MU firings. Variabilities of the force and CST are represented by the coefficient of variation (CoV). Spearman's rank correlation coefficient was used to assess the association between CoV of force and CoV of CST. For 15 and 30 % MVC trials, CoV of force was 1.86 ± 1.59 and 1.57 ± 1.26%, and CoV of CST was 5.01 ± 3.24 and 4.51 ± 2.78%, respectively. The correlation was significant at 15% (rho = 0.27, p < 0.001) and 30% (rho = 0.32, p < 0.001) MVC. Our findings suggest that in peroneus muscles, force fluctuation weakly to moderately correlates with neural drive variability.

Longitudinal development of muscle strength and relationship with motor unit activity and muscle morphological characteristics in youth athletes

Neural and morphological adaptations determine gains of muscle strength. For youth athletes, the importance of morphological adaptation is typically highlighted based on the change in maturity status. However, the long-term development of neural components in youth athletes remains unclear. The present study investigated the longitudinal development of muscle strength, muscle thickness (MT), and motor unit firing activity of the knee extensor and their relationships in youth athletes. Seventy male youth soccer players (mean ± SD age = 16.3 ± 0.6 years) performed neuromuscular, maximal voluntary isometric contraction (MVC), and submaximal ramp contraction (at 30 and 50% MVC) tests with knee extensors, two times with a 10-month measurement interval. High-density surface electromyography was recorded from the vastus lateralis and decomposed to identify each individual motor unit activity. MT was evaluated by the sum of the vastus lateralis and vastus intermedius thicknesses. Finally, sixty-four participants were employed to compare MVC and MT, and 26 participants were employed to analyze motor unit activity. MVC and MT were increased from pre to post (p < 0.05, 6.9 and 1.7% for MVC and MT, respectively). Y-intercept of the regression line between median firing rate vs. recruitment threshold was also increased (p < 0.05, 13.3%). Multiple regression analysis demonstrated that the gains of both MT and Y-intercept were explanatory variables for the gain of strength. These findings suggest that the neural adaptation could also make the important contribution to the strength gain for the youth athletes over a 10-month training period.

Consensus for experimental design in electromyography (CEDE) project: Single motor unit matrix

The analysis of single motor unit (SMU) activity provides the foundation from which information about the neural strategies underlying the control of muscle force can be identified, due to the one-to-one association between the action potentials generated by an alpha motor neuron and those received by the innervated muscle fibers. Such a powerful assessment has been conventionally performed with invasive electrodes (i.e., intramuscular electromyography (EMG)), however, recent advances in signal processing techniques have enabled the identification of single motor unit (SMU) activity in high-density surface electromyography (HDsEMG) recordings. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, provides recommendations for the recording and analysis of SMU activity with both invasive (needle and fine-wire EMG) and non-invasive (HDsEMG) SMU identification methods, summarizing their advantages and disadvantages when used during different testing conditions. Recommendations for the analysis and reporting of discharge rate and peripheral (i.e., muscle fiber conduction velocity) SMU properties are also provided. The results of the Delphi process to reach consensus are contained in an appendix. This matrix is intended to help researchers to collect, report, and interpret SMU data in the context of both research and clinical applications.

Acute changes in motor unit discharge property after concentric versus eccentric contraction exercise in knee extensor

This study aimed to investigate the motor unit firing property immediately after concentric or eccentric contraction exercise. Eighteen healthy men performed repetitive maximal isokinetic knee extension exercises with only concentric or eccentric contraction until they exerted less than 80% of the baseline strength. Before and after the fatiguing exercise, high-density surface electromyography of the vastus lateralis was recorded during submaximal ramp-up isometric contraction and individual motor units were identified. Only motor units that could be tracked before and after exercise were analyzed. Muscle cross-sectional area of the vastus lateralis was measured using ultrasound, and electrically evoked torque was recorded before and after the exercise. Sixty-five and fifty-three motor units were analyzed before and after the concentric and eccentric contractions, respectively. The results showed that motor units with moderate to high recruitment thresholds significantly decreased recruitment thresholds under both conditions, and the motor unit discharge rates significantly increased after concentric contraction compared to eccentric contraction. A greater muscle cross-sectional area was observed with concentric contraction. The evoked torque was significantly decreased under both conditions, but no difference between the conditions. These results suggest that fatiguing exercise with concentric contraction contributes to greater neural input to muscles and metabolic responses than eccentric contraction.

Blind identification of the spinal cord output in humans with high-density electrode arrays implanted in muscles

Invasive electromyography opened a new window to explore motoneuron behavior in vivo. However, the technique is limited by the small fraction of active motoneurons that can be concurrently detected, precluding a population analysis in natural tasks. Here, we developed a high-density intramuscular electrode for in vivo human recordings along with a fully automatic methodology that could detect the discharges of action potentials of up to 67 concurrently active motoneurons with 99% accuracy. These data revealed that motoneurons of the same pool receive common synaptic input at frequencies up to 75 Hz and that late-recruited motoneurons inhibit the discharges of those recruited earlier. These results constitute an important step in the population coding analysis of the human motor system in vivo.

Neuromuscular characteristics of front and back legs in junior fencers

In elite fencers, muscle strength and muscle mass of the front leg (FL) are greater than those of the back leg (BL) due to characteristic physiological and biomechanical demands placed on each leg during fencing. However, the development of laterality in their neural and muscular components is not well-understood. The present study investigated neuromuscular characteristics of FL and BL in junior fencers. Nineteen junior fencers performed neuromuscular performance tests for FL and BL, separately. There were no significant differences in the isometric knee extension strength (MVC), unilateral vertical jump (UVJ), vastus lateralis muscle thickness (MT), or motor unit firing rate of the vastus lateralis muscle (MUFR) between FL and BL (p > 0.05). In subgroup analyses, a significantly greater MUFR in FL than BL was noted only in fencers with > 3 years of fencing experience, and significantly greater UVJ in FL than BL was observed solely in fencers with < 3 years of fencing experience (p < 0.05). Strong positive correlations between FL and BL were identified in MVC, MT, and MUFR in fencers with > 3 years of fencing experience, but not in those with < 3 years of experience. These findings suggest that in junior fencers, laterality in neuromuscular performance has not manifested, whereas longer fencing experience induces fencing-dependent laterality in neural components, and laterality in dynamic muscle strength is decreased with fencing experience.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods

Abstract

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole‐body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well‐established paired motor unit (MU) technique. High‐density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20‐s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33‐s Achilles tendon vibration and simultaneous 20‐Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post‐vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non‐pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low‐magnitude involuntary torque combined with the negative effects of the interventions.

Key points

Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties such as the strength of persistent (long‐lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input.

In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole‐body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions.

However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions.

These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non‐pharmacological interventions such as electrical stimulation or relaxation could attenuate unwanted PIC‐induced muscle contractions in conditions characterised by motoneuron hyperexcitability.

Association between effective neural drive to the triceps surae and fluctuations in plantar-flexion torque during submaximal isometric contractions

What is the central question of this study? What is the association between the fluctuations in various estimates of effective neural drive to the triceps surae muscles and fluctuations in net plantar-flexion torque during steady submaximal contractions? What is the main finding and its importance? The fluctuations in estimates of effective neural drive to the triceps surae were moderately correlated with fluctuations in net torque at light and moderate plantar-flexion torques. Significant variability was observed in the association between neural drive and torque across participants, trials, short epochs of individual contractions, and varying motor unit number. ABSTRACT: The influence of effective neural drive on low-frequency fluctuations in torque during steady contractions can be estimated from the cumulative spike train (CST) or first principal component (FPC) of smoothed motor unit discharge rates obtained with high-density electromyography. However, the association between these estimates of total neural drive to synergist muscles and the fluctuations in net torque has not been investigated. We exposed the variability and compared the correlations between estimates of effective neural drive to the triceps surae muscles and fluctuations in plantar-flexion torque during steady contractions at 10% and 35% of maximal voluntary contraction (MVC) torque. Both neural drive estimates were moderately correlated with torque (CST, 0.55 ± 0.14, FPC, 0.58 ± 0.16) and highly correlated with one another (0.81 ± 0.1) during the 30-s steady contractions. There was substantial variability in cross-correlation values across participants, trials, and the 1-s and 5-s epochs of single contractions. Moreover, epoch duration significantly influenced cross-correlation values. Motor unit number was weakly associated with cross-correlation strength at 35% MVC (marginal R² 0.09 - 0.11; all p < 2.2×10^-5 ), but not at 10% MVC (all p > 0.37). Approximately one fifth of the variance in the coefficient of variation (CV) for torque was explained by CV for the CST estimate of neural drive (p = 6.6×10^-13 , R² = 0.21). Estimates of total neural drive to the synergistic triceps surae muscles obtained by pooling motor unit discharge times were moderately correlated with fluctuations in net plantar-flexion torque. This article is protected by copyright. All rights reserved.

Inter-Person Differences in Isometric Coactivations of Triceps Surae and Tibialis Anterior Decrease in Young, but Not in Older Adults After 14 Days of Bed Rest

We examined activation patterns of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), soleus (SO), and tibialis anterior (TA) muscles in eight older (58.4 ± 3.3 years) and seven young (23.1 ± 2.9 years) participants, before and after 14 days of horizontal bed rest. Visual feedback on the exerted muscle torque was provided to the participants. The discharge patterns of individual motor units (MUs) were studied in three repetitions of isometric plantar flexion at 30 and 60% of Maximum Voluntary Contraction (MVC), before, and 1 day after the 14-day bed rest, respectively. In the GL and GM muscles, the older participants demonstrated higher MU discharge rates than the young, regardless of the contraction level, both before and after the bed rest. In the TA and SO muscles, the differences between the older and young participants were less consistent. Detailed analysis revealed person-specific changes in the MU discharge rates after the bed rest. To quantify the coactivation patterns we calculated the correlation coefficients between the cumulative spike trains of identified MUs from each muscle, and measured the root mean square difference of the correlation coefficients between the trials of the same session (intra-session variability) and between different sessions (inter-session variability) in each participant (intra-person comparison) and across participants (inter-person comparison). In the intra-person comparison, the inter-session variability was higher than the intra-session variability, either before or after the bed rest. At 60% MVC torque, the young demonstrated higher inter-person variability of coactivation than the older participants, but this variability decreased significantly after the bed rest. In older participants, inter-person variability was consistently lower at 60% than at 30% MVC torque. In young participants, inter-person variability became lower at 60% than at 30% MVC torque only after the bed rest. Precaution is required when analyzing the MU discharge and coactivation patterns, as individual persons demonstrate individual adaptations to aging or bed rest.

Motor unit firing patterns on increasing force during force and position tasks

Different neurophysiological strategies are used to perform angle adjustments during motor tasks such as car driving and force-control tasks using a fixed-rigid pedal. However, the difference in motor unit behavior in response to an increasing exerted force between tasks is unknown. This study aimed to investigate the difference in motor unit responsiveness on increasing force between force and position tasks. Twelve healthy participants performed ramp and hold contractions during ankle plantarflexion at 20% and 30% of the maximal voluntary contraction using a rigid pedal (force task) and a free pedal with an inertial load (position task). High-density surface electromyograms were recorded of the medial gastrocnemius muscle and decomposed into individual motor unit firing patterns. Ninety and hundred and nine motor units could be tracked between different target torques in each task. The mean firing rate increased and firing rate variability decreased on 10% maximal voluntary contraction force gain during both force and position tasks. There were no significant differences in these responses between the two tasks. Our results suggest that the motor unit firing rate is similarly regulated between force and position tasks in the medial gastrocnemius muscle with an increase in the exerted force.NEW & NOTEWORTHY Different neurophysiological strategies are used to perform a force control task and angle adjustment task. Our results showed that motor unit firing rate is similarly regulated between the two tasks in the medial gastrocnemius muscle with an increase in the exerted force. Although it is reported that position tasks contribute to early fatigue, it does not seem to be a particular problem for the increase in force.

Analysis of motor unit spike trains estimated from high-density surface electromyography is highly reliable across operators

There is a growing interest in decomposing high-density surface electromyography (HDsEMG) into motor unit spike trains to improve knowledge on the neural control of muscle contraction. However, the reliability of decomposition approaches is sometimes questioned, especially because they require manual editing of the outputs. We aimed to assess the inter-operator reliability of the identification of motor unit spike trains. Eight operators with varying experience in HDsEMG decomposition were provided with the same data extracted using the convolutive kernel compensation method. They were asked to manually edit them following established procedures. Data included signals from three lower leg muscles and different submaximal intensities. After manual analysis, 126 ± 5 motor units were retained (range across operators: 119-134). A total of 3380 rate of agreement values were calculated (28 pairwise comparisons × 11 contractions/muscles × 4-28 motor units). The median rate of agreement value was 99.6%. Inter-operator reliability was excellent for both mean discharge rate and time at recruitment (intraclass correlation coefficient > 0.99). These results show that when provided with the same decomposed data and the same basic instructions, operators converge toward almost identical results. Our data have been made available so that they can be used for training new operators.

Quercetin ingestion modifies human motor unit firing patterns and muscle contractile properties

Quercetin is a polyphenolic flavonoid that has reported to block the binding of adenosine to A1 receptors at central nervous system and increase calcium release from the sarcoplasmic reticulum at skeletal muscle. The aim of the present study was to investigate the acute effect of quercetin ingestion on motor unit activation and muscle contractile properties. High-density surface electromyography during submaximal contractions and electrically elicited contraction torque in knee extensor muscles were measured before (PRE) and 60 min after (POST) quercetin glycosides or placebo ingestions in 13 young males. Individual motor units of the vastus lateralis muscle were identified from high-density surface electromyography by the Convolution Kernel Compensation technique. Firing rates of motor units recruited at 30–50% of the maximal voluntary contraction torque (MVC) were increased from PRE to POST only with quercetin (9.0 ± 2.3 to 10.5 ± 2.0 pps, p = 0.034). Twitch torque during doublet stimulation was decreased from PRE to POST with placebo (77.1 ± 17.1 to 73.9 ± 17.6 Nm, p = 0.005), but not with quercetin (p > 0.05). For motor units recruited at < 10% of MVC, normalized firing rate were decreased with quercetin (1.52 ± 0.33 to 1.58 ± 0.35%MVC/pps, p = 0.002) but increased with placebo (1.61 ± 0.32 to 1.57 ± 0.31%MVC/pps, p = 0.005). These results suggest that ingested quercetin has the functional roles to: mitigate reduction in the muscle contractile properties, enhance activations of relatively higher recruitment threshold motor units, and inhibit activation of relatively lower recruitment threshold motor units.

Fish Protein Ingestion Induces Neural, but Not Muscular Adaptations, Following Resistance Training in Young Adults

Purpose: Nutritional supplementation in conjunction with exercise is of interest for the prevention or improvement of declines in motor performances in older adults. An understanding of the effects on both young and older adults contributes to its effective application. We investigated the effect of fish protein ingestion with resistance training on neural and muscular adaptations in young adults using interventions and assessments that have already been tested in older adults.

Methods: Eighteen young adults underwent 8 weeks of isometric knee extension training. During the intervention, nine participants ingested 5 g of fish protein (n = 9, Alaska pollack protein, APP), and the other nine participants ingested casein as a control (n = 9, CAS) in addition to daily meals. Before, during, and after the intervention, the isometric knee extension force, lower extremity muscle mass, and motor unit firing pattern of knee extensor muscles were measured.

Results: Maximum voluntary contraction (MVC) was significantly increased in both APP and CAS groups from 0 weeks to 4, 6, and 8 weeks of intervention (p < 0.001), but there were no significant differences between the groups (p = 0.546–0.931). Muscle mass was not significantly changed during the intervention in either group (p = 0.250–0.698). Significant changes in motor unit firing rates (p = 0.02 and 0.029 for motor units recruited at 20–40% of MVC and at 40–60%) were observed following the intervention in the APP but not CAS (p = 0.120–0.751) group.

Conclusions: These results suggest that dietary fish protein ingestion changes motor unit adaptations following resistance training in young adults.

Modulation of Neural and Muscular Adaptation Processes During Resistance Training by Fish Protein Ingestions in Older Adults

Assessments of both neural and muscular adaptations during interventions would provide valuable information for developing countermeasures to age-related muscle dysfunctions. We investigated the effect of fish protein ingestion on training-induced neural and muscular adaptations in older adults. Twenty older adults participated 8 weeks of isometric knee extension training intervention. The participants were divided into two groups who took fish protein (n = 10, Alaska pollack protein, APP) or casein (n = 10, CAS). Maximal muscle strength during knee extension, lower extremity muscle mass (body impedance method), and motor unit firing pattern of knee extensor muscle (high-density surface electromyography) were measured before, during, and after the intervention. Muscle strength were significantly increased in both CAS (124.7 ± 5.8%) and APP (117.1 ± 4.4%) after intervention (p < .05), but no significant differences between the groups were observed (p > .05). Significant increases in lower extremity muscle mass from 0 to 8 weeks were demonstrated only for APP (102.0 ± 3.2, p < .05). Greater changes in motor unit firing pattern following intervention were represented in CAS more than in APP. These results suggest that nutritional supplementations could modulate neural and muscular adaptations following resistance training and fish protein ingestion preferentially induces muscular adaptation without the detectable neural adaptation in older adults.

Identification of the laterality of motor unit behavior in female patients with parkinson's disease using high-density surface electromyography

Patients with Parkinson's disease (PD) have greater laterality of muscle contraction properties than other people with parkinsonism diseases. However, few studies have reported the laterality of MU activation properties of the lower extremity muscles in patients with PD. The aim of the present study was to identify the laterality of MU behavior in PD patients using high-density surface electromyography (HD-SEMG). Eleven female patients with PD (age, 69.2 ± 6.2 years, disease duration, 2.7 ± 0.9 years, Unified Parkinson's disease Rating Scale score, 13 (9-16)), and 9 control female subjects (age, 66.8 ± 3.5 years) were enrolled in the present study. All subjects performed a sustained isometric knee extension in a 30% maximal voluntary contraction (MVC) task for 20 s. HD-SEMG signals were used to record and extract single MU firing behavior in the vastus lateralis muscle during submaximal isometric knee extensor contractions with 64 electrodes and decomposed with the convolution kernel compensation technique to extract individuals MUs. Compared to the control subjects, the patients with PD exhibited laterality of the MU firing rate and an absence of a relationship between the mean MU firing rate and MU threshold. Patients with PD exhibit laterality of MU behavior and experience MU behavioral abnormalities even with mild symptoms such as Hoehn & Yahr stage ≤ 3 and disease duration = 2.7 ± 0.9. These findings suggest the importance of considering the detection of abnormal muscle properties in PD patients beginning in the early phase of the disease.

Public database for validation of follicle detection algorithms on 3D ultrasound images of ovaries

BACKGROUND AND OBJECTIVE

Automated follicle detection in ovarian ultrasound volumes remains a challenging task. An objective comparison of different follicle-detection approaches is only possible when all are tested on the same data. This paper describes the development and structure of the first publicly accessible USOVA3D database of annotated ultrasound volumes with ovarian follicles.

METHODS

The ovary and all follicles were annotated in each volume by two medical experts. The USOVA3D database is supplemented by a general verification protocol for unbiased assessment of detection algorithms that can be compared and ranked by scoring according to this protocol. This paper also introduces two baseline automated follicle-detection algorithms, the first based on Directional 3D Wavelet Transform (3D DWT) and the second based on Convolutional Neural Networks (CNN).

RESULTS

The USOVA3D testing data set was used to verify the variability and reliability of follicle annotations. The intra-rater overall score yielded around 83 (out of a maximum of 100), while both baseline algorithms pointed out just a slightly lower performance, with the 3D DWT-based algorithm being better, with an overall score around 78.

CONCLUSIONS

On the other hand, the development of the CNN-based algorithm demonstrated that the USOVA3D database contains sufficient data for successful training without overfitting. The inter-rater reliability analysis and the obtained statistical metrics of effectiveness for both baseline algorithms confirmed that the USOVA3D database is a reliable source for developing new automated detection methods.

Motor unit filter prelearning strategies for decomposition of compound muscle action potentials in high-density surface electromyograms

We analyze the efficiency of motor unit (MU) filter prelearning from high-density surface electromyographic (HDEMG) recordings of voluntary muscle contractions in the identification of the motor unit firing patterns during elicited muscle contractions. Motor unit filters are assessed from 10 s long low level isometric voluntary contractions by gradient-based optimization of three different cost functions and then applied to synthetic HDEMG recordings of elicited muscle contractions with dispersion of motor unit firings ranging from 13 ms to 1 ms. We demonstrate that the number of identified MUs and the precision of MU identification depend significantly on the selected cost function. Regardless the selected cost function and MU synchronization level, the median precision of motor unit identification in elicited contraction is ≥ 95 % and is comparable to the precision in voluntary contractions. On the other hand, median miss rate increases significantly from < 1 % to ~ 3 % with the tested level of MU synchronization.Clinical Relevance-The identification of MU firings from HDEMG in elicited muscle contractions provides a new tool for in vivo investigation of motor excitability in humans.

Consensus for experimental design in electromyography (CEDE) project: Amplitude normalization matrix

The general purpose of normalization of EMG amplitude is to enable comparisons between participants, muscles, measurement sessions or electrode positions. Normalization is necessary to reduce the impact of differences in physiological and anatomical characteristics of muscles and surrounding tissues. Normalization of the EMG amplitude provides information about the magnitude of muscle activation relative to a reference value. It is essential to select an appropriate method for normalization with specific reference to how the EMG signal will be interpreted, and to consider how the normalized EMG amplitude may change when interpreting it under specific conditions. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, presents six approaches to EMG normalization: (1) Maximal voluntary contraction (MVC) in same task/context as the task of interest, (2) Standardized isometric MVC (which is not necessarily matched to the contraction type in the task of interest), (3) Standardized submaximal task (isometric/dynamic) that can be task-specific, (4) Peak/mean EMG amplitude in task, (5) Non-normalized, and (6) Maximal M-wave. General considerations for normalization, features that should be reported, definitions, and "pros and cons" of each normalization approach are presented first. This information is followed by recommendations for specific experimental contexts, along with an explanation of the factors that determine the suitability of a method, and frequently asked questions. This matrix is intended to help researchers when selecting, reporting and interpreting EMG amplitude data.

Effect of milk fat globule membrane supplementation on motor unit adaptation following resistance training in older adults

This study aimed to investigate the effect of milk fat globule membrane (MFGM) supplementation on motor unit adaptation following resistance training in older adults. Twenty-five older males and females took MFGM (n = 12) or a placebo (PLA; n = 12) while performing 8 weeks of isometric knee extension training. During the training, the motor unit firing pattern during submaximal contractions, muscle thickness, and maximal muscle strength of knee extensor muscles were measured every 2 weeks. None of the measurements showed significant differences in muscle thickness or maximal muscle strength (MVC) between the two groups (p > .05). Significant decreases in motor unit firing rate following the intervention were observed in PLA, that is, 14.1 ± 2.7 pps at 0 weeks to 13.0 ± 2.4 pps at 4 weeks (p = .003), but not in MFGM (14.4 ± 2.5 pps to 13.8 ± 1.9 pps). Motor unit firing rates in MFGM were significantly higher than those in PLA at 2, 4, 6, and 8 weeks of the intervention, that is, 15.1 ± 2.3 pps in MFGM and 14.5 ± 3.3 pps in PLA at 70% of MVC for motor units recruited at 40% of MVC at 6 weeks (p = .034). Significant differences in firing rates among motor units with different recruitment thresholds were newly observed following the resistance training intervention in MFGM, indicating that motor unit firing pattern is changed in this group. These results suggest that motor unit adaptation following resistance training is modulated by MFGM supplementation in older adults.

Consensus for experimental design in electromyography (CEDE) project: Electrode selection matrix

The Consensus for Experimental Design in Electromyography (CEDE) project is an international initiative which aims to guide decision-making in recording, analysis, and interpretation of electromyographic (EMG) data. The quality of the EMG recording, and validity of its interpretation depend on many characteristics of the recording set-up and analysis procedures. Different electrode types (i.e., surface and intramuscular) will influence the recorded signal and its interpretation. This report presents a matrix to consider the best electrode type selection for recording EMG, and the process undertaken to achieve consensus. Four electrode types were considered: (1) conventional surface electrode, (2) surface matrix or array electrode, (3) fine-wire electrode, and (4) needle electrode. General features, pros, and cons of each electrode type are presented first. This information is followed by recommendations for specific types of muscles, the information that can be estimated, the typical representativeness of the recording and the types of contractions for which the electrode is best suited. This matrix is intended to help researchers when selecting and reporting the electrode type in EMG studies.

High-density electromyographic data during isometric contractions of the ankle joint in children with cerebral palsy pre and post BoNT-A treatment

Understanding the underlying mechanisms leading to progressive muscle pathologies in spastic Cerebral Palsy remains a challenging field of research. Furthermore, Botulinum Neurotoxin-A (BoNT-A) is a frequent intervention to treat spasticity in CP but its effects on neuromuscular properties are not yet fully explored. High-density Electromyographic (HD-EMG) data have been collected before and after BoNT-A injections from children aged 5-15 years during isometric contractions of the ankle joint together with torque output, clinical assessments and demographic details. Data collected from a total of 13 children with and 29 children without spastic CP allow for between-group comparisons and are made available using Mendeley Data (https://doi.org/10.17632/3sbptrk54c.2 and https://doi.org/10.17632/3b98g5fyff.1).

Effect of Resistance Training and Fish Protein Intake on Motor Unit Firing Pattern and Motor Function of Elderly

We investigated the effect of resistance training and fish protein intake on the motor unit firing pattern and motor function in elderly. Fifty healthy elderly males and females (69.2 ± 4.7 years) underwent 6 weeks of intervention. We applied the leg-press exercise as resistance training and fish protein including Alaska pollack protein (APP) as nutritional supplementation. Subjects were divided into four groups: fish protein intake without resistance training (APP-CN, n = 13), placebo intake without resistance training (PLA-CN, n = 12), fish protein intake with resistance training (APP-RT, n = 12), and placebo intake with resistance training (PLA-RT, n = 13). Motor unit firing rates were calculated from multi-channel surface electromyography by the Convolution Kernel. For the chair-stand test, while significant increases were observed at 6 weeks compared with 0 week in all groups (p < 0.05), significant increases from 0 to 3 weeks and 6 weeks were observed in APP-RT (18.2 ± 1.9 at 0 week to 19.8 ± 2.2 at 3 weeks and 21.2 ± 1.9 at 6 weeks) (p < 0.05). Increase and/or decrease in the motor unit firing rate were mainly noted within motor units with a low-recruitment threshold in APP-RT and PLA-RT at 3 and 6 weeks (12.3 pps at 0 week to 13.6 pps at 3 weeks and 12.1 pps at 6 weeks for APP-RT and 12.9 pps at 0 week to 13.9 pps at 3 weeks and 14.1 pps at 6 weeks for PLA-RT at 50% of MVC) (p < 0.05), but not in APP-CN or PLA-CN (p > 0.05). Time courses of changes in the results of the chair-stand test and motor unit firing rate were different between APP-RT and PLA-RT. These findings suggest that, in the elderly, the effect of resistance training on the motor unit firing rate is observed in motor units with a low-recruitment threshold, and additional fish protein intake modifies these adaptations in motor unit firing patterns and the motor function following resistance training.

On the Impact of Muscle Shortening on Non-Negative Matrix Factorization of Dynamic Surface Electromyograms

We analyzed the sensitivity of Non-negative Matrix Factorization (NMF) of dynamic surface electromyograms (EMG) to muscle shortening. We first identified Motor unit action potentials (MUAPs) by decomposing experimentally recorded EMG signals during slow shortening of biceps brachii muscle in five young healthy males. We then used these MUAPs to generate different synthetic EMG signals with different muscle shortening and excitation profiles. Afterwards, we applied NMF to the synthetic EMG signals and calculated Pearson correlation coefficient (CC) between the extracted NMF components and a) muscle shortening and b) muscle excitation profiles. The results demonstrated good match between NMF components and muscle excitation profiles, but only when the muscle excitation level changed for at least 10 % during the muscle shortening. During constant muscle excitation, the resulting NMF components correlated significantly with the muscle shortening profiles. These results demonstrate that NMF components reflect not only the muscle excitation profiles but also muscle shortening profiles. Therefore, the results of NMF analysis of dynamic EMG signals need to be interpreted with caution.

Motor Unit-Driven Identification of Pathological Tremor in Electroencephalograms

Background: Traditional studies on the neural mechanisms of tremor use coherence analysis to investigate the relationship between cortical and muscle activity, measured by electroencephalograms (EEG) and electromyograms (EMG). This methodology is limited by the need of relatively long signal recordings, and it is sensitive to EEG artifacts. Here, we analytically derive and experimentally validate a new method for automatic extraction of the tremor-related EEG component in pathological tremor patients that aims to overcome these limitations. Methods: We exploit the coupling between the tremor-related cortical activity and motor unit population firings to build a linear minimum mean square error estimator of the tremor component in EEG. We estimated the motor unit population activity by decomposing surface EMG signals into constituent motor unit spike trains, which we summed up into a cumulative spike train (CST). We used this CST to initialize our tremor-related EEG component estimate, which we optimized using a novel approach proposed here. Results: Tests on simulated signals demonstrate that our new method is robust to both noise and motor unit firing variability, and that it performs well across a wide range of spectral characteristics of the tremor. Results on 9 essential (ET) and 9 Parkinson's disease (PD) patients show a ~2-fold increase in amplitude of the coherence between the estimated EEG component and the CST, compared to the classical EEG-EMG coherence analysis. Conclusions: We have developed a novel method that allows for more precise and robust estimation of the tremor-related EEG component. This method does not require artifact removal, provides reliable results in relatively short datasets, and tracks changes in the tremor-related cortical activity over time.

Simulations of high-density surface electromyograms in dynamic muscle contractions

We describe the extension of pre-existing cylindrical volume conductor model to synthetic high-density surface electromyograms (hdEMG), simulated during dynamic contractions of fusiform skeletal muscles. Its modular structure comprises two main parts. First, dynamic changes of motor unit action potentials (MUAPs) during 36 discrete steps of muscle shortening are simulated. Second, the increase in depth of simulated motor units (MUs) due to shortening and thickening of muscle fibers is simulated. MU firing patterns are generated with the model proposed by Fuglevand et al. and convolved with simulated MUAPs. In this way, the hdEMG simulator can be used to generate dynamic hdEMG of arbitrary muscle shortening, thickening and excitation profiles. In order to demonstrate the value of the aforementioned simulator we independently analyze the impact of muscle shortening and muscle thickening on MU identification by Convolution Kernel Compensation (CKC) technique.

On the impact of spike segmentation on motor unit identification in dynamic surface electromyograms

We discuss the adaptation of preexisting Convolution Kernel Compensation (CKC) surface electromyogram (EMG) decomposition technique to dynamic muscle contractions. In particular, three different algorithms for segmentation of motor unit (MU) spike trains into MU firings are discussed and mutually compared on synthetic dynamic surface EMG. The first segmentation algorithm employs a priori knowledge of the regularity of MU firings. The second one builds on K-means classification of MU spikes, whereas the third one combines both the regularity of MU firings and the previously introduced Pulse-to-Noise Ratio (PNR). On average, 5.5 ± 0.6 MUs were identified with sensitivity of 88.4 % ± 17.0 %, 83.8 % ± 16.7 % and 90.7 % ± 15.1 % for the first, the second and the third segmentation algorithm, respectively, demonstrating the feasibility of MU identification in moderate dynamic muscle contractions. In our tests, the third segmentation approach demonstrated superior accuracy in MU identification.

Age-related changes in motor unit firing pattern of vastus lateralis muscle during low-moderate contraction

Age-related changes in motor unit activation properties remain unclear for locomotor muscles such as quadriceps muscles, although these muscles are preferentially atrophied with aging and play important roles in daily living movements. The present study investigated and compared detailed motor unit firing characteristics for the vastus lateralis muscle during isometric contraction at low to moderate force levels in the elderly and young. Fourteen healthy elderly men and 15 healthy young men performed isometric ramp-up contraction to 70 % of the maximal voluntary contractions (MVC) during knee extension. Multichannel surface electromyograms were recorded from the vastus lateralis muscle using a two-dimensional grid of 64 electrodes and decomposed with the convolution kernel compensation technique to extract individual motor units. Motor unit firing rates in the young were significantly higher (~+29.7 %) than in the elderly (p < 0.05). There were significant differences in firing rates among motor units with different recruitment thresholds at each force level in the young (p < 0.05) but not in the elderly (p > 0.05). Firing rates at 60 % of the MVC force level for the motor units recruited at <20 % of MVC were significantly correlated with MVC force in the elderly (r = 0.885, p < 0.0001) but not in the young (r = 0.127, p > 0.05). These results suggest that the motor unit firing rate in the vastus lateralis muscle is affected by aging and muscle strength in the elderly and/or age-related strength loss is related to motor unit firing/recruitment properties.

The extraction of neural information from the surface EMG for the control of upper-limb prostheses: emerging avenues and challenges

Despite not recording directly from neural cells, the surface electromyogram (EMG) signal contains information on the neural drive to muscles, i.e., the spike trains of motor neurons. Using this property, myoelectric control consists of the recording of EMG signals for extracting control signals to command external devices, such as hand prostheses. In commercial control systems, the intensity of muscle activity is extracted from the EMG and used for single degrees of freedom activation (direct control). Over the past 60 years, academic research has progressed to more sophisticated approaches but, surprisingly, none of these academic achievements has been implemented in commercial systems so far. We provide an overview of both commercial and academic myoelectric control systems and we analyze their performance with respect to the characteristics of the ideal myocontroller. Classic and relatively novel academic methods are described, including techniques for simultaneous and proportional control of multiple degrees of freedom and the use of individual motor neuron spike trains for direct control. The conclusion is that the gap between industry and academia is due to the relatively small functional improvement in daily situations that academic systems offer, despite the promising laboratory results, at the expense of a substantial reduction in robustness. None of the systems so far proposed in the literature fulfills all the important criteria needed for widespread acceptance by the patients, i.e. intuitive, closed-loop, adaptive, and robust real-time ( 200 ms delay) control, minimal number of recording electrodes with low sensitivity to repositioning, minimal training, limited complexity and low consumption. Nonetheless, in recent years, important efforts have been invested in matching these criteria, with relevant steps forwards.

A modelling study on transmission of the central oscillator in tremor by a motor neuron pool

In spite of decades of intense research, pathological tremors still constitute unknown disorders. This study addresses, based on a multi-scale model, the behavior of an entire pool of motor neurons in tremor, under the hypothesis that tremor is an oscillation of central origin commonly projected to all motor neurons that innervate a muscle. Our results show that under such conditions both paired discharges and enhanced motor neuron synchronization, two of the characteristic landmarks of tremor, emerge. Moreover, coherence and correlation analyses suggest that the central tremor oscillator is transmitted linearly by the motor neuron pool given that a small set (7 or 8) of motor neurons are sampled.

Accuracy assessment of CKC high-density surface EMG decomposition in biceps femoris muscle

The aim of this study was to assess the accuracy of the convolution kernel compensation (CKC) method in decomposing high-density surface EMG (HDsEMG) signals from the pennate biceps femoris long-head muscle. Although the CKC method has already been thoroughly assessed in parallel-fibered muscles, there are several factors that could hinder its performance in pennate muscles. Namely, HDsEMG signals from pennate and parallel-fibered muscles differ considerably in terms of the number of detectable motor units (MUs) and the spatial distribution of the motor-unit action potentials (MUAPs). In this study, monopolar surface EMG signals were recorded from five normal subjects during low-force voluntary isometric contractions using a 92-channel electrode grid with 8 mm inter-electrode distances. Intramuscular EMG (iEMG) signals were recorded concurrently using monopolar needles. The HDsEMG and iEMG signals were independently decomposed into MUAP trains, and the iEMG results were verified using a rigorous a posteriori statistical analysis. HDsEMG decomposition identified from 2 to 30 MUAP trains per contraction. 3 ± 2 of these trains were also reliably detected by iEMG decomposition. The measured CKC decomposition accuracy of these common trains over a selected 10 s interval was 91.5 ± 5.8%. The other trains were not assessed. The significant factors that affected CKC decomposition accuracy were the number of HDsEMG channels that were free of technical artifact and the distinguishability of the MUAPs in the HDsEMG signal (P < 0.05). These results show that the CKC method reliably identifies at least a subset of MUAP trains in HDsEMG signals from low force contractions in pennate muscles.

Mechanisms of cramp contractions: peripheral or central generation?

We analysed the cramp threshold (i.e. the minimum frequency of electrical stimulation capable of inducing a cramp) and the behaviour of individual motor units during cramps electrically elicited in the absence (intact condition) and presence (blocked condition) of a peripheral nerve block in eight healthy subjects. The cramp threshold was significantly greater in the blocked than in the intact condition (18 ± 3 Hz vs. 13 ± 3 Hz; P = 0.01). Cramp duration and peak EMG amplitude in the intact condition (55.6 ± 19.2 s and 47.5 ± 24.8 μV, respectively) were significantly greater compared to the blocked condition (2.6 ± 1.3 s and 13.9 ± 8.8 μV; P < 0.01). All motor units identified in the blocked condition (n = 38) had a shorter interval of activity and a greater discharge rate compared to the intact condition (n = 37) (respectively, 1.1 ± 1.0 s vs. 29.5 ± 21.8 s, P < 0.0001; 25.7 ± 11.6 pulses s(-1) vs. 20.0 ± 5.9 pulses s(-1); P < 0.05). The motor unit activity detected during the blocked condition corresponded to spontaneous discharges of the motor nerves, while in the intact condition the motor unit discharge patterns presented the typical characteristics of motor neuron discharges. These results indicate a spinal involvement at the origin of cramps and during their development.

Noninvasive analysis of motor unit behavior in pathological tremor

A robust surface EMG decomposition tool, referred to as tremor-optimized Convolution Kernel Compensation (CKC) technique, is described. This technique modifies and extends the previously published CKC method in order to circumvent the typical assumption on regularity and asynchrony of motor unit firings in normal condition and adapt to the discharge patterns in pathological tremor. The results on synthetic and experimental surface EMG signals demonstrate high performance of decomposition. In the case of simulated surface EMG with 20 dB SNR, excitation level of 20% maximum voluntary contraction (MVC) and simulated tremor frequency of 8 Hz, the newly proposed method identified 8 ± 2 motor units with sensitivity of motor unit discharge identification ≥ 95 % and false alarm and miss rates ≤ 5%. The performance worsened with increasing noise power, with 5 ± 2 motor units identified at 10 dB SNR and 3 ± 1 at 0 dB SNR. In 24 recordings of high-density surface EMG signals from four tremor-affected patients, the modified CKC technique identified 134 motor units (6 ± 4 motor units per contraction).

Assessment of artefact suppression by ICA and spatial filtering on reduced sets of EEG signals

In recorded EEG signals, the signal components under interest are typically embedded in noise and artefacts. Independent Component Analysis has been demonstrated to be very successful at signal-to-noise ratio enhancement and artefact suppression, but mainly on a large set of EEG channels (20 or more) and typically on signals from healthy young subjects. In this paper, we assess the artefact suppression performance of five different ICA methods (AMUSE, FASTICA, RUNICA, SOBI and THINICA) combined with four different spatial filters on reduced sets of EEG channels from elderly tremor patients. Results demonstrate that a suitable combination of ICA and spatial filtering can effectively suppress artefacts in clinical EEG signals, even on very small sets with only three EEG channels.

On the selection of the cost function for gradient-based decomposition of surface electromyograms

Recently, gradient Convolution Kernel Compensation method was introduced for blind assessment of sparse pulse sequences (PS) out of their convolutive mixtures. This method employs multichannel recordings, is fully automatic and is minimally biased by assumptions about underlying mixing process. In the first step, the unknown mixing channels (convolution kernels) are compensated, whereas in the second step the gradient algorithm is used to blindly optimize the estimated PSs. This paper discusses the selection of the cost functions for aforementioned gradient-based optimization and provides analytical framework for their mutual comparison. Theoretical derivations are validated on both synthetic signals with random mixing matrices and experimental surface electromyograms from abductor pollicis brevis muscle. The analytical derivations agree very well with the results obtained from numerical simulations and establish theoretical guidelines for developing new gradient-based decomposition methods.

Building interactive virtual environments for simulated training in medicine using VRML and Java/JavaScript

Medicine is a difficult thing to learn. Experimenting with real patients should not be the only option; simulation deserves a special attention here. Virtual Reality Modelling Language (VRML) as a tool for building virtual objects and scenes has a good record of educational applications in medicine, especially for static and animated visualisations of body parts and organs. However, to create computer simulations resembling situations in real environments the required level of interactivity and dynamics is difficult to achieve. In the present paper we describe some approaches and techniques which we used to push the limits of the current VRML technology further toward dynamic 3D representation of virtual environments (VEs). Our demonstration is based on the implementation of a virtual baby model, whose vital signs can be controlled from an external Java application. The main contributions of this work are: (a) outline and evaluation of the three-level VRML/Java implementation of the dynamic virtual environment, (b) proposal for a modified VRML Timesensor node, which greatly improves the overall control of system performance, and (c) architecture of the prototype distributed virtual environment for training in neonatal resuscitation comprising the interactive virtual newborn, active bedside monitor for vital signs and full 3D representation of the surgery room.