Prediction of 1-year change in knee extension strength by neuromuscular properties in older adults

Improving muscle strength and preventing muscle weakness are important for older adults. The change in strength can be effectively explained by skeletal muscle mass and neural factors. Neural factors are important for older adults because the variation of neural components is greater in older than in young adults, and any decline in strength cannot solely be explained by a decrease in skeletal muscle mass. The purpose of the present study was to investigate whether skeletal muscle mass or motor unit firing properties could explain the change in muscle strength after 1 year. Thirty-eight older adults (75.0 ± 4.7 years, 156.6 ± 7.7 cm, 55.5 ± 9.4 kg, 26 women) performed maximum voluntary knee extension and their skeletal muscle mass was measured using a bioimpedance device. During a submaximal contraction task, high-density surface electromyography was recorded and the signals were decomposed into individual motor unit firing. As an index of motor unit firing properties, the slope and y-intercept (MU intercept) were calculated from the regression line between recruitment thresholds and firing rates in each participant. After 1 year, their maximum knee extension torque was evaluated again. A stepwise multiple regression linear model with sex and age as covariates indicated that MU intercept was a significant explanation with a negative association for the 1-year change in muscle strength (β =  - 0.493, p = 0.004), but not skeletal muscle mass (p = 0.364). The results suggest that neural components might be predictors of increasing and decreasing muscle strength rather than skeletal muscle mass.

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.

Association between physical fitness tests and neuromuscular properties

PURPOSE

While various fitness tests have been developed to assess physical performances, it is unclear how these tests are affected by differences, such as, in morphological and neural factors. This study was aimed to investigate associations between individual differences in physical fitness tests and neuromuscular properties.

METHODS

One hundred and thirty-three young adults participated in various general physical fitness tests and neuromuscular measurements. The appendicular skeletal muscle mass (ASM) was estimated by bioelectrical impedance analysis. Echo intensity (EI) was evaluated from the vastus lateralis. During submaximal knee extension force, high-density surface electromyography of the vastus lateralis was recorded and individual motor unit firings were detected. Y-intercept (i-MU) and slope (s-MU) from the regression line between the recruitment threshold and motor unit firing rate were calculated.

RESULTS

Stepwise multiple regression analyses revealed that knee extension strength could be explained (adjusted R2 = 0.712) by ASM (β = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R2 = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R2 = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R2 = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R2 = 0.631). On the other hand, i-MU and s-MU could be explained by five-sec stepping and counter-movement jump, respectively, but the coefficients of determination were low (adjusted R2 = 0.100 and 0.045).

CONCLUSION

Generally developed physical fitness tests were mainly explained by morphological factors, but were weakly affected by neural factors involved in performance.

Acute intermittent hypoxia enhances strength, and modulates spatial distribution of muscle activation in persons with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for facilitating neural plasticity in individuals with chronic incomplete spinal cord injury (iSCI). A single sequence of AIH enhances hand grip strength and ankle plantarflexion torque, but underlying mechanisms are not yet clear. We sought to examine how AIH-induced changes in magnitude and spatial distribution of the electromyogram (EMG) of the biceps and triceps brachii contributes to improved strength. Seven individuals with iSCI visited the laboratory on two occasions, and received either AIH or Sham AIH intervention in a randomized order. AIH consisted of 15 brief (∼60s) periods of low oxygen (fraction of inspired O2 = 0.09) alternating with 60s of normoxia, whereas Sham AIH consisted of repeated exposures to normoxic air. High-density surface EMG of biceps and triceps brachii was recorded during maximal elbow flexion and extension. We then generated spatial maps which distinguished active muscle regions prior to and 60 min after AIH or Sham AIH. After an AIH sequence, elbow flexion and extension forces increased by 91.7 ± 88.4% and 51.7 ± 57.8% from baseline, respectively, whereas there was no difference after Sham AIH. Changes in strength were associated with an altered spatial distribution of EMG and increased root mean squared EMG amplitude in both biceps and triceps brachii muscles. These data suggest that altered motor unit activation profiles may underlie improved volitional strength after a single dose of AIH and warrant further investigation using single motor unit analysis techniques to further elucidate mechanisms of AIH-induced plasticity.

Mental fatigue impairs physical performance but not the neural drive to the muscle: a preliminary analysis

Mental fatigue (MF) does not only affect cognitive but also physical performance. This study aimed to explore the effects of MF on muscle endurance, rate of perceived exertion (RPE), and motor units' activity. Ten healthy males participated in a randomised crossover study. The subjects attended two identical experimental sessions separated by 3 days with the only difference of a cognitive task (incongruent Stroop task [ST]) and a control condition (watching a documentary). Perceived MF and motivation were measured for each session at baseline and after each cognitive task. Four contractions at 20% of maximal voluntary contraction (MVIC) were performed at baseline, after each cognitive and after muscle endurance task while measuring motor units by high-density surface electromyography. Muscle endurance until failure at 50% of MVIC was measured after each cognitive task and the RPE was measured right after failure. ST significantly increased MF (p = 0.001) reduced the motivation (p = 0.008) for the subsequent physical task and also impaired physical performance (p = 0.044). However, estimates of common synaptic inputs and motor unit discharge rates as well as RPE were not affected by MF (p > 0.11). In conclusion, MF impairs muscle endurance and motivation for the physical task but not the neural drive to the muscle at any frequency bands. Although it is physiologically possible for mentally fatigued subjects to generate an optimal neuromuscular function, the altered motivation seems to limit physical performance. Preliminarily, our results suggest that the corticospinal pathways are not affected by MF.

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.

Effects of home-based bodyweight squat training on neuromuscular properties in community-dwelling older adults

BACKGROUND

It is important to investigate neural as well as muscle morphological adaptations to evaluate the effects of exercise training on older adults.

AIMS

This study was aimed to investigate the effects of home-based bodyweight squat training on neuromuscular adaptation in older adults.

METHODS

Twenty-five community-dwelling older adults (77.7 ± 5.0 years) were assigned to squat (SQU) or control (CON) groups. Those in the SQU group performed 100 bodyweight squats every day and the others in the CON group only performed daily activities for 4 months. Maximum knee extension torque and high-density surface electromyography during submaximal contraction were assessed. Individual motor units (MUs) were identified and divided into relatively low or high-recruitment threshold MU groups. Firing rates of each MU group were calculated. The muscle thickness and echo intensity of the lateral thigh were assessed using ultrasound. As physical tests, usual gait speed, timed up and go test, grip strength, and five-time chair stand test were performed.

RESULTS

While no improvements in muscle strength, muscle thickness, echo intensity, or physical tests were noted in either group, the firing rate of relatively low recruitment threshold MUs significantly decreased in the SQU group after intervention.

CONCLUSIONS

These results suggest that low-intensity home-based squat training could not improve markedly muscle strength or physical functions even if high-repetition and high frequency exercise, but could modulate slightly neural activation in community-dwelling older adults.

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.

Differences in motor unit behavior during isometric contractions in patients with diabetic peripheral neuropathy at various disease severities

The aim of this study was to determine whether HD-sEMG is sensitive to detecting changes in motor unit behavior amongst healthy adults and type 2 diabetes mellitus (T2DM) patients presenting diabetic peripheral neuropathy (DPN) at different levels. Healthy control subjects (CON, n = 8) and T2DM patients presenting no DPN symptoms (ABS, n = 8), moderate DPN (MOD, n = 18), and severe DPN (SEV, n = 12) performed isometric ankle dorsiflexion at 30 % maximum voluntary contraction while high-density surface EMG (HD-sEMG) was recorded from the tibialis anterior muscle. HD-sEMG signals were decomposed, providing estimates of discharge rate, motor unit conduction velocity (MUCV), and motor unit territory area (MUTA). As a result, the ABS group presented reduced MUCV compared to CON. The groups with diabetes presented significantly larger MUTA compared to the CON group (p < 0.01), and the SEV group presented a significantly lower discharge rate compared to CON and ABS (p < 0.01). In addition, the SEV group presented significantly higher CoV_force compared to CON and MOD. These results support the use of HD-SEMG as a method to detect peripheral and central changes related to DPN.

High-density Surface Electromyography as Biomarker of Muscle Aging

Sarcopenia is a muscle disease with adverse changes that increase throughout the lifetime but with different chronological scales between individuals. Addressing "early muscle aging" is becoming a critical issue for prevention. Through the CHRONOS study, we demonstrated the ability of the high-density surface electromyography (HD-sEMG), a noninvasive, wireless, portable technology, to detect both healthy muscle aging and accelerated muscle aging related to a sedentary lifestyle, one of the risk factors of sarcopenia. The HD-sEMG signals were analyzed in 91 healthy young, middle-aged, and old subjects (25-75 years) distributed according to their physical activity status (82 active and 9 sedentary; International Physical Activity Questionnaire) and compared with current methods for muscle evaluation, including muscle mass (dual-energy X-ray absorptiometry [DXA], ultrasonography), handgrip strength, and physical performance. The HD-sEMG signals were recorded from the rectus femoris during sit-to-stand trials, and 2 indexes were analyzed: muscular contraction intensity and muscle contraction dynamics. The clinical parameters did not differ significantly across the aging and physical activity levels. Inversely, the HD-sEMG indexes were correlated to age and were different significantly through the age categories of the 82 active subjects. They were significantly different between sedentary subjects aged 45-54 years and active ones at the same age. The HD-sEMG indexes of sedentary subjects were not significantly different from those of older active subjects (≥55 years). The muscle thicknesses evaluated using ultrasonography were significantly different between the 5 age decades but did not show a significant difference with physical activity. The HD-sEMG technique can assess muscle aging and physical inactivity-related "early aging," outperforming clinical and DXA parameters.

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.

Differences between vastus medialis and lateralis excitation onsets are dependent on the relative distance of surface electrodes placement from the innervation zone location

Conflictual results between the onset of vastus medialis (VM) and vastus lateralis (VL) excitation may arise from methodological aspects related to the detection of surface electromyograms. In this study we used an array of surface electrodes to assess the effect of detection site, relative to the muscle innervation zone, on the difference between VM and VL excitation onsets. Ten healthy males performed moderate isometric knee extension at 40 % of their maximal voluntary isometric contraction. After the actual VM-VL onset was defined (estimated when action potentials were generated at the neuromuscular junctions of both muscles), we calculated the largest bias that the detection site may introduce in the VM-VL onset estimation. We also assessed whether the location often considered for positioning bipolar electrodes on each muscle leads to VM-VL onset estimations comparable to the actual VM-VL onset. Our main results revealed that a maximum absolute bias of 20.48 ms may be introduced in VM-VL onset estimations due to the electrodes' detection site. In addition, mean differences of ∼ 12 ms in VM-VL onset estimations were attributable to largest possible discrepancies in the paired position of channels with respect to the innervation zone for VL and VM. When considering the classical location for positioning the bipolar electrodes over these muscles, differences error was subtle (∼3.4 ms) when compared with the actual VM-VL onset. Nonetheless, when accounting for the effect of relative differences in electrode position between muscles is not possible, our results suggest that a systematic absolute error of ∼ 12 ms should be considered in future studies regarding VM-VL onset estimations, suggesting that onset differences lower than that might not be clinically relevant.

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.

Maintenance of standing posture during multi-directional leaning demands the recruitment of task-specific motor units in the ankle plantarflexors

The purpose of this study is to investigate whether regional modulation of the ankle plantarflexors during standing was related to the recruitment of motor units associated with force direction. Fourteen participants performed a multi-directional leaning task in standing. Participants stood on a force platform and maintained their center of pressure in five different target directions. Motor unit firings were extracted by decomposition of high-density surface electromyograms recorded from the ankle plantarflexor muscles. The motor unit barycentre, defined as the weighted mean of the maximal average rectified values across columns and rows, was used to evaluate the medio-lateral and proximo-distal changes in the surface representation of single motor units across different leaning target directions. Using a motor unit tracking analysis, groups of motor units were identified as being common or unique across the target directions. The leaning directions had an effect on the spatial representations of motor units in the medial gastrocnemius and soleus (p < 0.05), but not in the lateral gastrocnemius (p > 0.05). Motor unit action potentials were represented in the medial and proximal aspects of the muscles during forward vs. lateral leans. Further analysis determined that the common motor units were found in similar spatial locations across the target directions, whereas newly recruited unique motor units were found in different spatial locations according to target direction (p < 0.05). The central nervous system may possess the ability to activate different groups of motor units according to task demands to meet the force-direction requirements of the leaning task.

Acute effect of inhibitory kinesio-tape of the upper trapezius on lower trapezius muscle excitation in healthy shoulders

INTRODUCTION

Shoulder pain increases excitation of the upper trapezius (UT) and reduces excitation in the lower trapezius (LT). Despite inconclusive evidence, kinesio-tape (KT) is often used to modify muscular excitation within the UT and/or LT to help correct alterations in scapular position and motion associated with shoulder pain/injury. The objective of the current study was to determine if inhibitory KT to the UT acutely increases LT excitation and if load alters the magnitude of change in the excitation observed.

METHODS

Twenty-two (N = 22, 11 female) individuals with healthy shoulders (24 ± 3 years) completed 10 repetitions of an arm elevation task during 3 taping conditions (no-tape, experimental KT, sham KT) and 2 loading conditions (no load and loaded). Whole-muscle (mean grid) and spatial distribution (grid row) of LT excitation (root mean squared; RMS) was measured using a single high-density surface electromyography 32-electrode grid.

RESULTS

There was a main effect for loading condition on whole-muscle LT RMS, F (1, 19) = 38.038, p < .001, partial η² = 0.667. Whole-muscle LT RMS was significantly higher in the loaded condition (0.055 V ±0 .005) compared to the no-load condition (0.038 V ±0 .004). No effect of tape condition was observed on whole-muscle or spatial distribution of RMS.

CONCLUSION

Our findings suggest that inhibitory KT to the UT does not alter whole-muscle excitation or shift the distribution of excitation within the LT during a repeated arm elevation task in healthy shoulders.

The knowns and unknowns of neural adaptations to resistance training

The initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is firmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g. limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude firm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of afferent and efferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron firing during different training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future.

Changes in supramaximal M-wave amplitude at different regions of biceps brachii following eccentric exercise of the elbow flexors

PURPOSE

Previous evidence from surface electromyograms (EMGs) suggests that exercise-induced muscle damage (EIMD) may manifest unevenly within the muscle. Here we investigated whether these regional changes were indeed associated with EIMD or if they were attributed to spurious factors often affecting EMGs.

METHODS

Ten healthy male subjects performed 3 × 10 eccentric elbow flexions. Maximal voluntary contraction (MVC), muscle soreness and ultrasound images from biceps brachii distal and proximal regions were measured immediately before (baseline) and during each of the following 4 days after the exercise. Moreover, 64 monopolar surface EMGs were detected while 10 supramaximal pulses were applied to the musculocutaneous nerve. The innervation zone (IZ), the number of electrodes detecting largest M-waves and their centroid longitudinal coordinates were assessed to characterize the spatial distribution of the M-waves amplitude.

RESULTS

The MVC torque decreased (~ 25%; P < 0.001) while the perceived muscle soreness scale increased (~ 4 cm; 0 cm for no soreness and 10 cm for highest imaginable soreness; P < 0.005) across days. The echo intensity of the ultrasound images increased at 48 h (71%), 72 h (95%) and 96 h (112%) for both muscle regions (P < 0.005), while no differences between regions were observed (P = 0.136). The IZ location did not change (P = 0.283). The number of channels detecting the greatest M-waves significantly decreased (up to 10.7%; P < 0.027) and the centroid longitudinal coordinate shifted distally at 24, 48 and 72 h after EIMD (P < 0.041).

CONCLUSION

EIMD consistently changed supramaximal M-waves that were detected mainly proximally from the biceps brachii, suggesting that EIMD takes place locally within the biceps brachii.

Linear mixed-effects models for the analysis of high-density electromyography with application to diabetic peripheral neuropathy

This article demonstrates the power and flexibility of linear mixed-effects models (LMEMs) to investigate high-density surface electromyography (HD-sEMG) signals. The potentiality of the model is illustrated by investigating the root mean squared value of HD-sEMG signals in the tibialis anterior muscle of healthy (n = 11) and individuals with diabetic peripheral neuropathy (n = 12). We started by presenting the limitations of traditional approaches by building a linear model with only fixed effects. Then, we showed how the model adequacy could be increased by including random effects, as well as by adding alternative correlation structures. The models were compared with the Akaike information criterion and the Bayesian information criterion, as well as the likelihood ratio test. The results showed that the inclusion of the random effects of intercept and slope, along with an autoregressive moving average correlation structure, is the one that best describes the data (p < 0.01). Furthermore, we demonstrate how the inclusion of additional variance structures can accommodate heterogeneity in the residual analysis and therefore increase model adequacy (p < 0.01). Thus, in conclusion, we suggest that adopting LMEM to repeated measures such as electromyography can provide additional information from the data (e.g. test for alternative correlation structures of the RMS value), and hence provide new insights into HD-sEMG-related work.

Regional modulation of the ankle plantarflexor muscles associated with standing external perturbations across different directions

Maintenance of upright standing posture has often been explained using the inverted pendulum model. This model considers the ankle plantarflexors to act as a single synergistic group. There are differences in muscle properties among the medial and lateral gastrocnemius (MG and LG, respectively) and the soleus that may affect their activation. Twelve volunteers participated in an investigation to determine whether the activation of the ankle plantarflexor muscles was modulated according to perturbation direction during unilateral standing perturbations of 1% body mass. High-density surface electromyography (HDS-EMG) was used to determine the amplitude and barycenter of the muscle activation and kinematic analysis was used to evaluate ankle, knee, and hip joint movement. The HDS-EMG amplitude and barycenter of MG and LG were modulated with the perturbation direction (MG p < 0.05; LG p < 0.01; one-way repeated-measures ANOVA). In soleus, the HDS-EMG barycenter modulated across the perturbation direction (p < 0.01 for X&Y coordinates), but the HDS-EMG amplitude did not change. A repeated-measures correlation was used to interpret the HDS-EMG pattern in the context of the kinematics. The relative contribution of MG activation compared to the total gastrocnemii activation was significantly associated with ankle dorsi/plantarflexion (r_rm = 0.620), knee flexion/extension and abduction/adduction (r_rm = 0.622 and r_rm = 0.547, respectively), and hip flexion/extension and abduction/adduction (r_rm = 0.653 and r_rm = 0.432, respectively). The findings suggest that the central nervous system activates motor units within different regions of MG, LG and SOL in response to standing perturbations in different directions.

Relationships between muscle strength and multi-channel surface EMG parameters in eighty-eight elderly

Background

Since age-related muscle strength loss cannot be explained solely by muscle atrophy, other determinants would also contribute to muscle strength in elderly. The present study aimed to clarify contribution of neuromuscular activation pattern to muscle strength in elderly group. From 88 elderlies (age: 61~ 83 years), multi-channel surface electromyography (EMG) of the vastus lateralis muscle was recorded with two-dimensional 64 electrodes during isometric submaximal ramp-up knee extension to assess neuromuscular activation pattern. Correlation analysis and stepwise regression analysis were performed between muscle strength and the parameters for signal amplitude and spatial distribution pattern, i.e., root mean square (RMS), correlation coefficient, and modified entropy of multi-channel surface EMG.

Results

There was a significant correlation between muscle strength and RMS (r = 0.361, p = 0.001) in the elderly. Muscle thickness (r = 0.519, p < 0.001), RMS (r = 0.288, p = 0.001), and normalized RMS (r = 0.177, p = 0.047) were selected as major determinants of muscle strength in stepwise regression analysis (r = 0.664 in the selected model).

Conclusion

These results suggest that inter-individual difference in muscle strength in elderly can be partly explained by surface EMG amplitude. We concluded that neuromuscular activation pattern is also major determinants of muscle strength on elderly in addition to indicator of muscle volume.

Evaluation of normal swallowing functions by using dynamic high-density surface electromyography maps

BACKGROUND

Swallowing is a continuous process with substantive interdependencies among different muscles, and it plays a significant role in our daily life. The aim of this study was to propose a novel technique based on high-density surface electromyography (HD sEMG) for the evaluation of normal swallowing functions.

METHODS

A total of 96 electrodes were placed on the front neck to acquire myoelectric signals from 12 healthy subjects while they were performing different swallowing tasks. HD sEMG energy maps were constructed based on the root mean square values to visualize muscular activities during swallowing. The effects of different volumes, viscosities, and head postures on the normal swallowing process were systemically investigated by using the energy maps.

RESULTS

The results showed that the HD sEMG energy maps could provide detailed spatial and temporal properties of the muscle electrical activity, and visualize the muscle contractions that closely related to the swallowing function. The energy maps also showed that the swallowing time and effort was also explicitly affected by the volume and viscosity of the bolus. The concentration of the muscular activities shifted to the opposite side when the subjects turned their head to either side.

CONCLUSIONS

The proposed method could provide an alternative method to physiologically evaluate the dynamic characteristics of normal swallowing and had the advantage of providing a full picture of how different muscle activities cooperate in time and location. The findings from this study suggested that the HD sEMG technique might be a useful tool for fast screening and objective assessment of swallowing disorders or dysphagia.

Between-day reliability of triceps surae responses to standing perturbations in people post-stroke and healthy controls: A high-density surface EMG investigation

The reliability of triceps surae electromyographic responses to standing perturbations in people after stroke and healthy controls is unknown. High-Density surface Electromyography (HDsEMG) is a technique that records electromyographic signals from different locations over a muscle, overcoming limitations of traditional surface EMG such as between-day differences in electrode placement. In this study, HDsEMG was used to measure responses from soleus (SOL, 18 channels) and medial and lateral gastrocnemius (MG and LG, 16 channels each) in 10 people after stroke and 10 controls. Timing and amplitude of the response were estimated for each channel of the grids. Intraclass Correlation Coefficient (ICC) and normalized Standard Error of Measurement (SEM%) were calculated for each channel individually (single-channel configuration) and on the median of each grid (all-channels configuration). Both timing (single-channel: ICC=0.75-0.96, SEM%=5.0-9.1; all-channels: ICC=0.85-0.97; SEM%=3.5-6.2%) and amplitude (single-channel: ICC=0.60-0.91, SEM%=25.1-46.6; ICC=0.73-0.95, SEM%=19.3-42.1) showed good-to-excellent reliability. HDsEMG provides reliable estimates of EMG responses to perturbations both in individuals after stroke and in healthy controls; reliability was marginally better for the all-channels compared to the single-channel configuration.