Surface EMG mapping of the human trapezius muscle: the topography of monopolar and bipolar surface EMG amplitude and spectrum parameters at varied forces and in fatigue

Excitation distribution of the trapezius changes in response to increasing contraction intensity, but not repeated contractions

Upper trapezius (UT) excitation redistributes with experimentally-induced muscle pain, fatigue, and repeated contractions. Excitation distribution variability is proposed to reduce the likelihood of shoulder pain and pathology by reducing cumulative stress on musculoskeletal structures. While the middle (MT) and lower (LT) trapezius are pivotal in scapular stabilization, it remains unclear whether they display similar excitation distribution variability with repeated or increasing contraction intensity. We determined if excitation distribution of the UT, MT, and LT differ: 1) during isometric contractions at different intensities (30 % and 60 % of maximum voluntary isometric contraction (MVIC)); and 2) with repeated contractions at 60 % MVIC. Nineteen individuals completed MVICs and submaximal contractions for the UT, MT, and LT while high-density electromyography was collected. Statistical parametric mapping t-tests were performed between intensities and the 1st and 5th repetition at 60 % MVIC. UT, MT, and LT excitation distribution changed with increasing contraction intensity in 358 (∼92 % of the map), 54 (∼14 %), and 270 pixels (∼70 %), respectively. No pixels exceeded significance with repeated contractions for any muscle. Barycentre analyses revealed no significant results. These results suggest that regions of the trapezius muscle use different neuromuscular strategies in response to changes in contraction intensity and repeated contractions.

Multipoint surface electromyography measurement using bull's-eye electrodes for wide-area topographic analysis

BACKGROUND

Surface electromyography (sEMG) is primarily used to analyze individual and neighboring muscle activity. However, using a broader approach can enable simultaneous measurement of multiple muscles, which is essential for understanding muscular coordination. Using the "bull's-eye electrode," which allows bipolar derivation without directional dependence, enables wide-area multipoint sEMG measurements. This study aims to establish a multipoint measurement system and demonstrate its effectiveness and evaluates forearm fatigue and created topographic maps during a grasping task.

METHODS

Nine healthy adults with no recent arm injuries or illnesses participated in this study. They performed grasping tasks using their dominant hand, while bull's-eye electrodes recorded their muscle activity. To validate the effectiveness of the system, we calculated the root mean squares of muscle activity and entropy, an indicator of muscle activity distribution, and compared them over time.

RESULTS

The entropy analysis demonstrated a significant time-course effect with increased entropy over time, suggesting increased forearm muscle uniformity, which is possibly indicative of fatigue. Topographic maps visually displayed muscle activity, revealing notable intersubject variations.

DISCUSSION

Bull's-eye electrodes facilitated the capture of nine homogeneous muscle activity points, enabling the creation of topographic images. The entropy increased progressively, suggesting an adaptive muscle coordination response to fatigue. Despite some limitations, such as inadequate measurement of the forearm muscles' belly, the system is an unconventional measurement method.

CONCLUSION

This study established a robust system for wide-area multipoint sEMG measurements using a bull's-eye electrode setup. This system effectively evaluates muscle fatigue and provides a comprehensive topographic view of muscle activity. These results mark a significant step towards developing a future multichannel sEMG system with enhanced measurement points and improved wearability.

TRIAL REGISTRATION

This study was approved by the Ethics Committee of Chiba University Graduate School of Engineering (acceptance number: R4-12, Acceptance date: November 04, 2022).

Searching for online information on the fit of children's footwear during the COVID-19 pandemic: an analysis of Google Trends data

BACKGROUND

Selecting footwear with appropriate fit in children is challenging due the changes with foot size and dimensions which occur throughout childhood. Access to appropriate footwear is important but recent challenges with the COVID-19 pandemic resulted in closure of retail stores for prolonged periods where parents/carers could not physically purchase footwear for their children and the footwear industry suffered disruption to their supply chain, and falls in retail sales. Simultaneously increased use of social media platforms for health information seeking throughout the pandemic have been documented. This likely would have included parents/carers seeking information online to support footwear purchases for their children. The primary aim of this work was to explore how searches for online fitting information for children changed throughout the COVID-19 pandemic lockdown periods. A secondary aim was to identify how searches were influenced by footwear style.

METHODS

 We employed Google Trends to obtain search engine traffic related to footwear fitting information for children. We collected data spanning the three years pre, during and post the main national lockdown for three eight-week windows: (1) first eight weeks of the U.K. national lockdown; (2) the first eight weeks of the calendaryear; (3) the eight weeks leading up to children going back-to-school for the new academic year in the U.K. The search terms reflected parents/carers searching for footwear fit information relating to children and were grouped by style of footwear: children, infants, babies and toddlers as well as school shoes.

RESULTS

We identified increased searching for footwear fit information for children during the pandemic, which reduced following post pandemic in all except the searches which related to school shoes. We saw reductions in searching related to fit of school shoes as schools closed indefinitely and an increase in online searches with the pandemic. This was also maintained post-pandemic despite shops reopening, suggesting that some of these changes in information reflect new consumer behaviours which may continue.

CONCLUSIONS

 Increased searches for online resources regarding footwear fit highlights the importance of ensuring high quality accessible online information on footwear fit is available to support those buying footwear for their children.

The Effect of Vibration Massage on Fatigue of the Upper Trapezius Muscle during Different Tasks

Background: Vibratory massage is now widely used to alleviate muscle fatigue. The effects of different vibration massage intensities on left and right upper trapezius (UT) fatigue have not been examined. Therefore, the present study first examined whether a vibration massage intervention had an effect on UT muscles and second compared the effects of 2 different levels (36 Hz and 46 Hz) of vibratory massage on the right and left oblique muscles under 3 different fatigue conditions. Methods: A total of 23 participants (12 female, 11 male; age: 26.5 ± 3.9 year, height: 170.5 ± 1.6 cm, mass: 57.5 ± 1.5 kg, BMI: 24.3 ± 1.6 kg/m2) were randomly divided into intervention and control groups. The 2 groups of subjects completed isometric contraction fatigue tasks of 30 s of fatigue, 60 s of fatigue and 90 s of fatigue in turn. The specific task of isotonic contraction was performed by subjects holding a 1 kg dumbbell in each hand and performing a straight arm weighted lateral supination exercise. After each exercise, the intervention group was randomized to apply a massage device with a vibration intensity of 36 Hz or 46 Hz on the left and right UT muscles for 5 min each. The control group did not receive any treatment. Both groups then repeated the same fatigue task as before. Then, the effects of different vibration massage interventions on UT muscles were derived by analyzing the changes in maximal voluntary contraction percentage (MVC%) of surface electromyography (sEMG) signals before and after the intervention, and the most effective vibration massage program for relieving left and right UT fatigue was summarized. Then, four classification algorithms were used to label and classify the collected sEMG data, and finally a UT muscle fatigue identification and vibration massage model was constructed. Results: After using the vibration massage level 1 (36 Hz) intervention, the MVC% of the right UT muscle showed significant reductions in the 30 s fatigue task, the 60 s fatigue task and the 90 s fatigue task (R1: p = 0.022, R2: p = 0.005, R3: p = 0.049). After using the vibration massage level 3 (46 Hz) intervention, the MVC% of the right UT muscle showed a significant decrease in both the 60 s fatigue task and the 90 s fatigue task (R2: p = 0.033, R3: p = 0.028). Significant decreases in MVC% for the left UT muscle were found only in the 90 s fatigue task (L3: p = 0.040). Then, by comparing the different performances of four commonly used classification algorithms, it was found that the bagging (accuracy = 0.860) algorithm had higher accuracy. Therefore, the bagging algorithm was used for the UT fatigue identification and vibration massage models. Conclusions: This was the first study to show the impacts of different levels of vibration massage on fatigue alleviation in the left and right UT muscles. Furthermore, the bilateral UT fatigue identification and vibration massage model developed in this study can help people to choose the most appropriate massage protocol for quick relief and relaxation of the UT muscles under three different fatigue tasks.

Recovery and Fatigue Behavior of Forearm Muscles during a Repetitive Power Grip Gesture in Racing Motorcycle Riders

Despite a reduction in the maximal voluntary isometric contraction (MVC_isom) observed systematically in intermittent fatigue protocols (IFP), decrements of the median frequency, assessed by surface electromyography (sEMG), has not been consistently verified. This study aimed to determine whether recovery periods of 60 s were too long to induce a reduction in the normalized median frequency (MF_EMG) of the flexor digitorum superficialis and carpi radialis muscles. Twenty-one road racing motorcycle riders performed an IFP that simulated the posture and braking gesture on a motorcycle. The MVC_isom was reduced by 53% (p < 0.001). A positive and significant relationship (p < 0.005) was found between MF_EMG and duration of the fatiguing task when 5 s contractions at 30% MVC_isom were interspersed by 5 s recovery in both muscles. In contrast, no relationship was found (p > 0.133) when 10 s contractions at 50% MVC were interspersed by 1 min recovery. Comparative analysis of variance (ANOVA) confirmed a decrement of MF_EMG in the IFP at 30% MVC_isom including short recovery periods with a duty cycle of 100% (5 s/5 s = 1), whereas no differences were observed in the IFP at 50% MVC_isom and longer recovery periods, with a duty cycle of 16%. These findings show that recovery periods during IFP are more relevant than the intensity of MVC_isom. Thus, we recommend the use of short recovery periods between 5 and 10 s after submaximal muscle contractions for specific forearm muscle training and testing purposes in motorcycle riders.

Effect of arm position on spatial distribution of upper trapezius muscle activity during simulated car driving

The present study aimed to investigate the upper trapezius muscle activity during simulated car driving while adopting three different arm positions. Ten participants were instructed to maintain the following positions: hands on the steering wheel (Hands-On), hands not on the steering wheel (Hands-Off) and hands not on the steering wheel but arms on armrests (Armrests). During the tasks, multi-channel surface electromyography (EMG) was recorded from the upper trapezius muscle with 64 two-dimensionally distributed electrodes. Amplitudes of surface EMG in Armrests were lower than in Hands-On (p = 0.004). The spatial distribution of surface EMG changed with time in Hands-Off and Armrests (p < 0.05), but not in Hands-On (p > 0.05). These findings suggest that being freed from steering leads to the recruitment of various muscle fibers/motor units within the upper trapezius muscle and the use of armrests may help reduce the physiological burden loaded on the muscle of drivers.

Hand Gesture Recognition based on Surface Electromyography using Convolutional Neural Network with Transfer Learning Method

This paper presents an effective transfer learning (TL) strategy for the realization of surface electromyography (sEMG)-based gesture recognition with high generalization and low training burden. To realize the idea of taking a well-trained model as the feature extractor of the target networks, 30 hand gestures involving various states of finger joints, elbow joint and wrist joint are selected to compose the source task, and a convolutional neural network (CNN)-based source network is designed and trained as the general gesture EMG feature extraction network. Then, two types of target networks, in the forms of CNN-only and CNN+LSTM (long short-term memory) respectively, are designed with the same CNN architecture as the feature extraction network. Finally, gesture recognition experiments on three different target gesture datasets are carried out under TL and Non-TL strategies respectively. The experimental results verify the validity of the proposed TL strategy in improving hand gesture recognition accuracy and reducing training burden. For both the CNN-only and the CNN+LSTM target networks, on the three target datasets from new users, new gestures and different collection scheme, the proposed TL strategy improves the recognition accuracy by 10%∼38%, reduces the training time to tens of times, and guarantees the recognition accuracy of more than 90% when only 2 repetitions of each gesture are used to fine-tune the parameters of target networks. The proposed TL strategy has important application value for promoting the development of myoelectric control systems.

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.

Statistical parametric mapping of three-dimensional local activity distribution of skeletal muscle using magnetic resonance imaging (MRI)

Analysis of the internal local activity distribution in human skeletal muscles is important for managing muscle fatigue/pain and dysfunction. However, no method is established for three-dimensional (3D) statistical analysis of features of activity regions common to multiple subjects during voluntary motor tasks. We investigated the characteristics of muscle activity distribution from the data of ten healthy subjects (29 ± 1 year old, 2 women) during voluntary teeth clenching under two different occlusal conditions by applying spatial normalization and statistical parametric mapping (SPM) to analysis of muscle functional magnetic resonance imaging (mfMRI) using increase in transverse relaxation time (T2) of the skeletal muscle induced by exercise. The expansion of areas with significant T2 increase was observed in the masticatory muscles after clenching with molar loss comparing with intact dentition. The muscle activity distribution characteristics common to a group of subjects, i.e., the active region in the temporal muscle ipsilateral to the side with the molar loss and medial pterygoid muscle contralateral to the side with the molar loss, were clarified in 3D by applying spatial normalization and SPM to mfMRI analysis. This method might elucidate the functional distribution within the muscles and the localized muscular activity related to skeletal muscle disorders.

Comparison of Facial Muscle Activation Patterns Between Healthy and Bell's Palsy Subjects Using High-Density Surface Electromyography

Facial muscle activities are essential for the appearance and communication of human beings. Therefore, exploring the activation patterns of facial muscles can help understand facial neuromuscular disorders such as Bell’s palsy. Given the irregular shape of the facial muscles as well as their different locations, it should be difficult to detect the activities of whole facial muscles with a few electrodes. In this study, a high-density surface electromyogram (HD sEMG) system with 90 electrodes was used to record EMG signals of facial muscles in both healthy and Bell’s palsy subjects when they did different facial movements. The electrodes were arranged in rectangular arrays covering the forehead and cheek regions of the face. The muscle activation patterns were shown on maps, which were constructed from the Root Mean Square (RMS) values of all the 90-channel EMG recordings. The experimental results showed that the activation patterns of facial muscles were distinct during doing different facial movements and the activated muscle regions could be clearly observed. Moreover, two features of the activation patterns, 2D correlation coefficient (corr2) and Centre of Gravity (CG) were extracted to quantify the spatial symmetry and the location of activated muscle regions respectively. Furthermore, the deviation of activated muscle regions on the paralyzed side of a face compared to the healthy side was quantified by calculating the distance between two sides of CGs. The results revealed that corr2 of the activated facial muscle region (classified into forehead region and cheek region) in Bell’s palsy subjects was significantly (p < 0.05) lower than that in healthy subjects, while CG distance of activated facial region in Bell’s palsy subjects was significantly (p < 0.05) higher than that in healthy subjects. The correlation between corr2 of these regions and Bell’s palsy [assessed by the Facial Nerve Grading Scale (FNGS) 2.0] was also significant (p < 0.05) in Bell’s palsy subjects. The spatial information on activated muscle regions may be useful in the diagnosis and treatment of Bell’s palsy in the future.

EMG map image processing for recognition of fingers movement

Electromyography (EMG) is the conventional noninvasive method for the estimation of muscle activities. We developed a new image processing method for the recognition of individual finger movements based on EMG maps. The maps were formed from the EMG recordings via an array electrode with 24 contacts connected to a multichannel wireless miniature digital amplifier. The task was to detect and quantify the high activity regions in the EMG maps in persons with no known motor impairment. The results show the temporal and spatial patterns within the images during well-defined finger movements. The average accuracy of the automatic recognition compared with the recognition by an expert clinician in persons involved in the tests was 97.87 ± 0.92%. The application of the technique is foreseen for control for an assistive system (hand prosthesis and exoskeleton) since the interface is wearable and the processing can be implemented on a microcomputer.

Effect of TMS coil orientation on the spatial distribution of motor evoked potentials in an intrinsic hand muscle

Previous reports on the relationship between coil orientation and amplitude of motor evoked potential (MEP) in transcranial magnetic stimulation (TMS) did not consider the effect of electrode arrangement. Here we explore this open issue by investigating whether TMS coil orientation affects the amplitude distribution of MEPs recorded from the abductor pollicis brevis (APB) muscle with a bi-dimensional grid of 61 electrodes. Moreover, we test whether conventional mono- and bipolar montages provide representative MEPs compared to those from the grid of electrodes. Our results show that MEPs with the greatest amplitudes were elicited for 45° and 90° coil orientations, i.e. perpendicular to the central sulcus, for all electrode montages. Stimulation with the coil oriented at 135° and 315°, i.e. parallel to the central sulcus, elicited the smallest MEP amplitudes. Additionally, changes in coil orientation did not affect the spatial distribution of MEPs over the muscle extent. It has been shown that conventional electrodes with detection volume encompassing the APB belly may detect representative MEPs for optimal coil orientations. In turn, non-optimal orientations were identified only with the grid of electrodes. High-density electromyography may therefore provide new insights into the effect of coil orientation on MEPs from the APB muscle.

A Modular, Smart, and Wearable System for High Density sEMG Detection

OBJECTIVE

The use of linear or bi-dimensional electrode arrays for surface EMG detection (HD-sEMG) is gaining attention as it increases the amount and reliability of information extracted from the surface EMG. However, the complexity of the setup and the encumbrance of HD-sEMG hardware currently limits its use in dynamic conditions. The aim of this paper was to develop a miniaturized, wireless, and modular HD-sEMG acquisition system for applications requiring high portability and robustness to movement artifacts.

METHODS

A system with modular architecture was designed. Its core is a miniaturized 32-channel amplifier (Sensor Unit - SU) sampling at 2048 sps/ch with 16 bit resolution and wirelessly transmitting data to a PC or a mobile device. Each SU is a node of a Body Sensor Network for the synchronous signal acquisition from different muscles.

RESULTS

A prototype with two SUs was developed and tested. Each SU is small (3.4 cm × 3 cm × 1.5 cm), light (16.7 g), and can be connected directly to the electrodes; thus, avoiding the need for customary, wired setup. It allows to detect HD-sEMG signals with an average noise of 1.8 μV_RMS and high performance in terms of rejection of power-line interference and motion artefacts. Tests performed on two SUs showed no data loss in a 22 m range and a ±500 μs maximum synchronization delay.

CONCLUSIONS

Data collected in a wide spectrum of experimental conditions confirmed the functionality of the designed architecture and the quality of the acquired signals.

SIGNIFICANCE

By simplifying the experimental setup, reducing the hardware encumbrance, and improving signal quality during dynamic contractions, the developed system opens new perspectives in the use of HD-sEMG in applied and clinical settings.

Surface electromyography (sEMG) of extradiaphragm respiratory muscles in healthy subjects: A systematic review

The aim of this systematic review was to examine procedures used and outcome measures reported from surface EMG (sEMG) of extradiaphragm inspiratory muscles in healthy people. Relevant articles were searched using the concepts "electromyography (EMG)", "respiratory muscles (sternocleidomastoid [SM], scalene, intercostal [IC] and parasternal)" and "healthy" in the electronic databases: MEDLINE, PubMed, EMBASE, Cochrane CENTRAL and Database of Systematic Reviews, CINAHL, SPORTDiscus, LILACS, and PEDro. Twenty-five papers were included and quality assessment was performed using an adapted Downs and Black checklist. Twenty-eight percent of included papers were classified as moderate quality and the rest were low quality. The SM was the muscle most often investigated. Description of EMG techniques were often incomplete for features such as the procedure before electrode placement, description of the surface electrodes, the EMG detection mode and amplification. Of note, descriptions of the IC muscle electrode positioning varied widely. Comparison of outcomes among studies was challenging because of the very diverse EMG outcomes reported. There are many controversies regarding methods and technique used to assess sEMG of extradiaphragm inspiratory muscles. Therefore, studies with higher methodological quality utilizing standardized EMG procedures including electrode positioning will enable accurate and reliable comparison among studies of the extradiaphragm inspiratory muscles.

Effects of cognitive loading and force on upper trapezius fatigue

Background

Musculoskeletal disorders (MSDs) are particularly common in the shoulder/neck region for some tasks that involve low force exertions, for example computer work. It has been demonstrated that muscle activity may be stimulated by cognitively demanding tasks. There is a lack of studies on the relationship between qualitative levels of cognitive loading, level of muscle activity, and muscle fatigue.

Aims

To investigate the effects of quantitative levels of cognitive loading on conduction velocity changes and isometric endurance times for the upper trapezius.

Methods

Participants performed a combination of three levels of a cognitively demanding computer task (0, 1.59 and 3 Bits) while simultaneously performing either of two isometric endurance tests (20 and 40% MVC) for the upper trapezius.

Results

Information load had a significant effect (P < 0.05) on normalized conduction velocity slopes for the 40% but not for the 20% maximum voluntary contraction conditions. Information load had a highly significant effect on endurance times for both exertion levels (P < 0.01).

Conclusions

This study found that performing a high-difficulty cognitive task while simultaneously performing increasingly higher levels of static isometric shoulder abduction exertions, fatigued the trapezius muscle quicker than performing an equivalent exertion with low cognitive load. This is particularly relevant to workplaces with increasing levels of automation that require high levels of decision making and computer use.

A pilot study on the evaluation of normal phonating function based on high-density sEMG topographic maps

Phonation is controlled by complex synergism of muscles over the front neck region. Proper evaluation of the muscular activities in this region would not only help to estimate phonation function, but may also provide characteristics to diagnose dysphonia. While surface electromyography (sEMG) technique has been used to study the physiological aspects of phonation in previous studies, it remains unclear if the phonating function could be dynamically characterized by the sEMG signals of the neck muscles associated with phonation. In this study, almost 80 channels of high-density (HD) sEMG signals were acquired from four healthy subjects when the vowel /a/ was phonated across different pitches by them. The root mean square (RMS) of the HD sEMG signals was computed within a series of segmented analysis windows and used to construct dynamic sEMG topographic maps. And the RMS maps represented the energy distribution of the front neck muscles, which would provide both the temporal and spatial information in accordance with the physiological and biomechanical principles of phonation. Our pilot results from the sEMG topographic maps across different pitch levels showed that the muscular activities consistently increased with the enhancement of the pitch levels. This pilot study suggests that HD sEMG might be a potential tool to visualize the distribution of the muscular activities and observe the coordination of muscular contractions during phonation. Also, it might pave way for proper screening and diagnosis of dysphonia as well as its associated pathologies.

Spatial Analysis of Multichannel Surface EMG in Hemiplegic Stroke

We investigated spatial activation patterns of upper extremity muscles during isometric force generation in both intact persons and in hemispheric stroke survivors. We used a 128-channel surface electromyogram (EMG) grid to record the electrical activity of biceps brachii muscles during these contractions. EMG data were processed to develop 2-D root mean square (RMS) maps of muscle activity. Our objective was to determine whether motor impairments following stroke were associated with changes in the muscle activity maps and in the spatial distribution of muscular activation. We found that, for a given subject, spatial patterns in muscle activity maps were consistent across all measured contraction levels differing only the RMS EMG. However, the maps from opposite arms (stroke-affected versus non-affected) of stroke survivors were significantly different from each other, especially when compared with the differences observed intact participants. Our analyses revealed that chronic stroke altered the size and location of the active region in these maps. The former is potentially related to disruption of fiber and tissue structure, possibly linked to factors such as extracellular fat accumulation, connective tissue infiltration, muscle fiber atrophy, fiber shortening, and fiber loss. Changes in spatial patterns in muscle activity maps may also be linked to a shift in the location of the innervation zone or the endplate region of muscles. Furthermore, the textural analysis of EMG activity maps showed a larger pixel-to-pixel variability in stroke-affected muscles. Alterations in the muscle activity maps were also related to functional impairment (estimated using Fugl-Meyer score) and to the degree of spasticity (estimated using the modified Ashworth scale). Overall, our investigation revealed that the muscle architecture and morphology were significantly altered in the chronic stroke.

Muscle Activity Map Reconstruction from High Density Surface EMG Signals With Missing Channels Using Image Inpainting and Surface Reconstruction Methods

OBJECTIVE

The aim of this study was to reconstruct low-quality High-density surface EMG (HDsEMG) signals, recorded with 2-D electrode arrays, using image inpainting and surface reconstruction methods.

METHODS

It is common that some fraction of the electrodes may provide low-quality signals. We used variety of image inpainting methods, based on partial differential equations (PDEs), and surface reconstruction methods to reconstruct the time-averaged or instantaneous muscle activity maps of those outlier channels. Two novel reconstruction algorithms were also proposed. HDsEMG signals were recorded from the biceps femoris and brachial biceps muscles during low-to-moderate-level isometric contractions, and some of the channels (5-25%) were randomly marked as outliers. The root-mean-square error (RMSE) between the original and reconstructed maps was then calculated.

RESULTS

Overall, the proposed Poisson and wave PDE outperformed the other methods (average RMSE 8.7 μV_rms ± 6.1 μV_rms and 7.5 μV_rms ± 5.9 μV_rms) for the time-averaged single-differential and monopolar map reconstruction, respectively. Biharmonic Spline, the discrete cosine transform, and the Poisson PDE outperformed the other methods for the instantaneous map reconstruction. The running time of the proposed Poisson and wave PDE methods, implemented using a Vectorization package, was 4.6 ± 5.7 ms and 0.6 ± 0.5 ms, respectively, for each signal epoch or time sample in each channel.

CONCLUSION

The proposed reconstruction algorithms could be promising new tools for reconstructing muscle activity maps in real-time applications.

SIGNIFICANCE

Proper reconstruction methods could recover the information of low-quality recorded channels in HDsEMG signals.

Gesture recognition by instantaneous surface EMG images

Gesture recognition in non-intrusive muscle-computer interfaces is usually based on windowed descriptive and discriminatory surface electromyography (sEMG) features because the recorded amplitude of a myoelectric signal may rapidly fluctuate between voltages above and below zero. Here, we present that the patterns inside the instantaneous values of high-density sEMG enables gesture recognition to be performed merely with sEMG signals at a specific instant. We introduce the concept of an sEMG image spatially composed from high-density sEMG and verify our findings from a computational perspective with experiments on gesture recognition based on sEMG images with a classification scheme of a deep convolutional network. Without any windowed features, the resultant recognition accuracy of an 8-gesture within-subject test reached 89.3% on a single frame of sEMG image and reached 99.0% using simple majority voting over 40 frames with a 1,000 Hz sampling rate. Experiments on the recognition of 52 gestures of NinaPro database and 27 gestures of CSL-HDEMG database also validated that our approach outperforms state-of-the-arts methods. Our findings are a starting point for the development of more fluid and natural muscle-computer interfaces with very little observational latency. For example, active prostheses and exoskeletons based on high-density electrodes could be controlled with instantaneous responses.

Filtered Virtual Reference: A New Method for the Reduction of Power Line Interference With Minimal Distortion of Monopolar Surface EMG

GOAL

This study tests and validates a new method to remove power line interference from monopolar EMGs detected by multichannel systems: the filtered virtual reference (FVR). FVR is an adaptation of the virtual reference (VR) method, which consists in referencing signals detected by each electrode in a grid to their spatial average. Signals may however be distorted with the VR approach, in particular when the skin region where the detection system is positioned does not cover the entire muscle.

METHODS

Simulated and experimental EMGs were used to compare the performance of FVR and VR in terms of interference reduction and distortion of monopolar signals referred to a remote reference.

RESULTS

Simulated data revealed the monopolar EMG signals processed with FVR were significantly less distorted than those filtered by VR. These results were similarly observed for experimental signals. Moreover, FVR method outperformed VR in removing power line interference when it was distributed unevenly across the signals of the grid.

CONCLUSION

Key results demonstrated that FVR improves the VR method as it reduces interference while preserving the information content of monopolar signals.

SIGNIFICANCE

Although the actual distribution of motor unit action potential is represented in monopolar EMGs, collecting high quality monopolar signals is challenging. This study presents a possible solution to this issue; FVR provides undistorted monopolar signals with negligible interference and is insensitive to muscle architecture. It is therefore relevant for EMG applications benefiting from a clean monopolar detection (e.g., decomposition, control of prosthetic devices, motor unit number estimation).

Re-evaluation of EMG-torque relation in chronic stroke using linear electrode array EMG recordings

The objective was to re-evaluate the controversial reports of EMG-torque relation between impaired and non-impaired sides using linear electrode array EMG recordings. Ten subjects with chronic stroke performed a series of submaximal isometric elbow flexion tasks. A 20-channel linear array was used to record surface EMG of the biceps brachii muscles from both impaired and non-impaired sides. M-wave recordings for bilateral biceps brachii muscles were also made. Distribution of the slope of the EMG-torque relations for the individual channels showed a quasi-symmetrical "M" shaped pattern. The lowest value corresponded to the innervation zone (IZ) location. The highest value from the slope curve for each side was selected for comparison to minimize the effect of electrode placement and IZ asymmetry. The slope was greater on the impaired side in 4 of 10 subjects. There were a weak correlation between slope ratio and strength ratio and a moderate to high correlation between slope ratio and M-wave ratio between two sides. These findings suggest that the EMG-torque relations are likely mediated and influenced by multiple factors. Our findings emphasize the importance of electrode placement and suggest the primary role of peripheral adaptive changes in the EMG-torque relations in chronic stroke.

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.

Spatial variability in cortex-muscle coherence investigated with magnetoencephalography and high-density surface electromyography

Cortex-muscle coherence (CMC) reflects coupling between magnetoencephalography (MEG) and surface electromyography (sEMG), being strongest during isometric contraction but absent, for unknown reasons, in some individuals. We used a novel nonmagnetic high-density sEMG (HD-sEMG) electrode grid (36 mm × 12 mm; 60 electrodes separated by 3 mm) to study effects of sEMG recording site, electrode derivation, and rectification on the strength of CMC. Monopolar sEMG from right thenar and 306-channel whole-scalp MEG were recorded from 14 subjects during 4-min isometric thumb abduction. CMC was computed for 60 monopolar, 55 bipolar, and 32 Laplacian HD-sEMG derivations, and two derivations were computed to mimic "macroscopic" monopolar and bipolar sEMG (electrode diameter 9 mm; interelectrode distance 21 mm). With unrectified sEMG, 12 subjects showed statistically significant CMC in 91-95% of the HD-sEMG channels, with maximum coherence at ∼25 Hz. CMC was about a fifth stronger for monopolar than bipolar and Laplacian derivations. Monopolar derivations resulted in most uniform CMC distributions across the thenar and in tightest cortical source clusters in the left rolandic hand area. CMC was 19-27% stronger for HD-sEMG than for "macroscopic" monopolar or bipolar derivations. EMG rectification reduced the CMC peak by a quarter, resulted in a more uniformly distributed CMC across the thenar, and provided more tightly clustered cortical sources than unrectifed sEMGs. Moreover, it revealed CMC at ∼12 Hz. We conclude that HD-sEMG, especially with monopolar derivation, can facilitate detection of CMC and that individual muscle anatomy cannot explain the high interindividual CMC variability.

Recording and assessment of evoked potentials with electrode arrays

In order to optimize procedure for the assessment of evoked potentials and to provide visualization of the flow of action potentials along the motor systems, we introduced array electrodes for stimulation and recording and developed software for the analysis of the recordings. The system uses a stimulator connected to an electrode array for the generation of evoked potentials, an electrode array connected to the amplifier, A/D converter and computer for the recording of evoked potentials, and a dedicated software application. The method has been tested for the assessment of the H-reflex on the triceps surae muscle in six healthy humans. The electrode array with 16 pads was positioned over the posterior aspect of the thigh, while the recording electrode array with 16 pads was positioned over the triceps surae muscle. The stimulator activated all the pads of the stimulation electrode array asynchronously, while the signals were recorded continuously at all the recording sites. The results are topography maps (spatial distribution of evoked potentials) and matrices (spatial visualization of nerve excitability). The software allows the automatic selection of the lowest stimulation intensity to achieve maximal H-reflex amplitude and selection of the recording/stimulation pads according to predefined criteria. The analysis of results shows that the method provides rich information compared with the conventional recording of the H-reflex with regard the spatial distribution.

Alteration of Surface EMG amplitude levels of five major trunk muscles by defined electrode location displacement

Exact electrode positioning is vital for obtaining reliable results in Surface EMG. This study aimed at systematically assessing the influence of defined electrode shifts on measured Surface EMG amplitudes of trunk muscles in a group of 15 middle aged healthy male subjects. The following leftsided muscles were investigated: rectus abdominis muscle, internal and external oblique abdominal muscles, lumbar multifidus muscle, and longissimus muscle. In addition to the recommended electrode positions, extra electrodes were placed parallel to these and along muscle fiber direction. Measurements were performed under isometric conditions in upright body position. Gradually changing, but defined loads were applied considering subject's upper body weight. For the abdominal muscles amplitude differences varied considerably depending on load level, magnitude, and direction. For both back muscles amplitudes dropped consistently but rather little for parallel electrode displacements. However, for the longissimus muscle a caudal electrode shift resulted in an amplitude increase of similar extent and independent from load level. Influence of electrode position variations can be proven for all trunk muscles but are more evident in abdominal than back muscles. Those muscle-specific effects confirm the necessity for an exact definition of electrode positioning to allow comparisons between individual subjects, groups of subjects, and studies.

The extraction of neural strategies from the surface EMG: an update

A surface EMG signal represents the linear transformation of motor neuron discharge times by the compound action potentials of the innervated muscle fibers and is often used as a source of information about neural activation of muscle. However, retrieving the embedded neural code from a surface EMG signal is extremely challenging. Most studies use indirect approaches in which selected features of the signal are interpreted as indicating certain characteristics of the neural code. These indirect associations are constrained by limitations that have been detailed previously (Farina D, Merletti R, Enoka RM. J Appl Physiol 96: 1486-1495, 2004) and are generally difficult to overcome. In an update on these issues, the current review extends the discussion to EMG-based coherence methods for assessing neural connectivity. We focus first on EMG amplitude cancellation, which intrinsically limits the association between EMG amplitude and the intensity of the neural activation and then discuss the limitations of coherence methods (EEG-EMG, EMG-EMG) as a way to assess the strength of the transmission of synaptic inputs into trains of motor unit action potentials. The debated influence of rectification on EMG spectral analysis and coherence measures is also discussed. Alternatively, there have been a number of attempts to identify the neural information directly by decomposing surface EMG signals into the discharge times of motor unit action potentials. The application of this approach is extremely powerful, but validation remains a central issue.

Greater glucose uptake heterogeneity in knee muscles of old compared to young men during isometric contractions detected by [(18)F]-FDG PET/CT

We used positron emission tomography/computed tomography (PET/CT) and [¹⁸F]-FDG to test the hypothesis that glucose uptake (GU) heterogeneity in skeletal muscles as a measure of heterogeneity in muscle activity is greater in old than young men when they perform isometric contractions. Six young (26 ± 6 years) and six old (77 ± 6 years) men performed two types of submaximal isometric contractions that required either force or position control. [¹⁸F]-FDG was injected during the task and PET/CT scans were performed immediately after the task. Within-muscle heterogeneity of knee muscles was determined by calculating the coefficient of variation (CV) of GU in PET image voxels within the muscles of interest. The average GU heterogeneity (mean ± SD) for knee extensors and flexors was greater for the old (35.3 ± 3.3%) than the young (28.6 ± 2.4%) (P = 0.006). Muscle volume of the knee extensors were greater for the young compared to the old men (1016 ± 163 vs. 598 ± 70 cm³, P = 0.004). In a multiple regression model, knee extensor muscle volume was a predictor (partial r = −0.87; P = 0.001) of GU heterogeneity for old men (R² = 0.78; P < 0.001), and MVC force predicted GU heterogeneity for young men (partial r = −0.95, P < 0.001). The findings demonstrate that GU is more spatially variable for old than young men and especially so for old men who exhibit greater muscle atrophy.

Spatial localization of electromyographic amplitude distributions associated to the activation of dorsal forearm muscles

In this study we investigated whether the spatial distribution of surface electromyographic (EMG) amplitude can be used to describe the activation of muscle portions with different biomechanical actions. Ten healthy subjects performed isometric contractions aimed to selectively activate a number of forearm muscles or muscle subportions. Monopolar electromyographic signals were collected with an electrode grid of 128 electrodes placed on the proximal, dorsal portion of the forearm. The monopolar EMG amplitude [root mean square (RMS) value] distribution was calculated for each contraction, and high-amplitude channels were identified through an automatic procedure; the position of the EMG source was estimated with the barycenter of these channels. Each of the contractions tested was associated to a specific EMG amplitude distribution, whose location in space was consistent with the expected anatomical position of the main agonist muscle (or subportion). The position of each source was significantly different from the others in at least one direction (ANOVA; transversally to the forearm: P < 0.01, F = 125.92; longitudinally: P < 0.01, F = 35.83). With such an approach, we could distinguish the spatial position of EMG distributions related to the activation of contiguous muscles [e.g., extensor carpi ulnaris (ECU) and extensor digitorum communis (EDC)], different heads of the same muscle (i.e., extensor carpi radialis (ECR) brevis and longus) and different functional compartments (i.e., EDC, middle, and ring fingers). These findings are discussed in terms of how forces along a given direction can be produced by recruiting population of motor units clustered not only in specific muscles, but also in muscle sub-portions. In addition, this study supports the use of high-density EMG systems to characterize the activation of muscle subportions with different biomechanical actions.

Surface EMG during the Push-up plus Exercise on a Stable Support or Swiss Ball: Scapular Stabilizer Muscle Exercise

[Purpose] Scapular stabilizer strengthening exercise is crucial for shoulder rehabilitation. The purpose of this study was to compare two types of push-up plus exercises, on a stable and unstable bases of support, using surface electromyography (EMG), to suggest an effective shoulder rehabilitation program. [Subjects and Methods] Ten healthy men volunteered for this study. All volunteers performed two sets of push-up plus exercise (standard push up and knee push up) on stable and unstable bases of support. The muscle activities of five important scapular stabilizer muscles (upper trapezius, middle trapezius, lower trapezius, serratus anterior, latissimus dorsi) were recorded during the exercise. [Results] The upper trapezius showed greater mean electric activation amplitude in the scapular retraction posture than in the scapular protraction posture, and the serratus anterior showed greater mean electric activation amplitude in the scapular protraction posture than in the scapular retraction posture. The root-mean-square normalized EMG values of the muscles were greater during the exercise performed on the unstable support than those on the stable support. [Conclusion] The standard push-up plus exercise on an unstable base of support helps to increase muscle activity, especially those of the upper/middle trapezius and serratus anterior.

Does interhemispheric communication relate to the bilateral function of muscles? A study of scapulothoracic muscles using transcranial magnetic stimulation

Interhemispheric connections have been demonstrated between the motor cortex controlling muscle pairs. However, these investigations have tended to concentrate upon hand muscles. We have extended these investigations to proximal muscles that control the scapula upon the trunk and help to move and stabilise the shoulder. Using a paired pulse transcranial magnetic stimulation protocol, the interhemispheric interactions between different shoulder girdle muscle pairs, serratus anterior, upper trapezius and lower trapezius were investigated. Test motor evoked potentials were conditioned using conditioning pulse intensities of 80% and 120% of active motor threshold at three different condition-test intervals, during three different tasks. Interhemispheric inhibition was observed in upper trapezius using a conditioning intensity of 120% and condition-test interval of 8 ms (17 ± 18%, p < 0.007). A trend towards inhibition was observed in lower trapezius and serratus anterior using a conditioning intensity of 120% and a condition-test interval of 8 ms (13 ± 22%; p < 0.07 and 10 ± 19% respectively; p < 0.07). No interhemispheric facilitation was evoked. The study demonstrates that a low level of interhemispheric inhibition rather than interhemispheric facilitation could be evoked between these muscle pairs.

Uneven spatial distribution of surface EMG: what does it mean?

The aim of this work is to show how changes in surface electromyographic activity (sEMG) during a repetitive, non-constant force contraction can be detected and interpreted on the basis of the amplitude distribution provided by high-density sEMG techniques. Twelve healthy male subjects performed isometric shoulder elevations, repeating five times a force ramp profile up to 25 % of the maximal voluntary contraction (MVC). A 64-electrode matrix was used to detect sEMG from the trapezius muscle. The sEMG amplitude distribution was obtained for the force levels in the range 5-25 % MVC with steps of 5 % MVC. The effect of force level, subject, electrode position and ramp repetition on the sEMG amplitude distribution was tested. The sEMG amplitude was significantly smaller in the columns of the electrode grid over the tendons (repeated measures ANOVA, p < 0.01). The barycentre of the distribution of sEMG amplitude was subject-specific (Kruskal-Wallis test, p < 0.01), and shifted caudally with the increase of force levels and cranially with the repetition of the motor task (both p < 0.01, repeated measures ANOVA). The results are discussed in terms of motor unit recruitment in different muscle sub-portions. It is concluded that the sEMG amplitude distribution obtained by multichannel techniques provides useful information in the study of muscle activity, and that changes in the spatial distribution of the recruited motor units during a force varying isometric contraction might partially explain the variability observed in the activation pattern of the upper trapezius muscle.

Comparison of an intermittent and continuous forearm muscles fatigue protocol with motorcycle riders and control group

Motorcycle races' long duration justify the study of forearm muscles fatigue, especially knowing the frequently associated forearm discomfort pathology. Moreover, while continuous fatigue protocols yield unequivocal results, EMG outcomes from an intermittent protocol are quite controversial. This study examined the forearm muscle fatigue patterns produced during these two protocols, comparing riders with a control group, and relating maximal voluntary contraction with EMG parameters (amplitude - NRMS and median frequency - NMF) of both protocols to the forearm discomfort among motorcycle riders. Twenty riders and 39 controls performed in separate days both protocols simulating the braking gesture and posture of a rider. EMG of flexor digitorum superficialis (FS) and carpi radialis (CR) were monitored. CR revealed more differences among protocols and groups compared to FS. The greater CR activation in riders could be interpreted as a neuromotor strategy to improve braking precision. When FS fatigue increased, the control group progressively shift toward a bigger CR activation, adopting an intermuscular activation pattern closer to riders. Despite the absence of NMF decrement throughout the intermittent protocol, which suggest that we should have shorten the recovery times from the actual 1 min, the superior number of rounds performed by the riders proved that this protocol discriminates better riders against controls and is more related to forearm discomfort.

Differential patterns of muscle modification in women with episodic and chronic tension-type headache revealed using surface electromyographic analysis

Tension-type headache (TTH) is a prototypical disorder in which muscular factors play a key role in the pathogenesis. This study was designed to understand muscular dysfunction in patients with episodic (ETTH) and chronic TTH (CTTH) using surface electromyography analysis (SEMG). Women with frequent ETTH (n = 14), CTTH (n = 14) and age-matched controls (n = 13) were recruited. SEMG data were recorded from the masseter, sternocleidomastoid, and upper trapezius muscles during maximum voluntary contraction and sustained voluntary isometric clenching, the neck flexion endurance test and shoulder elevation for 30s. The root mean square (RMS) and median frequency (MDF) of the SEMG signal were measured throughout the test. The fatigue index, which is the MDF slope during sustained muscle contraction, decreased significantly faster in the ETTH and CTTH groups compared with that in the control (p < 0.05). The mean absolute RMS and relative percentage values at the initial and final period during sustained isometric contraction decreased significantly in the CTTH group (p < 0.05). Furthermore, headache clinical parameters (frequency and duration) were negatively correlated with the amplitude values (p < 0.05). A different muscle firing pattern or some muscle modifications in patients with CTTH may reflect reorganization of the motor-control strategy.

EMG-based detection of muscle fatigue during low-level isometric contraction: effects of electrode configuration and blood flow restriction

This study assessed muscle fatigue during low-level contraction, which is sometimes difficult to observe. Surface EMG signals were recorded with monopolar and bipolar configurations during low-level isometric muscle contraction under a blood flow restriction condition (BFR condition) and a non-blood flow restriction condition (CON condition). As indices of fatigue, center frequency (CF) of the power spectral density (PSD) of the surface EMG and rating of perceived exertion (RPE) with the Borg CR-10 Scale were used. Results suggested that the monopolar configuration, which has a wider detection area relative than the bipolar one, was well suited for obtaining wave slowing accompanied by muscle fatigue. In addition, the monopolar configuration could detect differences in muscle condition in the BFR and CON conditions.

On functional motor adaptations: from the quantification of motor strategies to the prevention of musculoskeletal disorders in the neck-shoulder region

BACKGROUND

Occupations characterized by a static low load and by repetitive actions show a high prevalence of work-related musculoskeletal disorders (WMSD) in the neck-shoulder region. Moreover, muscle fatigue and discomfort are reported to play a relevant initiating role in WMSD.

AIMS

To investigate relationships between altered sensory information, i.e. localized muscle fatigue, discomfort and pain and their associations to changes in motor control patterns.

MATERIALS & METHODS

In total 101 subjects participated. Questionnaires, subjective assessments of perceived exertion and pain intensity as well as surface electromyography (SEMG), mechanomyography (MMG), force and kinematics recordings were performed.

RESULTS

Multi-channel SEMG and MMG revealed that the degree of heterogeneity of the trapezius muscle activation increased with fatigue. Further, the spatial organization of trapezius muscle activity changed in a dynamic manner during sustained contraction with acute experimental pain. A graduation of the motor changes in relation to the pain stage (acute, subchronic and chronic) and work experience were also found. The duration of the work task was shorter in presence of acute and chronic pain. Acute pain resulted in decreased activity of the painful muscle while in subchronic and chronic pain, a more static muscle activation was found. Posture and movement changed in the presence of neck-shoulder pain. Larger and smaller sizes of arm and trunk movement variability were respectively found in acute pain and subchronic/chronic pain. The size and structure of kinematics variability decreased also in the region of discomfort. Motor variability was higher in workers with high experience. Moreover, the pattern of activation of the upper trapezius muscle changed when receiving SEMG/MMG biofeedback during computer work.

DISCUSSION

SEMG and MMG changes underlie functional mechanisms for the maintenance of force during fatiguing contraction and acute pain that may lead to the widespread pain seen in WMSD. A lack of harmonious muscle recruitment/derecruitment may play a role in pain transition. Motor behavior changed in shoulder pain conditions underlining that motor variability may play a role in the WMSD development as corroborated by the changes in kinematics variability seen with discomfort. This prognostic hypothesis was further, supported by the increased motor variability among workers with high experience.

CONCLUSION

Quantitative assessments of the functional motor adaptations can be a way to benchmark the pain status and help to indentify signs indicating WMSD development. Motor variability is an important characteristic in ergonomic situations. Future studies will investigate the potential benefit of inducing motor variability in occupational settings.

Endurance time is joint-specific: a modelling and meta-analysis investigation

Static task intensity-endurance time (ET) relationships (e.g. Rohmert's curve) were first reported decades ago. However, a comprehensive meta-analysis to compare experimentally-observed ETs across bodily regions has not been reported. We performed a systematic literature review of ETs for static contractions, developed joint-specific power and exponential models of the intensity-ET relationships, and compared these models between each joint (ankle, trunk, hand/grip, elbow, knee, and shoulder) and the pooled data (generalised curve). 194 publications were found, representing a total of 369 data points. The power model provided the best fit to the experimental data. Significant intensity-dependent ET differences were predicted between each pair of joints. Overall, the ankle was most fatigue-resistant, followed by the trunk, hand/grip, elbow, knee and finally the shoulder was most fatigable. We conclude ET varies systematically between joints, in some cases with large effect sizes. Thus, a single generalised ET model does not adequately represent fatigue across joints. STATEMENT OF RELEVANCE: Rohmert curves have been used in ergonomic analyses of fatigue, as there are limited tools available to accurately predict force decrements. This study provides updated endurance time-intensity curves using a large meta-analysis of fatigue data. Specific models derived for five distinct joint regions should further increase prediction accuracy.

Analysis of motor units with high-density surface electromyography

Although the behaviour of individual motor units is classically studied with intramuscular EMG, recently developed techniques allow its analysis also from EMG recorded in multiple locations over the skin surface (high-density surface EMG). The analysis of motor units from the surface EMG is useful when the insertion of needles is not desirable or not possible. Moreover, surface EMG allows the measure of motor unit properties which are difficult to assess with invasive technology (e.g., muscle fiber conduction velocity or location of innervation zones) and may increase the number of detectable motor units with respect to selective intramuscular recordings. Although some limitations remain, both the discharge pattern and muscle fiber properties of individual motor units can currently be analyzed non-invasively. This review presents the conditions and methodologies which allow the investigation of motor units with surface EMG.

Effect of electrode array position and subcutaneous tissue thickness on conduction velocity estimation in upper trapezius muscle

Muscle fiber conduction velocity (CV) was estimated in 20 healthy subjects with a multichannel technique using a linear electrode array placed on the upper trapezius (UT) muscle during isometric voluntary contractions at 20% of the maximal voluntary effort. The array had 16 contacts spaced by 5mm and was centered in the middle of the line connecting C7 and the acromion. The effect of array displacement was investigated by shifting it either cranially or caudally by 1, 2 and 3cm. The effect of array misalignment was investigated by rotating it either clockwise or counterclockwise, by 10 degrees , 20 degrees and 30 degrees . The thickness of subcutaneous fat was measured by ultrasonic echography (range was 3-18mm). The average location of the innervation zone of the UT is in the middle of the line connecting C7 to the acromion, however, individual differences of up to 2cm were observed. The lateral side, near the acromion, should be preferred for electrode placement because of the thinner fat layer. The double differential (DD) montage should be preferred to the single differential (SD) electrode configuration. In this case, displacements of +/-1cm or misalignments of +/-10 degrees do not imply statistically significant bias in the estimate of CV. The same applies to amplitude and frequency variables estimated from the SD signals. CV estimates based on DD signals are lower because they are less affected by the thickness of the subcutaneous fat than estimates based on SD signals.