Complexity Analysis of Surface Electromyography for Assessing the Myoelectric Manifestation of Muscle Fatigue: A Review

The surface electromyography (sEMG) records the electrical activity of muscle fibers during contraction: one of its uses is to assess changes taking place within muscles in the course of a fatiguing contraction to provide insights into our understanding of muscle fatigue in training protocols and rehabilitation medicine. Until recently, these myoelectric manifestations of muscle fatigue (MMF) have been assessed essentially by linear sEMG analyses. However, sEMG shows a complex behavior, due to many concurrent factors. Therefore, in the last years, complexity-based methods have been tentatively applied to the sEMG signal to better individuate the MMF onset during sustained contractions. In this review, after describing concisely the traditional linear methods employed to assess MMF we present the complexity methods used for sEMG analysis based on an extensive literature search. We show that some of these indices, like those derived from recurrence plots, from entropy or fractal analysis, can detect MMF efficiently. However, we also show that more work remains to be done to compare the complexity indices in terms of reliability and sensibility; to optimize the choice of embedding dimension, time delay and threshold distance in reconstructing the phase space; and to elucidate the relationship between complexity estimators and the physiologic phenomena underlying the onset of MMF in exercising muscles

Complexity analysis of surface electromyography for assessing the myoelectric manifestation of muscle fatigue: A review

The surface electromyography (sEMG) records the electrical activity of muscle fibers during contraction: one of its uses is to assess changes taking place within muscles in the course of a fatiguing contraction to provide insights into our understanding of muscle fatigue in training protocols and rehabilitation medicine. Until recently, these myoelectric manifestations of muscle fatigue (MMF) have been assessed essentially by linear sEMG analyses. However, sEMG shows a complex behavior, due to many concurrent factors. Therefore, in the last years, complexity-based methods have been tentatively applied to the sEMG signal to better individuate the MMF onset during sustained contractions. In this review, after describing concisely the traditional linear methods employed to assess MMF we present the complexity methods used for sEMG analysis based on an extensive literature search. We show that some of these indices, like those derived from recurrence plots, from entropy or fractal analysis, can detect MMF efficiently. However, we also show that more work remains to be done to compare the complexity indices in terms of reliability and sensibility; to optimize the choice of embedding dimension, time delay and threshold distance in reconstructing the phase space; and to elucidate the relationship between complexity estimators and the physiologic phenomena underlying the onset of MMF in exercising muscles

CEPS: An Open Access MATLAB Graphical User Interface (GUI) for the Analysis of Complexity and Entropy in Physiological Signals

Background: We developed CEPS as an open access MATLAB® GUI (graphical user interface) for the analysis of Complexity and Entropy in Physiological Signals (CEPS), and demonstrate its use with an example data set that shows the effects of paced breathing (PB) on variability of heart, pulse and respiration rates. CEPS is also sufficiently adaptable to be used for other time series physiological data such as EEG (electroencephalography), postural sway or temperature measurements. Methods: Data were collected from a convenience sample of nine healthy adults in a pilot for a larger study investigating the effects on vagal tone of breathing paced at various different rates, part of a development programme for a home training stress reduction system. Results: The current version of CEPS focuses on those complexity and entropy measures that appear most frequently in the literature, together with some recently introduced entropy measures which may have advantages over those that are more established. Ten methods of estimating data complexity are currently included, and some 28 entropy measures. The GUI also includes a section for data pre-processing and standard ancillary methods to enable parameter estimation of embedding dimension m and time delay τ (‘tau’) where required. The software is freely available under version 3 of the GNU Lesser General Public License (LGPLv3) for non-commercial users. CEPS can be downloaded at https://bitbucket.org/deepak_panday/ceps/src/pipeline_v2/. In our illustration on PB, most complexity and entropy measures decreased significantly in response to breathing at 7 breaths per minute, differentiating more clearly than conventional linear, time- and frequency-domain measures between breathing states. In contrast, Higuchi fractal dimension increased during paced breathing. Conclusions: We have developed CEPS software as a physiological data visualiser able to integrate state of the art techniques. The interface is designed for clinical research and has a structure designed for integrating new tools. The aim is to strengthen collaboration between clinicians and the biomedical community, as demonstrated here by using CEPS to analyse various physiological responses to paced breathing

Unsupervised Classification of Uterine Contractions Recorded Using Electrohysterography

Pregnancy still poses health risks that are not attended to by current clinical practice motorization procedures. Electrohysterography (EHG) record signals are analyzed in the course of this thesis as a contribution and effort to evaluate their suitability for pregnancy monitoring. The presented work is a contributes with an unsupervised classification solution for uterine contractile segments to FCT’s Uterine Explorer (UEX) project, which explores analysis procedures for EHG records. In a first part, applied processing procedures are presented and a brief exploration of the best practices for these. The procedures include those to elevate the representation of uterine events relevant characteristics, ease further computation requirements, extraction of contractile segments and spectral estimation. More detail is put into the study of which characteristics should be chosen to represent uterine events in the classification process and feature selection methods. To such end, it is presented the application of a principal component analysis (PCA) to three sets: interpolated contractile events, contractions power spectral densities, and to a number of computed features that attempt evidencing time, spectral and non-linear characteristics usually used in EHG related studies. Subsequently, a wrapper model approach is presented as a mean to optimize the feature set through cyclically attempting the removal and re-addition of features based on clustering results. This approach takes advantage of the fact that one class is known beforehand to use its classification accuracy as the criteria that defines whether the modification made to the feature set was ominous. Furthermore, this work also includes the implementation of a visualization tool that allows inspecting the effect of each processing procedure, the uterine events detected by different methods and clusters they were associated to by the final iteration of the wrapper model

Uterine contractions clustering based on surface electromyography: an input for pregnancy monitoring

Tese de mestrado em Bioestatística, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, em 2018Inicialmente a investigação da contratilidade uterina recorria à utilização de dois métodos: o tocograma externo e o cateter de pressão intrauterino. Ambos os métodos apresentam limitações ao nível da avaliação do risco de parto prematuro e na monitorização da gravidez. O EHG (Electrohisterograma) é um método alternativo ao tocograma externo e ao cateter de pressão intrauterino. Este método pode ser aplicado de forma invasiva no músculo uterino, ou de forma não invasiva através de elétrodos colocados no abdómen. O EHG tem sido considerado uma ferramenta adequada para a monitorização da gravidez e do parto. O índice de massa corporal tem um impacto quase impercetível no EHG, sendo esta uma das principais características deste método. O EHG pode também ser utilizado para identificar as mulheres que vão entrar em trabalho de parto e ainda auxiliar na tomada de decisão médica quanto à utilização da terapia tocolítica (antagonista da oxitocina), evitando deste modo a ingestão de medicação desnecessária e os consequentes efeitos secundários. Na literatura existem apenas cinco casos publicados em que foi realizada uma separação dos principais eventos do sinal EHG: contrações, movimentos fetais, ondas Alvarez e ondas LDBF (Longue Durée Basse Fréquence). Em três das publicações a separação dos eventos foi feita manualmente e nos restantes casos algoritmos, como redes neuronais, foram aplicados ao EHG. As ondas Alvarez e as Braxton-Hicks são as mais reconhecidas. As ondas Alvarez foram descritas pela primeira vez nos anos cinquenta e as Braxton-Hicks foram descritas pela primeira vez em 1872 sendo detetadas através de palpação. As ondas Alvarez são ocasionalmente sentidas pela mulher. Estas ondas estão localizadas numa pequena área do tecido uterino sem propagação e podem levar a contrações com maior intensidade e, consequentemente, ao parto pré-termo. As Braxton-Hicks são contrações ineficientes registadas a partir da 20ª semana de gravidez que se tornam mais frequentes e intensas com o decorrer da gravidez. Estas contrações são menos localizadas que as ondas Alvarez e, durante o parto, propagam-se por todo o tecido uterino num curto período de tempo. As Braxton-Hicks estão associadas a uma diminuição do ritmo cardíaco fetal. As ondas LDBF são contrações de longa duração associadas a hipertonia uterina, quando há contração do tecido uterino sem retorno ao relaxamento muscular, o que representa um risco na gravidez. Neste trabalho foram utilizadas duas bases de dados. Na base de dados da Islândia existem 122 registos de 45 mulheres, dos quais apenas 4 correspondem a partos pré-termo. Na base de dados TPEHG (Term-Preterm EHG) existem 300 registos, dos quais 38 correspondem a partos pré-termo. Neste trabalho foram escolhidos canais bipolares, visto que estes reduzem o ruído idêntico, como o ECG (Eletrocardiograma) materno ou movimentos respiratórios. Para ambas as bases de dados os sinais originais de EHG foram processados e filtrados. Na estimação espetral foram considerados dois métodos: paramétricos e não paramétricos. O método Welch foi escolhido pois representa um bom compromisso entre ambos. Este método foi utilizado para calcular o espectro de cada evento detetado no sinal EHG. Para detetar os eventos no sinal EHG foram considerados cinco métodos baseados na energia ou amplitude. O método Wavelet foi o escolhido pois após uma inspeção visual, este era o método que delineava melhor as contrações. Na base de dados da Islândia foram identificadas 3136 contrações e na TPEHG foram encontradas 4622 contrações. O objetivo principal desta tese é obter clusters de contrações detetadas no sinal EHG. No entanto, as contrações são séries temporais não estacionárias, e a sua classificação visual é inviável a longo termo e também difícil de aplicar na prática clínica. Existem vários parâmetros que podem ser extraídos do sinal EHG, mas o espectro das contrações foi o método escolhido visto que este representa o sinal EHG e tem sempre a mesma dimensão, independentemente da duração da contração. As distâncias espetrais têm sido utilizadas com sucesso no reconhecimento áudio. Neste trabalho foi realizada uma aplicação desse método ao processamento do EHG, no qual foram realizados os ajustes necessários. Para comparar os espectros foram estudadas 8 distâncias diferentes: Itakura-Saito, COSH, Itakura, Itakura simétrica, Kullback-Leibler, Jeffrey, Rényi e Jensen-Rényi. Apenas as distâncias simétricas foram selecionadas para um estudo mais detalhado visto que estas são, segundo a literatura, as distâncias mais adequadas aquando do clustering. Após comparação das distâncias simétricas, a divergência de Jeffrey foi a selecionada para a comparação dos espectros. Nesta tese foram avaliados três métodos diferentes de clustering: o linkage, o K-means e o K-medoids. O linkage é um método hierárquico. Os clusters que resultam do agrupamento hierárquico estão organizados numa estrutura chamada dendrograma. No agrupamento hierárquico, não é necessário predeterminar o número de clusters, o que torna este um método ideal na exploração dos dados. O K-means e o K-medoids são métodos de partição, nos quais os dados são separados em k clusters decididos previamente. Os clusters são definidos de forma a otimizar a função da distância. No algoritmo K-means, os clusters baseiam-se na proximidade entre si de acordo com uma distância predeterminada. A diferença entre o K-medoids e o K-means é que o K-medoids escolhe pontos de dados como centros, chamados de medoides, enquanto K-means usa centróides. Após uma comparação dos diferentes métodos de clustering foi escolhido neste trabalho foi o average linkage, visto que este apresentava melhores resultados quer na separação dos espectros quer na silhueta. É então apresentado um método inovador no qual se utiliza todo o espectro das contrações detetadas automaticamente no EHG para o clustering não supervisionado. Esta técnica é uma contribuição para a classificação automática das diferentes contrações, especialmente aquelas mais reconhecidas na literatura: Alvarez e Braxton-Hicks. Era expectável encontrar um cluster isolado com as ondas LDBF, visto que estas representam um risco para o feto. O principal objetivo era juntar num cluster os espectros semelhantes das contrações, e relacioná-lo com o respetivo tipo de contração. Essa tarefa foi concluída através da identificação positiva de Alvarez e Braxton-Hicks. O clustering forneceu ainda algumas pistas sobre ondas Alvarez que não foram encontradas com o algoritmo de deteção de contrações, situação para a qual um método alternativo é apresentado. É sugerido que as ondas Alvarez sejam detetadas com métodos baseados na frequência, como, por exemplo, a frequência instantânea, no entanto este método não foi desenvolvido neste trabalho. Em relação às ondas LDBF, estas foram encontradas no cluster das Braxton-Hicks. É sugerido que a deteção das ondas LDBF seja baseada na sua caraterística mais distinta: a longa duração. Verificou-se que os casos pré-termo e os registos pré-parto não ficaram isolados num cluster, não se tendo encontrado uma relação entre a idade gestacional e o tipo de contração. Conclui-se que as contrações mais curtas apresentam maior amplitude do que as contrações com maior duração. Baseado em estudos anteriores sobre a eletrofisiologia do útero, supõem-se que o início do trabalho de parto pré-termo e termo esteja associado a sequências específicas de diferentes tipos de contrações, nas quais as ondas Alvares desempenham um papel importante. As contrações identificadas como Alvarez e Braxton-Hicks não são usadas como tal na prática clínica apesar de a maioria das contrações detetadas pelo tocograma serem Braxton-Hicks. O interesse pelas ondas Alvarez diminuiu rapidamente visto que estas ondas são praticamente indetetáveis pelo método de referência de deteção de contrações: o tocograma. As capacidades e a resolução do EHG levaram à renovação do estudo das contrações mais subtis, incluindo as Alvarez. Este trabalho é uma contribuição para a investigação nesta área.An innovative technique is introduced wherein where an unsupervised clustering method using as feature the whole spectrum of automatically detected contractions on the EHG (Electrohysterogram) is presented as a contribution to the automatic classification of the different uterine contractions, at least those that have been most recognized in the literature: Alvarez and Braxton-Hicks. It was expected to also be able to cluster the LDBF (Longue Durée Basse Fréquence) components, as these pose a fetal risk. The main task was to have the spectral contractions descriptions clustered and linked to the respective contraction type. That task was completed with positive identification of the Alvarez and Braxton-Hicks. The clustering process also provided clues regarding the missed Alvarez waves in the contraction detection algorithm, for which an alternative technique is suggested but not developed in this work. Regarding the LDBF they were found in the Braxton-Hicks cluster. It is suggested the LDBF´s to be detected based in their most prominent feature: the long duration. It is presented the rationale behind the selection of a cost function to be used in the spectral distance’s algorithm. Spectral distances have been successfully used in audio recognition and this works represents an application to the EHG processing, for which the necessary adjustments have to be implemented. It was found that no single cluster pointed to the preterm cases, or indeed to the pre-labor subject recordings. It is hypothesized, based on previous studies in uterine electrophysiology, that the initiation of pre-term or term labor should be associated with triggering contraction sequences of different types, where the Alvarez waves play a major role. Alvarez and Braxton-Hicks, labeled as such, are not typically used in the clinical environment despite most of the Tocogram detected contractions being the latter. Alvarez waves are not usually detectable by the Tocogram. Alvarez were firstly detected invasively in the early fifties, and Braxton-Hicks in 1872 using routine palpation techniques. The interest in Alvarez components declined rapidly since being practically undetectable by the de facto reference in the contraction detection: the Tocogram. The EHG capabilities and resolution made it possible to revive the research on the most subtle uterine contractions, Alvarez included and this work is a contribution in this research area

Engineering Dynamics and Life Sciences

From Preface: This is the fourteenth time when the conference “Dynamical Systems: Theory and Applications” gathers a numerous group of outstanding scientists and engineers, who deal with widely understood problems of theoretical and applied dynamics. Organization of the conference would not have been possible without a great effort of the staff of the Department of Automation, Biomechanics and Mechatronics. The patronage over the conference has been taken by the Committee of Mechanics of the Polish Academy of Sciences and Ministry of Science and Higher Education of Poland. It is a great pleasure that our invitation has been accepted by recording in the history of our conference number of people, including good colleagues and friends as well as a large group of researchers and scientists, who decided to participate in the conference for the first time. With proud and satisfaction we welcomed over 180 persons from 31 countries all over the world. They decided to share the results of their research and many years experiences in a discipline of dynamical systems by submitting many very interesting papers. This year, the DSTA Conference Proceedings were split into three volumes entitled “Dynamical Systems” with respective subtitles: Vibration, Control and Stability of Dynamical Systems; Mathematical and Numerical Aspects of Dynamical System Analysis and Engineering Dynamics and Life Sciences. Additionally, there will be also published two volumes of Springer Proceedings in Mathematics and Statistics entitled “Dynamical Systems in Theoretical Perspective” and “Dynamical Systems in Applications”

Vibration transmission through the human spine during physical activity

Osteoporosis causes bone to become fragile. Pharmacological treatments of osteoporosis are burdened with adverse effects and increase bone mineral density (BMD) only between 1% and 15% depending on the drug and time used. Thus non pharmacological treatments are needed to complement pharmacological ones. Physical activity is a non pharmacological treatment of osteoporosis and is essential for maintaining bone health at any age. However, physical activities have been identified to produce a modest improvement of spinal strength or just preserve it. In addition, it is not known how much exercise is optimal and safe for people with spinal osteoporosis. Most research employs conflicting definitions of physical activity and measure the effect of exercise on BMD alone instead of combining it with measurements of three dimensional bone strength. There is the need to offer a technique to measure the effect of physical activity on the overall strength of the spine, not only on its bone mineral content. Vibration transmissibility is a measurement of the mechanical response of a system to vibration expressed as stiffness or damping, thus offering a variable that represents structural strength. It can be employed to measure the mechanical response of the human spine during physical activity by attaching inertial sensors over the spine. However, it has not been employed to characterize the way vibration is transmitted through the osteoporotic spine during physical activity. Understanding the effects of osteoporosis and ageing on vibration transmission is important since such effects are related to the stiffness of the spine and thus very likely to the incidence of vertebral fractures. It is also often recommended that fast walking is beneficial to the bone, yet it is not known if fast walking affects the mechanical response of the spine of people with osteoporosis. The aims of this study were (1) to evaluate the feasibility of employing inertial sensors and a skin correction method to measure vibration transmission through the spine during physical activity (2) to characterize the transmission of vibration in the lumbar and thoracic spines of people with and without osteoporosis during physical activities, (3) to characterize the effect of osteoporosis on vibration transmissibility at levels of the thoracic spine which are known to fracture and (4) to investigate the effects of fast walking on vibration transmissibility. 100 young and healthy and older volunteers with and without osteoporosis were recruited. Participants were asked to perform straight walking, stair negotiation and turning while having inertial sensors attached to the skin over the spinous process of the first sacral (S1), twelfth (T12), eighth (T8) and first thoracic vertebrae (T1). Vibration transmissibility was calculated as the square root of the acceleration of the output (T12 for the lumbar and T1 for the thoracic spine) over the input (S1 for the lumbar and T12 for the thoracic spine) in the frequency spectrum. Vibration transmissibility was corrected for the movement of the skin-sensor interface and for the inclination of the sensor over the spine of every subject. All physical activities were performed at self selected normal and fast walking speeds. Lumbar and thoracic curvatures were determined with an electromagnetic device and BMD was measured through quantitative ultrasound. Skin measurement of transmission of vertical vibration is feasible with the inertial sensors and correction method presented. Vibration transmissibility through the human spine is significantly different between dissimilar physical activities and frequency dependent. Ageing significantly alters the vibration transmissibility of the spine. Osteoporosis has a minimal effect on vibration transmissibility of the spine. The effect of ageing and osteoporosis are frequency dependent. Older lumbar spines may receive greater stimulation than young and healthy ones, whereas older thoracic spines may receive lower stimulation during fast walking. There are significant differences in vibration transmissibility between lumbar and thoracic spines. A percentage of vibration transmission of the lumbar and thoracic spines is determined by their curvatures. This thesis has provided a technique that future research can employ to correlate vibration transmissibility with mechanotransduction signals in bone as well as volumetric bone health measurements and the risk of vertebral fractures. Until then it will be possible to prescribe physical activity taking into account individual capabilities, bone strength and differences in mechanical response between lumbar and thoracic sections