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Classify Arm Motions Using EMG Signals and Deep Learning

This example shows how to classify forearm motions based on electromyographic (EMG) signals. An EMG signal measures the electrical activity of a muscle when it contracts.

Thirty subjects each participated in four data collection sessions, during which they performed six individual trials of different forearm motions while EMG signals were recorded from eight muscles. The data set consists of 1440 MAT-files: 720 of these files contain signal data and the other 720 files contain corresponding label data. The label data consists of a motion variable, motion, and an index variable, data_indx. The data set is organized into subject folders that contain a subfolder for each session. Each session subfolder contains six signal data files and six label data files, corresponding to each trial.

The motion variable is a numeric array that represents seven different motions:

  1. Hand Open

  2. Hand Close

  3. Wrist Flexion

  4. Wrist Extension

  5. Supination

  6. Pronation

  7. Rest

Each motion was held for three seconds and repeated four times in random order. The first and last element in motion are equal to –1 and correspond to an extended rest period that was performed at the start and end of each trial. The data_indx variable contains the start indices of each motion.

You can download the files at this location: https://ssd.mathworks.com/supportfiles/SPT/data/MyoelectricData.zip.

Create Datastores to Read Signal and Label Data

To access the files, create a signal datastore that points to the location where the files are downloaded. The files containing signal data have names that end with "d" and the files containing label data have names that end with "i". The sample rate is 3000 Hz. Create a subset of the datastore containing only signal data.

fs = 3000;

localfile = matlab.internal.examples.downloadSupportFile("SPT","data/MyoelectricData.zip");
datasetFolder = fullfile(fileparts(localfile),"MyoelectricData");
if ~exist(datasetFolder,"dir")
unzip(localfile,datasetFolder)
end

sds1 = signalDatastore(datasetFolder,IncludeSubFolders=true,SampleRate=fs);
p = endsWith(sds1.Files,"d.mat");
sdssig = subset(sds1,p);

Create a second datastore that points to the same file location and specify the names of the two variables in the label files. Create a subset of this datastore containing only label data.

sds2 = signalDatastore(datasetFolder,SignalVariableNames=["motion";"data_indx"],IncludeSubfolders=true);
p = endsWith(sds2.Files,"i.mat");
sdslbl = subset(sds2,p);

Plot all eight channels of the first EMG signal to visualize the activation of each muscle during one trial.

signal = preview(sdssig);

for i = 1:8
    ax(i) = subplot(4,2,i);
    plot(signal(:,i))
    title("Channel"+i)
end

linkaxes(ax,"y")

Create ROI Table

Define region-of-interest (ROI) limits for each motion based on the indices in data_indx. Remove the first and last label values (equal to –1) and convert the remaining numeric labels into a categorical array. Create a table containing the ROI limits in the first column and the labels in the second column.

lbls = {};

i = 1;
while hasdata(sdslbl)

    label = read(sdslbl);
    
    idx_start = label{2}(2:end-1)';
    idx_end = [idx_start(2:end)-1;idx_start(end)+(3*fs)];
    
    val = categorical(label{1}(2:end-1)',[1 2 3 4 5 6 7], ...
          ["HandOpen" "HandClose" "WristFlexion" "WristExtension" "Supination" "Pronation" "Rest"]);
    ROI = [idx_start idx_end];

    % In some cases, the number of label values and ROIs are not equal.
    % To eliminate these inconsistencies, remove the extra label value or ROI limits. 
    if numel(val) < size(ROI,1)
        ROI(end,:) = [];
    elseif numel(val) > size(ROI,1)
        val(end) = [];
    end

    lbltable = table(ROI,val);
    lbls{i} = {lbltable};

    i = i+1;
end

Prepare Datastore

Create a new datastore containing the modified label data and display the ROI table from the first observation.

lblDS = signalDatastore(lbls);
lblstable = preview(lblDS);
lblstable{1}
ans=27×2 table
              ROI                    val      
    ________________________    ______________

          9509         19763    WristFlexion  
         19764         29343    HandOpen      
         29344         38862    Rest          
         38863         47592    WristExtension
         47593         59070    HandOpen      
         59071         68993    Pronation     
         68994         78781    Supination    
         78782         88403    HandClose     
         88404         98015    HandOpen      
         98016    1.0741e+05    HandClose     
    1.0741e+05    1.1778e+05    Supination    
    1.1778e+05    1.2803e+05    WristFlexion  
    1.2803e+05    1.3728e+05    Rest          
    1.3728e+05    1.4721e+05    WristExtension
    1.4722e+05    1.5714e+05    Rest          
    1.5714e+05    1.6736e+05    WristFlexion  
      ⋮

Combine the signal and label data into one datastore.

DS = combine(sdssig,lblDS);
combinedData = preview(DS)
combinedData=1×2 cell array
    {304640×8 double}    {27×2 table}

Create a signal mask and call plotsigroi to display the labeled motion regions for the first channel of the first signal. The start and end of the signal shown in black represent the extended rest periods that are removed in the next preprocessing step.

figure
msk = signalMask(combinedData{2});
plotsigroi(msk,combinedData{1}(:,1))

Preprocess Data

Transform the combined datastore using the preprocess function that performs these preprocessing tasks.

  • Remove extended rest periods from start and end of each signal.

  • Remove pronation and supination motions. Since EMG was not recorded from the main muscles that enable forearm pronation, and was recorded for only one of the muscles involved in supination, the function excludes these motions from the data.

  • Filter signal using bandpass filter with lower cutoff frequency of 10 Hz and higher cutoff frequency of 400 Hz.

  • Downsample signal and label data to 1000 Hz.

  • Create signal mask for regions of interest (motions) and labels, where each signal sample has a corresponding label to enable sequence-to-sequence classification.

  • Break the signals into shorter segments that are 12000 samples in length.

tDS = transform(DS,@preprocess);
transformedData = preview(tDS)
transformedData=5×2 cell array
    {8×12000 double}    {[WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion    WristFlexion   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WristFlexion    WristFlexion    …              ]}
    {8×12000 double}    {[WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    WristExtension    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WristExtension    WristExtension    WristExtension    …    ]}
    {8×12000 double}    {[HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    HandClose    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…           ]}
    {8×12000 double}    {[Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest   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Rest    …      ]}
    {8×12000 double}    {[Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest    Rest   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Divide Data into Training and Testing Sets

Use 80% of the data to train the network and 20% to test the network. Multiply the random indices by 24 (6 trials x 4 sessions = 24 files for each subject) to avoid including data from a single subject in both training and testing sets.

rng default
[trainIdx,~,testIdx] = dividerand(30,0.8,0,0.2);

trainIdx_all = {};
m = 1;

for k = trainIdx
    
    if k == 1
       start = k;
    else
       start = ((k-1)*24)+1;
    end
    l = start:k*24;
    trainIdx_all{m} = l;
    m = m+1;
end

trainIdx_all = cell2mat(trainIdx_all)';
trainDS = subset(tDS,trainIdx_all);

testIdx_all = {};
m = 1;

for k = testIdx
    if k == 1
       start = k;
    else
       start = ((k-1)*24)+1;
    end
    l = start:k*24;
    testIdx_all{m} = l;
    m = m+1;
end

testIdx_all = cell2mat(testIdx_all)';
testDS = subset(tDS,testIdx_all);

Train Network

Define a network that uses a long short-term memory (LSTM) layer with the "sequence" output mode and 80 hidden nodes. Specify a fullyConnectedLayer with an output size of 5 corresponding to one for each type of motion.

layers = [ ...
    sequenceInputLayer(8)
    lstmLayer(80,OutputMode="sequence")
    fullyConnectedLayer(5)
    softmaxLayer
    classificationLayer];

Specify options for network training. Use the Adam optimizer and a mini-batch size of 32. Set the initial learning rate to 0.001 and the maximum number of epochs to 70. Shuffle the data every epoch.

options = trainingOptions("adam", ...
    MaxEpochs=70, ...
    MiniBatchSize=32, ...
    Plots="training-progress",...
    InitialLearnRate=0.001,...
    Verbose=0,...
    Shuffle="every-epoch",...
    GradientThreshold=1e5,...
    DispatchInBackground=true);

To avoid repeating the preprocessing steps at every training epoch and thus reduce training time, read all the data into memory before training the network. You can speed up this process by reading the data in parallel (requires Parallel Computing Toolbox™).

traindata = readall(trainDS,"UseParallel",true);
rawNet = trainNetwork(traindata(:,1),traindata(:,2),layers,options);

Classify Testing Signals

Use the trained network to classify the motions for the testing data set. Display the results with a confusion chart.

testdata = readall(testDS);
predTest = classify(rawNet,testdata(:,1),MiniBatchSize=32);
confusionchart([testdata{:,2}],[predTest{:}],Normalization="column-normalized")

Conclusion

This example showed how to perform sequence-to-sequence classification to detect different arm motions based on EMG signals. An overall accuracy of about 80% was achieved using an LSTM network with 80 hidden units. Some misclassification occurred between hand open and wrist extension, and between hand close and wrist flexion. The hand open motion involves the same muscle as one of those used in wrist extension. Similarly, the hand close and wrist flexion motions can activate the same muscles. Further, the placement of EMG electrodes on the arm targeted mostly musles used in wrist flexion which had the highest classification accuracy.

The data for this example was collected by Professor Chan of Carleton University and can be found here: https://www.sce.carleton.ca/faculty/chan/index.php?page=matlab [1].

References

[1] Chan, Adrian D.C., and Geoffrey C. Green. 2007. "Myoelectric Control Development Toolbox." Paper presented at 30th Conference of the Canadian Medical & Biological Engineering Society, Toronto, Canada, 2007.

preprocess Function

function Tsds = preprocess(inputDS)

sig = inputDS{1};
roiTable = inputDS{2};

% Remove first and last rest periods from signal
sig(roiTable.ROI(end,2):end,:) = [];
sig(1:roiTable.ROI(1,1),:) = [];

% Shift ROI limits to account for deleting start and end of signal
roiTable.ROI = roiTable.ROI-(roiTable.ROI(1,1)-1);

% Create signal mask
m = signalMask(roiTable);
L = length(sig);

% Obtain sequence of category labels and remove pronation and supination motions
mask = catmask(m,L);
idx = ~ismember(mask,{'Pronation','Supination'});
mask = mask(idx);
sig = sig(idx,:);

% Create new signal mask without pronation and supination categories
m2 = signalMask(mask);
m2.SpecifySelectedCategories = true;
m2.SelectedCategories = [1 2 3 4 7];
mask = catmask(m2);

% Filter and downsample signal data
sigfilt = bandpass(sig,[10 400],3000);
downsig = downsample(sigfilt,3);

% Downsample label data
downmask = downsample(mask,3);

targetLength = 12000;
% Get number of chunks
numChunks = floor(size(downsig,1)/targetLength);

% Truncate signal and mask to integer number of chunks
sig = downsig(1:numChunks*targetLength,:);
mask = downmask(1:numChunks*targetLength);

% Create a cell array containing signal chunks
sigOut = {};
step = 0;

for i = 1:numChunks
    sigOut{i,1} = sig(1+step:i*targetLength,:)';
    step = step+targetLength;
end

% Create a cell array containing mask chunks
lblOut = reshape(mask,targetLength,numChunks)';
lblOut = num2cell(lblOut,2);
    
% Output a two-column cell array with all chunks
Tsds = [sigOut,lblOut];
end