The Physionet EMG signals can be downloaded as .mat, .txt, or other format files that you can read into your workspace. You just have to search Physionet for the appropriate file options.
I just downloaded the attached file. To load and plot it, save it to a directory on your MATLAB path, then run this:
emg = load('emg_healthy.txt');
figure(1)
plot(emg(:,1), emg(:,2))
grid
