Typical power spectrum density (PSD) of the EMG signals. In this case, EMG data from the left tibialis muscles of subject #7 was used. PSD of each epoch were obtained using FFT and then the useless frequency powers (around 60 Hz, lower than 10 Hz, greater than 75 Hz) were ignored. (a) PSD of the EMG signals measured during wake epoch. (b) PSD of the EMG signals measured during sleep epoch.