**9.2. Processing the frequency domain – Spectral analysis**

The EMG signal's frequencies are distributed between 1 and 500 Hz, with a great concentration between 20 and 250 Hz in the case of simple muscular activity. The distribution of energy at different frequencies (power spectral density) reflects the predominance of the low or high frequency components in the signal and has been used in kinesiological research. Factors that influence the spectral profile of the EMG signal have been listed by various authors.

EMG can be considered an overlapping of the action potentials of all the active motor units. The spectrum of EMG frequencies thus contains information about the characteristics of different fibers that contribute to the signal. Spectral analysis can provide information about the mean duration of the active fiber potentials, which in turn can be used to determine the mean velocity of muscle fiber conduction [3,4].
