Superposition of MUAPs
In kinesiological studies the motor unit action
potentials of all active motor units detectable
under the electrode site are electrically superposed
(Fig. 11) and observed as a bipolar
signal with symmetric distribution of positive
and negative amplitudes (mean value equals
to zero). It is called an Interference pattern.
Recruitment and Firing Frequency
The two most important mechanisms influencing
the magnitude and density of the observed
signal are the Recruitment of MUAPs and their
Firing Frequency.
These are the main control strategies to adjust the contraction process and modulate the force output of
the involved muscle. Because the human connective tissue and skin layers have a low pass filter effect on
the original signal, the analyzed firing frequency e.g. of a surface EMG does not measure the original firing
and amplitude characteristics. For simplicity, one can say that the EMG signal directly reflects the recruitment
and firing characteristics of the detected motor units within the measured muscle