The EMG signal is the algebraic sum of the motor units’ potential activity where the electrode is placed. These
motor units are usually not active simultaneously and are function at different times. Motor units are controlled by a
group of neurons. These neurons can transmit either excitatory or inhibitory impulses. The contraction or expansion
of muscle cords depends on the signals that reach a motor unit at that time. A motor unit will become active and its
muscles will contract only when the amount of excitatory impulses are more than inhibitory impulses and higher
than the level of threshold (Wilmer & Castle, 2008).