Digital Acquisition of EMG data

EMG activity is typically recorded and analysed by a computer digital-acquisition system, called A/D. The analogue signal is converted to a digital signal by an A/D converter. Essentially, an A/D converter maps the analogue signal into equivalent digital steps. These steps then allows us to perform a number of analyses on the data stream. The schematic presentation is shown below.

When you examine the recording site, it is clear that the electrodes do NOT record ionic movement within the muscle. Rather, they record electrical signals that arise from these internal ionic movements. Therefore, the recording of the electrical activity is actually a recording of NET activity. You can imagine a number of motor units within the muscle being active. Each motor unit will have a unique spatial relationship with the recording electrodes and therefore a unique shape. That is, some motor units will be closer to the electrodes than others. The electrodes respond to the arrival of all signals, summing them. Thus you could have a situation where a positive and a negative signal of the same magnitude arrive and yet no activity is reported. They sum to zero. However, in the typical recording we are looking at activity over some period of time. The diagram below shows five motor unit action potentials and the sum of these - this is what the surface elctrodes would record.