The standard single differential sensor has 2 muscle site contacts, 1mm in diameter and separated by 10 mm. This sensor performs a differential measurement of the voltages appearing under the contacts as determined by the reference potential (at the Reference Electrode). This type of measurement is shown in Figure 1 below, where the output of the sensor is the difference between the voltages at contact “V1” and contact “V2”.
DE-2.1 EMG Sensor
Figure 1: The DE-2.1 Differential EMG sensor performance a subtraction of the voltages detected by the 2 contact bars.
The Double Differential Sensor contains three contacts, each separated by 10 mm. The sensor performs a two-stage subtraction: the first stage establishes the voltage between contact “V1” and contact “V2” as well as the voltage between contact “V2” and contact “V3” (Figure 2). The second stage then performs the subtraction between these differences.
DE-3.1 EMG Sensor
Figure 2: The DE-3.1 Double Differential Sensor performs a two stage subtraction: the first stage establishes the differential voltages at the input, the second stage removes those components of the signals that are common.
The second differential subtraction will remove those signals which are common to all sensor contacts, while propagating those signals that exhibit potential differences across the contacts. EMG signals originating from muscles that are not immediately below the surface of the skin will have a larger latency than those immediately below the surface. These will appear in a similar fashion to all bars, and will thus be subtracted from the final sensor measurement (Figure 3).
Figure 3 : Removing EMG crosstalk- the signals originating from deep muscles, depicted by location “C” disperse as they travel to the skin surface and are detected by all sensor contacts. The signals originating from the fibers immediately below the skin surface (depicted by locations “A” and “B”), are only detected by the contact-pair directly above. The signal components originating from location “C” are common to all bars, and are removed in the double differential subtraction, while those components from locations “A” and “B ” are preserved. The figure on the right side demonstrates the effectiveness of the DE-3.1 sensor in removing EMG crosstalk from flexor and extensor activity.
The DE-3.1 sensor is recommended for those situations where EMG muscle crosstalk is considered to be problematic. Note that care must be taken to ensure proper and well-balanced skin contact with the sensor. In general, managing the two-point contact of the DE-2.1 sensor over curved surfaces is more natural than the three-point contact of the DE-3.1 (see Figure 4). Note also that the effective contact area of the DE-2.1 sensor is 10x10mm, while the effective contact area of the DE-3.1 sensor is 10x20mm. For those situation where EMG crosstalk is not a significant consideration, the user may benefit from the user-friendly characteristics of the DE-2.1 sensor.
Figure 4: Care must be taken when applying the DE-3.1 sensor to ensure that all three sensor inputs maintain event distributed contact on the skin throughout the recording.