Schematic of appropriate sensor location
The location of the sensor on the muscle is the single most important factor for good SNR. The placement can maximize the physiological EMG signal and minimize cross-talk interference from neighboring muscles.
The best location of the sensor on the muscle is generally on the midline of the muscle, far from the tendon origins and the innervation zones, with the sensor arrow parallel to the direction of the muscle fibers.
In the midline of the muscle, fibers have a bigger diameter and generally yield greater amplitude EMG signals. In contrast, locating the EMG sensor close to the tendon origins reduces the amplitude of the detected EMG signal because, as the muscle fibers approach the tendon, they become thinner and fewer in number. Smaller diameter fibers generate lower amplitude action potentials. Also, in this region the muscle is physically smaller, which makes it difficult to accurately place the sensor and avoid crosstalk interference from adjacent muscles. When possible, innervation zones should be avoided because in these areas the electrical activity propagates in opposite directions along the muscle fibers and can be attenuated.
Muscle fibers generally run longitudinally in a muscle. As a result, the electrical activity propagates predominantly along the length of the muscle. Each Delsys EMG sensor has an arrow on top that should be aligned along the length of the muscle so that the parallel-bar detection sites transect the muscle fibers.
A significant advantage of the Delsys surface EMG sensors is that they can be used as sensor probes on the muscle of interest prior to final placement. As the muscle is contracted, the location of the sensor can be shifted using real-time feedback software to check for signal quality.