Effect of electrode array position and subcutaneous tissue thickness on conduction velocity estimation in upper trapezius muscle

Muscle fiber conduction velocity (CV) was estimated in 20 healthy subjects with a multichannel technique using a linear electrode array placed on the upper trapezius (UT) muscle during isometric voluntary contractions at 20% of the maximal voluntary effort. The array had 16 contacts spaced by 5 mm and was centered in the middle of the line connecting C7 and the acromion. The effect of array displacement was investigated by shifting it either cranially or caudally by 1, 2 and 3 cm. The effect of array misalignment was investigated by rotating it either clockwise or counterclockwise, by 10°, 20° and 30°. The thickness of subcutaneous fat was measured by ultrasonic echography (range was 3–18 mm).The average location of the innervation zone of the UT is in the middle of the line connecting C7 to the acromion, however, individual differences of up to 2 cm were observed. The lateral side, near the acromion, should be preferred for electrode placement because of the thinner fat layer.The double differential (DD) montage should be preferred to the single differential (SD) electrode configuration. In this case, displacements of ±1 cm or misalignments of ±10° do not imply statistically significant bias in the estimate of CV. The same applies to amplitude and frequency variables estimated from the SD signals. CV estimates based on DD signals are lower because they are less affected by the thickness of the subcutaneous fat than estimates based on SD signals.