The origin of skin-stretch-caused motion artifacts under electrodes.

The outside of the skin of the forearm is typically 15 mV more negative than the inside. Stretching the skin causes a reduction in the magnitude of this skin potential V, which we observe as a motion artifact delta V. We seek to determine the origin of this motion artifact by successively stripping 12 layers of the skin using Scotch Tape. Between each stripping we measure artifact delta V, 13 Hz impedance Z, and change in impedance delta Z. On the interior surface of the forearm, Z decreases with number of strippings. delta Z can be first either positive or negative, then is always negative and decreases linearly with Z. delta V first remains constant and then decreases with Z and delta Z. delta V and delta Z increase with stretch force following a logarithmic relationship. delta Z has a rectangular shape waveform, whereas the rising edge of delta V shows a fast followed by a slow component and its falling edge decays exponentially with a large time constant. We have expanded the model of Thakor and Webster to best fit the waveform of delta V and delta Z caused by stretch.