Electrode matrix placement. The red dot indicates an MTrP in the upper trapezius and the blue line shows the IZ location over the area covered by the electrode matrix. The IZ was typically located medially to the Y-axis. TrP-IZ is the distance between SP in the MTrP region and the IZ line. For each of the five columns of the matrix EMG signals are reported and IZ locations indicated using a blue arrow. The IZ location was detected where EMG signals showed minimal amplitude and/or phase reversal. AC, acromial angle; C7, spinous process of the seventh vertebrae; SP, spot tenderness.