Fig. 1

Locations of the electrode attachments. A Channel 1 (right) and channel 2 (left) electrodes were placed superior to the hyoid bone and posterior to the mandible 1 cm away from the midline, and the targeted muscles were the bilateral digastric and mylohyoid muscles. Channel 3 electrodes were placed on the bilateral superior pole of the thyroid cartilage to target the bilateral thyrohyoid muscles, and channel 4 electrodes were placed medial to the sternocleidomastoid muscles and inferior to the thyroid cartilage, and the targeted muscles were the other infrahyoid muscles (sternohyoid, omohyoid, and sternothyroid muscles). B In the 2-channel NMES system, channel 1 and 2 electrodes were attached to the suprahyoid and thyrohyoid muscles, respectively. Other electrodes were attached to the same locations as performed for the 4-channel NMES system, but stimulations were only applied to channel 1 and 2 electrodes