This study was conducted in order to determine the input visual effect on electromyographic (EMG) activity of the sternocleidomastoid and masseter muscles in the supine and lateral decubitus positions. The study was performed on 22 patients with myogenic cranio-cervical-mandibular dysfunction (CMD) and 18 healthy subjects. EMG activity at rest and during swallowing of saliva and maximal voluntary clenching was recorded in the supine and lateral decubitus positions in the following conditions: 1. with eyes open; and 2. with eyes closed after 5 minutes in a dark room. A significant decrease of EMG activity at rest with closed eyes in both groups was observed in the sternocleidomastoid (lateral decubitus position) and in the masseter muscle (supine position). During swallowing of saliva a significant decrease of EMG activity with closed eyes was observed only in the sternocleidomastoid muscle (lateral decubitus position) in healthy subjects. During maximal voluntary clenching any significant differences were observed upon variation in the visual input. The significant change in EMG activity, mainly observed at rest, suggests that the visual input effect is weak. The absence of a significant change in EMG activity during maximal voluntary clenching upon variation in the visual input could be clinically relevant in patients with myogenic CMD who habitually brux.