Positional changes of anatomical structures surrounding the upper airway are known to affect pharyngeal mechanics and collapsibility. We hypothesized that these alterations also affect the ability of the upper airway dilator muscles to enlarge the pharynx by altering their ability to shorten when activated. Using sonomicrometry, we evaluated in seven anesthetized dogs the effects of changes in tracheal and head position on the length of the genioglossus (GG) and the geniohyoid (GH), and the effects of these positional changes on the magnitude of shortening of the two muscles in response to electro- (ES) and chemostimulation (CS). Caudal traction of the trachea lengthened the GG and GH in all dogs, while cranial displacement of the trachea and flexion of the head to a vertical position shortened the muscles. Compared to the magnitude of ES-induced shortening in the baseline position, ES-induced shortening of the GG was 144.7±14.6, 49.3±4.3, and 33.5±11.6 % during caudal and cranial displacement of the trachea and during head flexion, respectively. Similar effects of the positional changes were found for the GH, as well as for both muscles during respiratory stimulation with PCO2 of 90mmHg at the end of CO2 rebreathing, although inspiratory muscle shortening during CS reached only a quarter to a third of the magnitude observed during ES. We conclude that positional alterations of anatomical structures in the neck have a dramatic effect on the magnitude of shortening of the activated GG and GH, which may reduce substantially their ability to protect pharyngeal patency.