After the swallowing cycle is complete, the nasopharynx begins to open with relaxation of the superior constrictor. The oropharynx likewise begins to relax. When the muscles that elevated the larynx and drew the tongue onto the epiglottis relax, the epiglottis flips back to its open position. During this phase, contraction of which of the following muscles would facilitate and accelerate the opening of the airway?