Previous studies have shown that contractions of the jaw-closing masseter muscle (MAS) often occur shortly after respiratory events during sleep in obstructive sleep apnea (OSA) patients. Although it has been hypothesized that such non-specific motor activations may contribute to restoration of a compromised upper airway during respiratory events, proper physiological understanding of MAS contractions in patients with OSA is lacking. MAS contractions are usually associated with the termination of respiratory events, but these contractions do not always occur after respiratory events. Therefore, the above-stated hypothesis that "non-specific motor activations of the jaw-closing masseter muscle (MAS) may contribute to restoration of a compromised upper airway during respiratory events" is not accepted yet. Further, Kato et al. concluded from a recent study that MAS contraction is an orofacial manifestation of a general motor reaction to arousal occurring during sleep in OSA patients. This suggests that MAS contraction after a respiratory event is dependent on the arousal response rather than on the respiratory events per se.