Changes in masseter muscle activity associated with swallowing

The purpose of this study was to investigate masseter muscle activity during swallowing in seven healthy humans. In order to observe both the increase and the decrease of masseter muscle activity, the subjects performed voluntary saliva swallowing while low-intensity tonic clenching was maintained. Electromyograms (EMG) were recorded from the right masseter muscle and the suprahyoid muscle complex. For six of seven subjects, the masseter EMG amplitude increased above the pre-swallow baseline level at the initial phase of swallowing. This increase was initiated 0.1 +/- 0.3 s before the suprahyoid EMG onset and was terminated 0.6 +/- 0.4 s after the suprahyoid EMG onset. For six of seven subjects, the masseter EMG amplitude decreased below the pre-swallow baseline level during swallowing. This decrease was initiated 0.6 +/- 0.3 s after the suprahyoid EMG onset and was terminated 1.3 +/- 0.4 s after the suprahyoid EMG onset. These findings indicate that biphasic modulation of masseter motoneuron excitability occurs during swallowing in humans. Further investigations are needed to determine whether these findings are extensively applicable to swallowing as it occurs under natural conditions.