Mechanomyography (MMG) is the measurement of the vibrations associated with muscle contraction. As an indicator of muscle activity in swallowing, MMG has several potential advantages over conventional electromyography (EMG), including robustness to variations in sensor placement, perspiration, and food spillage. The objective of this study was to investigate the effects of participant age and stimulus on submental muscle activity as measured by MMG. Nasal airflow was utilized as a reference signal. Four liquid stimuli were investigated: water, nectar-thick and honey-thick apple juices, and a thin-liquid barium suspension. Each of 15 healthy adults completed nine swallowing sequences, each consisting of four discrete swallows via a self-administered cup-drinking task, with an MMG sensor at a midline submental location and a nasal cannula at the nares. Muscle activity and swallowing apneas in the signals were identified with pseudo-automatic segmentation algorithms. Various timing and amplitude features were extracted from each segmented swallow. Muscle activity onset preceded the onset of swallow apnea. Significant main effects of stimulus were found for the duration of muscle activity and for the time difference between the offsets of muscle activity and swallow apnea. No other main or interaction effects were significant. In general, the timing and amplitude variations of submental muscle activity revealed by MMG seem to agree with previously reported findings using EMG. The minor discrepancies between the results of this study and those of previous EMG studies are likely due to differences in experimental tasks. MMG may serve as an alternative measure of muscle activity during swallowing and further investigation is warranted.