The tongue enacts complex mechanical events during swallowing, the most important of which is the propulsion of a bolus from the oral cavity to the pharynx. Successful bolus transport requires the tongue to interact with other oral structures, especially the hard palate, to generate sufficient impulsive force or pressure gradients that drive the bolus toward the oropharynx. Our knowledge of deglutitive lingual pressure dynamics is at best incomplete. The available data on oral tongue pressure phenomena are based exclusively on commanded single swallows. Oral pressure changes during other important everyday eating activities (e.g., cup drinking) have not been studied to date. Past investigations revealed that rapid sequential swallowing during continuous drinking, in contrast to commanded discrete swallows, had unique tongue-palate contact patterns, surface electromyographic response characteristics, and hyoid displacement profiles. Given the different biomechanical properties and motor strategies, we hypothesize that oral lingual pressure profiles for sequential swallowing are also different, that sequential swallows require less impulsive force, and that selected dysphagic patients, especially those whose swallowing deficits are associated with reduced tongue strength, will perform sequential swallows more efficiently than they do discrete swallows. This protocol, therefore, proposes to test these hypotheses in healthy individuals of different ages, and in patients with reduced tongue strength and oral/oropharyngeal dysphagia associated with neurologic disorders, musculoskeletal diseases, or head and neck cancer. Our goals are to: (a) acquire a better and more complete understanding of normal tongue pressure phenomena as a function of swallowing tasks, (b) characterize the interrelationship between task-induced lingual pressure differences and results of clinical diagnostic tests of swallowing function in patient populations, and (c) differentially identify the profiles of dysphagic patients who can and those who cannot benefit from sequential swallowing as a compensatory/rehabilitative strategy. To date, we have studied the oral pressure as well as suprahyoid and infrahyoid surface EMG (sEMG) profiles of 54 healthy volunteers and 5 patients as a function of discrete and rapid sequential swallowing tasks. In the area of oral pressure dynamics, our measurements include: pressure bulb activation duration, peak pressure distribution ratios, peak mmHg, start-to-peak and peak-to-end slopes, and area under the curve. MANOVA with Subject as experimental unit and Task and Bulb as factors has revealed significant main effects of Task (p < .0124) and Bulb (p < .0001) with no significant interaction. Most of the significant contrasts can be attributed to the posterior bulb placed just before the end of the hard palate. For rapid sequential swallows, peak pressure is reached proportionally 1.5-fold farther into the swallow than for discrete tasks, and the rate of pressure change from peak to baseline is twice as rapid. Bulb activation pattern is not uniform for rapid sequential swallows. In addition, we have found: (1) considerable intra- and inter-subject variability across all tasks, but relatively more so for discrete than for rapid sequential swallows; (2) no striking task-based difference in peak pressure; and (3) a trend for rapid sequential swallows to be shorter in total bulb activation duration (thus smaller in area under the curve). Our findings suggest task-induced differences in pressure distribution strategy and a more efficient way of swallowing for rapid sequential by delaying the peak, thus devoting a greater portion of the swallow to bolus propulsion. This supports our thesis of the potential clinical application of rapid sequential swallowing in selected patient populations. We have also examined the effect of swallowing task on the temporal coordinatiion and response characteristics of suprahyoid and infrahyoid muscle activities. Our measurements include: max, mean, and ending amplitudes; start-to-max and max-to-end slopes; area under the curve; total sEMG activity duration; and suprahyoid-infrahyoid temporal differences in activity onset, offset, and time of max amplitude. Results to date show no significant task-induced differences in suprahyoid-infrahyoid timing coordination measures. In addition, for all swallowing tasks, max amplitude is greater for suprahyoid than for infrahyoid sEMG responses (p < .0001). Rapid sequential swallows have steeper suprahyoid start-to-max slope, steeper infrahyoid max-to-end slope, higher suprahyoid and infrahyoid ending amplitude, and shorter suprahyoid and infrahyoid total sEMG activity duration than any discrete tasks (p < .0001 in each case). There are significant subject differences across all variables assessed, especially for the rapid sequential task in max amplitude and area under the curve. A trend for age-related differences is noted in selected measures (e.g., lower max/mean amplitude and reduced slopes). The muscles targeted by our study clearly show a pattern of co-activation regardless of task nature. The finding of greater suprahyoid than infrahyoid peak tension suggests recruitment of a greater number of motor units and greater amount of energy for the suprahyoid muscle complex that plays a leading role in swallow initiation. The finding of partial suprahyoid and infrahyoid muscle relaxation during cyclical drinking may reflect motor neuron response summation as a function of shortened task execution time.
