Swallowing is a semi-volitional sensory-motor act that facilitates movement of food/water (known as the bolus) from oral cavity to the stomach. The process of swallow is often described in three stages (sometimes four) that include oral preparatory (only when mastication is involved), oral phase, pharyngeal phase and oesophageal phase. Physiological processes involved in this function are rapid and internal, and are masked from direct visualization. Speech and swallow therapists are in constant search for simple, affordable, minimally skilled, non-invasive technologies that can assist them in quick, but appropriate clinical decision making. Surface electromyography (sEMG) has been extensively debated for its application in swallowing sciences and has emerged as a reliable non-invasive tool for understanding human swallowing [1–3], and also as a bio-feedback tool for training persons with swallowing disorders [4–7]
The sub mental group of muscles represent a subset of muscles involved in swallowing that are peripheral, accessible for sEMG procedure, and closely related to lingual propulsion and hyo-laryngeal elevation and marks the transition from oral phase to pharyngeal phase of swallow [5, 6, 8–11]. Located in the anterior neck region, between the mandible and hyoid, the sub mental triangle or the suprahyoid triangle is formed by the anterior belly of right and left digastrics as lateral borders, geniohyoid and the mylo-hyoid muscles as floor of the triangle. The apex of this unpaired triangle is located at the chin and the base is on the hyoid bone. Study of sEMG characteristics at the sub mental triangle (SM-sEMG) provide speech and swallow clinicians with valuable insights related to swallow function in typical and in persons with swallowing difficulties [11].
Prevailing literature suggests that the SM-sEMG for swallow of non-masticated boluses obtained from healthy individuals vary with age [12–17], bolus volume [13],bolus consistency [13, 18], and ingestion method [19], but were not different across gender [1, 14]. The sequence and latency of muscle activation were independent of age [13, 14]. These studies incorporated a range of boluses for reporting their findings on SM-sEMG with majority using saliva and controlled liquid swallows [8, 14, 15, 19]. Little is known about the SM-sEMG performance in thicker boluses [13, 18].Complex eating processes that include mastication has not been studied in detail, except for the report by Hiramatsu[12]. Their study compared the muscle activity before and after consumption of a full meal in young and older individuals and reported no significant difference in peak SM-sEMG amplitude or duration in these individuals. Clearly, there is little information on the SM-sEMG performance before, during and after individual solid bolus intake during more natural, and complex eating behaviours.
Mastication or chewing involves repeated movements of jaw, tongue and cheeks that aim at mixing the food with saliva, and positioning the food between the occlusal surfaces for preparing a cohesive bolus that can be safely transported into the pharynx with a quick anterior-posterior lingual sweep. Compared to liquid bolus, the sub mental muscles are active for a longer period in masticated bolus swallows as they stabilize the floor of the mouth[20] during oral preparation followed by lingual propulsion and hyo-laryngeal elevation [5, 8–10]. It is not known if the SM-sEMG characteristics during swallow post-mastication of solids are any different from that of non-masticated boluses such as liquids.
Aging is a significant factor that alters swallow physiology and performance in healthy individuals. Age associated changes in muscle properties were seen as variations in the number of sEMG peaks [13], increase in pre-reflex peak [13] and reduction in peak SM-sEMG amplitude [14–17, 21]. These differences have not been gender specific [14] but varied with the consistency of bolus swallowed. The increase in number of chewing cycles and chewing duration [22–24], and the changes in sub mental muscle properties [25] associated with aging may lead to differences in SM-sEMG measures during swallow of masticated solid boluses in typical aging population, but is not well understood. Therefore, the current study aimed at comparing the sub mental sEMG measures in young and older individuals during swallow of masticated solid boluses. For the purpose of this study, a ‘swallow’ is initiated when the masticated solid bolus is propelled into the pharynx with a lingual sweep (oral phase) and hyolaryngeal elevation (pharyngeal phase). The change in myogenic potentials of the sub-mental muscle group during this process was recorded, analyzed and compared across age and gender to meet the objectives of the study.