[Effect of fluid viscosity on the biomechanical sequence of oropharyngeal swallowing in healthy males]

Objective: To confirm the effect of fluid with different viscosity on the normal biomechanical sequence of oropharyngeal swallowing in healthy males. Methods: Fifteen healthy male subjects [(27.7±1.8) years old] were recruited from November 2011 to February 2012 and instructed to swallow 15 ml of water (W), nectar-like fluid (N), and honey-like fluid (H) in an upright sitting position. The sensing system was consisted of tongue pressure sensor sheet, bend sensor, surface electrodes and microphone. They were used to monitor tongue pressure, hyoid activity, surface electromyography (EMG) of swallowing-related muscles and swallowing sound, respectively. The swallowing sound was chosen as the reference time. The significance of biomechanical sequence of structural events was determined by repeated-measures analysis of variance (ANOVA). Results: When swallowing liquid of any consistency, hyoid premotor and suprahyoid muscle electromyography (EMG) appeared synchronously (P>0.05), followed by the simultaneous appearances of hyoid rapid movement, peak time of suprahyoid muscle EMG, onset of infrahyoid muscle EMG, and anterior tongue pressure production (P>0.05). The peak time of infrahyoid muscle EMG was very close to the peak time of anterior tongue pressure (P>0.05), and both of them were earlier than the time that the hyoid reaching the highest position (P<0.05). At last, the time that the hyoid departing the highest position was synchronized with the disappearances of suprahyoid muscle EMG, infrahyoid muscle EMG, and tongue pressure (P>0.05). The tongue pressure production and peak time of tongue pressure arose from anterior to posterior along the midline of hard palate during normal swallowing, with the significances for tongue pressure production between the anterior site and the middle site (W: P=0.035, N: P=0.027, H: P=0.013) as well as the anterior site and the posterior site (W: P<0.001, N: P<0.001, H: P<0.001), while the appearance and peak time of the circumferential tongue pressure were very close (P>0.05). The increase of fluid viscosity did not affect the biomechanical sequence of the above structural physiological movements during normal swallowing. There were statistically significant differences between the hyoid premotor and the onset of suprahyoid muscle EMG when swallowing the honey-like liquid [(-1.03±0.47) and (-0.90±0.50) s] and water[(-0.87±0.32) and (-0.74±0.31) s] (P<0.001). Among the delayed structural events, except for the onset of infrahyoid muscle EMG and the tongue pressure production on the anterior site (P>0.05), the occurrences of all the parameters in swallowing honey-like fluid were significantly later than those in swallowing water (onset of hyoid rapid movement, P=0.007; time of hyoid reaching the highest position, P=0.034; time of hyoid departing the highest position, P=0.041; offset of hyoid movement, P=0.035; peak time of suprahyoid muscle EMG: P=0.040; offset of suprahyoid muscle EMG, P=0.014; peak time of infrahyoid muscle EMG: P=0.042; offset of infrahyoid muscle EMG, P=0.028; peak time of Ch.1: P=0.045; offset of Ch.1: P=0.012; onset of Ch.2: P=0.038; peak time of Ch.2: P=0.009; offset of Ch.2: P=0.034; onset of Ch.3: P=0.043; peak time of Ch.3: P=0.011; offset of Ch.3: P=0.026;onset of Ch.4: P=0.040; peak time of Ch.4: P=0.038; offset of Ch.4: P=0.033; onset of Ch.5: P=0.046; peak time of Ch.5:P=0.028; offset of Ch.5: P<0.001), but not for those between nectar-like fluid and honey-like fluid (P>0.05). Conclusions: The alteration of fluid viscosity did not affect healthy male biomechanical sequence of tongue, hyoid and swallowing-related muscles during normal swallowing. The biomechanics of the oropharyngeal structures is physiologically regulated with the alteration of fluid viscosity to ensure swallowing safely and smoothly.