Biomechanics of yawning: insights into cranio-cervical fluid dynamics and kinematic consistency
Yawning is a stereotyped orofacial-respiratory behaviour whose physiological role remains uncertain. Because cerebrospinal fluid (CSF) movement contributes to solute transport and waste removal and is strongly influenced by respiratory pressure dynamics, the study evaluated whether yawning alters neurofluid flow relative to normal and deep breathing, and whether yawning kinematics are reproducible within individuals. In a single MRI session in healthy adults, real-time phase-contrast imaging at the upper cervical level (C3) was combined with mid-sagittal real-time cine imaging to quantify CSF and internal jugular venous flows during normal breathing, forceful oral inspirations (deep breaths), yawns, and stifled yawns, and to derive tongue-motion trajectories. Both deep breaths and yawns increased CSF and venous flow compared with normal breathing; despite similar flow magnitudes, yawns more frequently produced co-directional caudal CSF and jugular outflow during inspiration, whereas deep breaths typically showed counter-directional CSF-venous flow. Yawning kinematics were highly reproducible within individuals across repeated events, indicating a stable motor sequence consistent with brainstem pattern-generator control. These observations show that yawning is not simply an intensified breath but a distinct cardiorespiratory manoeuvre that reorganizes neurofluid flow. The inspiratory alignment of CSF with venous outflow during yawns suggests a transient caudal advection that could influence solute transport and heat exchange within the cranial-cervical system, motivating targeted mechanistic studies with simultaneous airway pressure, thoraco-abdominal motion, and cervical venous pressure measurements.