Ultrasound Assessment of End-Expiratory Diaphragm Thickness Across Recumbent Positions: Implications for Measurement Standardization

Background Ultrasound-based diaphragm thickness assessment is increasingly used in respiratory and rehabilitation settings. However, measurements may be influenced by body position, potentially confounding clinical interpretation. Objective To investigate whether common recumbent positions (supine, prone, side-lying) alter end-expiratory right hemidiaphragm thickness at the end of quiet expiration in healthy young men. Methods In this randomized-order, within-subject cross-sectional study, 41 healthy male participants (18–24 years) underwent B-mode ultrasound assessment of right hemidiaphragm thickness at the zone of apposition during end-expiration. Three measurements per position were averaged. Differences across positions were analyzed using the Friedman test. Results Diaphragm thickness differed significantly across positions ( p < 0.001), with greatest values observed in prone, followed by side-lying and supine positions. Conclusion End-expiratory diaphragm thickness is posture-dependent in healthy young men. These findings demonstrate that diaphragm thickness measurements are strongly posture-dependent and highlight the necessity of strict positioning standardization for reliable ultrasound-based clinical assessment.