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Background The diaphragm contributes to respiration, postural stabilization, and regulation of esophageal sphincter pressures through its crural portion. Although its dual respiratory–postural role is established, the effects of physical load and arm position on esophageal sphincter pressures remain unclear. This study investigated how postural demand and external load influence upper (UES) and lower (LES) esophageal sphincter pressures under varying stabilization conditions. Methods Twenty-eight healthy adults underwent high-resolution manometry (HRM) to measure UES and LES pressures in standing, supine leg raise, and during standing while holding 3, 6, and 9 kg loads with arms alongside the body or raised to 45° shoulder flexion. Paired-samples t -tests and one-way ANOVA were applied, with effect sizes calculated. Results LES pressure significantly increased in the supine leg raise and when loads were held with arms elevated at 45° shoulder flexion ( p < 0.001), indicating enhanced activation of the esophagogastric high-pressure zone involving the crural diaphragm. No significant LES change occurred when weights were held parallel to the body. UES pressure significantly increased only during the supine leg raise ( p < 0.001), whereas load magnitude had no effect in standing. Conclusion Esophageal sphincter pressures are modulated by posture and load, supporting an integrated respiratory–postural–sphincteric function of the diaphragm. LES pressure rises with increasing postural demand, consistent with crural diaphragm recruitment, while UES responses appear position-dependent. These findings extend current understanding of the diaphragm’s coordinated role in esophagogastric junction and upper sphincter control.