Objective <p>In obstructive sleep apnea (OSA), recurrent apneic and hypopneic episodes may affect the inspiratory and expiratory muscles. This study aimed to evaluate the relationship between respiratory muscle shear wave elastography (SWE) and disease severity in obstructive sleep apnea (OSA), and to assess the effects of 12-week PAP therapy.</p> Methods <p>A total of 105 participants, including 70 OSA patients (32 severe, 38 mild-to-moderate) and 35 healthy controls, were enrolled. Anthropometric data, pulmonary function tests, maximal respiratory pressures, and diaphragmatic/intercostal SWE were evaluated. A subgroup of OSA patients (<i>n</i> = 45) underwent reassessment after 12 weeks of regular PAP use.</p> Results <p>Diaphragm and intercostal SWE values were significantly higher, while MIP and MEP values were lower in severe OSA patients compared to others (<i>p</i> &lt; 0.001). Diaphragmatic and intercostal SWE showed positive correlations with apnea-hypopnea index (AHI) and negative correlations with MIP and MEP (<i>p</i> &lt; 0.001 for all). FVC and FEV1 were inversely correlated with both AHI and BMI. After 12 weeks of PAP therapy, significant reductions in SWE values and improvements in MIP and MEP were observed in severe OSA patients (<i>p</i> &lt; 0.05). In mild-to-moderate OSA patients, a trend toward improved respiratory strength and reduced stiffness was also noted, though not statistically significant.</p> Conclusion <p>OSA and obesity are associated with increased respiratory muscle stiffness and reduced respiratory muscle strength. SWE appears to be a promising noninvasive tool for assessing both disease severity and muscle function. Improvements after PAP suggest potential reversibility of muscle changes with effective treatment.</p>