Background/Objectives: Duchenne muscular dystrophy (DMD) is the most prevalent progressive muscular dystrophy, and the guidelines recommend the regular assessment of respiratory muscle function. This study aimed to assess the relationship between maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP) and sniff nasal inspiratory pressure (SNIP) measurements and upright-supine spirometry parameters in children with DMD, the predictability of upright–supine spirometry in terms of diaphragm involvement, and the impact of nutrition on muscle strength. Methods: This prospective cross-sectional study examined patients with DMD by comparing upright and supine FVC, MIP, MEP, and SNIP measurements. The effects of the ambulatory status, kyphoscoliosis, chest deformity, and low BMI on respiratory parameters were investigated. Results: Forty-four patients were included in the study. The mean patient age was 10.8 ± 2.9 years. Twenty-five patients were ambulatory. A significant decrease in FVC, FEV1, and FEF25–75 values was detected in the supine position in both ambulatory and non-ambulatory patients (p 2O). MIP, MEP, and SNIP values were significantly lower in patients with a low BMI than in those without (p < 0.05). Conclusions: To accurately assess respiratory muscle strength, supine FVC should be combined with upright FVC, MIP, MEP, and SNIP measurements. It is crucial to regularly screen patients for nutrition, as this can significantly affect respiratory muscle function during pulmonology follow-up.
