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Introduction Reduced maximum inspiratory mouth pressure (PImax) indicates inspiratory respiratory muscle weakness. Drop in vital capacity (ΔFVC) greater than 20% from seated to supine posture is considered abnormal, whereas drop greater than 30% is considered pathognomonic for diaphragmatic paralysis.Aim To assess whether drop of ΔFVC>20% can be used as surrogate of severe respiratory muscle weakness.Methods A retrospective study which included data of MND patients from the medical registry of a tertiary lung function laboratory. Complete pulmonary function testing including FVC measurements in the seated and supine positions, respiratory muscle strength (RMS) assessment and pulse oximetry were performed in all patients.Results Ninety-four MND patients (age: 60±11 yrs, height: 166±9 cm, BMI: 27.5±4.4 kg·m -2) were included. The vital capacity was measured in the seated (FVCseated: 95±23%pred) and supine position (FVCsupine: 86±25 %pred). PImax was 90±38%pred and PEmax was 90±35%pred. Forty-five patients had abnormal PImax%pred. while PEmax%pred. was abnormal in thirty-nine patients. Oxygen saturation in the seated position (SpO2seated: 96±2%) and supine position (SpO2supine: 94±2%) were also recorded. SpO2 drop from the seated to the supine position was 2.4±2.2%. Seventeen patients had ΔFVC>20%. ΔFVC>20%was associated with SpO2drop (r=-0.542, p=0.025). ΔFVC>20% was associated with PImax%pred (r=-0.343, p=0.002) and PEmax%pred (r=-0.333, p=0.003). Neither PImax (ΔFVC>20%, 76±37% vs ΔFVC<20%, 92±36%, p=0.115) nor PEmax (ΔFVC>20%, 81±35% vs ΔFVC<20%, 91±35%, p=0.268) were significantly different between the two groups.Conclusion ΔFVC is related to respiratory muscle strength in patients with MND, and its use as an alternative when RMS assessment is not available might worth exploring.