BACKGROUND Decreased respiratory muscle strength and muscle mass is key in diagnosing respiratory sarcopenia. However, the role of reduced diaphragm activity, expressed as the maximal level of diaphragmatic excursion (DEmax), in diagnosing respiratory sarcopenia in patients with chronic obstructive pulmonary disease (COPD) remains unclear. This study aimed to characterize patients with COPD and low DEmax and maximal inspiratory pressure (MIP), a measure of inspiratory muscle strength, and assess the role of DEmax in respiratory sarcopenia. METHODS Patients with COPD underwent spirometry, exercise tolerance (VO2peak) test, and MIP measurement. DEmax and sternocleidomastoid thickness at the maximal inspiratory level (TscmMIL) were assessed using ultrasound sonography. RESULTS Overall, 58 patients with COPD (median age, 76 years; median %FEV1, 51.3 %) were included, 28 of whom showed a %MIP of ≥80 %, defined as having preserved MIP. Based on the %MIP of 80 % and median value of DEmax (48.0 mm) as thresholds, the patients were stratified into four groups: both-high (n = 18), %MIP-alone low (n = 11), DEmax-alone low (n = 10), and both-low (n = 19) groups. The both-low group exhibited the lowest %FEV1, Δinspiratory capacity, VO2peak, and TscmMIL, and these values were significantly lower than those of the both-high group. Except for %FEV1, these values were significantly lower in the both-low group than in the %MIP-alone low group despite adjusting DEmax level for body mass index. CONCLUSIONS Measuring DEmax along with MIP can characterize patients with COPD, reduced exercise capacity, and decreased accessory respiratory muscle mass and can help diagnose respiratory sarcopenia.