Functional Exercise Capacity and Respiratory Muscle Strength in Pre-COPD, PRISm, and COPD: A Cross-Sectional Study

Background and Objectives: Respiratory muscle strength and exercise capacity remain incompletely characterized in pre-COPD and preserved ratio impaired spirometry (PRISm). We compared respiratory pressures and functional exercise capacity across pre-COPD, PRISm, and COPD. Materials and Methods: This prospective single-center cross-sectional study included 150 symptomatic adults (50 per group) and was registered as NCT06650150. Assessments included post-bronchodilator spirometry, maximal inspiratory and expiratory pressures (MIP and MEP), dyspnoea, and two six-minute walk tests. Valid walking results were available for 148 participants. The registered primary outcome was MIP < 80% predicted. Group comparisons were supplemented by exploratory post hoc multinomial regression and a sensitivity analysis using sex-specific respiratory pressure cut-offs. Results: The proportion with MIP < 80% predicted did not differ significantly across pre-COPD, PRISm, and COPD (52.0%, 54.0%, and 56.0%; p = 0.923). Median percentage-predicted MIP (79.2%, 77.5%, and 77.1%; p = 0.881) and MEP (74.0%, 66.7%, and 65.8%; p = 0.289) also did not differ significantly. The post hoc cut-off analysis found no statistically significant group differences. Median percentage-predicted six-minute walk distance was lower in COPD (74.0%) than in pre-COPD (80.8%; adjusted p < 0.001) and PRISm (78.3%; adjusted p = 0.038). FEV1 and FEF25–75 differed overall (both p < 0.001). In exploratory regression, cumulative smoking exposure was associated with PRISm and COPD classification; the significance of age depended on model specification, and functional measures were not significantly associated with classification. Conclusions: The registered MIP outcome and continuous respiratory pressures did not differ significantly among the groups, whereas exercise capacity was lower in COPD. These findings do not establish equivalence or preserved respiratory muscle strength in any group.