Examining Functional Exercise Capacity and Respiratory Muscle Strength in COPD

Key Takeaways

• Among symptomatic adults in a Turkish chest-diseases outpatient clinic, MIP below 80% predicted occurred in 52% with pre-COPD, 54% with PRISm, and 56% with COPD, without a significant group difference.
• Median percentage-predicted six-minute walk distances were 80.8% in pre-COPD, 78.3% in PRISm, and 74% in COPD; the COPD distance was significantly lower than each comparator.
• Greater cumulative smoking exposure was associated with PRISm and COPD classification relative to pre-COPD in exploratory adjusted models, without establishing causation.

Respiratory symptoms can accompany different post-bronchodilator spirometric patterns, but those categories alone do not describe respiratory pressures or walking capacity. Adults classified as pre–chronic obstructive pulmonary disease (pre-COPD), preserved ratio impaired spirometry (PRISm), or chronic obstructive pulmonary disease (COPD) may have distinct functional profiles. Respiratory pressure measurements and walking tests allow investigators to assess those profiles separately.

Investigators recruited 150 symptomatic adults for the Medicina study of respiratory pressures and walking capacity, a prospective, single-center cross-sectional assessment at Muğla Training and Research Hospital’s chest diseases outpatient clinic in Türkiye from October 2024 through July 2025. Enrollment was capped at 50 adults in each post-bronchodilator category: pre-COPD, PRISm, and COPD. Investigators used the ratio of forced expiratory volume in one second (FEV1) to forced vital capacity (FVC) and FEV1 percent predicted to assign mutually exclusive groups; the study was registered as NCT06650150. Its registered primary outcome was maximal inspiratory pressure (MIP) below 80% predicted; maximal expiratory pressure (MEP) below that threshold was secondary. Dyspnea and six-minute walking were also assessed. Pressure testing preceded two same-day walks separated by at least 30 minutes; the longer distance was retained. Valid walking results were available for 148 adults: 50 with pre-COPD, 49 with PRISm, and 49 with COPD. Equal enrollment quotas and smoking eligibility differed from the registered plan; regression and sex-specific pressure-cutoff analyses were post hoc.

The registered MIP-threshold comparison showed no significant difference across categories (overall p = 0.923). Continuous percentage-predicted MIP and MEP also did not differ significantly, nor did the registered secondary MEP-threshold comparison or the post hoc sex-specific cutoff analysis.

Percentage-predicted walking distance was lower in COPD than in pre-COPD (adjusted p < 0.001) or PRISm (adjusted p = 0.038); pre-COPD and PRISm did not differ significantly. Walking below 80% predicted was most common in COPD and least common in pre-COPD. Dyspnea ratings were higher in COPD than pre-COPD, while PRISm differed significantly from neither. FEV1 and midexpiratory flow differed overall. In exploratory adjusted models, greater smoking pack-years were associated with PRISm and COPD classification relative to pre-COPD; age associations varied by model, and functional measures were not independently significant.

Nonsignificant pressure comparisons do not establish equivalent or preserved strength, particularly because sample-size planning addressed continuous MIP rather than the registered threshold outcome. Cross-sectional recruitment at one Turkish center cannot establish causation or movement between categories. Equal quotas cannot estimate category prevalence, and the symptomatic comparison groups were not healthy controls. Pressure reference equations required extrapolation for some adults, maximal pressures depend on effort, and lung volumes and peripheral-muscle function were unmeasured. Unavailable walking distances and complete-case exploratory modeling further constrain interpretation.

According to the authors, respiratory-pressure comparisons did not reveal significant between-group differences, whereas walking capacity was lower in COPD than in either symptomatic comparator. They did not interpret the pressure findings as evidence of equivalent strength, and the exploratory classification associations do not establish effects.

Clinician Questions

How were symptomatic pre-COPD, PRISm, and COPD distinguished by post-bronchodilator spirometry?

Among symptomatic adults, pre-COPD required FEV1/FVC ≥ 0.70 and FEV1 ≥ 80% predicted; PRISm required FEV1/FVC ≥ 0.70 and FEV1 < 80% predicted; COPD required FEV1/FVC < 0.70. Imaging was not required for pre-COPD classification.

Does percentage-predicted six-minute walk distance in pre-COPD, PRISm, and COPD account for body size?

For symptomatic adults classified as pre-COPD, PRISm, and COPD, investigators calculated percentage-predicted six-minute walk distance using the Gibbons equation, which incorporates age and sex but not height or weight. Those body-size characteristics therefore were not incorporated into the percentage-predicted comparison.