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Comparison of incremental and constant load tests of inspiratory muscle endurance in COPD

Depts of ¹ Pulmonary Physiology, and ² Physiotherapy, Sir Charles Gairdner Hospital, ⁴ Lung Institute of Western Australia, ⁵ School of Anatomy and Human Biology, University of Western Australia, Nedlands, and ³ School of Physiotherapy, Curtin University of Australia, Bentley, Perth, Western Australia, Australia.

CORRESPONDENCE: P. R. Eastwood, Dept of Pulmonary Physiology, Sir Charles Gairdner Hospital, Hospital Avenue, Nedlands, Western Australia, Australia 6009. Fax: 61 893462034. E-mail: peter.eastwood{at}health.wa.gov.au

Keywords: Assessment, endurance, lung disease, respiratory muscles

Received: July 20, 2006
Accepted April 28, 2007

The aim of the present study was to determine the relative value of incremental and constant load tests in detecting changes in inspiratory muscle endurance following high-intensity inspiratory muscle training (H-IMT) in chronic obstructive pulmonary disease.

In total, 16 subjects (11 males; forced expiratory volume in one second (FEV₁) 37.4±12.5%) underwent H-IMT. In addition, 17 subjects (11 males; FEV₁ 36.5±11.5%) underwent sham inspiratory muscle training (S-IMT). Training took place three times a week for 8 weeks. Baseline and post-training measurements were obtained of maximum threshold pressure sustained during an incremental load test (P_th,max) and time breathing against a constant load (t_lim). Breathing pattern was unconstrained.

H-IMT increased P_th,max and t_lim relative to baseline and to any change seen following S-IMT. The effect size for P_th,max was greater than for t_lim. Post-training tests were accompanied by changes in breathing pattern, including decreased duty cycle, which may have served to decrease inspiratory work and thereby contribute to the increase in P_th,max and t_lim in both groups.

When assessing inspiratory muscle function in chronic obstructive pulmonary disease via tests in which the pattern of breathing is unconstrained, the current authors recommend incremental load tests be used in preference to constant load tests. However, to attribute changes in these tests to improvements in inspiratory muscle endurance, breathing pattern should be controlled.