Imposed expiratory resistance, dynamic hyperinflation and locomotor power and fatigue.

Abstract

Expiratory flow limitation results in dynamic hyperinflation, dyspnoea and premature exercise intolerance. We aimed to measure whether expiratory resistance reduces locomotor power via limiting maximal voluntary motor activity, exacerbating muscle fatigue, or both. Healthy volunteers (n = 14; 23 (3) years) performed a series of very heavy-domain constant power cycling exercise tests with and without an imposed expiratory flow resistance (7 cmH₂O/L/s). The decline in maximal evocable isokinetic power at intolerance during each experimental condition was apportioned to: (1) the power equivalent from a reduction in maximum voluntary muscle activation (termed 'activation fatigue'); and (2) the deficit in expected power at a given isokinetic muscle activity (muscle fatigue). Imposed expiratory resistance reduced exercise tolerance (487 (145) vs. 575 (137) s; 95% confidence interval of the difference (CI_diff) 52, 125 s; P = 0.0002). At isotime-control, imposed expiratory resistance resulted in a greater decline in inspiratory reserve volume (CI_diff 0.20, 0.94 L; P = 0.007), and increased dyspnoea (Borg CR-10; CI_diff 0.7, 3.0; P = 0.006) than without. Muscle fatigue was unaffected (CI_diff -20, 17 W; P = 0.873), but activation fatigue was greater with expiratory resistance (CI_diff 1, 49 W; P = 0.044) and related to the reduction in inspiratory reserve volume (r² = 0.53; P = 0.028). As a result, locomotor power reserve was reduced with expiratory resistance (253 (83) vs. 201 (92) W; CI_diff -10, 113; P = 0.09). Imposed expiratory resistive loading initiated a cascade of abnormal lung mechanics and symptoms. These abnormalities conflate to reduce exercise tolerance through limiting maximal voluntary motor activity.