Smokers without airflow obstruction have reduced exercise capacity, but the underlying physiological mechanisms are not fully understood. We aimed to compare quadriceps function assessed using nonvolitional measures and ventilatory requirements during exercise, between smokers without airway obstruction and never-smoker controls. Adult smokers (n = 20) and never-smoker controls (n = 16) aged 25-50 yr with normal spirometry, underwent incremental cycle cardiopulmonary exercise testing to exhaustion with measurement of symptoms and dynamic lung volumes. Quadriceps strength and endurance were assessed nonvolitionally using single and repetitive magnetic stimulation. Quadriceps bulk was assessed using ultrasound, as rectus-femoris cross-sectional area. Physical activity level was quantified using the SenseWear armband worn for 5 days. Smokers had lower peak exercise workload, peak oxygen consumption, and anaerobic threshold than controls (170 ± 46 vs. 256 ± 57 W, 2.20 ± 0.56 vs. 3.18 ± 0.72 L/min, 1.38 ± 0.33 vs. 2.09 ± 0.7 L/min, respectively; P < 0.01 for all). Quadriceps endurance was lower in smokers (Δforce-time integral 54.9% ± 14.7% vs. 40.4% ± 14.7%; P = 0.007), but physical activity, quadriceps strength, and bulk were similar between groups. Smokers displayed higher ventilation (120 W: 52.6 ± 11.8 vs. 40.7 ± 6.0 L/min; P < 0.001), decreased ventilatory efficiency (higher V̇e/V̇co2), and were more breathless with greater leg fatigue at iso-workloads and iso-ventilation levels compared with never-smoker controls. Smokers showed no mechanical constraints on tidal volume expansion during exercise or ventilatory limitation at peak exercise. Adult smokers without airflow obstruction have reduced skeletal muscle endurance and ventilatory efficiency compared with never-smoker controls, despite similar daily physical activity levels, which contributed to reduced peak exercise capacity.NEW & NOTEWORTHY In adult smokers without airflow obstruction, the contributions of pulmonary and skeletal muscle functions to reduced exercise capacity are unclear. We found that non-COPD smokers had decreased exercise capacity and muscle endurance although strength was preserved compared with never-smoking controls. Exercise endurance was associated with quadriceps endurance and CO transfer factor. Despite similar physical activity levels, smokers developed leg fatigue, breathlessness, and displayed increased ventilation with reduced ventilatory efficiency at lower workloads, without exhibiting ventilatory constraint.
Keywords: exercise; magnetic stimulation; skeletal muscle; smoking; ventilatory efficiency.