BACKGROUND: Smokers without airflow obstruction have reduced exercise capacity, but the underlying physiological mechanisms are not fully understood. AIM: To compare quadriceps function assessed using non-volitional measures, and ventilatory requirements during exercise, between smokers without airway obstruction and never-smoker controls. STUDY DESIGN AND METHODS: Adult smokers (n=20) and never-smoker controls (n=16) aged 25-50 years 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 non-volitionally using single and repetitive magnetic stimulation. Quadriceps bulk was assessed using ultrasound, as rectus-femoris cross-sectional area (QRF-CSA). Physical activity level was quantified using the SenseWearTM armband worn for 5 days. RESULTS: Smokers had lower peak exercise workload, peak oxygen consumption and anaerobic threshold (AT) compared to 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 (D force-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 (120W: 52.6±11.8 vs. 40.7±6.0 L/min; p<0.001), decreased ventilatory efficiency (higher ⩒E/⩒CO2) and were more breathless with greater leg fatigue at iso-workloads and iso-ventilation levels compared to never-smoker controls. Smokers showed no mechanical constraints on tidal volume expansion during exercise or ventilatory limitation at peak exercise. CONCLUSION: Adult smokers without airflow obstruction have reduced skeletal muscle endurance and ventilatory efficiency compared to never-smoker controls, despite similar daily physical activity levels, which contributed to reduced peak exercise capacity.