The functional capacity of patients with coronary artery disease is often limited by breathlessness. The intensity of breathlessness is primarily determined by respiratory muscle effort, which increases as the pressures generated by the inspiratory muscles approach maximum capacity. Exercise training can potentially decrease breathlessness by inducing adaptations that reduce the ventilatory demand and increase the maximum capacity of the respiratory muscles. This study examines the extent to which the physiological adaptations occurring with exercise training contribute to reductions in breathlessness in patients with coronary artery disease. The pre- and post-training results were compared in 21 patients participating in the McMaster University Cardiac Rehabilitation program. Respiratory parameters were measured at rest and during a progressive maximal exercise test. The intensity of breathlessness was measured by psychophysical techniques using the Borg scale. Following training, the capacity to generate maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), and maximum inspiratory flow (Vimax) improved by 22%, 18% and 5%, respectively, with no significant change in vital capacity, forced expiratory volume in one second and maximum expiratory flow. Exercise capacity improved by 14%, with an 11% decrease in the maximum intensity of breathlessness experienced. Although maximum ventilation (Ve) did not change significantly, Ve per unit workload decreased by 10% and breathlessness per unit Ve decreased by 13%. At the highest similar workload between the 2 tests (850 kpm), breathlessness decreased by 34%, with a 19% reduction in Ve. Breathing frequency, mean inspiratory flow (Vt/Ti), inspiratory flow generated during exercise in relation to maximum inspiratory flow (Vi%max), and time of inspiration in relation to total duty cycle (Ti%Ttot) decreased significantly by 20%, 15%, 20% and 4%, respectively. The decreases in breathlessness were significantly related to the reduction in ventilation associated with training. A significant decrease in breathlessness remained after removing the variance accounted for by Ve, reflecting the improvements in respiratory muscle performance. The decreases in breathlessness were better correlated with improvements in the dynamic measure of Vimax than the static measure of MIP. The decreases in breathlessness were best correlated to the changes in Vi%max, which accounts for decreases in ventilatory demand and increases in respiratory muscle performance. Decreases in breathlessness primarily occurred in patients who achieved a training effect. Exercise training decreases breathlessness in patients with coronary artery disease largely through decreases in Ve, with improvements in respiratory muscle performance contributing to a lesser degree. / Thesis / Master of Science (MSc)