During the 5 minutes of breathing normally at rest and during the resistance breathing, the following non-invasive measurements will be made: inspiratory mouth pressure (Pm); expired gas will be analyzed breath-by-breath for minute ventilation (VE), oxygen consumption (VO2), and carbon dioxide production (VCO2) using a metabolic measurement system (MedGraphics); oxygen saturation will be recorded using a pulse oximeter; and end-tidal partial pressure of CO2.