Visit 3: Visit 3 will involve steady-state semi-recumbent cycling exercise at two steady-state exercise intensities; 40 percent of peak work and (after a brief rest) an intensity eliciting a heart rate of 110 beats per minute (to standardize diastolic duration). Before the exercise begins, participants will have one or two small balloon(s) (2 inches long, deflated) attached to a small plastic tube (the width of a pencil tip) inserted through the nasal cavity and into the esophagus. This is done to measure respiratory muscles at work. Participants will receive a numbing gel (2% lidocaine) to numb the nasal passage and upper esophagus prior to insertion of the balloon(s). During the insertion of the esophageal balloons, participants will also be asked to swallow water to minimize gagging and assure correct balloon placement in the esophagus. Participants will also be asked to breathe a mixture of gases containing acetylene (0.6%), dimethyl ether (1.8%), oxygen (21%, same as room air), helium (9%) and nitrogen (69.4%). The mixture of gases will be inhaled at various time points over the course of the exercise session for 8 to 10 breaths at a time. This is done to non-invasively measure cardiac output. Also during the session, a sonographer will use ultrasound to measure cardiac pressures and volumes.