1. Chronic stable tetraplegia (duration of injury > 1 year) matched for age, gender, and BMI with healthy control subjects, and clinically stable mild asthmatic subjects. The subjects approached will have already consented to, or be actively participating in "Effects of Nitric Oxide Synthase Inhibitor on Levels of Exhaled NO and Airway Tone in Subjects with Chronic Cervical Spinal Cord Injury" study.