The Applied Exercise Science Lab is seeking 18-35 year-old volunteers: 1) cyclists who train >8 hrs/wk, or 2) non-cyclists who participate in >3 exercise sessions/wk of >30 min totally 2.5-5 hrs/wk. Participants must have been at an altitude similar to Boulder for >3 wks. Activity screening questions will be asked upon inquiry. You will be asked to cycle at submaximal and supramaximal intensities. The study will require 8 hrs: 4 visits, 2 hrs/visit. VO2max, lactate threshold, efficiency and training zone break downs provided.