The aim of this randomized, single-blind, placebo-controlled crossover trial is to determine the effects of graded hypercapnia (0, 2, and 4% inspired CO2) on endurance performance. Twelve healthy and fit young adults (men and women) will complete this study. Participants will perform experimental trials on 3 separate days (one each breathing 0%, 2%, and 4% inspired CO2-all with normal 21% oxygen). During each trial, they will perform 10 minutes of submaximal treadmill walking exercise and then complete a self-paced 2-mile treadmill run time-trial.

Participants will undergo baseline measurements of pulmonary function and maximal oxygen uptake (VO2max). They will then complete two familiarization trials, with 10 minutes of submaximal exercise (~40%VO2max) followed by a self-paced 2-mile treadmill time trial. Lastly, they will complete three experimental trials, repeating the same submaximal and time-trial exercise procedures while breathing the test gas (0%, 2%, or 4% CO2-all with normal 21% oxygen). During the experimental trials, arterialized capillary blood samples will be collected at rest breathing room air, at rest breathing the test gas, and during submaximal exercise at 40% VO2max breathing the test gas. The primary outcome is 2-mile treadmill time-trial performance. Secondary outcomes include ventilation, metabolic rate, heart rate, arterialized capillary pH/PCO2/HCO3, perceived exertion, dyspnea, leg discomfort, breathing descriptors, and headache ratings.

Inclusion Criteria:

• Male or female, age 18-45
• In good health as determined by medical screening
• Passed his/her most recent Army Combat Fitness Test (ACFT) (Military Volunteers Only) or performs aerobic exercise at least 2 times per week and able to complete two mile run in ≤ 21 minutes (Civilian Volunteers)
• Willing to not participate in any exercise or drink alcoholic beverages for 24 hours before each study visit.
• Willing to not take part in any strenuous exercise in the 36 hours before each visit.
• Willing not to drink caffeine or eat food until after testing on familiarization and performance testing visits (visits 2-6).
• Willing to maintain a similar dietary pattern in the 2 days before each familiarization and performance visits.
• Have supervisor approval if permanent party military or a federal employee at U.S. Army Natick Soldier System Center
• Able to speak and read English fluently

Exclusion Criteria:

• Females who are pregnant or planning to become pregnant during the study
• Taking dietary supplements, prescriptions, or over the counter medication, other than a contraceptive (unless approved by medical staff & PI)
• Musculoskeletal injuries that compromise ability to run on a treadmill
• Abnormal blood count, presence of abnormal blood chemicals (hemoglobin S or sickle cell traits)
• Smokers or tobacco/nicotine users (unless have quit > 1 month prior)
• Any history of asthma
• Current or recent respiratory tract or sinus infections (< 1 month prior)
• Allergy to skin adhesive
• Any history of migraine or recurrent headaches
• Any history of panic disorder
• Blood donation in the previous 8 weeks
• Positive Covid-19 test within the last month

Single-blinded