This study evaluates oxygen prebreathe protocols and exercise conditions to reduce risk of decompression sickness during simulated extravehicular activity. Healthy adult volunteers will undergo oxygen prebreathe followed by hypobaric chamber exposure with monitoring for symptoms and ultrasound detection of bubbles.

Inclusion Criteria:

• Non-smoking volunteers aged 30-60 years
• Able to meet VO2 peak requirements (>35 ml/kg/min males, >30 ml/kg/min females)

Exclusion Criteria:

• Pregnancy
• Lung disease, heart disease, hypertension, or diabetes
• Musculoskeletal pain conditions
• Requirement for regular medications such as analgesics