The goal of this study is to characterize associations between cardiorespiratory fitness, as measured by VO₂max/VO₂peak, and blood-based molecular measurements in adults. This is an observational study where research staff measures VO₂max/VO₂peak and collects fasted, resting blood samples on the same day. The main questions it aims to answer are:

1. Can a multi-analyte blood panel accurately predict directly measured VO₂max/VO₂peak?
2. How do these blood-based features vary across age, sex, and self-reported or documented physical activity profiles?
3. Are the predictor analytes able to capture changes in VO₂max/VO₂peak caused by a self-directed intervention (e.g., training for a marathon)?
4. What is the analytic reliability of the predictor biomarkers?

Prospective observational cohort with baseline cross-sectional assessment; optional longitudinal follow-up with repeat VO₂max and blood collection in a subset. No intervention is assigned by the investigators. The investigators are amassing a large dataset of diverse individuals with directly measured VO2max paired with blood samples to support robust biomarker development.

Inclusion Criteria:

• Be ≥18 years old
• Be able to understand and provide informed consent (English; additional languages may be added via amendment with translated materials)
• Be willing to complete VO₂max testing, blood draw, and questionnaires.
• Be able to safely perform maximal/symptom-limited exercise testing as determined by: PAR-Q+ screening and investigator review (included as an attachment) and resting vitals screening (heart rate)

Exclusion Criteria:

• Conditions that, in the investigator's judgment, or a medical doctor, makes maximal exercise testing unsafe (e.g., unstable cardiovascular symptoms)
• Contraindications or unwillingness to undergo venipuncture
• Acute illness (fever, significant respiratory illness) at the time of testing
• Failure to comply with procedures.

The study population includes adults from the general community who are interested in health and fitness assessment. Participants are anticipated to primarily be athletes or recreational exercisers. This study does not specifically target clinical populations; however, individuals with stable medical conditions may participate if they have documented medical clearance to safely undergo maximal exercise testing.

Individual participant data will not be shared in order to protect participant privacy and confidentiality. The study was not designed with participant consent for public IPD sharing, and the data include sensitive physiological and molecular measurements. In addition, the collected data include proprietary measurements, algorithms, and analyses that are considered confidential and commercially sensitive. Only aggregate, de-identified results will be disseminated.