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?