BACKGROUND: Testing to determine physical activity (PA) and aerobic fitness levels is time-consuming and costly. Hypoxanthine (Hx) is a product of ATP breakdown that has been shown by previous literature to respond to exercise and indicate training adaptation. The purpose of this study is to determine the relationship between hypoxanthine concentration at rest, average daily physical activity levels, and peak fitness measures. METHODS: n=30 male and females ages 18-25 will be recruited to participate. Participants will wear an ActiGraph GT9X Link accelerometer on the non-dominant wrist for three consecutive days, including one weekend day. Time spent in moderate and vigorous intensity PA will be determined using standard Freedson cut-points. Participants will then visit the laboratory for a single testing session and will be asked to refrain from exercise and caffeine consumption for 12 hours prior to the visit. Capillary blood samples will be collected at rest as well as 15 minutes after the VO2 max test using a finger prick on the third or fourth finger with a disposable lancet. Blood will then be centrifuged, and plasma will be separated and stored until biochemical analysis. Participants will complete a VO2max test to volitional exhaustion on a Concept 2 rowing ergometer; expired gases will be collected throughout the testing session via the Parvo metabolic system, and heart rate (HR) will be continuously monitored with a Polar HR monitor. Each test stage will last 2 minutes before increasing intensity by 50 watts and proceeding to the next stage, with starting power output of 50 Watts. Within the first 15 seconds of each stage, the participant will increase their power to the wattage instructed and will maintain the output for the remaining time of the stage; failure to maintain power output will result in test cessation. Oxygen consumption (VO2), HR, and respiratory exchange ratio (RER) will be recorded each minute. Immediately after exercise cessation, a fingerstick will be conducted to measure lactate concentration in the blood. Repeat blood sampling for Hx will be taken at 15 minutes post-exercise. After data acquisition is complete, plasma concentrations of Hx will be determined using a commercially available enzyme activity xanthine/hypoxanthine assay kit. Correlations between physical activity, aerobic fitness, and resting and post-exercise Hx levels will be calculated. ANTICIPATED RESULTS: We anticipate, based upon previous literature, that Hx levels will decrease from rest to post-exercise in all participants, but that aerobic fitness and physical activity will be positively associated with Hx levels. Further, we hypothesize that post-exercise Hx will have a stronger correlation with PA and aerobic fitness compared to resting Hx levels.

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