VO2max and Substrate Crossover Point

This study evaluates the relationship between maximal oxygen uptake (VO2max) and the substrate crossover point during incremental treadmill exercise in young, physically active individuals. Substrate utilization kinetics and metabolic flexibility represent critical components of cardiovascular conditioning and endurance performance; however, the impact of continuous variations in aerobic capacity on the onset of metabolic transition requires detailed investigation.

Participants undergo anthropometric testing followed by a maximal graded exercise test on a motor-driven treadmill following the standard Bruce protocol. Breath-by-breath expired gas analysis using indirect calorimetry continuously monitors oxygen uptake (VO2), carbon dioxide production (VCO2), and minute ventilation (VE) throughout the test. Respiratory exchange ratio (RER) values are calculated continuously. The substrate crossover point is operationally defined as the exercise intensity corresponding to an RER threshold of 0.85, representing the metabolic transition where carbohydrate oxidation becomes the predominant contributor to total energy expenditure over lipid oxidation.

Key outcome measures extracted at this threshold include:

Time to reach the crossover point (minutes)

Absolute oxygen uptake at crossover (liters per minute [L/min])

Relative oxygen uptake at crossover (milliliters per kilogram per minute [mL/kg/min])

Percentage of maximal oxygen uptake at crossover (%VO2max)

Heart rate at crossover (beats per minute [bpm])

Statistical associations between maximal aerobic capacity (VO2max) and the primary outcome measures at the crossover point are analyzed using linear correlation and regression models to determine how cardiorespiratory fitness alters substrate switching kinetics under progressive physical strain.

Inclusion criteria

• Healthy male or female physical education students aged 18 to 30 years
• Regularly physically active (participating in structured physical activity at least 3 days per week)
• Medically cleared and capable of performing maximal graded treadmill exercise testing to voluntary exhaustion
• Willing and able to provide written informed consent

Exclusion criteria

• History of cardiovascular, pulmonary, metabolic, renal, or neuromuscular disorders
• Acute musculoskeletal injury or orthopedic condition that impairs treadmill running or maximal exertion
• Current use of prescription medications, stimulants, or dietary supplements known to alter metabolic rate, heart rate, or substrate oxidation
• Use of tobacco, nicotine products, or alcohol within 24 hours prior to testing
• Consumption of caffeine or vigorous physical exercise within 24 hours prior to the exercise trial