VO2max and Substrate Crossover Point

Celil Kaçoğlu20 enrolled

Overview

This study aims to investigate the relationship between maximal oxygen uptake (VO2max) and the substrate crossover point during incremental exercise. A total of 20 physical education students are recruited to complete a maximal incremental treadmill test using the standard Bruce protocol. Indirect calorimetry is utilized to assess oxygen uptake (VO2), carbon dioxide production (VCO2), and energy substrate utilization patterns. The substrate crossover point is operationally defined at a respiratory exchange ratio (RER) threshold of 0.85, indicating the metabolic transition toward predominant carbohydrate oxidation. The primary objective is to evaluate how variations in aerobic capacity influence the timing, oxygen uptake, and heart rate (HR) responses at this metabolic transition threshold.

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.

Study Type

Eligibility

Sex: ALLMin age: 18 YearsMax age: 30 YearsHealthy volunteers:

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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

Outcomes

Primary Outcomes

Time to reach the substrate crossover point

Measure Description: Elapsed time (in minutes) from the start of the incremental treadmill test (Bruce protocol) to the point where the respiratory exchange ratio (RER) reaches 0.85, indicating the metabolic crossover where carbohydrate oxidation becomes the predominant fuel source. Time Frame: Baseline (single exercise test session, Day 1) Unit of Measure: Minutes

Time frame: Time Frame: Baseline (single test session, Day 1)

Relative oxygen uptake at the crossover point

Measure Title: Relative oxygen uptake at the substrate crossover point Measure Description: Oxygen uptake (VO2) normalized to body weight (mL/kg/min) recorded at the moment the substrate crossover point (respiratory exchange ratio \[RER\] = 0.85) is reached during the maximal incremental treadmill test (Bruce protocol). Time Frame: Baseline

Time frame: Measure Description: Measured continuously during the incremental exercise test using indirect calorimetry, recorded specifically at the point where the respiratory exchange ratio (RER) reaches 0.85. Time Frame: Baseline

Secondary Outcomes

Percentage of VO2max at the crossover point

Measure Title: Percentage of maximal oxygen uptake at the substrate crossover point Measure Description: Relative oxygen uptake (VO2) at the substrate crossover point (respiratory exchange ratio \[RER\] = 0.85) expressed as a percentage (%) of the participant's maximal oxygen uptake (VO2max) during the Bruce protocol treadmill test. Time Frame: Baseline

Time frame: Measure Description: Relative oxygen uptake (VO2) at the substrate crossover point (respiratory exchange ratio [RER] = 0.85) expressed as a % of the participant's maximal oxygen uptake (VO2max), calculated upon completion of the Bruce protocol

Heart rate at the crossover point

Measure Title: Heart rate at the substrate crossover point Measure Description: Heart rate (HR) recorded in beats per minute (bpm) using a telemetric heart rate monitor at the point where the substrate crossover threshold (respiratory exchange ratio \[RER\] = 0.85) is achieved during the maximal incremental treadmill test (Bruce protocol). Time Frame: Baseline

Time frame: Measure Description: Heart rate (HR) in beats per minute (bpm) measured continuously during the incremental treadmill test (Bruce protocol), recorded at the substrate crossover threshold (respiratory exchange ratio [RER] = 0.85). Time Frame: Baseline

Maximal oxygen consumption (VO2max)

Measure Description: The highest rate of relative oxygen uptake (VO2 in mL/kg/min) attained during the maximal graded exercise test (Bruce protocol) to voluntary exhaustion via indirect calorimetry. Time Frame: Baseline

Time frame: Measure Description: Relative maximal oxygen uptake (VO2max in mL/kg/min) assessed at maximal exertion at the end of the Bruce protocol treadmill test to voluntary exhaustion via indirect calorimetry. Time Frame: Baseline

VO2max and Substrate Crossover Point

Overview

Conditions

Interventions

Eligibility

Locations (1)

Outcomes

Primary Outcomes

Secondary Outcomes