Cardiorespiratory Fitness, Maximal Oxygen Uptake, Metabolic Flexibility, Substrate Metabolism During Exercise
Conditions
Keywords
vo2max, Substrate Crossover Point, Metabolic Flexibility, Bruce Protocol, Indirect Calorimetry, Substrate Oxidation, Incremental Exercise, Respiratory Exchange Ratio (RER)
Brief summary
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.
Detailed description
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.
Interventions
BEHAVIORALIncremental Graded Exercise Test (Bruce Protocol)
Participants perform a maximal treadmill exercise test following the standardized Bruce Protocol. The test begins with a warm-up, followed by progressive 3-minute stages with continuous increases in speed and incline until voluntary exhaustion. Indirect calorimetry is used continuously via breath-by-breath gas analysis to measure VO2VCO2, and RER for substrate crossover determination.
Sponsors
Celil Kaçoğlu
Study design
Allocation
NA
Intervention model
SINGLE_GROUP
Primary purpose
BASIC_SCIENCE
Masking
NONE
Masking description
Not applicable. This is an unblinded (Open Label) physiological performance study where all participants and investigators are fully aware of the exercise testing protocol and procedures.
Intervention model description
This study utilizes a single-arm, continuous-variable design. All eligible participants undergo a single-visit testing session consisting of anthropometric measurements followed by a standardized incremental graded exercise test using the Bruce treadmill protocol until voluntary exhaustion. Because the main objective is to evaluate physiological relationships-specifically, how varying levels of maximal oxygen uptake (VO2max) correlate with the timing and oxygen cost of the substrate crossover point-all participants receive the same exercise stimulus and assessment procedures, serving as their own control across exercise intensity stages. No comparative control group or parallel group stratification is required.
Eligibility
Sex/Gender
ALL
Age
18 Years to 30 Years
Healthy volunteers
Yes
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
Design outcomes
Primary
| Measure | Time frame | Description |
|---|---|---|
| Time to reach the substrate crossover point | Time Frame: Baseline (single test session, Day 1) | 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 |
| Relative oxygen uptake at the crossover point | 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 | 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 |
Secondary
| Measure | Time frame | Description |
|---|---|---|
| Percentage of VO2max at the crossover point | 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 | 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 |
| Heart rate at the crossover point | 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 | 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 |
| Maximal oxygen consumption (VO2max) | 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 | 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 |
Countries
Turkey (Türkiye)
Contacts
PRINCIPAL_INVESTIGATORcelil kaçoğlu, Ph.D.
Eskişehir Teknik Üniversitesi
Outcome results
None listed