VO2

What limits our ability to do aerobic exercise

  • O2 delivery to muscles and how well the muscles utilize O2

Energy generation during physical activity

  • Immediate energy system is the first system that is used and is fully depleted after ~30 seconds

  • Short-term energy is second and kicks in after around 15 seconds and last for about 2 minutes until starts to deplete

  • Long term energy does not kick in until around the 2 minute mark where its becomes the main source of energy

What is VO2

VO2 oxygen consumption

  • Amount of O2 taken up, transported and used at the cellular level

  • Can be measured at rest and during exercise

VO2max: Maximal oxygen consumption

  • Highest oxygen consumption achieved, usually during a graded exercise test

  • Indicates individual’s capacity to re-synthesize ATP aerobically

Why do we care about VO2max

  • Excellent measure of overall fitness

  • Excellent measure for accurately prescribing exercise and monitoring adaptations

  • Excellent clinical marker of mortality risk

Complicated process

O2 → Air → Lungs → Blood → Muscle → Mitochondria → ATP

  • Need to consider

    • Percent O2 in the air

    • Amount of air inspired

    • Transfer of O2 from alveoli to blood

      • Transit time, pH, CO

    • O2 transport in blood to tissues

      • Hemoglobin, Carrying capacity, capillary network

    • O2 use at muscle

      • Mitochondria, enzymes

Factors implicated in increasing VO2max with exercise training


VO2max

  • Maximal oxygen consumption

    • Highest oxygen consumption achieved, usually during a graded exercise test

    • Indicated Individual’s capacity to re-synthesize ATP aerobically

Measurement of VO2

FICK PRINCIPLE:

  • The amount of substrate that goes into an organ in a given period of time minus the amount that comes out must be equal that tissue utilization rate of that substrate

    • VO2 = amt of inspired oxygen - amt expired oxygen (Respiratory)

    • VO2 = amt delivered oxygen x amt oxygen used (Cardiovascular)

VO2 max rest

  • Graded exercise test to exhaustion

  • Use large muscle mass

  • Many types exist 

  • Highly reproducible

Physiological tests of the aerobic energy system

  • Direct calorimetry → measure heat

  • Indirect calorimetry → measure oxygen uptake

  • Metabolic carts → computer calculates data rapidly and continuously

VO2max vs VO2peak

VO2max

  • plateau in oxygen uptake during increasing exercise intensity

  • Chance of seeing a plateau influenced by anaerobic capacity, VO2-kinetics, and metabolite accumulation

  • Not necessarily seen more in athletes

VO2peak

  • Highest oxygen uptake reached during a test

Determinants of finding a VO2 plateau

Upper limit of the cardiovascular and respiratory system to transport oxygen to the muscles

  • Potential causes

  1. Motivation and pain tolerance

  2. Anthropometric, age, and gender

  3. Aerobic fitness and endurance training

  4. Anaerobic power and capacity

  5. Oxygen uptake kinetics

  6. Accumulation of anaerobic metabolites

Test criteria

VO2max:

  • Leveling off or slight decrease in oxygen uptake during increasing exercise intensity

Other criteria (peak)

  • Blood lactate > 70-80mg/100ml blood

  • RER > 1.0 - 1.1

  • Age predicted HR max (+/- 10bpm)

  • Volitional fatigue (ratings of perceived exertion) not considered a criterion

Types of VO2max protocols

Continious:

  • No rest between exercise increments (graded or ramped)

  • Usually lasts 7-12 mins

Discontinuous:

  • Several minutes rest between stages

  • Opportunity to get feedback from subject or make measurements

  • Test can last up to an hour

Treadmail vs Bile:

  • Want to ensure large muscle mass utilize (50% +)

  • Treadmill greater by 10%

  • Adjust incline/speed or adjust resistance (maintain cadence)

Verification test to confirm you reached VO2max

  • Complete a short verification phase at a 10% higher work rate to ensure VO2 does not rise any further 

Factors affecting maximal oxygen uptake

  1. Exercise mode → reflects quantity of muscle mass used

  2. Genetics → twin studies - 20-40%

  3. Training state

    1. VO2 can vary by 5-25% based on training

    2. Sedentary the best responders to training

    3. Plasma volume

  4. Biological sex

    1. Women typically 15-30% lower

    2. Body composition and hemoglobin

  5. Body Composition

    1. Mass explains ~70% of difference among individuals

  6. Age

    1. Cross sectional data, hard to draw inferences

    2. Habitual activity better predictor than age

Absolute vs Relative VO2

You can also have subjects exercise at absolute VO2 or relative VO2

Assume VO2max = 4.0L/min for a male and 3.0L/min for a female

  • Absolute VO2 of 2.5 L/min

    • 65% for a male, or 80% for a female

  • Relative VO2 of 50%

    • 2.0L/min for a male, or 1.5L/min for a female

Predicting VO2max

Valuable for:

  • Those unable (or unwilling) to exert maximum effort

  • Large group

  • When no equipment is available

Various methods exist:

  • Graded sub-max exercise → measure HR and VO2

  • Endurance runs → Measure distance in set time

  • Walking test → measure time for set distance

Predicting VO2max using heart rate

  • For prediction based on HR data, several assumptions exist

    • Linear relationship between HR and VO2

    • Similar max HR for all subjects

    • Constant economy and mechanical efficiency

    • Limited day to day variation in HR

Predicting VO2max from Submax exercise heart rate

  • Graded, submax treadmill exercise test

  • NOTE: both relationship are linear but different slopes

CAUTION

  • All prediction involve error

  • Predicted maximal HR can differ by +/- 10-15 BPM