Lab #4: Maximum Oxygen Consumption

Basal metabolic rate

• amount of energy (in calories) your body needs to maintain basic life-sustaining functions at rest

• measured under strict conditions

  • in the morning, fully rested, supine position

  • 12-18 hours after eating, drinking

  • no strenuous exercise for 24-48 hours

  • free of all physical disturbances (medications, stress, etc.)

Resting metabolic rate

• less strict conditions

  • 4 hour fasting

  • 1 hour without strenuous activity

ATP turnover in muscle from slowest to fastest

ATP-CP → anaerobic glycolysis → aerobic glycolysis (carbs + lipids)

What is maximal oxygen consumption?

• maximum amount of O₂ consumed per minute

• indicates functional ability of:

  • lungs to take in O₂ from surrounding air and transfer it to blood

  • heart to pump blood carrying O₂ to working muscles (limiting factor)

  • skeletal muscle cells to extract and utilize O₂ from blood (AVO₂ difference)

What are factors impacting VO2 max?

• body size (~70%)

• age

• sex (15–30%)

• heredity (20–30%)

• FFM (fat-free mass)

• training status (5–20%)

• exercise mode

How does body size relate to VO2 max?

• ↑ body mass = ↑ absolute VO₂max (L/min)

• to account for differences in body mass, VO₂max is also expressed in relative terms:

  • ml/kg/min

• therefore, relative VO₂max is a better indicator of fitness level than absolute VO₂max 

How does sex impact body mass and therefore VO2 max?

• 70% of the difference in maximal aerobic power between genders may be explained by differences in body mass

• however, if maximal VO₂ is corrected for body weight, and expressed as:

  • ml/kg FFM/min, the difference becomes smaller since men and women may have different amounts of fat but similar lean tissue

• the difference becomes even smaller—or disappears— when maximal aerobic power is expressed as:

  • ml/kg active muscle mass/min

  • muscle tissue in both men and women is equally capable of using oxygen efficiently

How will you be measuring VO2 max?

open circuit indirect calorimetry

Describe the criteria to reach VO2 max

• a plateau in VO₂ with increasing work rate is ideal, but occurs in less than 50% of subjects tested

• peak heart rate within 10 bpm of the age-predicted maximum (220 − age)

• respiratory exchange ratio (RER) > 1.10 at highest VO₂

• blood lactate concentration > 8.0 mmol/L within the first 5 minutes of recovery

• 2/4 of these satisfied = max!

What is the respiratory exchange ratio?

VCO2/VO2

What is lactate threshold?

• the point at which blood lactic acid rises systematically during incremental exercise

  • appears at approximately 50–60% VO₂max in untrained subjects

  • appears at higher work rates (65–80% VO₂max) in trained subjects

• also called:

  • anaerobic threshold

  • onset of blood lactate accumulation (OBLA)

• blood lactate levels reach 4 mmol/L at this point

What are explanations for lactate threshold?

• low muscle oxygen (hypoxia)

• accelerated glycolysis

  • NADH produced faster than it is shuttled into mitochondria

  • excess NADH in cytoplasm converts pyruvic acid to lactic acid

• recruitment of fast-twitch muscle fibers

  • LDH isozyme in fast fibers promotes lactic acid formation

• reduced rate of lactate removal from blood