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

What is an individual’s basal metabolism dependent on?

• amount of lean body mass

• amount of BSA

• thyroxine levels

• nutritional state (starvation)

• growth (pregnancy/lactation, puberty, infancy)

What are things that increase BMR to above normal levels?

• caffeine/tobacco

• stress/increase in SNS activity/sleep loss

• high body temperature/fever

• injury, sepsis, burns

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

How do you convert L/min to mL/kg/min?

you take the value that is already in L/min

• numerator: multiply by 1,000 (to convert L to mL)

• denominator: divide by x kg (to add kg to the denominator)

How exactly do you determine if plateau is reached?

• 1) calculate the expected VO2 for the last two stages

• 2) calculate the expected increase from one stage to the next (basically take the difference between the values from step 1)

• 3) calculate half of the expected increase (divide value from step 2 by 2)

• 4) calculate actual increase

• 5) compare the actual half increased to expected

  • if the actual increase is greater than expected increase, then you did not reach plateau (makes sense, because this means VO2 can still increase rather than plateau)

How would you locate the pneumotach on a printout?

VE, which comes from the pneumotach

How would you locate the gas analyzer on a printout?

fraction of expired O2 and CO2 in percentages, which comes from the gas analyzer

Describe the physiology behind VO2 plateau (criterion 1)

• vo₂ increases as exercise intensity increases

• vo₂ measures how much oxygen the body uses

• at high intensities, vo₂ eventually stops rising → plateau

• plateau occurs because:

  • oxygen delivery can’t increase further to meet the demands

  • oxygen extraction reaches its limit

Describe the physiology behind peak heart rate (criterion 2)

• cardiac output must increase to meet high metabolic demands during exercise

• cardiac output = stroke volume × heart rate

• sympathetic activation raises heart rate as exercise intensity increases

• stroke volume eventually plateaus, but heart rate keeps rising

• heart rate rises until it reaches its max (≈220 − age)

• once HR is at max, the heart is working at full capacity

Describe the physiology behind RER (criterion 3)

• RER is the ratio of co₂ produced to o₂ consumed

• the body uses fat or carbs as fuel; they require different amounts of o₂

  • at high intensity, carbs become the primary fuel source

• intense exercise produces lactate → extra co₂ is generated to buffer acidity

• this added co₂ pushes rer above 1

  • an RER > 1.1 indicates heavy carb use

  • fat metabolism would give an rer around 0.7

Describe the physiology behind lactate production (criterion 4)

• muscles produce lactate during exercise

• at high intensities, fast-twitch fibers are recruited and rely on anaerobic metabolism

• anaerobic metabolism generates lactate

• eventually, lactate production exceeds lactate removal

• the lactate shuttle and liver can’t keep up with rising lactate levels

• this is why blood lactate concentration is measured after intense exercise