Exercise Thresholds

• Lactate concentration:

  • lactate has an acidifying effect
  • lactate separates and releases a proton
  • increased protons (H) = increased acidity/decreased pH
• Buffering protons:

  • bicarbonate (HCO3) binds with H to form H2O and CO2
  • prevents increase in protons
  • reaction = carbonic anhydrase (goes both ways)
  • body’s main defence against acidity
• CO2 production and breathing:

  • ventilation stops CO2 from accumulating
  • drives carbonic anhydrase reaction toward production of CO2 (to breathe it out) and consumption of protons (H)
• Metabolic Boundaries:

  lactate threshold:

  • metabolic rate (VO2) where blood lactate is maintained at resting levels
  • separates moderate and heavy domains

  critical intensity:

  • highest metabolic rate (VO2) where lactate production in muscle is stabilized by blood
  • separates heavy and severe domains
• Incremental exercise:

  • purpose = progressively increase intensity until we can determine VO2max
  • as we move from rest to VO2max, we cross LT and CI
• Below LT:

  • moderate intensity
  • 2 metabolic CO2 paths = PDH rxn and Krebs Cycle
  • always some lactate being produced
  • at moderate intensity, everything works perfectly
  • VO2 and VCO2 rise together; VE rises with need to resupply O2 and remove CO2
  • ratio if variables does not change
  • no change in: La, HCO3, pH
  • pressure in gas after expiration tells us what the arterial pressures of O2 and CO2 are
• Lactate at resting concentrations:

  • rate of lactate appearance equal to its disappearance
  • any tissue that is oxidative will take lactate
  • concentration does not change
• Above LT:

  • heavy intensity
  • pyruvate becomes lactate to ensure glycolysis continues
  • remove lactate via blood
  • lactate concentration in blood increases (goes to other tissues as pyruvate for oxidative systems)
  • increase lactate = increase proton production = increase acidity (HCO3 mops up H to produce H2O and CO2)
• Above LT:

  • VE follows along with CO2 as CO2 increases
  • O2 increases with ATP demand (start buffering)
  • detect changes at mouth
  • La increases slightly (HCO3 mops up H from La to keep pH normal)
  • but CO2 starts to rise as we buffer H
  • therefore: moderate rise in La and HCO3
• Lactate elevated but stable:

  • rate of lactate production equals disappearance
  • La increases but stabilizes
  • increase muscle engagement = more La dumped into blood
  • VO2 and VCO2 rise together above LT
  • VCO2 rises more than VO2; this is our respiratory threshold
• Above LT:

  • VE increases with VCO2 (maintains arterial CO2 levels)
  • La, gas exchange, and first ventilatory threshold all detect transition from moderate to heavy domain
  • no change in pH
• Above CI:

  • severe intensity
  • exercise not maintained by oxidative systems; must use anaerobic
  • decrease exercise tolerance (because we are not able to replace ATP)
  • have not reached VO2max yet
  • too much H; HCO3 cannot handle it, so it fails at CI
  • therefore, we remove CO2 by breathing faster (hyperventilation/respiratory alkalosis)
  • arterial pH decreases, meaning we accumulate more H = acidic
  • increase VE even more to decrease CO2 pressure
• Lactate elevated and unstable:

  • rate of lactate appearance exceeds disappearance
  • point of hyperventilation
• 2 metabolic thresholds:

  GET/VT1:

  • VO2 at onset of HCO3 buffering where VCO2 and VE begin to rise at greater rate than VO2

  RCP/VT2:

  • VO2 at which HCO3 buffering cannot prevent acidosis; hyperventilation to compensate
• Variability:

  • thresholds are important for predicting performance and fitness levels
  • not static
  • variability of threshold between men and women
  • normalizing metabolic response = NO
  • the only way to prescribe exercise is to identify where thresholds are and place individuals within those thresholds
• RCP and performance:

  • the higher the RCP, the faster you can complete a task
  • therefore, RCP is an indicator of performance
  • higher RCP = cover distance in less time