Exercise — Energy Systems and Exercise Zones.

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Key Takeaways:

  • Aim for regular moderate intensity exercise:
    • Uses and builds low twitch muscle fibers.
      • These fibers have the highest number of mitochondria – you want more of them.
      • These fibers also help clear lactate.
    • More low twitch fibers = better mitochondrial health = better fuel partitioning.
      • Rely more on burning fat.
      • Save your glycogen stores.
    • Also known as “Zone 2” training.
      • Zone 2 = “you are still able to have a conversation” (or more sophisticated heart rate or lactate levels measurements).
      • 2 times a week for about one hour.

Introduction

  • All tissues require adenosine triphosphate (ATP) to function normally
    • ATP = universal energy currency.
      • Adenosine plus three phosphate groups.
      • Phosphate groups have high energy (electron) bonds between them.
  • The release of phosphate groups by ATP creates free energy to be used by the tissue.
    • Losing one phosphate group at a time (see “AMPK” for more detail):
      • ATP -> ADP -> AMP.
    • Needs an enzyme to get the process started.
      • ATPase.
  • ATP can be produced:
    • With or without oxygen (aerobically or anaerobically).
    • From various substrates (mainly fats or carbohydrates).
    • Inside the cell’s cytosol or mitochondria.
  • Three main energy systems generate energy and replenish ATP.
    • Phosphagen system.
      • When: for immediate, ultra-short term ATP demand.
      • How: anaerobic.
      • Using: no substrate (uses existing ATP stores in the muscles).
      • Where: mitochondria.
    • Glycolysis.
      • When: for short-term (high intensity) ATP demand.
      • How: anaerobic.
      • Using: glucose.
      • Where: cytosol.
    • Mitochondrial respiration.
      • When: for longer-term (low intensity) ATP demand.
      • How: aerobic.
      • Using: glucose or fats.
      • Where: mitochondria.
  • All energy systems contribute to varying degrees to the coordinated metabolic cellular response.
    • Depending on intensity and duration of the exercise.

Phosphagen system: anaerobic use of existing ATP stores in the muscle.

  • Immediate response to high ATP demand.
    • Able to support extremely high muscle force application and power outputs.
      • ATP demands exceed the capacity of mitochondrial oxidation.
    • Short-term singular or a limited number of repeated intense muscle contractions.
      • Throwing, hitting, jumping, sprinting.
    • Limited capacity.
  • Existing ATP or phosphocreatine stores in the muscle.
    • ATP store:
      • Small, allows for 1-2 seconds of maximum effort.
    • Phosphocreatine store:
      • Another 5-8 seconds of exercise.
    • Together 10-15 seconds of exercise of maximal activity.
      • Seldom near complete reliance on phosphagen system.
    • Rapidly replenished during recovery.

Glycolysis: anaerobic use of glucose in the cytosol.

  • High intensity ATP demand.
    • When requirements exceed maximum oxygen uptake.
    • Kicks in after the first few seconds and up to 2-3 minutes.
  • Breaks up blood glucose.
    • Glycogen stores in the muscles (80%) and in the liver (15%)
    • One glucose molecule breaks into two pyruvate molecules.
  • Produces pyruvate and NADH.
    • Pyruvate: produced through the breakdown of glucose.
    • NADH: produced through the reduction of NAD+.
  • Produces modest amount of ATP:
    • Net yield of 2 ATP produced in two phases:
      • Phase 1: costs 2 ATP (breaking glucose into 2 intermediate molecules).
      • Phase 2: generates 4 ATP (formation of 2 pyruvate molecules).
  • In presence of oxygen:
    • Pyruvate and NADH absorbed by mitochondria for further ATP production.
  • If oxygen is absent:
    • NADH donates its electrons to (excess) pyruvate.
    • Pyruvate converts into lactate.
      • This process allows NADH to be onverted back into NAD+.
      • (Similar to process of fermentation)
  • Production of lactate is important.
    • Helps to remove excess pyruvate.
    • Helps to regenerate NAD+.
    • NAD+ allows process of glycolysis to continue.
  • Lactic acid taken back to the liver.
    • Through the bloodstream where it is converted back to pyruvate.
  • Perhaps, some lactic acid taken back to the mitochondria.
    • Lactate shuttle theory (see below).

Mitochondrial respiration: aerobic using fats and carbs in the mitochondria.

  • When ATP demand is slow enough.
    • Cellular respiration or oxidative phosphorylation.
    • Combustion of fuel in the presence of sufficient oxygen.
  • In multiple steps, oxygen takes electrons from fuel to generate free energy.
    • Energy is then stored in the form of ATP.
  • Step 1: fuel enters the mitochondria.
    • Fatty acids, glucose / pyruvate enter the matrix of the mitochondria.
  • Step 2: taking electrons from fuel.
    • Taking away electrons.
      • Oxidation of pyruvate.
      • Beta-oxidation of fatty acids.
    • Giving electrons.
      • Reduction of NAD+ into NADH (see “NAD” for more detail)
      • Reduction of FAD into FADH2.
    • Formation of acetyl CoA.
  • Step 3: taking electrons from acetyl CoA.
    • A series of reactions called the Krebs or Citric Acid Cycle.
      • Oxidation of acetyl CoA.
      • Again, reduction of NAD+ and FAD into NADH and FADH2.
    • Generation of 2 ATP.
  • Step 4: release of electrons.
    • Process takes places at the inner membrane space.
      • The “electron transport chain”.
    • NADH and FADH2 are ready to release their electrons.
      • Oxidative phosphorylation.
        • Oxidation of NADH (into NAD+).
        • Oxidation of FADH2.
    • Electrons are gradually released through the transport chain.
      • As electrons are released, protons are pumped across the membrane.
        • At the end of the chain, electrons are donated to oxygen (forming water).
        • Oxygen is needed as a final acceptor of electrons.
        • Without oxygen, the process would back up.
    • A proton gradient forms on one side of the membrane (see also “The Vital Question”).
      • The flow-back of protons across the membrane back to the matrix helps form ATP.
      • ADP -> ATP.
    • Produces about 27-38 ATP.
      • Depends on efficiency of the cell.
  • See also MedCram video for explanation of mitochondrial process.

Examples of estimated contribution of energy systems:

  • 10 second sprint estimated ATP contribution:
    • Phosphagen: 53%
    • Glycolysis: 44%
    • Mitochondrial respiration: 3%
  • 30 second sprint estimated ATP contribution:
    • Phosphagen: 23%
    • Glycolysis: 49%
    • Mitochondrial respiration: 28%
  • 75 second sprint estimated ATP contribution:
    • Anaerobic (phosphagen & glycolysis): 50%
    • Mitochondrial respiration: 50%

Exercise intensity and energy systems and fuels

  • Exercise = sustained muscle contraction = sustained need for ATP.
  • Muscle metabolic rates vary to a greater extent than any other tissue.
    • High intensity exercise -> 1,000-fold increase in ATP demand compared to at rest.
  • Muscle tissue can vary its metabolic rate to a greater extent than any other tissue.
    • Depending on the demands placed upon it.
  • Low to moderate intensity exercise (majority).
    • Up to 55-75% of VO2max.
    • Mitochondrial respiration.
    • Burning mostly fats, limited carbohydrates.
  • Higher exercise intensity exercise.
    • Beyond 75% of VO2max.
    • Mitochondrial respiration.
    • Less fats (ATP generation too slow), more carbohydrates.
  • Very high intensity.
    • Beyond 100% of VO2max.
    • Anaerobic (glycolysis and phosphagen system).
    • Burn glucose.
  • Importance of lactate.
    • We could not sustain high-intensity exercise for more than 15 seconds without lactate production.

Exercise intensity and muscle fibers.

  • Two kinds of muscle fibers.
    • Type I or slow twitch (less forceful muscle contractions, fatigue slower).
    • Type II or fast twitch (more forceful, fatigue faster).
      • Sub-divided into Type IIa and IIb.
  • Sequential recruitment.
    • First Type I, then Type IIa, then Type IIb.
  • Slow twitch, Type 1:
    • Used in low intensity exercise.
    • Have the highest mitochondrial density.
    • High oxidative capacity (mitochondrial respiration), using fatty acids.
    • Low glycolytic capacity.
  • Fast twitch, Type 2A:
    • Used in higher intensity exercise.
    • Lower mitochondrial density.
    • Fatigue faster, but still relatively slowly.
    • High oxidative and high glycolytic capacity.
  • Fast twitch, Type 2B:
    • Used in highest intensity (short duration) exercise.
    • Little mitochondrial density.
    • Fatigues fastest.
    • Low oxidative and high glycolytic capacity (also using existing ATP stores).

Exercise zones: combining substrates and muscle fibers

Zone 2

  • Iñigo San Millan’s six training zones (image credit: Trainingpeaks.com)

Exercise: benefits of zone 2 training

  • Stimulates Type I fibers.
    • Have the highest density of mitochondria.
    • Clear lactate.
  • Stimulates mitochondrial growth and function.
    • This in turn will improve the ability to utilize fat.
    • Key in athletic performance.
      • By improving fat utilization, preserve glycogen utilization for longer duration.
      • Beneficial at later stages of exercise as intensity increases and glucose utilization is needed.
  • Provides better lactate clearance.
    • Type II fibers create lactate (during glycolysis).
    • Type I fibers clear lactate (by transporting it back to mitochondria for further use)
  • Frequency and duration:
    • 3-4 days a week of zone 2 training in the first 2-3 months of pre-season training.
    • 2-3 days a week as the season gets closer.
    • 2 days of maintenance once the season is in full blown.

Exercise: lactate as fuel

  • Old thesis: lactate is a toxic metabolic by-product.
    • Excess lactate goes into the bloodstream, then back to the liver where it is cleared.
    • Lactate gives rise to fatigue and muscle pain during anaerobic exercise.
  • Complementary new thesis: lactate shuttle.
    • The accumulation of Hydrogen ions drives fatigue, not lactate.
    • Lactate is formed under both aerobic and anaerobic conditions.
    • Type I fibers clear lactate by transporting it back to mitochondria.
    • The oxidation of lactate (back to pyruvate) in the mitochondria can be a major energy source.
  • So, lactate can be a major fuel source.
    • Faster fuel than glucose.

Exercise and fatigue.

  • Decrease in force production during muscle contraction despite constant or increasing effort.
    • Preventive mechanism.
    • Stops ATP from falling to levels that could cause muscle rigor or irreversible muscle damage.

Sidenote: aerobic glycolysis (as opposed to “usual” anaerobic glycolysis).

  • Glucose can also be converted to lactate in the presence of oxygen.
    • Usually it happens anaerobically.
  • Cancer stem cells within a tumor are notorious for aerobic glycolysis
    • Warburg Effect.
    • Even in the presence of sufficient oxygen, cancer cells use glucose for fuel.
    • Cancer cells produce a lot of lactate due to:
      • Injured mitochondria -> increased glycolysis -> increased lactate.
      • High cell growth drives high metabolic demand -> increased lactate.
      • Lactate is a signaling model -> over-express oncogenes.

Key Sources

 

 

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