Metabolism During Intense Exercise


In this lecture, we look at all the ways metabolism is altered by the various types of exercise that we undertake. We explore how muscle activation signals to the cells to start running glycogenolysis and glycolysis, and how, under intense exercise, glycolysis requires an additional step using anaerobic metabolism to produce lactate. We also explore the Cori cycle, which shuttles lactate to the liver for conversion BACK to glucose.

From there, we discuss the creatine phosphate system, and how taking supplemental creatine boosts your internal energy stores.

Lastly, we discuss aerobic exercise, specifically understanding when it is fueled by fatty acids, and when it is fueled by glycogen/glucose.

Key Takeaways

  • Know which types of exercise are fueled by which energy sources (glucose, fatty acid, ATP/Creatine)
  • Explain how muscle firing activates glycogen breakdown
  • Explain how lactate is created during anaerobic exercise
  • Describe what happens to the lactate that is produced during anaerobic exercise
  • Explain how creatine is used to restore ATP levels immediately
  • Recognize when aerobic exercise uses fatty acids and when it uses glucose as the major fuel source
  • Know the patterns of how growth hormone and cortisol are produced post exercise

Key Terms

Types of Exercise

  • Anaerobic Exercise
    • Very short: ATP - Creatine phosphate
    • Short: Anaerobic Glycolysis
    • Type || muscle fiber (fast twitch) dominant
  • Aerobic Exercise
    • Aerobic Glycolysis
    • Type | muscle fiber dominant

Anaerobic Exercise

• The high intensity nature of the exercise requires near immediate replenishment of ATP • Creatine phosphate pathway exhausts itself in seconds • Use of the glycolysis pathway increases 1000X!!!

• The blood stream supply of glucose is too slow, and therefore, glycogen breakdown is considered the primary source • Muscle firing itself stimulates glycogen breakdown to glucose

Muscle contractions are what actually stimulate the glucose breakdown pathway

calcium is liberated from muscle contractions -> ATP turns into ADP + P -> APK -> Activates glycogen phosphorylase -> Glycogen -> Glucose-1p

NADH builds up because there isn’t enough oxygen in your muscle

Lactate frees up NADH and turns it back into NAD+ which lets you run the pathway again.

Increase in Lactate.. but the cause of breakdown in performance is thought to be Hydrogen ions

Disruption of Actin and Myosin (muscles cant contract) decreased enzymatic function of PFK-1 (cells cant run glycolysis)

Sodium Bicarbonate (Baking soda) can neutralize this build up

(Glycolysis) Glucose -> Pyruvate -> Lactate

goes to the liver

(Gluconeogenesis) Lactate -> Pyruvate -> Glucose

lactate can be picked up by other muscles and turned into pyruvate -> ATP

Contract muscles around the one that has build up to ‘sink’ the lactate into the surrounding muscles

Anaerobic / Aerobic metabolism isn’t an on / off switch.. it’s which one is more dominant at one time

Creatine Phosphate

  • When ATP is “used” by the muscle contraction process, it results in ADP + P
  • Creatine Phosphate can donate a phosphate to ADP very rapidly to restore ATP for additional muscle firing
  • However, the intracellular supply of creatine phosphate is relatively small, only allowing for SHORT bursts of exercise at maximal force

Creatine Kinase turns creatine into creatine phosphate (and works the other way as well)

During repeated muscle contraction creatine phosphate loans the phospate to ADP

Creatine supplementation increases the intracellular stores of creatine, leading to increased creatine phosphate

Aerobic Exercise

  • Simply, aerobic exercise is done at a pace that allows for the complete oxidation of substrates
    • Provides a MUCH higher ATP output
    • O2 levels must be adequate and CO2 must be appropriately removed
  • Both fatty acids and carbs can be used during aerobic workouts
    • Which is used is predominantly determined by the intensity

Can run the full process of glycogen moving into the mitochondria and running the full citric acid cycle

  • Oxidation of fatty acids requires efforts to be maintained below 60% VO2 max
  • Above that, carbohydrate must be used
  • Time to exhaustion runs directly correlate with preloaded muscle and liver glycogen levels

total system response

Sympathetic nervous system signal wins during exercise

increased cardiac output, insulin suppression, vasodilation glycogen breakdown, lipolysis, adrenaline induction

‘fight or flight’ response

(parasympathetic nervous system is the rest and digest)

  • Activity of the sympathetic nervous system, and adrenaline, cause large changes in physiology
  • Blood flow to muscles can increase by 100-fold
  • Cardiac output increases as well
  • Both Growth Hormone and Cortisol are secreted in response to exercise

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