Some notes I took based on a very cool website (with citations).

• Exercise can be maintained so long as ATP resynthesis equals ATP usage
• ATP resynthesis needs two ingredients:
  • Oxygen supply
  • Fuel supply
    • Glycogen: stored in muscles and not shared
    • Fatty acids: circulating in the bloodstream, from fat
• Types of muscle fibers:
  • Type I (slow-twitch): slow to fatigue, high blood flow capacity, high capillary density (for delivering oxygen), high mitochondrial concentration, low glycogen content.
  • Type IIa (fast-twitch): fatigues faster than Type I
  • Type IIx (fast-twitch): low blood flow capacity, low capillary density, low mitochondrial concentration, but more forceful contractions.
  • More intense exercise recruits Types IIa and IIx fibers, but these can’t last very long.
  • Exercise does not seem to be able to change the relative composition of each type of fiber. So you’re either genetically a fast-twitch or slow-twitch type.
• Mitochondria:
  • Exercise generally increases the mitochondrial content of cells, allowing fuels to be processed faster.
  • Additionally, having more mitochondria favors the use of fatty acids over glycogen at sub-maximal levels of exercise. This is good because glycogen stores can run out, and slow-twitch fibers don’t have high glycogen stores (compared to the other types).
• Muscle capillarity:
  • Exercise increases the number of capillaries in the muscle, providing more blood flow.
• Blood flow capacity:
  • This is different from muscle capillarity since it refers only to the amount of blood that is delivered through regular blood vessels. In general this capacity does not seem to be binding, so it’s unclear whether the small increases in blood flow capacity we observe from training are very helpful.
• Training:
  • All the above effects are muscle-specific; muscles that aren’t trained won’t benefit.
  • Mitochondrial adaptations occur relatively quickly, and detraining also occurs RAPIDLY.
    • In the below chart, the x-axis is in weeks.
    • it takes only 4-5 weeks to max out mitochondrial capacity, a 1 week to lose half of your gains.  (This seems hard to believe… if true, can it really be good to take a rest week before a marathon?).
    • Higher intensity exercise over shorter periods seems to lead to a steady state with higher mitochondrial content.
    • The higher intensity exercise recruits the “fast-twitch” fibers and causes their mitochondrial content to increase too. Lower intensity exercise just leaves them out. (Which may not necessarily be bad, since it’s unclear that they would be used much in an endurance race.)