Physiology Summary

Zone 2 changes the muscles themselves to be more efficient at extracting oxygen and fuel then burning it for function. The longer time at a lower exertion level allows time for the adaptation to take place. 

Zone 4 and 5 changes the heart and lungs to deliver oxygen faster and improve the body's ability to tolerate long or very intense exercise bouts.

Here is the physiology behind these effects:

Zone 2 Adaptations

• Mitochondrial Density & Efficiency
  Zone 2 is the most effective intensity for stimulating mitochondrial biogenesis (increasing the number of mitochondria) and improving their function. Mitochondria are the "power plants" of the cell; having more of them means you can produce more energy aerobically before fatigue sets in.

• Capillary Density:
  Low-intensity training promotes angiogenesis, the growth of new capillary beds (tiny blood vessels) around muscle fibers. This increases the surface area for oxygen delivery and waste removal. If the heart is the pump, capillaries are the delivery roads; Zone 2 builds more roads.

• Fat Oxidation (Metabolic Flexibility):
  Zone 2 maximizes fat oxidation rates. Training here increases the enzymes responsible for breaking down fat. This preserves your limited glycogen (carbohydrate) stores for efforts later in a race or training.

• Lactate Clearance:
  Lactate is a byproduct of the energy burning process. Zone 2 increases the density of proteins that transport lactate into the mitochondria to be reused as fuel. Zone 2 teaches your body to "eat" lactate rather than just drowning in it.

Zone 4 and 5 Adaptations

• Increased Stroke Volume (Cardiac Output):
  While Zone 2 builds the heart's volume slowly, Zone 5 forces the heart to pump at its absolute limit. This maximizes stroke volume—the amount of blood ejected with every beat. This is the primary driver of increasing your VO2 Max (the ceiling of your aerobic performance).

• Lactate Buffering & Tolerance:
  Unlike Zone 2, which improves clearance, Zone 4/5 improves buffering. High intensity creates an acidic environment (H+ ions). Training here increases the body's ability to neutralise this acid and "tolerate" the pain of high-intensity efforts, allowing you to hold a fast pace longer.

• Neuromuscular Recruitment:
  High intensity forces the body to recruit fast-twitch muscle fibres that are less active during easy exercise. Training these fibers makes them more aerobic and fatigue-resistant. For endurance athletes, this means the fast twitch can more easily become active to help out during long endurance bouts (ie over 90 minutes). 

• Glycogen Utilization:
  These zones increase the number and efficiency of enzymes in the mitochondria that focus on breaking down glycogen (stored carbohydrate) for energy.