Why is it commonly accepted that zone 4 training is more effective at building fatigue resistance than zone 3, when both zones are characterized by a significant increase in lactate production and a high level of physiological stress?
Is it due to the longer periods of time spent at or near VO2 max in zone 4, which forces the body to adapt by increasing mitochondrial density and improving muscle buffering capacity? Or is it the result of the higher intensity of zone 4, which recruits more fast-twitch muscle fibers and stimulates greater improvements in neuromuscular power and endurance?
If zone 4 is indeed more effective at building fatigue resistance, why do many coaches and athletes still prioritize zone 3 training for building endurance and increasing lactate threshold? Is it because zone 3 is perceived as a more sustainable and less injurious intensity, or because it allows for longer periods of training at a moderate to high intensity?
Can anyone provide some insight into the physiological mechanisms underlying the supposed benefits of zone 4 training for building fatigue resistance, and explain why it is often prioritized over zone 3 training for this specific goal?