Does the common practice of incorporating zone 3 training into an athletes regimen during the base-building phase actually undermine the effectiveness of aerobic development in zone 2, or are the benefits of higher-intensity work sufficient to offset any potential drawbacks? Specifically, is it possible that the inclusion of zone 3 efforts shifts the physiological focus away from the slow and efficient aerobic adaptations that are the hallmark of zone 2, and towards a more anaerobic, fast-twitch dominant profile? Are there any studies or data that suggest a hierarchical structure to aerobic development, where the more intense work in zone 3 somehow trumps the lower-intensity efforts of zone 2, or can the two coexist in a single training phase without interfering with each other?