[here is one of several reasons why “weekly long runs” are –not– superior to high intensity short stuff for building blood vessels in distance runners. And this is among the reasons why there are no “weekly long runs” in TheETG training program]
Exercise Physiology 101…..

“…..endothelial progenitor cells contribute to vascular repair process by differentiating into endothelial cells. This study investigates how high-intensity interval and moderate-intensity continuous exercise training affect circulating endothelial progenitor cell levels and endothelial progenitor cells functionality…..”

“60 healthy sedentary males were randomized to engage in either HIT (3-minute intervals at 40 and 80 % VO2max for five repetitions) or MCT (sustained 60% VO2max) for 30 min/day, 5 days/week for 6 weeks, or to a control group that did not received exercise intervention.”
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“High intensity interval training is superior….”
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“Moreover, high intensity interval training effectively enhances endothelial progenitor cell functionality and suppresses endothelial injury…..”
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Hsing-Hua Tsai, et al
High-intensity Interval training enhances mobilization/functionality of endothelial progenitor cells and depressed shedding of vascular endothelial cells undergoing hypoxia
European Journal of Applied Physiology — Volume 116 #11 –December 2016 — page 2375