I am currently working on improving my VO2 max in preparation for the upcoming football (soccer) season. I've been playing around with the Norwegian 4x4 protocol for the past 3 weeks or so now and although extremely brutal in the moment, have been feeling much more fit than before. And my HR has gradually gone down when engaging in football training and games. Still early days, but something I have incorporated into my overall training regimen and have already begun to observe benefits.