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Worked for me. I didnt really like doing it though, took me around 6 minutes to do the sets of 30 breaths. Was happy when at 4 weeks they say you dont have to do it as often. Definately improved my interval running though, i found after 1 week i could suddendly keep with people that i used to be just behind and after the 4 weeks i was passed them and have never looked back.