Okay there seems to be some confusion in the last few posts. Cardiac output work, i.e. lower intensity higher volume, trains the heart itself and the cardiac sysem itself to deliver more oxygen to the working muscles because it increases the size of the left ventricle of the heart. You keep the HR in the 120-150 or so range because this is where maximum stroke volume occurs. Above those ranges stroke volume tends to go down and you lose the effect.
I read somewhere another coach gave an answer to a question about cardio and said there wasn't much of a need to do cardio because strength training also increases the size of the heart. This is bad information and is nonsense. Strength training increases the size of the heart but in a different way and only increases it relative to body size. In other words strength training makes all of your muscles bigger including your heart but relatively speaking your volume of your left ventrical does not improve just from strength training.
The addage about long slow aerobic work killing your strength is nonense unless you're talking about a huge volume of aerobic work. Yes of course if you spent hours a day running it has a negative impact on strength development to some level, but a few times a work of GPP work in the cardiac output ranges is not going to hurt your maximum strength.
HICT is not targeting the heart so much as the muscles oxidative abilities themselves, specifially the fast twitch oxidative-glycolytic fibers to be exact. It's different than cardiac output work. Remember there is two sides to energy production, supply from the cardiovascular system and demand from the neuromuscular system. You need to develop both sides of the equation together in the right order and with the right methods to really improve your conditioning
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