HIIT vs steady-state

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Endocrine
High-intensity interval training (HIIT) briefly drives up hormones such as growth hormone and adrenaline during a session, but these short peaks are thought to contribute little to long-term adaptation. HIIT's main gains are in the heart, lungs and mitochondria: in the controlled trials pooled by Milanović 2015, HIIT raised VO2max about as much as continuous endurance training, possibly slightly more.

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What is HIIT

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Sprint for a short stretch, slow down and catch your breath, and repeat for a few rounds: that is high-intensity interval training (). Its best-known selling point is saving time, but saving time is the result, not the principle. The principle is in those tens of seconds of hard effort.

On an easy jog, most of the work is done by endurance-type slow-twitch fibers. They are rich in mitochondria, burn fat slowly with oxygen, produce little force, and tire slowly. Once the effort rises to the point where you cannot finish a sentence, the slow-twitch fibers cannot supply enough force, and the nervous system calls up the fast-twitch fibers that usually sit idle. Those fibers do not wait for oxygen. They break down the sugar stored in muscle for energy, and within tens of seconds they can spend most of the cell's ready-to-use energy.

Fibers that usually never clock in are pushed into an energy shortfall. That is the main training signal HIIT sends, not a nice number on a heart-rate watch. So what it saves is not training time as such, but the time it takes to reach those fibers.

Educational content, not medical advice — consult a clinician.

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