Among adults with stable coronary artery disease, 55% of participants undergoing high-intensity interval training and 34% of participants undergoing moderate-intensity steady-state training demonstrate a true positive response in cardiorespiratory fitness exceeding the minimal clinically important difference after accounting for measurement error.

When the muscles work very hard in short bursts, the energy demand spikes, causing a drop in cellular energy levels. This triggers a signal that turns on a master regulator, which tells the muscle cells to make more energy-producing factories called mitochondria and more tiny blood vessels to deliver oxygen. More of these factories and vessels let the muscles use oxygen better, which increases how much oxygen the whole body can use during exercise.