Let’s begin with a simple definition. HIT is usually defined as exercise consisting of repeated bouts high-intensity work performed above the lactate threshold (a perceived effort of “hard” or greater) or critical speed/power, interspersed by periods of low- intensity exercise or complete rest. There are many way that definition can be achieved, however, the graph below provides a broad illustration of the concept. The general basis of high-intensity interval training can be described simply as follows:

Imagine performing a bout of exercise at an intensity above your lactate threshold, or critical velocity/power. To be clear here, this is an exercise intensity that is unsustainable, and one at which your brain would eventually force you to lower your intensity if you were to sustain it for as long as you could. It feels hard, and you know that fatigue would be inevitable at this pace if you were to hold on. Higher levels of sugar-burning glycolysis are needed generally to sustain the energy demand, and lactate accumulates to high levels typically at the point of fatigue. Now, if we took that same high intensity effort and separate it with pauses that include periods of complete rest or lower levels of active recovery, that glycolytic energy rate is eased so that lactate production is more in check, whilst the cardiovascular strain remains high, and perceived effort, although still high, is reduced and manageable.