THE HIIT “AFTERBURN”
Even though HIIT workouts are short they are much more effective at burning calories than, say, a long, steady run. The main reason for this is that the combination of bursts of maximal effort with frequent short rests, causes an extended recovery period of what scientists call Excess Post-exercise Oxygen Consumption (EPOC), aka “the afterburn effect”.
what is EPOC?
As we have seen, to power muscle contractions needed to perform the movements in HIIT exercises, the body converts stored glucose to molecules of ATP (see Powering your HIIT Workout). The primary mode of conversion requires the presence of oxygen in the chemical reaction, which is supplied via the cardiovascular system. Yet even after exercise, your body has an elevated requirement for energy – and therefore oxygen – in order to fuel various processes that replenish lost glycogen stores and generally return the body to homeostatic balance. This recovery period is when Excess Post-exercise Oxygen Consumption occurs to facilitate the body’s raised metabolism while it adjusts down to a state of rest. As the graphs opposite demonstrate, the duration of EPOC after a short HIIT workout is far longer than after an extended session of even moderate steady-state aerobic exercise.
HIIT oxygen consumption
The period of EPOC recovery is more than twice as long as the workout. This is because the body does not have time for metabolic adjustment during the workout, and the anaerobic strength training element of HIIT leaves an oxygen deficit and lactate accumulation beyond the ability of VO2 max to clear.
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steady exercise oxygen consumption
Exercising for a prolonged period where consumption of oxygen (VO2) meets your raised energy needs, allows the body to adjust and achieve metabolic efficiency during exercise. This results in a much shorter period of EPOC recovery.
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Fuelling the recovery
With HIIT, the extended period of raised metabolism can continue beyond EPOC for up to 24 hours after a workout. To maintain metabolic efficiency it is important to consider what you eat and when. Skipping meals or eating at widely spaced intervals, back-loaded towards the end of the day, can cause your metabolism to slow, with a drop in blood sugar levels leading to low energy. Poor nutrition can also impede physical adaptations promoted by exercise, such as muscle growth and increased glycogen stores. I would recommend eating four to five small meals balanced with the right macronutrients and evenly spaced through a workout day. I like to use the analogy of feeding a fire with a solid piece of wood every few hours to keep it burning brightly all day.
processes during epoc
During EPOC recovery, various physiological processes kick in to bring the body back to a state of rest, in which the basal metabolic rate is sufficient for energy needs. Higher oxygen levels and raised metabolism are required to reduce your heart rate, to bring down your breathing rate, and to bring your core body temperature back down to a normal 37°C (99°F). In addition, EPOC is required to restock depleted energy stores and start the work of physiological adaptation to the stimulus of exercise, including muscle growth and improved respiratory efficiency.
energy stores in muscle cells
Muscle cells contain small stores of ATP and CP molecules, which supply the chemical energy for short bursts of physical effort, and these are replenished during EPOC.
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lactate converts to glycogen
Once sufficient oxygen becomes available, lactate produced as a result of anaerobic respiration is converted first to pyruvate and thence to granules of glycogen stored in the liver.
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reoxygenating the “-Globins”
During EPOC the proteins haemoglobin in your blood and myoglobin in muscles – vital to the transportation and uptake of oxygen – are resupplied with oxygen.
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