HIIT is becoming an increasingly popular form of exercise and with good reason. HIIT stands for high intensity interval training and as the name suggests, this form of exercise combines high intensity intervals of near maximal work with periods of rest. This is what separates it from steady state cardio, which can be maintained at a moderate intensity for longer periods.
It is the high intensity nature of HIIT, working at over 80% of our maximal aerobic capacity that requires there to be extended periods of rest. As a general rule the longer the ‘work’ interval, the longer the rest. However, some protocols will demand a work rest ratio of 2:1. For example ‘Tabata’ requires 20 seconds work to 10 seconds rest. As you can imagine at the high intensity nature of the 20 second work period, 10 seconds is an ineffective time period to fully recover so performance on each interval is likely to deteriorate fairly rapidly and exhaustion comes quickly. It is for this reason that Tabata, or any type of HIIT where work periods are longer or even just as long as rest periods have to be very short in duration. Tabata for example is designed to last for just eight twenty second intervals with the whole exercise bout lasting just four exhausting minutes.
More typical work:Rest intervals are 1:2 and 1:3, again the longer the work interval, the longer the rest; for example, 30 seconds of truly maximal effort can take up to three minutes to fully recover from, the aim for most HIIT training is usually to not fully recover to really push the body to its limits. It should be pretty obvious by now that HIIT is very, very hard work, but what are its benefits over other forms of exercise?
Well, the most obvious one is that you can burn a lot of calories in a short space of time, making it a really efficient way to get in a workout for those who may struggle for time. The high intensity nature of this kind of exercise also means that although the duration of exercise might be short, the ‘calorie burn’ goes on for much longer after you have finished exercising. This is associated with what is known as excessive post exercise oxygen consumption (EPOC). This means that as HIIT is mostly anaerobic, meaning not enough oxygen can get to the muscles to support energy production, there is an accumulation of bi-products of energy breakdown such as lactate (otherwise known as lactic acid) and disruption to hormonal and cellular functions. This causes what is often referred to as an ‘oxygen debt’.
In order to recycle lactate back into a usable form for energy and rebalance many of the body’s systems, there is an increased demand for oxygen and energy to make up for this debt, so your body is still working over-time for long periods of up to (and possibly over) 24 hours after completion of a demanding HIIT style workout.
There are also other adaptations that HIIT causes, for example the mitochondria, the main energy producing parts of a cell, are increased in number, meaning that our bodies adapt to use fuel more efficiently, therefore we can improve performance. It may also have some positive effects on fat loss, by increasing the amount of body fat that we can use for energy during this type of exercise.
One of the main benefits of HIIT is the variety of ways in which it can be performed, using different pieces of equipment and exercises both indoors or outdoors. It really is a any-place-any-time type of exercise that allows an almost infinite variety of training methods. One branch of HIIT that is becoming increasingly popular is high intensity functional training (HIFT); this combines the same ethos of high intensity bouts of exercise but performs these using resistance equipment, not just focusing on cardio machines.
Combining elements of weightlifting, powerlifting and strongman, using forms of resistance in a HIIT format, HIFT allows a person to develop strength and fitness simultaneously, which means that for those looking to develop muscle and get the benefits of cardiovascular work in one session this can be a very effective tool. It is for this reason that many trainers use HIFT as a foundation for their clients, especially those looking to lose weight, due to the benefits of increased muscle mass supporting metabolic rate and a high and sustained amount of energy demand to tap into body fat reserves.
HIIT and HIFT are great ways to train, especially for those on a tight schedule, giving both muscle building and fat loss benefits. Its main drawback is that it can be very physically and therefore mentally demanding, so like all forms of exercise it is important to progress workouts properly in order to avoid injury, make sure adequate nutrition and rest is in place to allow recovery to help reduce risk of injury and support improvements in performance.
Paul Rimmer (BSc, MSc, PhD).
