The Science Of HIIT, Understanding Your Body So You Can Get More Out of It
Lt’s first consider how the body gets energy and manages that energy during exertion.
First, in order to exercise, the body needs energy. This energy comes from a source known as ATP or Adenosine Triphosphate which is described in scientific circles as the ‘energy currency of life’. This substance is a nucleotide made up of three phosphagen molecules, bonded together by a powerful force. That’s what the name literally means tri- meaning three, phosphate – meaning phosphagen.
All types of energy in the body are ultimately converted into ATP, so when you eat a big cake, the sugar and glucose will ultimately need to be converted into this molecule before it can be of any use to your muscles or your cells. In real terms, any one ‘mole’ of ATP energy will provide 7.3 calories. It would take just over 190 micromoles to move your index finger enough to click a mouse button on a computer and this would equate to around 1.42 calories!
The power in this substance however doesn’t come from the phosphagen itself, but from the powerful bonds that bind it together, and it’s when these bonds break that they unleash the energy that the body can utilise. An athlete needs to be able to supply their muscles with a lot of ATP then in order to perform the necessary movements for running or weightlifting – and there are three ways in which they can do this.
The Three Energy Systems
The first way the body gets ATP is through the phosphagen system, also known as the ATP-CP system, which uses the ATP stored in the muscles to supply that energy. The body can store enough ATP at any one time to allow for around 3 seconds of full powered exertion (a little more or a little less depending on your physical fitness and various other factors), at which point it will need to look elsewhere.
Fortunately breaking the ATP molecules results in some useful bi products – ADP (andenosinediphosphate) and AMP (andenosine monophosphate) with two and one bonded phosphagen molecules respectively. So if you imagine you have three bonded molecules and they break you will understandably be left with a one and a two, or three single molecules. It’s basic maths… The good news is that using a substance called creatine phosphate (hence the CP!) can then recombine these molecules to make them back into ATP ready to be broken once more for extra energy.
The body can store enough creatine for roughly 8-10 seconds of continued exertion, meaning that in total the body can use the phosphagen system for around 13 seconds maximum of continued exertion.That is enough to sprint just over 100 metres. It is thought however that through the use of creatine supplements that this maximum time can be increased marginally.
At this point if exertion continues the body needs to get its ATP from somewhere else and this is when it looks to its stored carbohydrates in the form of glycogen. This represents the shift to what is known as the ‘glycogen lactic acid system’. This system is a slightly slower and less efficient means of supplying energy, which requires the body to split the glycogen first into glucose and then again into ATP.
This unfortunately creates a number of unwanted by-products called metabolites including lactic acid (from which the substance takes its name).This metabolic build-up creates the uncomfortable, mildly painful ‘burning’ sensation we get in our muscles when we push ourselves in the gym. The body can sustain itself using the glycogen lactic acid system for a further one minute and thirty seconds until this build up becomes too much to tolerate. If we continue to try and push ourselves at MHR past this point, it can lead to nausea and even fainting.
It was long believed that lactic acid was actually responsible for this failure and for the burning sensation. However, more recent research has shown us that lactate is not harmful in itself but rather seems to correlate with other factors that fatigue the glycogen lactic acid system. Thus high level athletes can still monitor their build-up of lactate in the blood in order to calculate a ‘lactate inflection point’. With training, it is possible to improve tolerance to metabolites and thus sustain maximum exertion for longer.
Guess what you can use to improve this aspect of your fitness? HIIT! Both these systems are anaerobic, meaning that for the first one minute and forty-three seconds the body won’t be using oxygen or burning fat.
In order to lose weight then the training must continue past this point and force the body to find its energy elsewhere. This is where the aerobic system comes in, relying on the oxidisation of foodstuffs in our mitochondria. In other words, the body looks to our supplies of glycogen (and so ATP) stored in our cells as fat and then uses the oxygen in our blood to break them down and carry them to our muscles. This is then what leads to fat being burned directly. This forces us to breathe more heavily in order to supply the necessary amount of oxygen and it increase our heartrate further to transport the oxygen to the fat stores and then to bring the energy to our muscles and brain.
The aerobic energy system can actually be used indefinitely and will continue until you completely exhaust all supplies of energy located around the body. During a typical prolonged endurance test, you will find you also breakdown protein for energy and even muscle. This in contrast to high intensity exercises that will use 100% carbohydrates for fuel, purely because they provide the quickest and most accessible source of ATP.
How We Progress Through Energy Systems
So if you head outside and start jogging, you’ll notice that at first, you don’t need to gasp for breath in order to maintain your speed and your heartrate doesn’t immediately go crazy. That’s because you are using your ATP-CP system. If you continue this exertion though, you will switch to your glycogen lactic acid system. This will use up energy stored as glycogen in the muscles. This will lead to an increase in lactate and metabolites
in the muscles and the blood stream, leading to nausea, muscle pain, cramping and more. It’s at this point that things become uncomfortable.
If you are running fast, you will continue to use this system until you eventually pass out – this is your ‘lactate threshold’ or your ‘lactate inflection point’. This is the point at which the build-up of lactate and metabolites becomes too great for you to maintain that level of exercise. This will happen before you have completely exhausted the stored glycogen in the muscles.
But most of us will instead find we are forced to slow down before we reach our inflection point and switch to the aerobic system. We’ll drop to sub-maximal exertion triggered by the physical symptoms and will find a steady pace at around 70% of our maximum heartrate. This will mean we have time to burn fat for fuel, which will require heavy breathing and a high heartrate but which won’t lead to the same levels of discomfort.
If you were training with steady state cardio, you would continue this level of exertion indefinitely and stop after you’d burned a satisfactory number of calories. Following this, your body would then continue to use a combination of all three systems for tasks throughout the remainder of the day.
Low blood sugar however would trigger a release of the hunger hormone ghrelin and this would be accompanied by cortisol (the stress hormone). This is why we’re always stressed when we’re hungry! This would also correlate with an increase in myostatin – an unpopular molecule that leads to an increased breakdown of muscle. This is on top of the increased protein breakdown during the exercise itself.
But if you utilize HIIT, you will use the aerobic system for a set period of time giving your body enough time to clear the lactate build-up in your bloodstream and then you would switch back to maximum exertion to further deplete the glucose stores. This would mean you were taking a small break from burning fat and blood sugar thus reducing the negative impact on your mood and muscle mass.Moreover, it would mean you could almost entirely empty your glycogen stores and thereby force your body to use blood sugar and fat stores for even the simplest movements for a long period afterward while it creates more glycogen!