Energy Systems
The body’s main “fuel” is a molecule called ATP (adenosine triphosphate). This molecule contains three high energy phosphate chemical bonds which, when broken, release a large quantity of energy. There are four main energy systems in the body which produce ATP, two which don’t use oxygen as part of the process (anaerobic) and two which do use oxygen (aerobic). These are described below:
The anaerobic phosphocreatine (PCr) system
Within muscles stored ATP is broken down to release phosphate and energy, immediately with the onset of exercise, but these stores only last for about 2 seconds. The resulting adenosine diphosphate (ADP) is replenished by the molecule PCr which donates its phosphate group to resynthesize ATP. Stores of PCr only last a further 8 seconds with maximal exercise, longer with gentler exercise. With exercise stops, PCr is replenished quickly after about 4 minutes. It is the most powerful energy system in the body, but has very limited in capacity. This energy system is used almost exclusively in high intensity explosive events such as the 100m sprint or shot putting. Specific short-term anaerobic training, for example involving multiple repetitions of 60m sprints with 4 minutes rest in between, can increase the capacity of this system, resulting in a slightly longer duration of action.
The anaerobic glycolysis system
Within muscle tissue glucose is stored as the polysaccharide glycogen. Following the commencement of exercise, and as the capacity of the PCr system is depleted, muscle glycogen is broken down to release glucose which is then metabolised to pyruvate via a series of steps to generate a net gain of 2 molecules of ATP. A by-product of this system is the production of lactic acid and hydrogen ions, whose build up causes muscle soreness and fatigue. This is the next most powerful energy system in the body and although it has a greater capacity than the PCr system, it is still limited compared to the aerobic system and during high intensity exercise it only lasts about 2 minutes. This system is very important for sporting performance where strong finishes are required – for example, in middle distance running or rowing, where aerobic systems supply most of the energy for the majority of the race, with the more powerful anaerobic glycolytic system kicking in to supply energy for the strong finish. One of the enzymes involved in the synthesis of pyruvate, phosphofructokinase, can be induced to function more efficiently by long-term anaerobic endurance training and, in addition, threshold training at an intensity just at the point where lactic acid begins to accumulate can delay the onset of muscle fatigue. Both of these training methods prolong the period of useful activity of this system.
The aerobic glycolysis system
After one to two minutes of exercise, increases in heart rate, and increases in blood flow and capillary dilation deliver oxygen to muscle tissue. This allows pyruvate formed from the breakdown of muscle glycogen or blood glucose to enter special intracellular “powerhouses” called mitochondria. Here the pyruvate is converted to a substance called acetyl co-A which then enters a sequence of reactions (known as Kreb’s cycle) which in turn generates electrons that are then fed into the mitochondrial electron transport chain – the final step resulting in about 32 molecules of ATP being produced for every molecule of glucose. Although less powerful than the anaerobic systems, the aerobic glycolysis stem has a much greater capacity and can therefore sustain exercise for much longer periods of time. The availability of glucose and muscle glycogen stores as a limiting factor for this energy system illustrates why it is important to consume carbohydrates before and after exercise.
The aerobic beta-oxidation system
After about two to four minutes of exercise, stored fat in muscle and in adipose tissue is broken down into free fatty acids. These are transported into the mitochondria where they are broken down to form acetyl co-A, a process known as beta-oxidation. The acetyl co-A is then fed into the Kreb’s cycle and electron transport chain as described above. Fat as an energy source provides in excess of 200 molecules of ATP per fatty acid molecule and therefore provides an almost limitless supply of energy for prolonged exercise. Aerobic training results in marked gains in cardiorespiratory endurance. EPA, the principal omega-3 fatty acid in OmegaFlex, has been shown to boost beta-oxidation of fats in mitochondria, helping with both aerobic fitness and weight loss. CLA, also contained in OmegaFlex, increases both fatty acid oxidation and the breakdown of fat so it also helps to reduce weight.
Interaction of these energy systems
It is important to realise that in fact all 4 energy systems operate to a greater or lesser extent during exercise, but that in general the more intense the exercise level, the more the body is forced to use the more powerful anaerobic systems.
Another important point about using exercise to lose weight needs to be made at this point. You may have seen the following diagram on running machines at your gym:
In fact this diagram is somewhat misleading. It is based upon research by Achten and Jeukendrup (2003)who found that fat breakdown was maximal at about 70% of maximum heart rate (HRmax) but that at 85% HRmax fat breakdown was negligible. This research led to the belief that prolonged steady state exercise within 60-70% of HRmax should have the most beneficial effect on weight loss. However, exciting recent work has suggested that High Intensity Interval Training (HIIT) has a greater effect on weight loss (Trapp et al, 2008). This form of exercise uses short periods of high intensity anaerobic exercise alternating with lower intensity aerobic periods in a fixed ratio over a 20-25 minute period. With just 2-3 sessions a week of this form of exercise, significant weight loss and reductions in body fat can be demonstrated over a 15 week period. It works by both increasing the capacity of muscles to use fat as a fuel source and by boosting resting metabolic rate after exercise, so that your body continues to burn fat long after exercise has finished. To find out if you are overweight for your height visit the Body Mass Index page. To find out what heart rate targets you should aim for during your cardiorespiratory workout visit the Heart Rate Page.
