Whether you are a basketball player, a Greco-Roman style wrestler,or even a marathoner, the way your body creates energy is universal. Although all these sports are very different, our bodies must follow the same physiological principles in their metabolism of food for energy. A basketball player is constantly running up and down the court in start and stop actions. Conversely, the wrestler needs explosive applications of force that must be sustained at times. The marathoner is known for their long, lean physique that fosters running along miles and miles of pavement. The long, slow distance and rythmicity of this sport plays an important role in their body type and composition. What we should realize is that all these different sports use the same fuel substrates to create energy. Carbohydrates, fats, and proteins are the body’s main fuels. For every gram of substrate metabolized, energy is created which is quantified in calories. Carbohydrates yield 4.2calories/gram, with protein close at 4.6 calories/gram. A fat molecule provides a whopping9.4 calories/gram. It is obvious which is the most caloric dense food substrate. Just to mention, protein is the fuel that is used the least amount during exercise. Protein is related to post-exercise consumption, and used mainly for recovery. It is necessary for the synthesis of all tissues in the body, especially skeletal muscle. Proteins would be a source of fuel for prolonged, continuous exercise such as triathlon, ultra-marathon, and mountaineering. To what extent each system is developed and trained is the key to specificity, peak performance, and the corner stone to any comprehensive exercise program. How our body chooses an energy source is based on several variables. The availability of different fuels, the ratios of high-energy phosphates (HE’s) present, both the intensity and duration of the exercise and, finally, the hormonal interactions all play a substantial role. HEP’s are comprised of specialized proteins called nucleotides that are constantly combining to produce ATP. The main goal is to produce energy rich ATP from the potential energy in food and use the chemical energy in ATP to create work. ATP is known as energy currency and stands for Adenosine Tri-Phosphate. It is of paramount importance to produce ATP for all of life’s functions. In fact, the only time your body is not producing ATP is when one is deceased. All the systems involved have one main goal: producing ATP via any fuel possible! Many believe in supplementing Creatine to increase energy production in order to enhance performance and increase muscle mass. Creatine is said to be of benefit in the HEP pool, but the research is equivocal at this time and beyond the scope of this article. The second major energy pathway has been called the short-term energy system, the glycolitic pathway, lactic acid cycle and, finally, anaerobic metabolism. I favor the latter term because it denotes anaerobic, meaning without oxygen. When we are not efficient enough to burn oxygen as our primary energy source during exercise, we burn sugar. Sugar is also known as glucose. We possess circulating glucose in our blood, stored glucose known as glycogen, and some glucose at the muscle site itself. The body can produce energy with this system up to about two minutes without needing to switch resources. We must not forget that lactic acid is a byproduct of glucose metabolism. At this point in the metabolic cycle, byproducts are a hindrance because they compete for binding sites at the muscle site. Lactic acid also severely lowers the Ph levels, making an acidic environment in which the muscle must function. Most know this as “The Burn.” A post-exercise recovery regimen is of benefit with this type of training in order to prevent excess byproducts from pooling and remaining in the muscles, inducing soreness. Albeit this system is not efficient for long-term work, it is the bridge to aerobic metabolism. Training the glycolitic cycle can be very tough and one must have a strong will to be able to function in this internal milieu. The anaerobic systems can produce energy for about two to three minutes, beyond that it becomes much more efficient to use oxygen as a primary energy source, not sugar. With aerobic metabolism, there are no byproducts like lactic acid that interfere with exercise. There are other limitations encountered during this process. Aerobic metabolism, or the long-term energy system, can burn fat and oxygen for energy to produce the most ATP’s, as compared to other metabolic pathways. It is well known that fat yields more calories than any other substrate and oxygen is infinite. Theoretically, when our bodies arrive at steady-state exercise, or the balanced aerobic state, there is a balance between the energy required by the working muscle and the ATP production through aerobic metabolism. Some variables that limit long-term aerobic exercise are: how efficiently we deliver and consume the oxygen available, fluid loss and electrolyte depletion, fuel reserves, especially glycogen, and, of course, the willpower to continue. Two major points should be made clear: First, the systems do not work independently of each other and they are alwayscontributing in some way to the physical task at hand. As the workload increases or the exercise intensity gets harder, your body actually resorts back to its glucose burning pathways to accomplish the task. This happens because you are pushing too hard to maintain aerobic metabolism. This is known as anaerobic threshold, OBLA (onset of blood lactate accumulation), a switch from Type I to II muscle fibers Second, we have many different muscle fibers in our body. For simplification of this subject, we have Slow (Type I), Medium (Type IIA), and Fast (Type IIb). Each fiber has a propensity for a certain type of metabolism; hence the associated substrates for fuel. For example, Type I is predominantly provided with augmented abilities for the use of O2 and lipids for energy. Type II or Fast Twitch have a proclivity toward glucose metabolism. The Slow Twitch fibers have greater enzymes specific for oxygen metabolism while Type IIb have better anaerobic enzyme capabilities. However, the Medium Twitch Fibers, the Type IIa, have been the only fibers known to develop traits of each slow or fast twitch through the adaptation of training. Otherwise, our model of muscle physiology does not allow a Slow Oxidative fiber to become a Fast Glycolitic. Furthermore, another common neuro-physiological principle is know as the size principle. This describes the timing and synchronicity of muscle fiber recruitment. It states that muscle fibers always contract from Slow to Fast twitch in a size order of recruitment. It is dependenton intensity and NOT the speed of movement. Conscious thought to these three energy systems and proper planning can help give order to your workouts and help create clear-cut goals. Think about your energy sources during the aerobic and anaerobic parts of your workout. This focus will change your thought patterns from external ones to internal ones and you will be on the way to a balanced exercise regime.