The subject of exercise physiology and energy systems can get quite boring and confusing, but as you begin to adopt a healthy lifestyle, it’s important to understand the systems and processes that are operating to keep us healthy, and how different exercise or nutrition inputs affect our performance and overall healthy.
This will be meant to provide a brief introduction to the systems at play, simply to provide the basic information in a non-technical way.
The cardiorespiratory system (heart, lungs, blood vessels) is responsible for a number of important functions within our body. First and foremost. It delivers oxygen and nutrients to our muscles and organs, and removes waste from the body.
When we breathe in, oxygen is absorbed into the deoxygenated blood flowing through pulmonary blood vessels. From there, the newly oxygenated blood is transported through the heart and then through the arteries to the rest of the body where the oxygen is delivered to the muscles/organs/skeletal system. Deoxygenated blood carrying waste is then transported via the veins back to the heart and lungs. As we exhale, we remove the waste in the form of carbon dioxide (CO2).
When we exert ourselves (in the form of exercise or other physical exertion), our muscles begin to require more oxygen to accommodate the increased demand we are putting on them. Therefore, the heart rate increases to pump more oxygen rich blood to the muscles. The harder we exert ourselves, the higher the demand for oxygen, the faster the heart rate. This is why we breathe more heavily the harder we work out, and one reason why breathing and heart rate are great indicators of effort level.
The fitter and healthier we become, the more efficient our cardiorespiratory system becomes. Our heart is able to pump more blood in every stroke, and therefore more oxygen can be delivered to the muscles per stroke. The effect becomes lower heart rates (resting and active) and lower perceived effort at higher paces. Both exercise and nutrition play a significant role in improving the efficiency of our cardiorespiratory system (see the Nutrition section for more information on how nutrition affects our bodies).
The human body operates primarily using two sources of energy: Carbohydrates (in the form of glucose and glycogen stores) and fat. Carbohydrates and fats are two of the three macronutrients (protein is the third) contained in our diets. For more information on how macronutrients are metabolized, see the Macronutrient section in the Nutrition module.
Carbohydrates and fats are both critical in providing energy, and they act in a complementary fashion, but the two energy systems are entirely different. Carbohydrates are broken down into glucose and stored as glycogen in our liver and muscles. There is a limited capacity for storing glycogen within our bodies, approximately 1,500 to 2,000 calories at any given time. On the other hand, our bodies have an unlimited capacity to store fats. Even the leanest of athletes can have up to 50,000 calories of stored fat available for the body to burn.
During physical activity, the body is always burning some combination of fats and carbohydrates (glycogen). This number varies depending on a number of factors including level of fitness, diet, intensity of workout, and stress. For example, someone who is in excellent physical condition and practices healthy dieting habit, will burn a higher proportion of fats to carbohydrate when going out for a run than someone who is out of shape and eats a diet high in refined sugars and processed foods.
As you can probably already guess, an athlete with an ability to burn fat in higher proportion to carbohydrates will have a capability, simply based on the energy stores in the body, to exercise for longer periods of time. This is where aerobic endurance comes from, our ability to endure in physical activity due to our employment of the “fat burning” aerobic engine. Alternatively, the person burning a higher proportion of carbohydrates for energy will burn out or “bonk” faster because they will run out of their finite glycogen stores.
In general, we burn a higher percentage of carbohydrate energy as a stress response. In that instance, our bodies are conditioned that our effort is a reaction to a stress (for example, we are running from something dangerous), thus we need access to fast burning energy. As we deplete glycogen without replacing it (with the ingestion of simple sugars), we run out of that energy and then lose our ability to function at higher levels.
Anaerobic vs. Aerobic
Aerobic respiration occurs at lower intensities, and is characterized by a utilization of oxygen and glucose in the muscles for energy. Because of the efficiency of the process, athletes operating “aerobically” can exercise for very long periods of time before fatiguing. Since oxygen is a key player in this process, it is converted to carbon dioxide and water.
Anaerobic respiration occurs at very high intensities of workout, or at high stress (such as during a strength training program). It is not nearly as efficient as aerobic respiration, and is therefore unsustainable as an energy system. During this process, glucose is used for energy (though not fully broken down, so glucose use is inefficient), but not oxygen, since oxygen can not meet the demand of the muscles. Glucose is converted to lactic acid in the cells, which needs to be removed to prevent buildup. The buildup of lactic acid in the muscle cells is what produces the “burning” sensation, and causes muscles to fatigue (a primary reason that anaerobic efforts are unsustainable for extended periods of time). The removal of lactic acid requires oxygen, which is not in sufficient quantity during anaerobic work, so an athlete needs to “recover” in order to convert the lactic acid stores to carbon dioxide (a waste product removed by the blood in the cardiorespiratory system).
Note: This is one of the key measures during a workout that you should back off the effort during an endurance workout. If you start feeling “the burn”, you are going too hard and creating too much lactic acid faster than it can be oxidized.
Putting it all Together
All of the above systems operate in harmony, and you can develop a good rhythm in determining which energy systems you are conditioning by measuring your biofeedback (your breathing and heart rate, for example). A controlled, conversational, and easy breathing rhythm during exercise combined with a low heart rate below your maximum aerobic function (the heart rate at which your body begins to burn more carbohydrates instead of fat) leads to a conditioning of the “fat burning” aerobic system. Alternatively, more frequent workouts at high heart rates, unsustainable paces, and high breathing rates will lead to a conditioning of the anaerobic systems.
We discuss the importance and elements of conditioning the aerobic system in relation to overall health and fitness in the next section.