Watch an image of an athlete in motion, and it is easy to see how dynamic the human body is. Each intricate movement is driven by billions of microscopic units, each with its unique function, fueled by a molecule called adenosine triphosphate (ATP). The human body carries enough ATP energy within the muscles (~100 g) to support 1-4 seconds of intense activity before needing to tap into its triple-line system of energy production to fuel sports performance. All lines of the energy system work together, yet the intensity of sports activity dictates the proportion of energy supplied by each.

 ATP-Creatine Phosphate Energy System

The smallest fuel tank, called alactic or ATP-CP, operates without oxygen and uses creatine phosphate (CP) in the muscles to generate ATP for 5-10 seconds of explosive activity, making it a commonly used, important fuel system for athletes engaged in sports that incorporate quick, all-out bursts of energy (e.g., jumping, sprinting, blocking). Because creatine is stored in limited capacity (~120 g), creatine supplements have become popular among power- and speed-oriented athletes wanting to enhance performance.

 Metabolic Demands of Sports 

Basketball High Moderate to High --
Boxing  High High Moderate 
Field Hockey High Moderate  Moderate 
Gymnastics High Moderate  --
Golf High -- --
Marathon Low Low High
Soccer High Moderate Moderate
Skating  Moderate High Low
Swimming Short Distance High Moderate  --
Swimming Long Distance -- Moderate High
Tennis High Moderate --

Track & Field - Short Distance

High Moderate --
Track & Field - Long Distance -- Moderate High
Volleyball High Moderate --
Wrestling High High Moderate
Weightlifting High Low Low


Anaerobic-Lactate Energy System

More sustained, moderate- to high-intensity efforts lasting up to 3 minutes (e.g., 800 m dash) or repeated explosive periods of activity (e.g., boxing) rely on the midsize fuel tank, called anaerobic-lactate (AN-LA) or the lactic acid energy system, which, in the absence of oxygen, breaks down glucose (carbohydrate) through a series of enzymatic reactions into pyruvic acid to help generate ATP energy. This metabolic process is called anaerobic glycolysis; it results in the formation of fatigue-inducing lactic acid as well as hydrogen ions that impair muscle contraction through several mechanisms.

Aerobic Energy System

The final and largest energy tank, called aerobic, breaks down carbohydrates (stored in the body as glycogen), fats, and at times protein in the presence of oxygen to generate a plentiful amount of ATP energy; it plays an important role during endurance sport activity as well as aiding recovery between periods of heightened exertion (e.g., interval training). Unlike anaerobic metabolism, where lactic acid and hydrogen ions can build up and potentially limit performance, the byproducts of aerobic metabolism, carbon dioxide and water, are easily disposed of through respiration or breathing, making it easier to sustain an aerobic effort for a longer time. During aerobic metabolism, glucose (carbohydrate), stored as glycogen in limited amounts within the liver and muscles, is broken down via glycolysis. The human body is capable of storing enough carbohydrate energy to fuel up to 2 hours of moderate- to high-intensity training before depletion is inevitable, and the onset of low blood sugars (known as “hitting the wall”) and muscle-fatigue ensue. This is why the use of carbohydrate supplementation from sources like sports drinks, energy gels, bars, and chews becomes a crucial practice for athletes during endurance events, such as a marathon, lasting longer than a couple of hours as well as team sports involving higher intensity exercise for prolonged durations, including basketball, football, soccer, and hockey. In addition, athletes may taper training volume while increasing dietary intake of carbohydrate, often with the help of carbohydrate supplements, for 3 days prior to competition to facilitate increased storage of carbohydrate, a practice known as carb-loading. When glycogen (i.e., carbohydrate) depletion occurs, training intensity must be reduced to facilitate increased oxygen consumption needed to break down fat, which is stored as triglycerides in large amounts within adipose tissue throughout the body. Like glucose, triglycerides can be broken down to form free fatty acids, which undergo a process called beta-oxidation to produce ATP. One of the primary physiological goals of an aerobic training program is to increase the reliance on fat metabolism, even when generating more power or speed, thus helping to spare glycogen stores and extend endurance. Several nutritional ingredients, including caffeine, have been explored for their potential role in boosting fat metabolism, thereby helping to spare muscle glycogen and enhance endurance performance. As the last-resort source of energy, protein can be broken down into amino acids (building blocks of protein) and converted into either glucose or other metabolic intermediates such as acetyl coenzyme A to generate ATP. While most aerobic activity uses minimal amounts of protein for energy, during times of low glycogen availability, such as in the later stages of an endurance event such as the Ironman triathlon, protein can contribute as much as 18% of total energy requirements, thus having implications for recovery. This is one reason protein supplementation has been researched as a plausible addition to intense or high-volume training regimens.