Energy systems and being fit
During exercise the energy system used to produce ATP will depend on the duration that the exercise continues, how intense the exercise is and how well developed the person's cardiorespiratory system is (aerobic fitness). Aerobic capacity is the most important component of fitness and is the ability to keep producing energy and using it to perform activities using the whole body for extended periods of time. The ability to consume oxygen is called oxygen uptake (VO2). The highest possible oxygen intake is VO2max or maximal oxygen uptake. Oxygen uptake is expressed as millilitres or litres per minute and it is related to body size. It is expressed in millilitres of oxygen consumed per kilogram of body weight per minute (mL/kg/min). It is affected by age, gender and genetics. Athletes who excel at endurance sports have a greater capacity for aerobic transfer - the ability to generate energy for the resynthesis of ATP aerobically. They have a greater capacity to take up, deliver and transfer oxygen to the working muscles and remove carbon dioxide and other wastes, and so it is a good measure of an individual's capacity to exercise [2].