THE HUMAN BODY AND EXERCISE

All human movement require energy and the method by which the body generates energy is determined by the rate of energy demand, that is, the intensity and duration of an activity. Activity that require sudden bursts of effort such as jumping and sprinting need a large production of energy over a short period of time. At the other extreme, activities like distance running and cycling call for continued energy production over a prolonged period at a slower rate.

The first of those types of movement are powered by energy systems that don't require oxygen. Energy comes from high-energy phosphate stores in muscle (the phosphate energy system) or via the lactate energy system through anaerobic glycolysis. More extended activities like jogging and cycling require oxygen to produce continued activation of muscles, hence these are called aerobic (with oxygen) activities.

The aerobic system
These is perhaps the most important energy pathway for active individuals to understand because it is the system the body uses for everyday living. The aerobic system is so called because it is energy produced in the presence of  sufficient oxygen. As exercise intensity increases the exercising muscles use increasing amounts of oxygen, and glucose becomes the preferred source of fuel,  because more energy is derived from the breakdown of glucose per litre of oxygen used. The by-products of the aerobic system are carbon dioxide and water.

The anaerobic system
Anaerobic energy is simply a way of producing energy without depending on oxygen.  This comes into play when the intensity of an activity increases to a point where the cardiorespiratory system can't supply sufficient oxygen to meet the body's energy demands. Anaerobic energy use is like taking money from the bank. If you continue to withdraw without making deposit you end up in debt. In the same way, an oxygen debt is quickly built up from anaerobic activity that must eventually be repaid: sprinter has to stop and catch his breath; the footballer has to slow down and jog between explosive sprints.

The phosphate system
The first procc for supplying energy to the muscles is the phosphate system or the ATP-CP system.  This comes into play when there is insufficient time for the body to break down glycogen for the manufacture of ATP, Although ATP serves as the energy currency for all cells, its quantity is limited, with about 85 grams being stored in the body at any one time. Thus ATP must be constantly re-synthesised to provide a continuous supply of energy. In almost all sports, the phosphate system play a role for short intense bursts of activity.

The lactate system
These system is extremely important as it provides a rapid supply of ATP for intense, short bursts of activity. It also acts as an energy reserve for the middle or long distance runner to 'kick' at the finish, or for the footballer to perform repeated bursts of high intensity activity to beat or chase down opponents. Running events ranging from 400 to 800 metre predominantly make use of the lactate system. Middle distance running on the other hand uses predominantly aerobic energy.