ENERGY SYSTEMS OF THE BODY

Soccer involves performing multiple actions at varying intensities over the course of 90 minutes
(sometimes less for youth players). Carrying out these movements requires energy in the form of a small
molecule known as adenosine triphosphate or ATP.

The body’s stores of ATP are quite small and there needs to be a system(s) in place to regenerate ATP in
times of high demand, such as during exercise. ATP required for muscular work can be generated either
anaerobically (does not require oxygen) or aerobically (requires oxygen to be present) and the body has
three metabolic systems that are responsible for this regeneration process: i) the phosphagen system, ii)
the glycolytic system and iii) the oxidative system.

The phosphagen system uses the breakdown of another high energy molecule known as phosphocreatine
(PCr) to produce ATP. This system provides ATP rapidly for high-intensity actions such as short sprints
and jumps. However, because the stores of PCr in the muscle are very limited, the phosphagen system can
only provide energy for very short periods (< 10 seconds) at a time.

Glycolysis refers to the breakdown of carbohydrates (either glucose in the blood, or glycogen in the
muscle) to produce a molecule known as pyruvate, which is further broken down by one of two processes.
The process by which pyruvate is converted to lactate (lactic acid) is referred to as anaerobic or fast
glycolysis and provides ATP for high intensity actions that last approximately 10 to 90 seconds. Aerobic
or slow glycolysis occurs when pyruvate is transported to a part of the muscle cell known as the
mitochondria, and provides ATP for exercises at moderate intensities that last approximately 90 seconds
to three minutes.

The oxidative or aerobic system provides ATP from the breakdown of fats and carbohydrates, and to a
lesser extent proteins, in the mitochondria. As the name suggests, the aerobic system depends on the
availability of oxygen and is the major provider of ATP for low to moderate intensity exercises that last
longer than three minutes.

The nature of soccer involves deriving ATP from all three metabolic systems. The phosphagen system
produces ATP at a rapid rate, but for very short durations. It is therefore the predominant supplier of
energy for very high-intensity soccer activities such as slide tackles, diving saves and sprints over short
distances. The glycolytic system produces ATP rapidly, but not as quickly as the phosphagen system. In
comparison to the phosphagen system however, it can produce ATP for durations lasting up to two and
half minutes. The glycolytic system is the predominant supplier of energy when your team chooses to
enforce high-pressure defending and players are asked to make repeated runs to win the ball as quickly as
possible. Of the three systems, the oxidative system has the greatest capacity to produce ATP, but does so
at the slowest rate. This system provides energy for low-intensity activities such as walking and jogging
that occur during the recovery period between high-intensity bouts. Table 1.1 summarizes the main
features of each energy system.