The following article by Steve Merrett was written in 2004 as part of a level 3 fitness award to be a personal trainer.

The body needs energy at a much greater rate when engaged in physical activity. This article will outline the basic requirements for a health diet and explain how the body uses the food that we eat to provide energy, it will explain as simply as possible the predominant energy system utilised.

By the end of this article you should be able to:

• Name the types of food and suggested amounts recommended per day.
• List the three ways the body produces energy:
• PC (phosphocreatine) system
• Lactic Acid system
• Aerobic System
• Have an understanding of when each system is predominantly being used.

Figure , Tortora, G and Grabowski, S.(2000). Principles of Anatomy & Physiology. 9^th Ed. John Wiley & Sons

Energy comes from the food we eat and the oxygen in the air we breathe. The four main components in food and drink that are needed to produce energy are: carbohydrate, protein, fat and alcohol. The food pyramid is a graphic representation of the types and amounts of those foods that should be eaten every day. It is important to include a variety of foods within each of the groups. Anita Bean in her book “Sports Nutrition” shows the pyramid with slightly different numbers aimed at the needs of the sportsman. Her recommendations are:

• Fruit and vegetables for health, immunity and optimum performance.
• 3-5 portions of vegetables a day
• 2-4 portions of fruit a day
• Healthy carbohydrates (wholegrain complex carbohydrates), to fuel hard training.
• 4-6 portions a day
• Calcium rich foods for strong bones
• Protein-rich foods, for muscle growth and repair
• 2-4 portions a day
• Healthy fats, to improve endurance, recovery and protect against heart disease.
• 2-4 portions a day
• Junk foods
• 0-1 portion a day.

Energy is actually created by the splitting away of phosphate molecule from a substance known as adenosine triphosphate or ATP for short, the breaking of this chemical bond produces a burst of energy that allows work to be carried out such as a muscle contraction.

This energy can be produced with and without the presence of Oxygen in a number of ways that are known as:

• ATP-PC (also known as phosphagen) system,
• Lactic Acid (or anaerobic glycolytic) system
• Aerobic System – Which comprises the glycolytic (carbohydrate) system and the lipolytic (fat) systems.

Type of exercise	Main energy system	Major storage fuels used
Maximal short bursts lasting less then 6 seconds	ATP-PC	ATP and PC
High intensity lasting up to 30 seconds	ATP-PC Anaerobic glycolytic	ATP and PC Muscle glycogen
High intensity lasting up to 15 minutes	Anaerobic glycolytic Aerobic System	Muscle glycogen
Moderate-high intensity lasting 15-60 minutes	Aerobic System	Muscle glycogen Adipose tissue
Moderate-high intensity lasting 60-90 minutes	Aerobic System	Muscle glycogen Liver glycogen Blood glycogen Intra-muscular fat Adipose tissue
Moderate intensity lasting longer then 90 minutes	Aerobic System	Muscle glycogen Liver glycogen Blood glycogen Intra-muscular fat Adipose tissue

Table , The main energy systems used during different types of exercise. Bean, A. (2003). Sports Nutrition, 4^th Ed. A&C Black

For exercise lasting longer then 30 seconds all three energy systems will be used to a greater or lesser extent, but one system will normally be the main supply of energy. As we start exercising for the first few seconds’ energy can be created without oxygen, ATP is used for fuel and lactic acid is produced. If the exercise continues the heart beats faster, air is breathed faster and oxygen is delivered to the working muscles and the lactic acid is recovered and reused to produce energy. If this exercise is continued for longer then 15 minutes, and is fairly steady with plenty of oxygen then fatty acids start to be broken down as well. The longer the exercise continues the less the body uses carbohydrate stores and instead to use more fat. If the duration is longer then 15 minutes and the intensity is higher and lactate acid increases until the athlete begins to suffer from fatigue.

 FATIGUE

Fatigue can be defined as the bodies inability to sustain a given power output or speed (Bean, 2003). As the body demands more fuel in the form of ATP and phospogen, it cannot keep up with the need and fatigue results. To the sportsman this means they can no longer sprint as fast and technical ability will suffer. In the gym that last rep can not be pushed out, whilst in the aerobics class it is difficult to maintain the pace and intensity. In activities lasting between 30 seconds and 30 minutes, ATP depletion is not the main cause of fatigue but the accumulation of lactic acid brings about fatigue. High intensity exercise cannot be continued as the excess acid in the muscle inhibits further contractions and causes cells to die and the athlete experiences the “burn” which is a protection mechanism to stop the muscle cells from being destroyed. Reducing intensity allows the muscles to switch to the aerobic system and the lactic acid build up to be reduced.

 REFERENCES

Bean, A. (2003). Sports Nutrition, 4^th Ed. A&C Black

Tortora, G and Grabowski, S.(2000). Principles of Anatomy & Physiology. 9^th Ed. John Wiley & Sons