ATP (Adenosine Triphosphate) is a complex chemical compound formed with the energy released from food and stored in all cells, particularly muscles. When our brain sends signals to our muscles to contract, energy stored in the form of ATP is converted into the energy to drive the movement of the muscles

Every muscle cell has a small amount of ATP floating around which can be used readily however this is used off very quickly and must constantly be replenished. In order for the ATP levels to be maintained the muscle relies on Aerobic and Anaerobic respiration

Anaerobic Respiration

Anaerobic replensihment of ATP can occur by the breakdown of creatine phosphate (CP) which is stored in the muscles like ATP, or by a process known as glycolysis

Creatine phosphate is the main supplier of energy for the formation of ATP. As it is broken down to create creatine and Pi, it releases energy in order to rapidly replenish ATP. There is a sufficient enough supply of CP inside our muscles to support maximal effort for up to 10 seconds. Without this system explosive fast movements could not be performed
Glycolysis is another anaerobic route in which ATP can be produced. Carbohydrate is broken down from the carbohydrate stores inside the muscle in the form of “glycogen”. Glycolysis cannot produce ATP as quickly as the breakdown of CP, but is still considerably quicker than aerobic ways of creating ATP. Unfortunatley glycolysis is not the most efficient way of producing ATP and leads to a byproduct named “lactic acid” created from incomplete carbohydrate breakdown, which interferes with muscle performance, however glycolysis is a very important process when oxygen availability is limited and when the energy demands are greater than the energy which can be produced through anaerobic metabolism

Aerobic Respiration

During exercise our muscles are in constant need of energy. In the prescence of oxygen, carbohdrates, fats and proteins can be broken down in order to generate ATP. The aerobic energy production occurs inside the muscle fibers in a place called the “mitochondria”. The mitochondria converts energy stored as fat and carbohydrates into ATP

The carbohydrate needed for use during aerobic metabolism can come from 3 places:

• Glycogen supplies inside the muscle
• The breakdown of liver glycogen, which gets to working muscle through the bloodstream
• The absorbtion of glucose from food in the intestines, which gets to working muscle through the bloodstream

Aerobic respiration can also use fatty acids from fat reserves in muscle and the body to produce ATP. Because the majority of fat is stored inside adipose tissue, its not readily available for fuelling muscles, and must be broken down into fatty acids and then transported to the muscle via the bloodstream. Only inside the mitochondria can fatty acids be used for ATP synthesis. This is not a very fast process and can only be used during low intensity exercise

When carbohydrate stores are low the body can also use protein for ATP production. Every protein is made up of amino acids. All the essential amino acids must be present in order for the body to repair damaged muscles. During exercise when glycogen stores in the muscles and liver become depleated, muscle proteins are broken down into amino acids and released into the blood. The amino acids are then transported to the liver where they are converted into glucose through a process known as “gluconeogenesis”. The glucose is then released back into the bloodstream and can be used as a fuel source for the muscle

References:

Dickson-Parnell, B.E., and A. Zeichner. Effects of a short-term exercise program on caloric consumption. Health Psychol. 4:437-448, 1985

John Ivy & Robert Portman. The Future Of Sports Nutrition. Nutrient Timing. 2:15-20

Gillette, C.A., R.C. Bullough, and C.L. Melby. Postexercise energy expenditure in response to acute aerobic or resistive exercise. Int. J. Sports Nutr. 4:347-360, 1994

Karlsson, J. Lactate and phosphagen concentrations in the working muscle of man. : 1-65, 1971