We need the energy to grow and repair tissues, maintain body temperature, and provide fuel for physical activity. The energy comes from foods rich in carbohydrates, proteins, and fats.
What are sources of energy during exercise?
The energy source used to drive the contraction movement in working muscles is adenosine triphosphate (ATP). However, ATP is not stored to any great extent in the cells.
There are three biochemical systems for producing ATP:
- use of creatine phosphate
- use of glycogen
- aerobic respiration.
Use of creatine phosphate
All muscle cells have some ATP in them that they can use immediately. A high-energy compound called creatine phosphate is broken down to make more ATP quickly. Creatine phosphate can provide energy to a working muscle at a very high speed, but only for 8-10 seconds.
Use of glycogen
Muscle also has large stores of carbohydrates, called glycogen, which can be used to make ATP from glucose. But this requires about 12 chemical reactions, so it adds energy more slowly than from creatine phosphate. The process will produce enough energy to last for about 90 seconds. Oxygen is not necessary, and it is good because it takes some time for the heart and lungs to get an increased supply of oxygen to the muscles. You feel the lactic acid when your muscles build up lactic acid because it causes fatigue and stiffness.
Shortly after exercise, the body supplies working muscles with oxygen. With oxygen present, aerobic respiration can help break down glucose for ATP. Glucose can come from several sources:
- from residual glucose supply in the muscle cells
- from glucose from food
- from glycogen in the liver
- from fat reserves in the muscles
- from the body’s protein (in case of hunger).
Aerobic respiration requires more chemical reactions to produce ATP than the other two systems, and it is slow but delivers ATP for several hours or longer.