Muscles use the stored chemical energy from food we eat and convert that to heat and energy of motion (kinetic energy). Energy is required to enable growth and repair of tissue, to maintain body temperature and to fuel physical activity. Energy comes from foods rich in carbohydrate, protein and fat.
The source of energy that is used to power the movement of contraction in working muscles is adenosine triphosphate (ATP), the body’s biochemical way to store and transport energy. ATP is a high-energy nucleotide which acts as an instant source of energy within the cell. When muscles contract, they break down ATP in a reaction that provides energy. However, muscle cells only store enough ATP to fuel a few seconds of maximal contraction. Once muscle contraction starts, the making of ATP must start quickly.
Since ATP production is so important, muscle cells have several different ways to make it. These systems work together in phases. The three biochemical systems for producing ATP are, in order:
Using Creatine Phosphate
To continue working, muscle cells must replenish their ATP supply. All muscle cells contain a high-energy compound, creatine phosphate, which is quickly broken down to make ATP. Because stores of creatine phosphate are also limited, this energy system can only sustain maximal muscle output for about 10 seconds. The phosphagen system is the primary energy source during very short, rapid bursts of activity, such as sprints.
Using Glycogen (Anaerobic Glycolysis)
To sustain exercise for more than 10 seconds, muscles must break down fuel sources such as carbohydrates and fats to provide the energy to re-synthesize ATP. Carbohydrate metabolism is faster than fat metabolism. Therefore, carbohydrates provide a high percentage of the energy during very high-intensity workouts. Because carbohydrates can be metabolized anaerobically, without oxygen, they become a vital energy source when oxygen supply to muscles cannot keep up with demand.
The breakdown of carbohydrates to provide energy without oxygen is called anaerobic glycolysis. This process releases energy very rapidly and will produce enough energy to last about 90 seconds. It is important that oxygen is not required because it takes the heart and lungs some time to get increased oxygen supply to the muscles. Glucose and stored carbohydrates in the form of glycogen in muscle cells are broken down through a series of reactions to form a compound called pyruvate. This process yields two to three molecules of ATP for each molecule of glucose. A by-product of making ATP without oxygen is lactic acid, which can accumulate in your muscles during rapid exercise causing tiredness and soreness.
Using Aerobic Respiration
Within two minutes of exercise, the body starts to supply working muscles with oxygen. When oxygen is available, pyruvate can be further broken down aerobically to produce as many as 30 additional molecules of ATP, making aerobic metabolism, although slower, much more efficient than anaerobic metabolism. Fats can be broken down aerobically to produce large quantities of ATP. After vigorous workouts, muscles restock ATP supplies aerobically.
Aerobic respiration can supply ATP for several hours or longer as long as a supply of glucose lasts. This glucose can come from several places:
Lactate (Lactic Acid) Production
When the body has plenty of oxygen, pyruvate is transferred to an aerobic pathway to be further broken down to ATP (pyruvate is produced by glycolysis from the breakdown of glucose). However, when oxygen is limited, the body temporarily converts pyruvate into lactate, which allows glucose breakdown – and thus energy production – to continue. The working muscle cells can continue this type of anaerobic energy production at high rates for one to three minutes, during which time lactate can accumulate to high levels.
A side effect of high lactate levels is an increase in the acidity of the muscle cells. The same metabolic pathways that permit the breakdown of glucose to energy perform poorly in this acidic environment. This is a natural defense mechanism for the body. It prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction. Once the body slows down, oxygen becomes available and lactate is converted back into pyruvate, allowing continued aerobic metabolism and energy for the body’s recovery from the strenuous event.
Lactate buildup is not responsible for the soreness felt in the days following strenuous exercise. Rather, the production of lactate and other metabolites during extreme exertion is the results in a burning sensation often felt in active muscles. This often-painful sensation also gets us to stop overworking the body, thus forcing a recovery period in which the body clears the lactate.