Energy Sources for Muscle Contraction

ENERGY SOURCES FOR MUSCLE CONTRACTION

Before discussing the contraction process itself, let us look first at how muscle fibers obtain the energy they need to contract. The direct source of energy for mus-cle contraction is ATP. ATP, however, is not stored in large amounts in muscle fibers and is depleted in a few seconds.

The secondary energy sources are creatine phos-phate and glycogen. Creatine phosphate is, like ATP, an energy-transferring molecule. When it is bro-ken down (by an enzyme) to creatine, phosphate, and energy, the energy is used to synthesize more ATP. Most of the creatine formed is used to resynthesize creatine phosphate, but some is converted to creati-nine, a nitrogenous waste product that is excreted by the kidneys.

The most abundant energy source in muscle fibers is glycogen. When glycogen is needed to provide energy for sustained contractions (more than a few seconds), it is first broken down into the glucose mol-ecules of which it is made. Glucose is then further bro-ken down in the process of cell respiration to produce ATP, and muscle fibers may continue to contract.

Recall from our simple reaction for cell respiration:

Glucose + O₂ → CO₂ + H₂O + ATP + heat

Look first at the products of this reaction. ATP will be used by the muscle fibers for contraction. The heat produced will contribute to body temperature, and if exercise is strenuous, will increase body temperature. The water becomes part of intracellular water, and the carbon dioxide is a waste product that will be exhaled.

Now look at what is needed to release energy from glucose: oxygen. Muscles have two sources of oxygen. The blood delivers a continuous supply of oxygen from the lungs, which is carried by the hemoglobin in red blood cells. Within muscle fibers themselves there is another protein called myoglobin, which stores some oxygen within the muscle cells. Both hemoglo-bin and myoglobin contain the mineral iron, which enables them to bond to oxygen. (Iron also makes both molecules red, and it is myoglobin that gives muscle tissue a red or dark color.)

During strenuous exercise, the oxygen stored in myoglobin is quickly used up, and normal circulation may not deliver oxygen fast enough to permit the completion of cell respiration. Even though the respi-ratory rate increases, the muscle fibers may literally run out of oxygen. This state is called oxygen debt, and in this case, glucose cannot be completely broken down into carbon dioxide and water. If oxygen is not present (or not present in sufficient amounts), glucose is converted to an intermediate molecule called lactic acid, which causes muscle fatigue.

In a state of fatigue, muscle fibers cannot contract efficiently, and contraction may become painful. To be in oxygen debt means that we owe the body some oxy-gen. Lactic acid from muscles enters the blood and circulates to the liver, where it is converted to pyruvic acid, a simple carbohydrate (three carbons, about half a glucose molecule). This conversion requires ATP, and oxygen is needed to produce the necessary ATP in the liver. This is why, after strenuous exercise, the respiratory rate and heart rate remain high for a time and only gradually return to normal. Another name proposed for this state is recovery oxygen uptake, which is a little longer but also makes sense Oxygen uptake means a faster and deeper respiratory rate. What is this uptake for? For recovery from stren-uous exercise.