How the Systems Interact

To better understand how each of these energy systems relate to each other, let’s take a look at what happens when muscles contract. First, we will take a look at the immediate energy systems. The brain sends a signal along the nerves which triggers a release of calcium ions in the muscles, which stimulates the muscles to contract, and in the process, the high energy molecule ATP (adenosine triphosphate) releases energy and is reduced to adenosine diphosphate plus one phosphate atom. In this way, the immediately available ATP stores are depleted very rapidly, the first few seconds of a maximum muscle contraction.

The second immediate source of cellular energy is creatine phosphate (CP). There are several more times CP molecules in the cell than ATP. Creatine phosphate serves to instantaneously regenerate ATP molecules. Therefore, the ATP that is broken down to ADP during muscle contraction is restored to the high energy ATP by CP. The third immediate energy system enables the cell to regenerate ATP from two ADP molecules, resulting in one ATP and one AMP (adenosine monophosphate) molecule. This immediate energy source is depleted in a matter of seconds under conditions of all-out effort, under conditions of maximum muscle contractions.

Figure 1-4

Fast twitch muscle fibers, those associated with strength and size, are also referred to as fast glycolytic muscle fibers because they house the metabolic machinery to get quick energy through fast glycolysis pathways. The fast twitch fibers have a low capacity for oxidative metabolism and are instead set up to run glucose through their fast glycolysis pathways. Lactic acid then builds up because it is being produced too rapidly to enter into the oxidative pathways. Lactic acid is then cleared from the muscle, fed into the bloodstream, taken to the liver, and there made into glucose and glycogen. Glycolysis takes place in the cytoplasm of the cell.

For physical activities lasting more than 2 minutes in duration, the oxidative metabolic pathways produce the majority of energy to maintain muscle contractions. Potential oxidative energy sources include glucose, glycogen, fats, and amino acids. Oxidative energy production takes place in the mitochondria of the cells. Far more energy is produced when glucose is completely broken down in the mitochondria. Glucose is still first split in half by glycolysis. The pyruvate molecules then enter into the mitochondria where they are completely broken down. The oxidative pathways are called the Krebs cycle and electron transport. Fatty acids, from fat, are a major energy source during endurance events. The processes of fat utilization are activated more slowly than carbohydrate metabolism and proceed at a lower rate. Fatty acids are activated and combined with the molecule carnitine, which enables them to be transported into the mitochondria.

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