When a person exercises, muscles experience an increased demand for energy to contract. That energy comes from one of two main sources: carbohydrates in the form of glucose and fats in the form of fatty acids. These energy substrates provide the fuel that humans need to keep moving.

Both glucose and fatty acids are rich in chemical energy, stored in the chemical bonds holding the molecules together. Before that energy can be tapped by the body, however, it must be converted to a form that cells can use to perform work. That usable form is called adenosine triphosphate (ATP). Commonly referred to as the cell’s energy currency, ATP stores chemical energy in the bonds of its three phosphate groups. When our cells need energy, they typically break the bond between the last two phosphates, releasing the stored energy and forming adenosine diphosphate (ADP). (The “di” in diphosphate means “two,” as in two phosphates; the “tri” in “triphosphate” refers to its three phosphates.)

You can think of the energy in glucose and fat as the value of a gold brick: It’s worth a lot of money, but you couldn’t buy even a cup of coffee with it. ATP, however, is like bills and coins—it’s the energy your cells can actually spend.

ATP is produced in the body by three separate energy systems, two of which are anaerobic (not requiring oxygen), and one of which is aerobic (requiring oxygen). The anaerobic energy systems reside in the cytosol, the intracellular fluid of the cell outside the mitochondria. The aerobic energy system resides in mitochondria, small organelles within cells that are often called the powerhouses of our cells because this is where the vast majority of all ATP is produced. The production of ATP in mitochondria is completely dependent on the availability and use of oxygen. These three energy systems differ in the speed at which they replenish ATP for use by cells and, in the case of the aerobic energy system, the fuel that is burned. (INFOGRAPHIC 16.3)

Question 16.1

Why have three separate ways to make ATP? These systems provide overlapping coverage to replenish ATP over the short, medium, and long terms. The most important source of energy for short bursts of highly intense exercise, such as the 15-second sprints that Hubbell does while training, is the anaerobic phosphagen system. The amount of ATP stored in resting muscle is limited and depleted after only a few seconds of such vigorous exercise. To continue to meet energy demands and produce more ATP, muscle contains creatine phosphate (also known as phosphocreatine), which can readily transfer its phosphate and stored bond energy to ADP to quickly make more ATP.

370

The other significant source of energy for high-intensity exercise is glycolysis, an anaerobic energy system that breaks down glucose (obtained from the blood or muscle glycogen) into a three-carbon molecule called pyruvate, producing ATP in the process. Like the phosphagen system, glycolysis occurs in cytosol, the intracellular fluid of the cell outside the mitochondria.

371

When energy demands are high, glycolysis produces pyruvate at a rate faster than it can be used as a fuel for aerobic metabolism in mitochondria. As pyruvate accumulates in the muscle, it must be converted to lactate (or lactic acid) for glycolysis to continue. Lactate then enters the blood to be delivered to the liver where it can be recycled into glucose and returned to contracting muscle for use as an energy source. The heart, brain, and nonworking muscles can also take up lactate and use it directly as a source of energy.

Lactate gets a lot of bad press, but it turns out that essentially none of it is true. It is often blamed for causing muscle fatigue or the muscle “burn” associated with intense exercise, but in reality neither lactate nor muscle becoming more acidic are significant contributors to fatigue or muscle soreness.

Slightly longer bursts of exercise lasting a few minutes, such as Hubbell’s short ice dance routines (lasting 2 minutes and 50 seconds), rely heavily on glycolysis, but the aerobic energy system will also begin to come into play. During the first minute or so of this routine, the dominant means of creating ATP will be through the phosphagen system and glycolysis. As the program continues, however, the aerobic energy system begins to dominate. And this is the case for recreational exercisers, as well: The first minute of a jog or run will rely primarily on anaerobic energy systems, and then as the run continues the oxygen-dependent aerobic energy system begins to take over.

Aerobic metabolism is a process that occurs within a muscle cell’s mitochondria, where oxygen is required to drive ATP production. (INFOGRAPHIC 16.4)

372

Question 16.2

Despite producing ATP more slowly than anaerobic energy systems, longer-lasting, lower-intensity activities such as long-distance running or cycling rely almost entirely on the aerobic energy system in the mitochondria. Mitochondria and the aerobic energy system are so critically important for endurance exercise performance that the primary adaptation to endurance training within skeletal muscle is to increase the number of mitochondria. Aerobic metabolism is relied on heavily during endurance exercise because it can be maintained for much longer periods than anaerobic metabolism (with much higher yields of ATP). The primary fuels for the aerobic energy system are pyruvate generated by glycolysis in the cytosol, as well as fatty acids that are released from triglycerides stored both in adipose tissue and skeletal muscle. (INFOGRAPHIC 16.5)

373

Question 16.3

Ultimately, the energy contributions made through anaerobic and aerobic pathways combine to provide muscles with enough ATP to meet demands. The percentage of each depends on the intensity and duration of the activity.

Both glucose and fats are burned via the aerobic energy system to make ATP. The relative contributions of glucose and fat depend on the intensity and duration of the activity. The lower the intensity of an activity is, the more likely it is to burn fat. The use of fat as an energy source decreases proportionally as energy intensity increases, until at maximal intensity the energy contribution of fat is negligible. This does not mean that exercising at low intensity is better for losing weight. As the percentage of fat’s contribution to energy use decreases with increasing exercise intensity, the total amount of energy being expended is increasing even faster.

374

Experts point out that if low-intensity exercise truly maximized fat utilization, then staying in bed would be the best way to lose weight since an even higher percent of energy use in muscle is derived from fat when one is completely at rest. Total fat use during both moderate-intensity and high-intensity exercise exceeds that of low-intensity exercise of the same duration because energy use is so much higher. Furthermore, the higher overall calorie expenditure will potentially lead to a greater energy deficit and more effectively promote gradual weight loss. (INFOGRAPHIC 16.6)

Question 16.4