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Exercise Energy Systems: A Primer

by Dr. Sheri Colberg, Ph.D., FACSM

It may have been a while since you’ve given much thought to how the body fuels the activities that a person does. The way that muscles make and use energy, which is affected by how fast people move, how much force their muscles produce, and how long the activity lasts, can affect blood glucose levels.

The body has three distinct energy systems to supply muscles with ATP (adenosine triphosphate), a high-energy compound found in all cells that directly fuels muscular work. The three systems act as a continuum, with one, then the next, and finally the third producing ATP as exercise continues. If anyone exercises long enough (even just a minute), he or she use all three systems to varying degrees. All of the energy systems work by increasing the production of ATP as it directly fuels all contractions. When a nerve impulse initiates a muscle contraction, calcium is released within recruited muscle cells, ATP “energizes” the muscle fibers, and they go into action. Without ATP, muscles can’t contract and people won’t be able to exercise.

Muscle cells contain only small quantities of ATP ready for use when starting, enough to fuel any activity for about a second, at best. If someone wants to keep going longer, the muscles need to get ATP from another source right away. Although all the systems can supply additional ATP, the rate at which they supply it varies. The fuels used to make the ATP and the amount of time needed to produce it also differ by system. Keep in mind that because of differences in how these energy systems work, the type of exercise that people do can affect their blood glucose responses differently.

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ATP–CP System: Short and Intense
For short and powerful activities, one energy system primarily provides all the requisite energy: the ATP-CP system. Also known as the phosphagen system, it consists of ATP that is already stored in muscle and creatine phosphate (CP), which rapidly replenishes ATP. This system requires no oxygen for energy production, making it anaerobic in nature. CP can’t fuel an activity directly, but the energy released from its rapid breakdown is used to resynthesize ATP for an additional five to nine seconds following depletion of the muscles’ initial one-second supply of ATP.

In total, all of the body’s phosphagen stores (ATP and CP) can fuel an all-out effort for only about 10 seconds before being depleted. Thus, any activity that lasts less than 10 seconds is fueled mainly by phosphagens, including a power lift, 40-meter sprint, long jump, baseball pitch, or basketball dunk. Generally, these types of activities don’t lower blood glucose levels because glucose isn’t used to produce the energy. In fact, they can raise levels because of an exaggerated release of glucose-raising hormones.

Lactic Acid System: Carbohydrate Use Only
The second energy system, the lactic acid (or glycolysis) system, supplies the additional energy for activities that last longer than 10 seconds and up to about 2 minutes. The lactic acid system also produces energy anaerobically (without using oxygen) through the breakdown of muscle glycogen, a process called glycogenolysis.1 After it has been released from storage, glycogen produces energy through the metabolic pathway of glycolysis, which forms lactic acid as a by-product. At rest, the muscle cells do some glycolysis, but since they’re not using up much ATP, carbohydrates are processed aerobically (using oxygen) and not much lactic acid builds up.

Due to the muscles’ immediate need for additional energy when exercise continues beyond 10 seconds, glycolysis proceeds rapidly to provide more ATP, and the system soon becomes limited by the accumulation of lactic and other metabolic acids. When large quantities are present in muscle, lactic acid drops the pH of muscle and blood, causing the associated “burn” in those muscles and fatigue. This system can make only a relatively small amount of ATP compared to aerobic energy systems. Consequently, it mainly supplies fuel for 800-meter runs, 200-meter swimming events, and stop-and-start activities like basketball, lacrosse, field hockey, and ice hockey.

Aerobic System: All Fuels
The other end of the spectrum is the aerobic energy system used for prolonged endurance or ultra-endurance exercise. Due to their duration, these activities mainly depend on aerobic production of energy by the oxygen system. Muscles require a steady supply of ATP during sustained activities like walking, running, swimming, cycling, rowing, and cross-country skiing, or anything done for longer than two minutes continuously. Running a marathon or ultramarathon, doing an Ironman triathlon, or participating in successive full days of long-distance cycling or backpacking are extreme examples of prolonged aerobic activities.

The fuels for these activities are mainly a mix of carbohydrate and fat, more of the latter than the former during rest and greater carbohydrate use during exercise.2 Protein can be used to fuel an activity, but it usually contributes less than five percent of the total energy. The body may use slightly more (up to 15 percent) protein during extremely prolonged endurance activities like running a marathon.

At rest, people’s diets and how recently they last exercised affect the mix of fuels that their bodies use, but most people use about 60 percent fat and 40 percent carbohydrate. If exercising with hyperglycemia, a person may use more blood glucose than normal, regardless of whether he or she has type 1 or type 2 diabetes.3,4

The body will rapidly begin to use more carbohydrate as soon as exercise starts, and its contribution rises further during harder exercise intensity.2 High-intensity or near-maximal activities use almost 100 percent carbohydrate. Muscle glycogen provides more—usually close to 80 percent—than blood glucose, unless a person is already glycogen depleted from long-duration exercise or a low-carbohydrate diet. The actual aerobic fuels that the body uses depends on training status, diet before and during the activity, intensity and duration of the activity, and circulating levels of insulin.5 More glucose appears to come from the liver making it from scratch (a process called gluconeogenesis), particularly if someone has poorly-controlled type 1 diabetes.6,7

Circulating hormones like adrenaline mobilize fats from fat cells (adipocytes), and those fats then circulate in the blood as free fatty acids that active muscles can take up and use during less intense or longer duration activities.5 The body will be able to use fats more during mild and moderate activities, along with some carbohydrates. The fats stored in the muscles themselves (intramuscular triglycerides) become more important in fueling your recovery from exercise or during prolonged exercise sessions (greater than two to three hours in length).5

Use of the aerobic system requires that going through the other two first. Both of the body’s anaerobic energy systems (ATP-CP and lactic acid systems) are important at the beginning of any longer-duration exercise before aerobic metabolism gears up to supply enough ATP. These first two systems are also important whenever someone picks up the pace or work harder, such as to run uphill or sprint to the finish line.
References cited:

  1. Wahren J, Ekberg K. Splanchnic regulation of glucose production. Annu Rev Nutr 2007;27:329-45.
  2. Mittendorfer B, Klein S. Physiological factors that regulate the use of endogenous fat and carbohydrate fuels during endurance exercise. Nutr Res Rev 2003;16:97-108.
  3. Jenni S, Oetliker C, Allemann S, et al. Fuel metabolism during exercise in euglycaemia and hyperglycaemia in patients with type 1 diabetes mellitus–a prospective single-blinded randomised crossover trial. Diabetologia 2008;51:1457-65.
  4. Colberg SR, Hagberg JM, McCole SD, Zmuda JM, Thompson PD, Kelley DE. Utilization of glycogen but not plasma glucose is reduced in individuals with NIDDM during mild-intensity exercise. J Appl Physiol 1996;81:2027-33.
  5. Watt MJ, Heigenhauser GJ, Dyck DJ, Spriet LL. Intramuscular triacylglycerol, glycogen and acetyl group metabolism during 4 h of moderate exercise in man. The Journal of physiology 2002;541:969-78.
  6. Petersen KF, Price TB, Bergeron R. Regulation of net hepatic glycogenolysis and gluconeogenesis during exercise: impact of type 1 diabetes. The Journal of clinical endocrinology and metabolism 2004;89:4656-64.
  7. Suh SH, Paik IY, Jacobs K. Regulation of blood glucose homeostasis during prolonged exercise. Mol Cells 2007;23:272-9.

As a leading expert on diabetes and exercise, I recently put my extensive knowledge to use in founding a new information web site called Diabetes Motion (www.diabetesmotion.com), the mission of which is to provide practical guidance about blood glucose management to anyone who wants or needs to be active with diabetes as an added variable. Please visit that site and my own (www.shericolberg.com) for more useful information about being active with diabetes.

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