Therapeutic exercise encompasses basic physiology and physiologic sciences as well as exercise science. This chapter briefly outlines the use of physical activity, muscular contraction, and physical exertion in order to prevent, treat, and rehabilitate physical conditions as well as improve performance or fitness. Exercise can be used to increase or improve strength, endurance, aerobic capacity and conditioning, flexibility, body mechanics, and proprioception, and can help a patient progress toward functional movements and activities. Although the chapter summarizes essential knowledge, readers are encouraged to explore additional sources for more comprehensive details.
AEROBIC & ANAEROBIC METABOLISM
Adenosine triphosphate (ATP) is the primary energy substrate used by muscle cells (myocytes) to generate contractile force during exercise. When the amount of intramuscular ATP stores is depleted after 10 seconds of exercise, it must be replenished. ATP regeneration occurs through three processes, using aerobic (ie, inclusive of O2) and anaerobic (exclusive of O2) avenues. All three ATP generation systems occur simultaneously. Activities that are shorter in duration and have greater intensity rely on anaerobic systems, whereas those of lower intensity and longer duration rely on aerobic systems.
The two anaerobic systems fueling muscle activity are the adenosine triphosphate–phosphocreatine (ATP-PC) system and anaerobic glycolysis. Of the two, the ATP-PC system regenerates ATP more quickly because it involves only a single chemical reaction. In this reaction, the bond between phosphate (P) and creatine (C) in phosphocreatine (PC) is broken, releasing energy and a phosphate group to resynthesize ATP from ADP: PC + ADP → ATP + P. Unfortunately, the PC stores of myocytes can sustain only about 30 seconds of activity. Activities involving short, powerful bursts (eg, the swing of a baseball bat, a 100-m sprint, or powerlifting) primarily rely on the ATP-PC system.
Glycolysis, the second aerobic system, involves a series of reactions whereby simple (glucose) and long-chain (glycogen) carbohydrates are metabolized into pyruvate. During both aerobic and anaerobic glycolysis, ATP and pyruvate are synthesized. However, during anaerobic conditions, pyruvate is preferentially fermented into lactate so that glycolysis, and its ATP generation, can continue. Anaerobic glycolysis is, nevertheless, limited in its ability to generate ATP because the acidification of cytosol by lactate eventually impedes glycolytic enzyme activity. Interestingly, although lactate can impede glycolysis, some lactate is transferred to the systemic circulation. There, lactate may be converted to glycogen in the liver through the Cori cycle. Alternatively, intracellular and extracellular lactate may be oxidized into pyruvate and then metabolized for ATP in a process known as the lactate shuttle. Anaerobic glycolysis produces more ATP over a longer time frame than the ATP-PC system; however, both anaerobic systems generate less than 10% of the body’s potential ATP. Highly intense, short-duration exercises of up to 2 minutes, such as 400-m sprints or repetitive weight lifting, will exhaust both systems.