Maximal Oxygen Uptake

Whether exercise is light, moderate, or heavy for a given person depends on that person's maximal capacity for aerobic exercise. The maximum rate of oxygen consumption (by aerobic respiration) in the body is called the maximal oxygen uptake, or the aero.bic capacity, and is often expressed in abbreviated form as the VO2 max. The maximal oxygen uptake is determined primarily by a person's age, size, and sex. It is from 15% to 20% higher for males than for females and highest at age 20 for both sexes. The VO2 max ranges from about 12 ml of O2 per minute per kilogram body weight for older, sedentary people to about 84 ml per minute per kilogram for young, elite male athletes. Some world-class athletes have maximal oxygen uptakes that are twice the average for their age and sex—this appears to be due largely to genetic factors, but training can increase the maximum oxygen uptake by about 20%.

The intensity of exercise can also be defined by the lac-tate (or anaerobic) threshold. This is the percentage of the maximal oxygen uptake at which a significant rise in blood lac-tate levels occurs. For average healthy people, for example, a significant amount of blood lactate ap. pears when exercise is performed at about 50% to 70% of the VO2 max.

During light exercise (at about 25% of the VO2 max), most of the exercising muscle's energy is derived from the aerobic respiration of fatty acids. These are derived mainly from stored fat in adipose tissue, and to a lesser extent from triglycerides stored in the muscle (fig. 12.21). When a person exercises just below the lactate threshold, where the exercise can be described as moderately intense (at 50% to 70% of the VO2 max), the energy is derived almost equally from fatty acids and glucose (obtained from stored muscle glycogen and the blood plasma). By contrast, glucose from these sources supplies two-thirds of the energy for muscles during heavy exercise above the lactate threshold.

During exercise, the carrier protein for the facilitated diffusion of glucose (GLUT4—chapter 6) is moved into the muscle fiber's plasma membrane, so that the cell can take up an increasing amount of blood glucose. The uptake of plasma glucose contributes 15% to 30% of the muscle's energy needs during moderate exercise and up to 40% of the energy needs during very heavy exercise. This would produce hypoglycemia if the liver failed to increase its output of glucose. The liver increases its output of glucose primarily through hydrolysis of its stored glycogen, but gluconeogenesis (the production of glucose from amino acids, lactate, and glycerol) contributes increasingly to the liver's glucose production as exercise is prolonged.

Clinical Investigation Clue

Remember that Maria has a high maximal oxygen uptake, consistent with her athletic lifestyle.

Is it possible, likely, or unlikely that Maria's muscle pain and fatigue are caused by her playing softball?

The enzyme that transfers phosphate between creatine and ATP is called creatine kinase, or creatine phosphokinase. Skeletal muscle and heart muscle have two different forms of this enzyme (they have different isoenzymes, as described in chapter 4). The skeletal muscle isoenzyme is found to be elevated in the blood of people with muscular dystrophy (degenerative disease of skeletal muscles). The plasma concentration of the isoenzyme characteristic of heart muscle is elevated as a result of myocardial infarction (damage to heart muscle), and measurements of this enzyme are thus used as a means of diagnosing heart disease.

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