5 C H A P T E R Bioenergetics of Exercise and Training Mike Conley
Chapter Outline Essential terminology Biological energy systems Substrate depletion and repletion Bioenergetic limiting factors in exercise performance Oxygen uptake and the aerobic and anaerobic contributions to exercise Metabolic specificity of training
E nergy stored in the chemical bonds of adenosine triphosphate (ATP) is used to power muscular activity. The replenishment of ATP in human skeletal muscle is accomplished by three basic energy systems: phosphagen, glycolytic, and oxidative.
Phosphagen (Anaerobic) System Occurs in the absence of molecular oxygen Provides ATP for short-term, high-intensity activities Is active in the start of all exercise regardless of intensity
Glycolytic System Breaks down carbohydrates to produce ATP that supplements the supply from the phosphagen system for high-intensity muscular activity May go in one of two ways: fast glycolysis and slow glycolysis
D uring fast glycolysis, pyruvate is converted to lactic acid, providing ATP at a fast rate compared with slow glycolysis, in which pyruvate is transported to the mitochondria for use in the oxidative system.
F ast glycolysis has commonly been called anaerobic glycolysis , and slow glycolysis, aerobic glycolysis , as a result of the ultimate fate of the pyruvate. However, because glycolysis itself does not depend on oxygen, these terms are not practical for describing the process.
Lactate Threshold (LT) and Onset of Blood Lactate Accumulation (OBLA)
Oxidative (Aerobic) System Requires molecular oxygen Uses primarily carbohydrates and fats as substrates Provides ATP at rest and during low-intensity activities
T he oxidative metabolism of blood glucose and muscle glycogen begins with glycolysis. If oxygen is present in sufficient quantities the end product of glycolysis, pyruvate, is not converted to lactic acid but is transported to the mitochondria, where it is taken up and enters the Krebs Cycle , or citric acid cycle.
I n general, an inverse relationship exists between the relative rate and total amount of ATP that a given energy system can produce. As a result, the phosphagen energy system primarily supplies ATP for high-intensity activities of short duration, the glycolytic system for moderate- to high-intensity activities of short to medium duration, and the oxidative system for low-intensity activities of long duration.
Table 5.3 Effect of Event Duration on Primary Energy System Used Duration Intensity Primary energy of event of event system(s) 0-6 s Very intense Phosphagen 6-30 s Intense Phosphagen and fast glycolysis 30 s-2 min Heavy Fast glycolysis 2-3 min Moderate Fast glycolysis and oxidative system > 3 min Light Oxidative system
Table 5.4 Rankings of Rate and Capacity of ATP Production System Rate of ATP Capacity of ATP production production Phosphagen 1 5 Fast glycolysis 2 4 Slow glycolysis 3 3 Oxidation of carbohydrates 4 2 Oxidation of fats and proteins 5 1 1 = fastest/greatest; 5 = slowest/least
T he extent to which each of the three energy systems contributes to ATP production depends primarily on the intensity of muscular activity and secondarily on the duration. At no time, during either exercise or rest, does any single energy system provide the complete supply of energy.
T he use of appropriate exercise intensities and rest intervals allows for the “selection” of specific energy systems during training and results in more efficient and productive regimens for specific athletic events with various metabolic demands.