INTRODUCTION

Objectives

By studying this chapter, you should be able to do the following:

1. Discuss the relationship between exercise intensity/duration and the bioenergetic pathways that are most responsible for the production of ATP during various types of exercise.

2. Graph the change in oxygen uptake during the transition from rest to steady-state exercise and then recovery from exercise. Identify the oxygen deficit, oxygen requirement, and oxygen debt (excess postexercise oxygen uptake).

3. Describe graphically the changes in oxygen uptake during an incremental (graded) exercise test and identify the maximal oxygen uptake ($V˙O2$ max).

4. Identify the criteria for having achieved ($V˙O2$ max).

5. Describe graphically the changes in the blood lactate concentration during an incremental (graded) exercise test and identify the lactate threshold.

6. Discuss several possible explanations for the sudden rise in blood-lactate concentration (i.e., lactate threshold) during incremental exercise.

7. Identify the factors that regulate fuel selection during exercise.

8. Describe the changes in the respiratory exchange ratio (R) with increasing intensities of exercise and during prolonged exercise of moderate intensity.

9. Discuss the fuels used during low-, moderate-, and high-intensity exercise and during prolonged exercise of moderate intensity.

10. Describe the lactate shuttle and discuss how this shuttle might be beneficial during exercise.

Outline

Energy Requirements at Rest

Rest-to-Exercise Transitions

Recovery from Exercise: Metabolic Responses

Metabolic Responses to Exercise: Influence of Duration and Intensity

• Short-Term, Intense Exercise

• Prolonged Exercise

• Incremental Exercise

Estimation of Fuel Utilization during Exercise

Factors Governing Fuel Selection

• Exercise Intensity and Fuel Selection

• Exercise Duration and Fuel Selection

• Interaction of Fat/Carbohydrate Metabolism

• Body Fuel Sources

Key Terms

anaerobic threshold

Cori cycle

excess postexercise oxygen consumption (EPOC)

free fatty acids (FFAs)

gluconeogenesis

incremental exercise test

lactate threshold

lipase

lipolysis

maximal oxygen uptake ($V˙O2$ max)

oxygen debt

oxygen deficit

respiratory exchange ratio (R)

Exercise poses a serious challenge to the bioenergetic pathways in the working muscle. For example, during heavy exercise, the body’s total energy expenditure may increase 15 to 25 times above expenditure at rest. Most of this increase in energy production is used to provide ATP for contracting skeletal muscles, which may increase their energy utilization >100 times over utilization at rest (1, 126). Clearly, skeletal muscles have a great capacity to produce and use large quantities of ATP during exercise. This chapter describes (a) the metabolic responses at the beginning of exercise and during recovery from exercise; (b) the metabolic responses to high-intensity, incremental, and prolonged exercise; (c) the selection of fuels used to produce ATP; and (d) how exercise metabolism is regulated.

We begin with an overview of the energy requirements of the body at rest, followed by a discussion of which bioenergetic pathways are activated at the onset of exercise.

ENERGY REQUIREMENTS AT REST

Recall that homeostasis is defined as a steady and ...

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