Chapter 4: Exercise Metabolism

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. Describe graphically 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 post-exercise 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. Describe the various 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 during incremental exercise.

7. List 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. Describe the types of carbohydrates and fats used during increasing intensities of exercise and during prolonged exercise of moderate intensity.

10. Describe the lactate shuttle, with examples of how it is used during exercise.

OUTLINE

• Energy Requirements at Rest 69

• Rest-to-Exercise Transitions 69

• Recovery from Exercise: Metabolic Responses 71

• Metabolic Responses to Exercise: Influence of Duration and Intensity 75

  • Short-Term, Intense Exercise 75

  • Prolonged Exercise 75

  • Incremental Exercise 76

• Estimation of Fuel Utilization during Exercise 80

• Factors Governing Fuel Selection 81

  • Exercise Intensity and Fuel Selection 81

  • Exercise Duration and Fuel Selection 82

  • Interaction of Fat/Carbohydrate Metabolism 84

  • Body Fuel Sources 84

KEY TERMS

anaerobic threshold

Cori cycle

excess post-exercise oxygen consumption (EPOC)

free fatty acids (FFAs)

gluconeogenesis

graded (or incremental) exercise test

incremental exercise test

lactate threshold

lipase

lipolysis

maximal oxygen uptake $(V˙O2 max)$

oxygen debt

oxygen deficit

respiratory exchange ratio (R)

INTRODUCTION

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 200 times over utilization at rest (1). 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 ...