It is one of the great marvels and mysteries of life that simple elements like carbon, hydrogen, and oxygen can be combined with a life force to produce human existence and movement. These elements are found in carbohydrate, fat, and protein, which are the food substrates that we consume. This chapter begins with quantification of energy expenditure and then describes the formation of energy substrates and their subsequent breakdown. Nutrition for optimum health will be explored. Adenosine triphosphate (ATP) will be appreciated as the link between the breakdown of food and the ability to perform physical activity. Exercise states will be described as a function of metabolic pathways. Obesity and measurement of body composition will be discussed. The chapter concludes with components of a physical fitness program and principles of exercise prescription.
This chapter is not intended to be an in-depth examination of exercise physiology. Rather, it is an extraction of select basic principles of exercise that has meaning and application to patients with disease. Understanding the normal physiological response to exercise will provide a firm foundation to appreciate abnormal responses.
MEASURES OF ENERGY EXPENDITURE
The energy value of the food that we eat can be quantified in terms of calorie. A kilocalorie (kcal) is the amount of heat necessary to raise the temperature of 1.0 kg of water by 1.0°C. The energy value of food is determined by placing a known quantity of food in a bomb calorimeter. This device uses oxygen to completely burn the substrate and measures the amount of heat liberated. Thus, the energy value of 4.0 oz of cheesecake is almost 350 kcal. As you will see, it takes a lot of exercise to burn off a slice of cheesecake!
Oxygen is utilized as an adjunct to substrate catabolism, or breakdown, in all metabolically active tissues. Oxygen consumption may be abbreviated as V̇O2, or the volume of oxygen consumed per minute, and expressed as either mL O2/min or mL O2/kg of body weight/min. The basal metabolic rate (BMR) is the minimal amount of oxygen utilized in order to support life. It is the sum total of cellular activity in all metabolically active tissues while under basal conditions. Skeletal muscle V̇O2 accounts for approximately 20% of the total BMR. The BMR is measured under strictly controlled laboratory conditions. The resting metabolic rate (RMR) is a more easily acquired measurement. Patients are instructed to avoid strenuous exercise for at least 24 hours before testing. Measurements are obtained at least 4 hours after a light meal and no caffeine.1 Its value is only slightly higher than the BMR. Measurement of the RMR was once costly and time-consuming, and it involved use of a metabolic cart or Douglas bag collection systems. Newer, handheld portable ...