Chapter 18:
The Physiology of Aerobic Exercise
The word ‘aerobic’ literally means ‘with oxygen’ and aerobic exercise is fueled by reactions
which require oxygen to proceed. Typical aerobic activities are walking, running, cycling and
swimming. While activities like basketball and soccer could be considered aerobic (as they rely
on aerobic energy systems), their stop and start nature would cause them to be more typically
referred to as interval training discussed in the next chapter.
We will define aerobic exercise as any activity that is fueled by aerobic energy sources and
only consider exercise such as walking, cycling, etc in this chapter. Aerobic energy pathways
include the breakdown of glycogen, blood glucose, free fatty acids, intramuscular triglyceride,
ketones and protein. The intensity and duration of exercise will determine which of these fuels is
the primary energy source.
Aerobic exercise typically causes heart rate to reach 50% to 80-85% of maximum heart
rate (or about the lactate threshold, defined below). In general, the adaptations to aerobic
exercise are for the body to become more efficient at producing energy aerobically. These
adaptations occur in the enzymes necessary for aerobic energy production as well as in the
muscle and heart. As a general rule, maximal strength and muscle size do not increase with
aerobic exercise.
Section 1: Adaptations to aerobic exercise
Aerobic exercise affects two major tissues in the body: the heart and the muscles. With
regular aerobic training, the heart becomes stronger and more efficient, pumping more blood with
every beat. Heart rate at rest and during exercise decreases indicating a greater efficiency.
Normal resting heart rate is around 70 to 80 beats per minute (bpm), but elite endurance
athletes may have resting heart rates of 40 bpm. (1)
The primary change in the muscle is an increase in the capacity to utilize fats for fuel
during exercise and at rest (2,3), and this adaptation only occurs in the muscles which are trained
(4). For example, when subjects trained the quadriceps of only one leg, that leg’s ability to use
free fatty acids (FFA) increased while the untrained leg did not. (4) This is why running (which
primarily uses the hamstrings and gluteal muscles) does not improve cycling (which primarily
uses the quadriceps) and vice versa.
The main site of aerobic energy production in the muscle is the mitochondria. Regular
training increases the number and activity of mitochondria. (2,5) Training also increases the
number of capillaries in the muscle which deliver blood, oxygen and nutrients to the muscles.
(2,4,6) Finally, the enzymes required for the oxidation (burning) of FFA all increase with regular
aerobic training (2,4,6). These adaptations in the mitochondria and capillaries may also occur as
a result of weight training, especially if high reps and short rest periods are used (see chapter 20
for more information).