Human Physiology, 14th edition (2016)

378 Chapter 12

form as the V

·

o 2 max. It can be measured accurately by hav-

ing a person exercise intensely on a treadmill or cycle with

devices that measure the ventilation and the oxygen content of

the inspired and expired air. More commonly, the V

·

o 2 max is

only estmated by using equations that relate it to the heart rate

and work rate during exercise.

The maximal oxygen uptake is determined primarily by a

person’s age, size, and sex. It is from 15% to 20% higher for

males than for females and highest at age 20 for both sexes.

The V

·

o 2 max ranges from about 12 ml of O 2 per minute per

kilogram body weight for older, sedentary people to about

84 ml per minute per kilogram for young, elite male athletes.

Some world-class athletes have maximal oxygen uptakes that

are twice the average for their age and sex—this appears to

be due largely to genetic factors, but training can increase the

About 70% of the energy (ATP) consumed by muscles is used maximum oxygen uptake by about 20%.
by myosin ATPase in the sarcomeres for contraction, and about
30% is used primarily for Ca^2 1 transport by the sarcoplasmic
reticulum to allow muscle relaxation. Skeletal muscles at rest
(not exercising) obtain most of their energy from the aerobic
respiration of fatty acids. During exercise, muscle glycogen
and blood glucose are also used as energy sources ( fig. 12.22 ).
Blood glucose can be used because skeletal muscle con-
tractions during exercise stimulate the insertion of GLUT4 car-
riers into the sarcolemma (chapter 6; see fig. 6.17). This occurs
primarily in the transverse tubules, which comprise most of the
surface area of the sarcolemma. The more intense the exercise,
the greater will be the number of GLUT4 carriers inserted and
thus the greater the rate of glucose uptake ( fig. 12.23 ). This
is similar to the action of insulin, which also stimulates the
insertion of GLUT4 carriers into the sarcolemma and glucose
uptake by skeletal muscles (chapter 11; see fig. 11.30). How-
ever, the signaling mechanisms by which exercise and insulin
stimulate GLUT4 insertion are different, so that their effects
can be additive. In addition to increased glucose uptake, exer-
cise promotes the inhibition of glycogen synthesis and the
increased uptake and oxidation of fatty acids.

Metabolism of Skeletal Muscles

Skeletal muscles metabolize anaerobically for the first 45 to
90 seconds of moderate-to-heavy exercise, because the cardio-
pulmonary system requires this amount of time to sufficiently
increase the oxygen supply to the exercising muscles. If exer-
cise is moderate, aerobic respiration contributes the major por-
tion of the skeletal muscle energy requirements following the
first two minutes of exercise.

Maximal Oxygen Uptake and Lactate

Threshold

Whether exercise is light, moderate, or heavy for a given person
depends on that person’s maximal capacity for aerobic exer-
cise. The maximum rate of oxygen consumption (by aerobic
respiration) in the body is called the maximal oxygen uptake,
or the aerobic capacity, and is often expressed in abbreviated

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Explain the roles of creatine and creatine phosphate
   in muscle physiology.


9. Distinguish the different types of skeletal muscle
   fibers.


10. Describe aerobic capacity, lactate threshold, and
    muscle fatigue.


11. Explain how exercise training affects skeletal
    muscles.

Figure 12.22 Muscle fuel consumption during

exercise. The relative contributions of plasma glucose, plasma

free fatty acids, muscle glycogen, and muscle triglycerides to

the energy consumption of exercising muscles. These are shown

during mild exercise (25% of V

·

o 2 max), moderate exercise (65%

of V

·

o 2 max), and heavy exercise (85% of V

·

o 2 max). Data for heavy

exercise performed at 90 to 120 minutes are not available.

See the Test Your Quantitative Ability section of the Review

Activities at the end of this chapter.

100

80

60

40

20

0

100

80

60

40

20

0

Intensity of exercise

Mild

exercise

Moderate

exercise

Heavy

exercise

Muscle

triglyceride

Muscle

glycogen

Key

Plasma

glucose

Plasma free

fatty acids

Percentage of energy expenditure

0–30 min of exercise

Percentage of energy expenditure

90–120 min of exercise