Human Physiology, 14th edition (2016)

Muscle 379

During exercise, the carrier protein for the facilitated dif-

fusion of glucose (GLUT4) is moved into the muscle fiber’s

plasma membrane, so that the cell can take up an increasing

amount of blood glucose ( fig. 12.23 ). The uptake of plasma

glucose contributes 15% to 30% of the muscle’s energy needs

during moderate exercise and up to 40% of the energy needs

during very heavy exercise. This would produce hypoglycemia

if the liver failed to increase its output of glucose. The liver

increases its output of glucose primarily through hydrolysis

of its stored glycogen, but gluconeogenesis (the production of

glucose from amino acids, lactate, and glycerol) contributes

increasingly to the liver’s glucose production as exercise is

prolonged.

Oxygen Debt

When a person stops exercising, the rate of oxygen uptake

does not immediately return to pre-exercise levels; it returns

slowly (the person continues to breathe heavily for some time

afterward). This extra oxygen is used to repay the oxygen debt

incurred during exercise. The oxygen debt includes oxygen

that was withdrawn from savings deposits—hemoglobin in

blood and myoglobin in muscle (chapter 16, section 16.6); the

extra oxygen required for metabolism by tissues warmed dur-

ing exercise; and the oxygen needed for the metabolism of the

lactic acid produced during anaerobic metabolism.

Phosphocreatine

During short, intense bouts of exercise, ATP may be used

faster than it can be replenished by anaerobic metabolism and

aerobic respiration. At these times the rapid renewal of ATP

is extremely important for the exercise to continue. The rapid

production of ATP is accomplished by combining ADP with

an inorganic phosphate derived from another high-energy

compound in the muscle cell known as phosphocreatine, or

creatine phosphate.

Within muscle cells, the phosphocreatine concentration is

more than three times the concentration of ATP and represents

a ready reserve of high-energy phosphate that can be donated

directly to ADP ( fig. 12.24 ). Production of ATP from ADP and

phosphocreatine is so efficient that, even though the rate of ATP

breakdown rapidly increases from rest to exercise, muscle ATP

concentrations decrease only slightly in aerobically adapted

muscle. During times of rest, the depleted reserve of phospho-

creatine can be restored by the reverse reaction—phosphory-

lation of creatine with phosphate derived from ATP. Both the

formation and the breakdown of phosphocreatine are catalyzed

by an enzyme known as creatine kinase ( CK ) or creatine phos-

phokinase ( CPK ).

Creatine is produced by the liver and kidneys, and a small

amount can be obtained by eating meat and fish. In addition,

some athletes take creatine monohydrate dietary supplements,

which have been found to increase muscle phosphocreatine by

15% to 40%. Most studies indicate that creatine supplementa-

tion can increase muscle weight (due to increased water entry

The intensity of exercise can also be defined by the lactate
(or anaerobic ) threshold. This is the percentage of the maxi-
mal oxygen uptake at which a significant rise in blood lactate
levels occurs. For average healthy people, for example, a sig-
nificant amount of blood lactate appears when exercise is per-
formed at about 50% to 70% of the V

·

o 2 max.

Energy Usage During Exercise

During light exercise (at about 25% of the V

·

o 2 max), most of
the exercising muscle’s energy is obtained from the aerobic
respiration of fatty acids. These are derived mainly from stored
fat in adipose tissue, and to a lesser extent from triglycerides
stored in the muscle (see fig. 12.22 ). When a person exer-
cises just below the lactate threshold, where the exercise can
be described as moderately intense (at 50% to 70% of the V

·

o 2
max), the energy is derived almost equally from fatty acids and
glucose (obtained from stored muscle glycogen and the blood
plasma). By contrast, glucose from these sources supplies two-
thirds of the energy for muscles during heavy exercise above
the lactate threshold.
Figure 12.22 shows that muscle glycogen is the primary
source of energy during heavy exercise, and indeed the gly-
cogen content of muscles helps determine how long heavy
exercise can be sustained. Glycogen is a highly branched poly-
saccharide of glucose (chapter 2; see fig. 2.15). A glycogen
molecule packages approximately 55,000 glucose molecules
together in a form that does not affect the cell’s osmolarity,
preventing osmotic damage. The highly branched form of gly-
cogen allows the bonds with glucose to be accessed and effi-
ciently hydrolyzed by the enzyme phosphorylase (chapter 5,
section 5.3), so that glucose can be quickly released from
stored glycogen.

Figure 12.23 Glucose uptake in leg muscle during

exercise with a cycle ergometer. Note that the uptake

of blood glucose increases with the intensity of the exercise

(measured in Watts) and with the exercise time. The increased

uptake is largely due to the ability of muscle contraction to

increase the amount of GLUT4 carriers in the sarcolemma.

2

3

4

1

0

0210 03040

Time (min)

Leg glucose uptake (mmoles/min.)

65 Watts

130 Wat

ts

200 Watts High-intensity

exercise

Medium-intensity

exercise

Low-intensity

exercise