NUTRITION IN SPORT

cortisol secretion is stress-induced release of
adrenocorticotrophic hormone from the anterior
pituitary gland.

Metabolic responses to exercise

Undoubtedly the most important factor influenc-
ing the metabolic response to exercise is the exer-
cise intensity. The physical fitness of the subject
also modifies the metabolic response to exercise
and other factors, including exercise duration,
substrate availability, nutritional status, diet,
feeding during exercise, mode of exercise, prior
exercise, drugs and environmental factors, such
as temperature and altitude, are also important.
Several of these factors are dealt with in subse-
quent chapters and here a brief discussion is
limited to consideration of the effects of exercise
intensity, duration and training on the metabolic
responses to exercise and the possible metabolic
causes of fatigue.

High-intensity exercise

ATP is the only fuel that can be used directly for

skeletal muscle force generation. There is suffi-

cient ATP available to fuel about 2 s of maximal

intensity exercise and therefore for muscle force

generation to continue it must be resynthesized

very rapidly from ADP. During high-intensity

exercise, the relatively low rate of ATP resynthe-

sis from oxidative phosphorylation results in the

rapid activation of anaerobic energy production

from PCr and glycogen hydrolysis. PCr break-

down is initiated at the immediate onset of con-

traction to buffer the rapid accumulation of ADP

resulting from ATP hydrolysis. However, the rate

of PCr hydrolysis begins to decline after only a

few seconds of maximal force generation (Fig.

2.7).

If high-intensity exercise is to continue beyond

only a few seconds, there must be marked

increases in the contribution from glycolysis to

biochemistry of exercise 33

ATP production (mmol.kg

–1 dm

.–1s

)

10

0

8

6

4

0–1.3

Exercise time (s)

2

0–2.6 0–5 0–10 10–20 20–30

Fig. 2.7Rates of anaerobic ATP
resynthesis from phosphocreatine
(PCr) hydrolysis ( ) and
glycolysis ( ) during maximal
isometric contraction in human
skeletal muscle. Rates were
calculated from metabolite
changes measured in biopsy
samples of muscle obtained
during intermittent electrically
evoked contractions over a period
of 30 s. Note that the rate of ATP
resynthesis from PCr hydrolysis is
highest in the first few seconds of
exercise, but falls to almost zero
after 20 s. The rate of ATP
resynthesis from glycolysis peaks
after about 5 s, is maintained for a
further 15 s but falls during the
last 10 s of the exercise bout. From
Maughanet al. (1997).