aerobic metabolism. For example, a marathon
runner can utilize over 12 000 repeated muscle
actions of each leg in completing the 42.2-km
course.
The characteristics and capabilities of the
muscle fibres can be substantially modified by
specific training programmes. Athletes engaging
in sports which involve wide ranges of power
and continuously varying amounts of aerobic
and anaerobic metabolism must utilize a pro-
gramme of conditioning that raises both the
anaerobic and aerobic capabilities of the three
fibre types. Examples of such sports are soccer,
basketball and tennis.
Physiological support systems
While muscle cells may obtain energy for force
and power production from both anaerobic
sources (the breakdown of ATP and PCr; anaero-
bic glycolysis) and aerobic sources (aerobic gly-
colysis and b-oxidation of fatty acids, both
leading to the provision of electrons to the elec-
tron transport system in the mitochondria), the
entire human organism and all of its component
cells are fundamentally aerobic. Exercise per-
formed at low enough intensities can be per-
formed entirely with energy from aerobic
metabolism. The provision of significant
amounts of energy for muscular activity by the
anaerobic mechanisms, however, is limited in
amount and therefore in time. Most importantly,
the return to the pre-exercise or resting state
following any amount of anaerobic energy
release is accomplished exclusively by aerobic
metabolism.
Therefore, the essential features in the provi-
sion of oxygen for metabolism during aerobic
exercise and recovery following anaerobic exer-
cise become pulmonary ventilation (air move-
ment into and out of the lungs), external
respiration (exchange of O 2 and CO 2 between
alveoli and pulmonary capillary blood), blood
circulation and internal respiration (exchange of
O 2 and CO 2 between systemic capillary blood
and interstitial fluid). The essential elements as
regards these processes are cardiac output, blood
10 nutrition and exercise
volume, blood composition and skeletal muscle
capillarization.Pulmonary ventilation and external respiration
Movement of air into and out of the lungs is
accomplished by the diaphragm and various
muscles of the neck and trunk. Pulmonary venti-
lation is usually accomplished as a subconscious
activity under the influence of chemical stimuli
provided by the systemic arterial blood to a
nervous centre in the brain stem. While this
centre serves the sole function of controlling the
minute volume of pulmonary ventilation (by
interaction of frequency of ventilation and mag-
nitude of tidal volume), it is interesting to note
that it is identified anatomically and physiologi-
cally as the ‘respiratory centre.’
For continuous aerobic activity that would
involve attainment of a ‘steady state’ of oxygen
uptake (and carbon dioxide elimination) via
the lungs, pulmonary ventilation corresponds
directly to oxygen uptake by an approximate
20 : 1 ratio (litres per minute are used in the
presentation of both variables). Starting at rest,
an 80-kg athlete would expect the values pre-
sented in Table 1.2 for oxygen uptake and pul-
monary ventilation.
The increase in the ratio for the highest level of
activity reflects the increased acidity of the blood
due to the production in the muscle and appear-Table 1.2Representative data for steady-state oxygen
uptake and ventilatory minute volume at rest and
during various intensities of constant-intensity
exercise (for an 80-kg athlete). The maximum aerobic
power is 4.5 l · min-^1.V.
o 2 V.
E
(l · min-^1 ) (l · min-^1 )Rest 0.25 5
- 1.00 20
- 2.00 40
- 3.00 60
Intense aerobic exercise 3.50 70
Intense aerobic exercise with 4.00 100
anaerobic contribution
- 3.00 60