Introduction
Answers to questions in exercise physiology and
sports nutrition, including the most fundamental
ones such as the causes of fatigue, can only be
obtained by an understanding of cellular, subcel-
lular and molecular mechanisms to explain how
the body responds to acute and chronic exercise.
Biochemistry usually refers to the study of events
at the subcellular and molecular level, and this is
where the emphasis is placed in this chapter. In
particular, this brief review describes the sources
of energy available for muscle force generation
and explains how acute exercise modifies energy
metabolism. For further details, see Maughan
et al. (1997) and Hargreaves (1995). Training also
modifies the metabolic response to exercise and
training-induced adaptations encompass both
biochemical responses (e.g. changes in enzyme
activities in trained muscles) and physiological
responses (e.g. changes in maximal cardiac
output and maximal oxygen uptake, V
.
o2max.)
(Saltin 1985).
Skeletal muscle
Individual muscles are made up of many parallel
muscle fibres that may (or may not) extend the
entire length of the muscle. The interior of the
muscle fibre is filled with sarcoplasm (muscle cell
cytoplasm), a red viscous fluid containing nuclei,
mitochondria, myoglobin and about 500 thread-
like myofibrils, 1–3 mm thick, continuous from
end to end in the muscle fibre. The red colour
is due to the presence of myoglobin, an intra-
cellular respiratory pigment. Surrounding the
myofibrils is an elaborate form of smooth endo-
plasmic reticulum called the sarcoplasmic reticu-
lum. Its interconnecting membranous tubules lie
in the narrow spaces between the myofibrils, sur-
rounding and running parallel to them. Fat (as
triacylglycerol droplets), glycogen, phosphocre-
atine (PCr) and adenosine triphosphate (ATP)
are found in the sarcoplasm as energy stores. The
myofibrils are composed of overlapping thin and
thick filaments and it is the arrangement of these
filaments that gives skeletal muscle its striated
appearance. The thin filaments are comprised of
the protein actin; located on the actin are two
other types of protein, tropomyosin and tro-
ponin. The thick filaments contain the protein
myosin.
When calcium and ATP are present in suffi-
cient quantities, the filaments interact to form
actomyosin and shorten by sliding over each
other. Sliding of the filaments begins when the
myosin heads form cross bridges attached to
active sites on the actin subunits of the thin fila-
ments. Each cross bridge attaches and detaches
several times during a contraction, in a ratchet-
like action, pulling the thin filaments towards the
centre of the sarcomere. When a muscle fibre
contracts, its sarcomeres shorten. As this event
occurs in sarcomeres throughout the cell, the
whole muscle fibre shortens in length.
The attachment of the myosin cross bridges
requires the presence of calcium ions. In the
relaxed state, calcium is sequestered in the sar-