Introduction
During exercise, the energy demands of muscle
contraction will fluctuate enormously. For
muscle contraction to occur, chemical energy
stored in the form of adenosine triphosphate
(ATP) must be converted into mechanical energy
at rates appropriate to the needs of the muscle.
However, the muscle store of ATP is relatively
small and therefore for exercise to continue
beyond a few seconds ATP must be resynthe-
sized from phosphocreatine, carbohydrate and
fat. It is generally accepted that carbohydrate is
the major substrate for ATP resynthesis during
intense exercise. The carbohydrate stores of the
body are principally located in skeletal muscle
and liver, with small amounts also being found
in the form of circulating glucose. The amount
of energy stored as glycogen amounts to ap-
proximately 6000 kJ (1430 kcal) and 1500 kJ
(360 kcal) in muscle and liver, respectively, which
is very small compared with the body store of tri-
acylglycerol (340 MJ, 81 200 kcal), the alternative
fuel for ATP resynthesis. Triacylglycerol is the
preferred substrate for energy production in
resting muscle and can cover the energy
demands of exercise up to 50% of maximal
oxygen consumption. At higher exercise inten-
sities, the relative contribution of fat to total
energy production falls and carbohydrate oxida-
tion increases, such that carbohydrate is the
sole fuel oxidized at the highest exercise intensi-
ties. This is due to an increasing recruitment of
the glycolytic type II muscle fibres and an activa-
tion of glycolytic enzymes when ATP turnover
rate is increased. The maximal rate of ATP pro-
duction from lipid is lower than that of carbohy-
drate. In addition, the ATP yield per mole of
oxygen utilized is lower for lipid compared with
carbohydrate. In contrast with lipid, carbohy-
drate can be metabolized anaerobically via gly-
colysis. The lactate accumulation that occurs
almost instantaneously at the onset of contrac-
tion demonstrates that the activation of this
pathway is extremely rapid. It should be noted
that the anaerobic utilization of carbohydrate
will be indispensable during the transition from
rest to steady-state exercise and during maximal
exercise. Furthermore, the relatively small store
of body carbohydrate will limit exercise perfor-
mance during prolonged intense exercise due to
the depletion of muscle and liver glycogen
stores.
The body store and maximal rates of ATP
resynthesis from phosphocreatine, carbohydrate
and lipid are shown in Table 6.1.Regulation of muscle carbohydrate
utilization during exercise
Glycogenolysis is the hydrolysis of muscle
glycogen to glucose-1-phosphate, which is
transformed to glucose-6-phosphate via a
phosphoglucomutase reaction. The glucose-6-
phosphate formed, together with that derived
from the phosphorylation of blood glucose by
hexokinase at the muscle cell membrane, enters
the glycolytic pathway which is a series of