after 20 s of contraction. This suggests that the
rapid utilization of PCr may buffer the momen-
tary lag in energy provision from glycolysis, and
that the contribution of the latter to ATP resyn-
thesis rises as exercise duration increases and
PCr availability declines. This point exemplifies
the critical importance of PCr at the onset of con-
traction. Without this large hydrolysis of PCr, it is
likely that muscle force production would almost
instantaneously be impaired, which is indeed the
case in muscles in which the PCr store has been
replaced with a Cr analogue (Meyer et al. 1986). It
is also important to note that ultimately there is a
progressive decline in the rate of ATP resynthesis
from both substrates during this type of exercise.
For example, during the last 10 s of exercise
depicted in Fig. 6.1, the rate of ATP production
from PCr hydrolysis had declined to approxi-
mately 2% of the peak rate. Similarly, the corre-
sponding rate of ATP resynthesis from glycogen
hydrolysis had fallen to approximately 40%.
The above example concerns exercise of
maximal intensity lasting about 30 s. However,
non-steady-state exercise, albeit less intense, can
be sustained for durations approaching 5–15 min
before fatigue is evident. Under these conditions,
carbohydrate oxidation can make a major con-
tribution to ATP production and therefore its
importance should not be underestimated.
It has been demonstrated that during 3.2 min
of fatiguing exercise, oxidative phosphorylation
can contribute as much as 55% of total energy
production (Bangsbo et al. 1990). This indicates
the importance of substrate oxidation during
high-intensity exercise, a point which is often
overlooked. Under these conditions, muscle
glycogen is the principal fuel utilized as muscle
glucose uptake is inhibited by glucose 6-
phosphate accumulation and adipose tissue
lipolysis is inhibited by lactate accumulation.Submaximal exercise
The term submaximal exerciseis typically used to
define exercise intensities which can be sustained
for durations falling between 30 and 180 min.carbohydrate metabolism in exercise 89
ATP production (mmol.kg–1 muscle–1.s
)1008640–1.3
Exercise time (s)20–2.6 0–5 0–10 10–20 20–30Fig. 6.1Rates of anaerobic ATP
formation from phosphocreatine
and glycolysis during maximal
intermittent electrically evoked
isometric contraction in man (see
Hultmanet al. 1991). Note that the
reference base for the muscle data
in the figures and text is dry
muscle. This is because the muscle
samples were freeze-dried prior to
biochemical analysis. To convert to
wet weight, values should be
divided by 4.3. This assumes 1 kg
of wet muscle contains 70 ml of
extracellular water and 700 ml of
intracellular water. ,
phosphocreatine; , glycolysis.