tion of 20–30 mmol · kg–1of wet weight, the rate
of energy production is reduced and perfor-
mance decreased (Costill 1988).
Carbohydrate loading and repeated sprints
Sprint training involves many sprints during
daily training sessions. The metabolic and physi-
ological responses to repeated sprints performed
in the laboratory (Gaitanos et al. 1993; Trump et
al.1996) provide some information on the glyco-
gen demands of a sprint training session, or in
sports such as soccer or rugby (Nicholas et al.
1994), where maximal sprints are performed
briefly between periods of less intense exercise
over an 80–90-min period.
Several studies have examined maximal
dynamic muscle power output and the associ-
ated metabolic changes in muscle during three to
four bouts of maximal cycling at 100 r.p.m.
(10.5 rad · s-^1 ), separated by 4-min recovery inter-
vals (McCartney et al. 1986; Spriet et al. 1989;
Trump et al. 1996). In these studies, power output
and work done decreased by 20% in both the
second and third exercise periods, but there was
no further decrement in performance in the
542 sport-specific nutrition
fourth bout (McCartney et al. 1986). Changes in
muscle glycogen, lactate and glycolytic interme-
diates suggested that the rate of glycogenolysis
was limited at the PFK level during the first and
second exercise periods, and at the phosphory-
lase level in the third and fourth exercise periods
(McCartneyet al. 1986). In agreement with these
findings, Spriet et al. (1989) reported that muscle
[H+] was higher and the glycolytic flux lower
after the third exercise bout than after the second,
even though ATP and PCr degradation was
similar in the two exercise bouts. As a conse-
quence of the reduction in the rate of glycolysis
during the third and fourth sprints, there is a
greater reliance on aerobic metabolism (Fig.
41.5), and possibly the intramuscular triglyceride
stores (McCartney et al. 1986).
The laboratory studies on repeated sprints of
30 s duration are relevant to training sessions
where sets of 200 or 300 m are performed.
However, muscle metabolism during repeated
shorter sprints (<6 s) has a wider applicability,
not only for sprint training, but also to the multi-
ple sprint sports. Gaitanos et al. (1993) examined
muscle metabolism during intermittent maximal
exercise. The exercise protocol consisted of 10 ¥
100
0
75
50
25
Glycogen concentration (mmol
.kg
–1
wet wt)
Sampling time
1800
1600
1400
1000
Peak power (W)
PPO
1200
800
Rest
18.2 14.8 –2 3
Sprint 1 Sprint 2 Sprint 3 Sprint 4
Fig. 41.5Post-sprint glycogen
concentration and power output
during four bouts of 30-s
isokinetic cycling (McCartney et
al. 1986). The amount of glycogen
utilized during each bout is also
shown (in mmol · kg-^1 wet wt).
PPO, peak power output.