NUTRITION IN SPORT

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.