Creatine and Creatinine Metabolism - Physiological Reviews

1180 MARKUS WYSS AND RIMA KADDURAH-DAOUK Volume 80
supplementation at a rate of 0.25 g � (kg body mass) �1
over 5 days significantly improved the 1,000-m performance
of competitive rowers by an average of 2.3 s (823).
Regression analysis revealed a positive relationship of
borderline statistical significance between performance
and Cr uptake.
It should not be ignored that some conflicting reports
suggested no improvement in muscle performance with
Cr supplementation (for a review, see Ref. 1106; see also
Refs. 62, 141, 426, 918, 1035). Several explanations can be
put forward for the discrepant findings which, however,
will have to be tested: e.g., too short or too long (possibly
causing downregulation of the Cr transporter) a period of
Cr supplementation, insufficient daily Cr dosage, or a high
proportion of nonresponders among the subjects analyzed.
In the study by Cooke et al. (142), a potentially
favorable effect of oral Cr supplementation on power
output during bicycle ergometry may have been masked
by the high standard errors of the data. The lack of a
favorable effect of Cr supplementation on supramaximal
running on a motor-driven treadmill for 3–6 min until
exhaustion, in a6kmterrain run (45), in 700-m maximal
running bouts (993) and in a series of 1-min supramaximal
cycling bouts (232) may be due to the longer duration of
exercise which inevitably results in a decrease in the
relative contribution of PCr hydrolysis to total work output.
Notably, Cr supplementation even resulted in a significant
increase in the run time for the 6-km terrain run
(45). The absence of a favorable effect of Cr supplementation
on repeated 10-s cycle ergometer sprint performance
(51) or on 60-m sprint performance (798) lacks an
explanation and is in direct contrast to the studies that
have shown an ergogenic effect of Cr supplementation in
high-intensity, intermittent exercise (see above).
Inconclusive data have been published on the effect
of Cr supplementation on swimming performance. An
improvement in performance parameters was seen in
some studies (312, 765), but not in others (103, 673, 999).
In the study of Thompson et al. (999) on trained swimmers,
the daily ingestion of only 2gCrfor14days may
have been insufficient to increase the muscle stores of
total Cr. On the other hand, the tendency toward worse
25- and 50-m performance times of competitive swimmers
after Cr supplementation (20 g/day over 5 days) was
suggested to be due to an increase in hydrodynamic resistance
which is associated with the body weight gain
caused by Cr ingestion (103, 441, 673). In elite competitive
male swimmers, Cr supplementation (9 g/day for 5 days)
had no effect on performance in a single 50-yard sprint
but significantly improved performance in a repeated
sprint set of 8 � 50 yards at intervals of 90 s (765). A study
on junior competitive swimmers performing three 100-m
freestyle sprint swims with 60-s recovery between heats
indicated that Cr supplementation (21 g/day for 9 days)
may improve performance already in heat 1 relative to the
placebo group and decreases swim time in heat 2 (312). In
conclusion, the results published so far suggest that Cr
supplementation has either no or even an adverse effect
on single-sprint swim performance but may have a slight,
favorable effect on repetitive swimming performance.
Cr supplementation (20 g/day for 5 days) caused an
acceleration of PCr recovery after intense electrically
evoked isometric contraction of the vastus lateralis muscle
in humans (Fig. 20) (306, 307; see also Refs. 47, 913).
The most pronounced effects were observed in those
subjects in whom Cr supplementation caused the largest
increase in muscle Cr concentration. Both Bogdanis et al.
(75) and Trump et al. (1017) demonstrated that PCr resynthesis
during recovery from intense muscular activity
is critical to the restoration of muscle power at the onset
of a next bout of maximal exercise. In a test protocol
consisting of two 30-s cycle ergometer sprints interspersed
with 4 min of recovery, a high correlation was
seen between the percentage of PCr resynthesis and the
percentage recovery of power output and pedalling speed
during the initial 10 s of sprint 2 (75). Consequently, an
increased rate of PCr recovery after Cr supplementation
FIG. 20. PCr resynthesis (E, F) and accompanying decrease in free
Cr concentration (�, ■) in human muscle biopsy samples obtained after
0, 20, 60, and 120 s of recovery from intense, electrically evoked isometric
contraction before (E, �) and after (F, ■) 5 days of Cr ingestion.
Only subjects with an increase in total Cr concentration in vastus
lateralis muscle of �19 mmol � (kg dry mass) �1 after Cr ingestion demonstrated
an accelerated rate of PCr resynthesis. *P � 0.05. **P � 0.01.
[From Greenhaff et al. (307).]