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School of Human Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada
Received 11 August 1995; accepted in final form 5 June 1996.
Jackman, M., P. Wendling, D. Friars, and T. E. Graham.
Metabolic, catecholamine, and endurance responses to caffeine during intense exercise. J. Appl.
Physiol. 81(4): 1658-1663, 1996.
This study
examined the possible effects of caffeine ingestion on muscle metabolism and endurance during brief intense exercise. We tested 14 subjects after they ingested placebo or caffeine (6 mg/kg) with an
exercise protocol in which they cycled for 2 min, rested 6 min, cycled
2 min, rested 6 min, and then cycled to voluntary exhaustion. In each
exercise the intensity required the subject's maximal
O2 consumption. Eight subjects had
muscle and venous blood samples taken before and after each exercise
period. The caffeine ingestion resulted in a significant increase in
endurance (4.12 ± 0.36 and 4.93 ± 0.60 min for placebo and
caffeine, respectively) and resulted in a significant increase in
plasma epinephrine concentration throughout the protocol but not in
norepinephrine concentration. During the first two exercise bouts, the
power and work output were not different; blood lactate concentrations
were not affected significantly by caffeine ingestion, but during the
exercise bouts muscle lactate concentration was significantly increased
by caffeine. The net decrease in muscle glycogen was not different
between treatments at any point in the protocol, and even at the time of fatigue there was at least 50% of the original glycogen
concentration remaining. The data demonstrated that caffeine ingestion
can be an effective ergogenic aid for exercise that is as brief as
4-6 min. However, the mechanism is not associated with muscle
glycogen sparing. It is possible that caffeine is exerting actions
directly on the active muscle and/or the neural processes that
are involved in the activity.
methylxanthines; epinephrine; norepinephrine; glycogen; ergogenic aids; fatigue
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