Caffeine and Exercise Performance - Caffeine: Chemistry, Analysis, Function and Effects - page 315

soft drinks, and energy drinks. It is also widely-used by athletes (Forman et al
1995), as up to 27% of athletes ingest caffeine to enhance performance.
Due to the widespread availability and acceptability of caffeine intake
worldwide, it is important to thoroughly examine how caffeine enhances
performance in various athletic activities. Its multiple actions on the body,
time course, dosing regimens, and effects on exercise performance will be
discussed.

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17.2 Mechanisms Explaining Ergogenic Effect of
Caffeine
Early studies (Costill et al 1978; Ivy et al 1979) showed that caffeine increased
lipolysis and spared glycogen during exercise. This finding has been
corroborated (Wiles et al 1992; Engels et al 1999), yet Jackman et al (1996)
and Graham et al (2000) revealed that caffeine does not spare glycogen during
exercise. Alternatively, increased intracellular calcium concentration (Doherty
et al 2004) and-or altered excitation-contraction coupling (Clausen 2003) have
been postulated. However, a study by Rosser et al (2009) concluded that when
ingested in physiological doses, caffeine's effects lie outside the muscle fiber.
Davis et al (2003) reported that caffeine delays fatigue by acting as an
adenosine antagonist. Adenosine is a normal component of the cell whose
concentration increases with muscular contraction. Via binding to its
receptors, it inhibits neuron excitability and synaptic transmission, thus
decreasing arousal and increasing fatigue (Porkka-Heiskanen 1999). In male
rats, run performance was 60% greater with caffeine versus an adenosine
agonist (Davis et al 2003).
Caffeine ingestion also modifies perceptual responses that may alter
performance. In a meta-analysis of 21 studies containing 202 subjects,
Doherty and Smith (2005) revealed that caffeine decreased rating of perceived
exertion (RPE) by 5.6% during prolonged exercise, which explained 33% of
improved performance. They speculated that caffeine augments performance
by masking the perception of fatigue and recruiting additional motor units
during exercise. A pain-reducing effect of caffeine has also been documented.
In young men (Motl et al 2003) and women (Gliottoni and Motl 2008)
completing submaximal cycling, 5-10 mg kg 21 caffeine ingested 1 h pre-
exercise significantly reduces leg pain versus placebo. Experimentally, pain
influences motor unit recruitment, so enhanced motor unit activation was
identified as a potential mechanism explaining the attenuated pain sensation
with caffeine. These data oppose findings from Astorino et al (2010) showing
improved force production, yet no change in pain perception or RPE, when
caffeine was ingested prior to maximal knee extension-flexion exercise.
However, high doses may not be practical as a pre-exercise intervention, and
these experimental protocols do not simulate the demands of sport, so further
study is merited.