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Consuming caffeine during a long, strenuous workout may prevent fatigue by activating brain chemicals that control muscle function, according to a new study from the Unversity of Auckland. The findings appear this month in Scientific Reports.

The World Doping Agency lifted the ban on caffeine use among Olympic contenders in 2004. It's not as though the powers that be decided to let athletes get jacked up on the world's most popular and socially accepted stimulant because scientists disclaimed its performance-enhancing effects. Quite the opposite; research depicts caffeine as having noticeable benefits for exercise. Consuming it before hitting the pavement (or court, or gym) is linked to more pep in one's step during and after workouts. The perks show up when researchers evaluate fatigue using physiological measures (e.g., post-exercise oxygen consumption levels) as well as subjective reports (e.g., "How tired are you, on a scale of feeling like an immobilized pile of taffy limbs to wishing your Tough Mudder had been tougher and muddier?").

But how exactly does caffeine work its working-out magic? It may be by counteracting a phenomenon called "central fatigue," which happens when the central nervous system doesn't effectively command bodily muscles. In the current study, researchers proposed that ingesting caffeine during a three-hour cycling session would protect participants against feeling as exhausted as they otherwise would following their ride. Researchers tested their hypothesis using an unorthodox barometer of fatigue: rapid eye movements. The "why" is a little complicated. 

Exercise, researchers believe, also leaves us drained because neurotransmitters fail to keep muscular systems chugging along.

The main idea is that post-exercise fatigue isn't solely a byproduct of direct biochemical changes inside limb muscles. Exercise, researchers believe, also leaves us drained because neurotransmitters fail to keep muscular systems chugging along. Past research offers evidence of central fatigue affecting skeletal muscles (the ones we use when we run and jump and bust out a pas de bouree). It's been less clear if central fatigue also affects motor systems that aren't directly involved in limb movement, namely the oculomotor muscles that control eye movement. But the researchers-in-question think it does. Among other evidence, studies on non-human primates have linked slowed eye movements with physical fatigue. And, they think caffeine helps combat exercise-induced exhaustion by activating neurotransmitters including dopamine, the pleasure chemical, and norepinephrine, involved in arousal. 

Caffeine stimulates activity in these neurotransmitters directly as well as indirectly, through blocking adenosine (the drowsiness-causing hormone that builds up over the course of the day). Remember, neurotransmitters don't have one function apiece. They're involved in both the mental (e.g. mood regulation) and the physical (e.g., leg movement). Among other jobs, norepinephrine appears to help control the rapid eye movements (called saccadic movements) we demonstrate when we're dreaming as well as during plenty of (waking) visual tasks.

To test their hunch, researchers needed to measure fatigue using a physiological process that might change as a result of brain chemistry, but not from direct muscle strain. Cycling would only affect eye-movement speed, they reasoned, as a side-effect of lagging worn-out neurotransmitters. 

Eye movements did not, however, slow down for participants who'd gotten caffeine.

So, researchers measured eye-movement speed in trained cyclists before and after they spent three hours pedaling away. (Researchers performed the experiment twice, with and without caffeine.) Fifteen minutes into their ride, researchers gave participants carb-heavy energy supplements, some of which were caffeinated. No one (researchers or participants) knew who got the goods and who got ripped off. Before and after the bike rides, researchers measured participants' eye movements, as well as physiological functions including heart rate and grip strength. Participants also rated their levels of exertion and arousal at the same intervals. 

If caffeine combats neurotransmitter changes as researchers predicted it would, then participants' eyes would have darted speedily even after they dismounted their bikes. (To be clear, researchers are not claiming that caffeine only enhances energy levels by combatting central fatigue, but rather that the phenomenon is part of the equation.) And that's what they found: Participants who got the de-caf treatment slowed down, eyeball-wise, after their workouts, whereas eye movements among caffeinated participants proceeded at the same (or slightly faster) pace. 

"This study provides evidence for an effect of central fatigue on the oculomotor system, whereby fatigue induced by prolonged cycling exercise influences oculomotor control."

If caffeine partially thwarts exercise-induced exhaustion through manipulating neurotransmitter levels, then we're staring down a conversation about treating physical fatigue using psychiatric drugs, researchers suggest. But it's hardly the first time we're discussing the line between endorsing caffeine and other, less JV performance-boosters.