Supplementing with caffeine can have a few key roles in affecting exercise. The three main roles it can play are that it can increase time to fatigue during submaximal isometric contractions, it can make alterations in fat metabolism, and it can cause increased excitement of the neuromuscular system as a consequence of adenosine receptor antagonism. For the purpose of this review, we will look at the role of caffeine on the neuromuscular system as it relates to exercise.
At dosages safe for human consumption, caffeine can act as an adenosine and benzodiazepine inhibitor. These two ligands are known to slow brain activity. Adenosine is classified in the human body as a peripheral and central nervous system inhibitor. Low levels of adenosine have been shown to decrease the firing and spontaneity of cortical neurons. These low levels of adenosine have also been shown to inhibit the release of other neurotransmitters, such as serotonin, dopamine, and norepinephrine. This being said, caffeine supplementation can reverse the effects of adenosine. This could explain the ergogenic effects that caffeine plays on the human body with ingestion. The study conducted contained 10 men who consumed less than 200 mg/wk of caffeine. Only subjects who presented low caffeine diets were selected, as high caffeine diets can upregulate adenosine receptors. Motor unit firing rates were looked at in the quadriceps during intermittent isometric contractions. The hypothesis was that the ergogenic effects of caffeine occur because of these firing rates and are either maintained or increased during a task that is proposed to fatigue the body. The studies showed that the firing rates did not decline during the contractions and that the increase in fatigue time could not be explained by the changes in firing rates. To conclude this study, caffeine extends the time to fatigue in repeated submaximal contractions with no change in central drive or neuromuscular transmission failure (Carafelli & Meyers, 2005).
Another study conducted by McLellan et al. looked at the role of caffeine in special force military units who routinely perform in sleep-deprived situations. It is in these situations where mental alertness and cognitive reasoning are crucial. The special force units were tested in areas where wakefulness was sustained for long periods of time as well as endurance tests. The soldiers were given anywhere from 600-800mg of caffeine, in the form of chewing gum, as it is thought to dissolve into the blood stream a lot quicker this way. In all areas, the caffeine was able to enhance the performance of the soldiers as well as the mental alertness (Goldstein, 2010).
Lieberman et al. studied the effects of caffeine supplementation in U.S. Navy Seals who deal with sleep deprivation as well. Random dosages (100mg, 200mg, and 300mg) were supplied to the participants at random. Some participants were also given a placebo. The results concluded much of the same information, with that caffeine had the most effect on the mental alertness and cognitive reasoning of the participants. The dosages that proved to show the most effect were the 200mg and 300mg dosages, with the 100mg dose not showing much of an effect (Goldstein, 2010).
The studies suggest that the performance enhancing effects of this caffeine supplementation on motor learning and memory may be correlated to an individual being able to be more mentally alert and sustain concentration. The effects of the caffeine can be attributed to the actions of the central nervous system in that the caffeine has the ability to change inhibitory actions, especially those of adenosine.
According to multiple studies, different levels of caffeine contribute different effects on an individual (Ruxton, 2008). Caffeine was shown to have the best effects with minimal detriment between 38 and 400mg/day. This is the low to medium range for caffeine consumption. Closer to 400mg was where mild sleep disturbances was seen in the studies. (Ruxton, 2008).
Obviously, if an individual has a pace maker or is on medications for heart disease or high blood pressure, then caffeine consumption must be monitored very closely if not cut out completely. If an individual with these problems performs physical activity, his/her blood pressure will rise naturally. When caffeine is thrown into the mix, it will raise blood pressure as well. This can cause the blood pressure to rise too much and the person can eventually have a heart attack or collapse. 300mg/day might be too high of a dose for these individuals to be consuming. Hypertension risk should take priority in these individuals if the client is consuming caffeine.
David Rynecki B.S., M.S., CPT, FNS, CES, CSS, Pn1 Sports, Fitness, and Rehab Specialist
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• Carafelli, E., & Meyers, B. M. (2005). Caffeine increases time to fatigue by maintaining force and not by altering ﬁring rates during submaximal isometric contractions. (3 ed., Vol. 99, pp. 1056-10633). Toronto: Journal of Applied Physiology. Retrieved from http://jap.physiology.org/content/99/3/1056.full.pdf html • Goldstein, E. (2010). International society of sports nutrition position stand: Caffeine and performance. Journal of the International Society of Sports Nutrition, 7(5), doi: 10.1186/1550-2783-7-5 • Ruxton, C. H. S. (2008), The impact of caffeine on mood, cognitive function, performance and hydration: a review of benefits and risks. Nutrition Bulletin, 33: 15–25. doi: 10.1111/j.1467-3010.2007.00665.x