Fancy a drink containing the most widespread psychoactive drug? Coffee,tea, soft drinks and even alcoholic beverages contain caffeine. This alkaloid,which occurs naturally in coffee beans, tea leaves and other plants, is a mild stimulant of the central nervous system. In addition to being lipolytic and increasing the level of circulating free fatty acids that can be used as alternative fuel during aerobic exercise, caffeine also plays a role in the release of calcium from the storage sites around the myofibrils, which may stimulate more powerful muscle contractions. Although athletes have used it in the past as an ergogenic aid with beneficial effects on their endurance and strength performance, caffeine is now banned by the International Olympic Committee. However, the precise nature of the effect of physiological caffeine concentrations on skeletal muscle was not clear until Rob James and colleagues decided to investigate the direct impact of caffeine on murine muscle performance during recovery from fatigue and found that caffeine can relieve the effects of fatigue, but only at extremely high concentrations.
In this ex vivo study, recently published in the Journal of Applied Physiology, the team selected the extensor digitorum longus (EDL)and the musculus soleus as typical fast-twitch and slow-twitch muscles,respectively, to test the effects of caffeine on muscle recovery from fatigue. The EDL is a thin muscle that extends about three-quarters of the way down the lower leg and is used to flex the ankle and move the 2nd to 5th toes, whilst the soleus is a thick muscle located on the back of the lower leg and is used to raise the heel or to point the foot.
The team started by placing the excised muscles in an oxygenated physiological salt solution and then determined their maximum pre-fatigue power output during cyclical length changes or `work loops'. Muscle fatigue was induced by 50 and 250 work loops for the EDL and soleus muscles,respectively. At this stage, the muscles were either placed in fresh salt solution (controls) or in a solution containing either 10 mmol l–1 or 70 μmol l–1 caffeine. The performance of the tired muscles during recovery was assessed by subjecting them to regular sets of four work loops.
James and his colleagues found that milimolar concentrations of caffeine were able to reverse the depressant effects of fatigue and restore contractile function. In fact, both the EDL and soleus muscles generated significantly higher power output during shortening than either the control or 70 μmol l–1 caffeine-treated muscles. However, the work required to extend the muscles bathed with 10 mmol l–1 caffeine increased, perhaps due to a greater stiffness of the muscle. The extra work required to lengthen the muscles in the presence of 10 mmol l–1 caffeine resulted in a decreased net power output of the soleus muscle and a transient increase in net power output of the EDL muscle. By contrast, the authors observed that the net power output of both EDL and soleus muscles was not affected by 70 μmol l–1 caffeine,and their responses to fatigue were similar to those of control muscles.
Although caffeine can have direct effects at the high concentrations tested by James, it's unlikely that the micromolar concentrations of caffeine found in human athletes' blood plasma directly improved the performance of their fatigued muscles. But the competitors in this year's Olympics will still have to steer clear of that tempting Greek coffee, as caffeine has many other ways of bumping them up from silver to gold.