Effects of caffeine on neuromuscular function

This double-blind, repeated-measures sturdy examined the effects of caffeine on neuromuscular function. Eleven male volunteers [22.3 +/- 2.4 (SD) yr] came to the laboratory for control, placebo, and caffeine (6 mg/kg dose) trials. Each trial consisted of 10 x 1-ms stimulation of the tibial nerve to elicit maximal H reflexes of the soleus, four attempts at a maximal voluntary contraction (MVC) of the right knee extensors, six brief submaximal contractions, and a 50% MVC held to fatigue. Isometric force and surface electromyographic signals were recorded continuously. The degree off maximal voluntary activation was assessed with the twitch-interpolation technique. Single-unit recordings were made with tungsten microelectrodes during the submaximal contractions. Voluntary activation at MVC increased by 3.50 +/- 1.01 (SE) % (P < 0.01), but there was no change in H-reflex amplitude, suggesting that caffeine increases maximal voluntary activation at a supraspinal level. Neither the force-EMC; relationship nor motor unit firing rates were altered by caffeine. Subjects were able to hold a 50% MVC for an average of 66.1 s in the absence of caffeine. Time to fatigue (T-lim) increased by 25.80 +/- 16.06% after caffeine administration (P < 0.05). There was no significant change in T-lim from pretest to posttest in the control or placebo trials. The increase in T-lim, was associated with an attenuated decline in twitch amplitude, which would suggest that the mechanism is, at least in part, peripheral.