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Department of Kinesiology and Health Science, Faculty of Pure and Applied Science, York University, Toronto, Ontario, Canada M3J 1P3
This double-blind, repeated-measures study 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 × 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 of 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-EMG 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 (Tlim) increased by 25.80 ± 16.06% after caffeine administration (P < 0.05). There was no significant change in Tlim from pretest to posttest in the control or placebo trials. The increase in Tlim was associated with an attenuated decline in twitch amplitude, which would suggest that the mechanism is, at least in part, peripheral.
activation; contractile properties; ergogenic; fatigue; H reflex; motor unit excitability; motor unit firing rates
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