Introduction It has been shown that caffeine promotes vigilance, attention, mood and arousal5. The potential mechanisms of caffeine’s action include its role as an adenosine receptor antagonist that reverses the tonic inhibitory influence of adenosine in the central nervous system. By antagonising the inhibitory effects of adenosine, caffeine can lower the threshold for neural activation and enhance excitatory neurotransmission3,7. There is evidence that caffeine increases endurance and short-term, high-intensity exercise performance2. However, the effect of caffeine on neural activation and maximal voluntary contraction strength during different contraction modes is less clear. Published data on caffeine’s effect on cortical and spinal excitability, assessed during isometric voluntary contractions, indicate a central modulation rather than changes in the excitability of spinal alpha-motoneurons1,6. Since the contribution of cortical neurons and spinal alpha-motoneurons to the neural drive differs depending on the type of muscle action4, caffeine could have distinct ergogenic effects on MVC strength.
Purpose Therefore, this study investigated the effects of caffeine ingestion (8 mg·kg-1) on neural activation and maximum voluntary torque (MVT) of the quadriceps muscle during isometric, concentric and eccentric muscle actions.
Methods Fourteen subjects ingested caffeine and placebo in a randomised, controlled, counterbalanced, double-blind crossover design. Neuromuscular tests were performed before and 1 h after oral caffeine and placebo intake. MVTs were measured and the interpolated twitch technique was applied during isometric, concentric and eccentric muscle actions (angular velocity 25°·s-1, 70° of knee flexion) to assess voluntary activation. Furthermore, normalised root mean square of the EMG signal (RMS·M-1) was calculated and evoked spinal reflex responses (H-reflex evoked at rest and during weak isometric voluntary contraction at 10% of MVT) as well as contractile properties of the quadriceps were analysed.
Results Caffeine increased MVT by 13.7% (26.4 N·m, 95% CI: 9.3-43.5 N·m, P = 0.004), 13.9% (22.5 N·m, 95% CI: 3.1-42.0 N·m, P = 0.025) and 11.7% (22.5 N·m, 95% CI: 2.2-42.7 N·m, P = 0.032) for isometric, concentric and eccentric muscle actions. Strength enhancements were due to increases in voluntary activation. Isometric explosive voluntary strength and neural activation at the onset of contraction were significantly increased following caffeine ingestion. Changes in spinal reflex responses and contractile properties were not observed.
Conclusion Data suggest that caffeine ingestion induced acute adaptations that were responsible for the increased neural activation and strength regardless of the contraction mode. These results and the unchanged peak twitch torque of the quadriceps muscle indicate that the acute adjustments were mainly of neural origin. Unfortunately, we were not able to measure changes in the excitability of cortical and/or spinal neurons by using the interpolated twitch technique. However, our data on the H-reflexes evoked at rest and during weak isometric voluntary contractions suggest that the acute modulations occurred at the supraspinal level.
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