Effects of caffeine on neuromuscular function in a non‐fatigued state and during fatiguing exercise

Abstract

Caffeine enhances exercise performance, but its mechanisms of action remain unclear. In this study, we investigated its effects on neuromuscular function in a non-fatigued state and during fatiguing exercise. Eighteen men participated in this randomized, double-blind, placebo-controlled crossover trial. Baseline measures included plantarflexion force, drop jump, squat jump, voluntary activation of triceps surae muscle, soleus muscle contractile properties, M-wave, -motoneuron excitability (H-reflex), corticospinal excitability, short-interval intracortical inhibition, intracortical facilitation, silent period evoked by transcranial magnetic stimulation (SP) and plasma potassium and caffeine concentrations. Immediately after baseline testing, participants ingested caffeine (6 mg⋅kg −1 ) or placebo. After a 1-h rest, baseline measures were repeated, followed by a fatiguing stretch-shortening cycle exercise (sets of 40 bilateral rebound jumps on a sledge apparatus) until task failure. Neuromuscular testing was carried out throughout the fatigue protocol and afterwards. Caffeine enhanced drop jump height (by 4.2%) and decreased the SP (by 12.6%) in a non-fatigued state. A caffeine-related decrease in SP and short-interval intracortical inhibition before the fatiguing activity was associated with an increased time to task failure. The participants who benefitted from an improved performance on the caffeine day reported a significantly lower sense of effort during exercise and had an accelerated postexercise recovery of M-wave amplitude. Caffeine modulates inhibitory mechanisms of the CNS, recovery of M-wave amplitude and perception of effort. This study lays the groundwork for future examinations of differences in caffeine-induced neuromuscular changes between those who are deemed to benefit from caffeine ingestion and those who are not.

The Influence of Recreational Substance Use in TMS Research

“…Acute caffeine intake has no reported effect on SAI and LAI [ 158 ], or SICI [ 154 , 157 , 158 ]. One study reported a reduction in SICI following caffeine intake, although a similar finding was also observed in the placebo condition [ 155 ]. This suggests that the change in SICI was not due exclusively to the effects of caffeine.…”

“…First, acute caffeine intake has no effect on threshold [ 154 , 155 , 156 , 157 , 158 ]. Multiple studies have reported no change in resting MEPs following caffeine intake, at intensities ranging from 100–150% RMT [ 154 , 155 , 156 , 157 , 158 , 159 , 160 , 161 ]. Bowtell et al [ 162 ] also found no change in resting MEPs following caffeine intake, while MEPs obtained during maximal contraction were potentiated by caffeine.…”

“…Most studies have reported no effect of caffeine on ICF [ 154 , 155 , 157 ]. However, Concerto et al [ 158 ] found a decrease in ICF after administration of a sugar-free energy drink, which contained caffeine as the main active ingredient.…”

“…The reduction in ICF reported by Concerto et al [ 158 ] may be attributed to time of testing, as TMS measures were acquired 45 min following ingestion of the energy drink. Studies reporting no effect of caffeine on ICF performed testing at least 1 h following caffeine administration [ 154 , 155 , 157 ].…”

“…CSP is either reduced [ 155 , 156 , 157 ] or not affected by caffeine [ 154 , 158 , 162 ]. Studies reporting a decrease following caffeine intake obtained CSP at low TMS intensities of 110% RMT [ 156 , 157 ].…”