F-12P Free Communication/Poster Motor Control/Neuroscience
Numerous observations in the literature suggest that caffeine and TMS could be used together to gain insight into the contributions of the central nervous system to neuromuscular fatigue.
The current study examined the effects of caffeine on central excitability during a fatigue protocol using the quadriceps femoris muscle group.
Subjects attended 2 laboratory sessions. Each session began with baseline data collection, including: maximal M wave and twitches, maximal voluntary contraction (MVC) verified with twitch interpolation, maximal surface EMG, and MEPs evoked during a 2.5% MVC. Subjects were then given a capsule containing caffeine (6 mg/kg) or placebo (randomized, double-blind, repeated measures). After a 1-h rest, baseline measures were repeated and a fatigue protocol commenced. The fatigue protocol consisted of sets of 10 isometric knee extension contractions. The 1st and 10th contraction of each set were maximal whereas contractions 2–9 were 50% MVC. Supramaximal shocks were applied to the femoral nerve on and immediately each maximal contraction. TMS was applied during contractions 2, 4, 6, and 8 at an output 10% above active motor threshold. Between each set 4 TMS stimuli were applied during a 2.5% MVC. This protocol continued until MVC fell by 40%. M wave, twitch, MVC and MEPs were recorded at 0, 2, 5, 10 and 15 min of recovery.
Caffeine increased the amplitude of MEPs elicited during the 50% contractions throughout the fatigue protocol (p < 0.01). However, the amplitude of MEPs evoked during the 2.5% MVC between sets was not affected by caffeine. The root mean square of the surface EMG during the 50% contractions was not affected by caffeine.
These data suggest that the caffeine enhances central excitability during voluntary effort, but that this effect is dependent on the extent of voluntary activation. Supported by NSERC (E.C.) and a Reebok Research Grant on Human Performance and Injury Prevention from the American College of Sports Medicine Foundation (J.K.)