Kalmar, J. M., & Cafarelli, E. (2003). Caffeine increases central excitability during a submaximal fatigue protocol. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 1560.

This study examined the effects of caffeine on central excitability during a fatigue protocol using the quadriceps femoris muscle group. Ss attended two test sessions and were given a capsule containing caffeine (6 mg/kg) or placebo. After 1-hour rest, baseline measures were repeated and a fatigue protocol (sets of 10 isometric knee extensions) commenced. The first and tenth contractions were maximal while contractions 2-9 were at 50% maximal voluntary contraction. Additional supramaximal shocks were applied to the femoral nerve until maximal voluntary contraction fell by 40%. Measurements were taken at 0, 2, 5, 10, and 15 minutes into recovery.

Caffeine significantly increased the amplitude of maximal excitatory potentials elicited during the 50% contractions throughout the fatigue protocol. However, the amplitude of maximal excitatory potentials at lower exertion levels was not affected by caffeine. The effects of caffeine are much greater, the higher the level of voluntary effort.

Implication. The effects of caffeine are most when voluntary effort is highest.

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