Doyle, A., & Cafarelli, E. (2003). Caffeine alters voluntary but not reflex activation of single human motor units. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 1559.

The purpose of this experiment was to compare the effects of caffeine on the recruitment threshold (RT) and firing rate (FR) of motor units in the vastus lateralis during reflex and voluntary contractions. It was hypothesized that there would be a decrease in the RT and an increase in the average FR in the caffeine condition during voluntary activation and that caffeine would have no effect during reflex activation. Caffeine, which is known to act centrally and is excitatory to the motor unit pool during voluntary contractions, was compared to a no-caffeine condition.

During reflex activation, RT and FR were significantly lower than in voluntary activation in both caffeine and no-caffeine conditions. In the caffeine condition, FR increased in the voluntary but not in the reflex condition. RT did not change in either caffeine condition.

". . . caffeine only exerts its effects during voluntary activation of muscle, and does not alter peripheral feedback from muscle spindles. Caffeine's potentiating effects on [motor unit firing rate] may be due to an increase in the central excitatory neurotransmitters serotonin and norepinephrine, which are known to increase with caffeine ingestion" (p. S281).

Implication. Caffeine only exerts its effects during voluntary activation and does not alter peripheral feedback from muscle spindles.

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