70 μM caffeine treatment enhances in vitro force and power output during cyclic activities in mouse extensor digitorum longus muscle

Caffeine ingestion by human athletes has been found to improve endurance performance primarily acting via the central nervous system as an adenosine receptor antagonist. However, a few studies have implied that the resultant micromolar levels of caffeine in blood plasma (70 mu M maximum for humans) may directly affect skeletal muscle causing enhanced force production. In the present study, the effects of 70 mu M caffeine on force and power output in isolated mouse extensor digitorum longus muscle were investigated in vitro at 35 degrees C. Muscle preparations were subjected to cyclical sinusoidal length changes with electrical stimulation conditions optimised to produce maximal work. 70 mu M caffeine caused a small but significant increase (2-3%) in peak force and net work produced during work loops (where net work represents the work input required to lengthen the muscle subtracted from the work produced during shortening). However, these micromolar caffeine levels did not affect the overall pattern of fatigue or the pattern of recovery from fatigue. Our results suggest that the plasma concentrations found when caffeine is used to enhance athletic performance in human athletes might directly enhance force and power during brief but not prolonged activities. These findings potentially confirm previous in vivo studies, using humans, which implied caffeine ingestion may cause acute improvements in muscle force and power output but would not enhance endurance.